AUTHORS

Stacy C Atchley Baylor University Depart-ment of Geology One Bear Place 97354 WacoTexas 76798-7354 stacy_atchleybayloredu

Stacy Atchley received his BS and MS degreesfrom Baylor University in 1984 and 1986 and PhDfrom the University of Nebraska Lincoln in 1990After working for ExxonMobil from 1986 to 1995he rejoined Baylor where he directs the appliedpetroleum studies program and is a researcherwithin the paleoclimate research group

Nathaniel H Ball Baylor University Departmentof Geology One Bear Place 97354 Waco Texas 76798-7354 present address Nexen Petroleum USA Inc 5601Granite Parkway Suite 1400 Plano Texas 75024-6654

EampP NOTE

Reservoir characterization andfacies prediction within the LateCretaceous Doe Creek MemberValhalla field west-centralAlberta CanadaStacy C Atchley Nathaniel H Ball and Luke E Hunt

Nathaniel Ball received both his BS (2006) and MS(2009) degrees in geology from Baylor UniversityHe recently began his career as a geologist withNexen Petroleum USA Inc and is currently workingin the Gulf of Mexico

Luke E Hunt Baylor University Departmentof Geology One Bear Place 97354 Waco Texas76798-7354 present address Husky Energy 7078th Ave SW Box 6525 Station D CalgaryAlberta Canada T20 3G7

Luke Hunt received his honors BS degree in geo-sciences from McMaster University in 2007 andhis MS degree from Baylor University in 2009He currently works for Husky Energy as a memberof the geological services exploration team

ACKNOWLEDGEMENTS

Results documented in this manuscript were com-pleted in association with the Applied PetroleumStudies program of Baylor University Financialsupport was provided by Husky Energy as admin-istered by Jim Beckie Craig Lamb and Dave StuartSpecial thanks are extended to Ben GrossberndtLaurie Wilcox and the other helpful staff members(particularly the core handlers) at the Alberta EnergyResources Conservation Board Core Research CenterCalgary Computational and analytical assistancewas provided by Kris Friedel Todd Gustafsson DorianHolgate John McCrossan Anne Young and JoannaZhou of Husky Energy Subsurface correlationmapping and data analysis were facilitated bysoftware donations to Baylor University by HalliburtonGeoGraphixreg IHS Energy AccuMapreg and ACD Sys-

ABSTRACT

Oil resources atValhalla field of west-central Alberta Canadaare stratigraphically trapped within the Upper CretaceousDoeCreekMember of the Kaskapau Formation The reservoiris subdivided into four thin (1ndash10 m [3ndash33 ft]) cyclic alter-nations of offshore mudrock and shoreface sandstone thatare designated the I minus 1 I I + 1 and I + 2 units The thickestandmost widespread I sandstone is the primary reservoir Op-timum reservoir quality corresponds to coarser grain shorefacesandstone however reservoir quality may be diminished bypostdepositional calcite cement commonly observed near thetop of shoreface sandstones Open-hole well logs are used topredict depositional facies and calcite cement occurrence inwells that lack core control Decreasing shale volume (Vsh) andincreasing deep resistivity values correspond to progressivelyshallower water deposits Zones of calcite-cemented shorefacesandstone greater than 05 m (16 ft) thick are interpreted whenthe neutron porosity exceeds the density porosity by morethan 7 Facies distributions predicted for the I sandstone close-ly match trends of the sandstone gross pore volume and dailytotal fluid production and suggest that open-hole well logsmay be used to anticipate reservoir quality and continuity

Regional and local observations support previous interpre-tations that attribute the Doe Creek to forebulge erosion andsouthwestward sediment transport toward a foredeep where

tems Canvastrade The manuscript benefited from re-views provided by Gretchen M Gillis Dale A LeckieWilliam C Stephens Jr and an anonymous reviewerThe AAPG Editor thanks the following reviewers fortheir work on this paper Dale A Leckie William CStephens Jr and an anonymous reviewer

Copyright copy2010 The American Association of Petroleum Geologists All rights reserved

Manuscript received March 25 2009 provisional acceptance May 19 2009 revised manuscript receivedJune 24 2009 final acceptance July 2 2009DOI10130607020909062

AAPG Bulletin v 94 no 1 (January 2010) pp 1ndash25 1

shoreface sandstones accumulated within a coastalembayment to theWestern Interior seaway Region-ally the Doe Creek interval thins northeast of Val-halla and is truncated beneath the K1 unconformityand shoreface sandstone bodies are encased withinoffshoremudrocks and detached from their contem-poraneous shoreline Locally at Valhalla the DoeCreek reservoir progrades toward the southwest andis extensively and commonly uniformly burrowedby a relatively diverse assemblage of trace makers

INTRODUCTION

Reserves and Objectives

Conventional in-place oil resourceswithin theWest-ern Canada sedimentary basin (WCSB) are esti-mated to total 48 billion bbl (77 million m3) andare primarily contained within reservoirs of Devo-nian and Cretaceous age (36 and 49 of in-placeoil resources respectively) (Allan and Creaney1991) Upper Cretaceous reservoirs account for23 of in-place oil resources that are highly prizedbecause of their characteristically low gravity (APIgravity greater than 30deg) and low sulfur content(generally less than 05) (Allan and Creaney1991) One such Upper Cretaceous reservoir is theDoe Creek Member of the Kaskapau Formationat Valhalla field (Figure 1) In-place oil resourcesat Valhalla are stratigraphically trapped withinshoreface sandstones of the Doe Creek and total279 million bbl (44370 103 m3) of light (APIgravity of 38deg) low-sulfur oil (Hogg et al 1998AERCB 2008) Oil was discovered within the DoeCreek at Valhalla in 1979 and widespread devel-opment drilling followed in 1982 An extensive40-ac vertical drilling program was initiated duringthe 1990s for secondary recovery of oil through apatternedwaterflood By the end of 1996more than90 of Valhalla was producing oil through sec-ondary recovery (Hogg et al 1998) To further en-hance oil recovery a fieldwide drilling program ofhorizontal production and water injection wells wasinitiated in 2004 and as of this writing is ongoing

Additional increases in recoverable reserveswithin the Doe Creek at Valhalla field will likely

2 EampP Note

require the application of tertiary recovery meth-ods The efficacy of both secondary and tertiary re-covery schemes is reliant upon the accurate depic-tion of preferred flow pathways within the reservoirinterval This study evaluates flow continuitywithintheDoeCreekMember atValhalla field by address-ing the following questions (1)What control do de-positional facies and diagenesis have on reservoirquality (2) Is it possible to predict the occurrence

Figure 1 Stratigraphic correlation chart and third-order sea lev-el history for the Late Cretaceous within west-central Alberta(lithostratigraphic designations are modified from Bhattacharya1994 age designations and sea level history are from Caldwell 1983Ogg et al 2004) Following sea level lowstand and DunveganFormation deposition the study interval was deposited duringthe ensuing third-order Greenhorn transgression (Caldwell 1983)

of depositional facies and diagenetic products inwells that lack core control (3) What is the spatialdistribution of depositional facies within a time-stratigraphic framework and does this distributioncorrespond to historic trends of fluid productionFinally do answers to these three applied questionsprovide serendipitous insight into the ongoing dis-cussions regarding the depositional and stratigraph-ic origins of the Doe Creek Member

Geologic Setting

TheValhalla field area is located within west-centralAlberta and during the LateCretaceous (Cenoma-nian) was situated along the westernmargin of theWestern Interior seaway (WIS) near its coastline

terminus against the northwest-trending Laramidedeformational front (Figures 1 2) (Varban andPlint 2005 Kreitner and Plint 2006) RegionallytheDoe CreekMember thickens to approximately115 m (377 ft) near the axis of theWCSB foredeepand thins northeastward to less than 30 m (98 ft)thick approximately 75 km (~47 mi) northeast ofValhalla field (Kreitner and Plint 2006) From itsoutcrop exposures near the Alberta-British Colum-bia border the Doe Creek Member pinches outapproximately 90 km (sim56 mi) to the southeast(Wallace-Dudley and Leckie 1993) Eastward thin-ning is related to both accommodation loss awayfrom the WCSB foredeep axis and erosional trun-cation beneath the regional K-1 intraformationalunconformity surface (Plint et al 1993 Plint

Figure 2 (A) Late Cretaceous (early Turonian) paleogeography of North America highlighting the position of the Western Interiorseaway in gray (modified from Williams and Stelck 1975 Irving et al 1993 as cited by Varban and Plint 2005) The area enlargedin panel B is indicated (B) Paleogeography of west-central Alberta and east-central British Columbia during the Late Cretaceous (upperCenomanian) Doe Creek deposition (modified from Kreitner and Plint 2006) Doe Creek sandstone bodies occur within the central partof a coastal embayment to the Western Interior seaway and are detached from their contemporaneous shoreline Panels A and B arereprinted with permission from the Bulletin of Canadian Petroleum Geology

Atchley et al 3

2000) At Valhalla field oil is produced from theDoe Creek I and N sandstones and gas and smallamounts of oil are produced from the A sandstone(Wallace-Dudley and Leckie 1993) Reservoir sand-stones range from less than 1 to 7 m (3 to 23 ft) inthickness and consist of very fine to fine-grainedmarine shoreface deposits that are detached fromtheir equivalent shoreline and coastal plain depos-its (Figure 2) As such sandstone bodies transitionboth laterally and vertically into basinal marineshales (Kreitner and Plint 2006) The isolatednature of the sandstones and the southwestwardregional structural dip account for the hydrocar-bon entrapment at Valhalla and define the fieldlimits (Figure 3)

4 EampP Note

Data and Methods

Data from Valhalla field incorporated into thestudy include wireline logs and historic fluid pro-duction data for 508 wells and descriptive and ac-companying porosity and permeability data for allavailable Doe Creek cores (approximately 1350m[4429 ft] of core from 120wells) (Figure 3) Wellsreferenced in this manuscript are designated bythe Canadian system of unique well identifiers eg0002-12-075-08W6 In this example the well islocated within legal subdivision 02 (of 16 possible)of section 12 within township 75 north range 08west of the 6th meridian The 00 designation pre-ceding 02 is the legal exception code that is used

Figure 3 Basemap for Valhalla field showing the distribution of data incorporated into the study including wells with open-hole logs(black well symbols) cased-hole logs (gray well symbols) and core from the Doe Creek sandstones (circled well symbols) Contours arefor the structural top of the Doe Creek I sandstone and the dashed line delimits the lateral extent of reservoir sandstone and the positionof the original oil-water contact (+46 m [151 ft]) within the southwestern part of the field

Table 1 Summary Table of Features Diagnostic to Sedimentary Facies Observed Within the Doe Creek Member Valhalla Field

Reservoir FaciesNo Designation 1a 1b 1c 2 3 4

Name Black-laminatedmudstone

Flaser- tolenticular-beddedmudrock

Burrowed mudrock Intercalated burrowedmudrock andwave-rippled tohummockycross-stratifiedsandstone

Laminated to hummockycross-stratified sandstone

Planar tabular totrough cross-stratifiedsandstone

Environment Offshore Offshore Offshore Distal lower shoreface Proximal lower shoreface Upper shorefaceTypical mud content gt90 60ndash80 55ndash85 10ndash50 0ndash10 0ndash5Typical grain size Silt and clay Silt and clay Silt and clay Lower very fine to lower

fine sandUpper very fine to upperfine sand

Lower fine to upperfine

Typical ichnofabricindex

1ndash2 1ndash2 4ndash5 sand = 1ndash2 mud = 2ndash4 1ndash5 1ndash2

Ichnofacies(representativeichnofauna)

Cruziana(ThallassinoidesPlanolites)

Cruziana(ThallassinoidesPlanolites)

Cruziana(ThallassinoidesPlanolitesZoophycos Scolicia)

Cruziana (ThallassinoidesPlanolites TeichichnusZoophycos ScoliciaPalaeophycus) Skolithos(Skolithos OphiomorphaPalaeophycus)

Skolithos (SkolithosOphiomorphaPalaeophycusBergaureria)

Skolithos (SkolithosOphiomorphaPalaeophycus)

Mechanicalsedimentarystructures

Millimeter-scalelamina

Millimeter-scalelamina flaserbedding

None observed Flaser bedding hummockycross-stratification waveripples

Hummocky cross-stratificationmillimeter-scale laminawave ripples

Trough cross-stratificationplanar tabularstratification

Representative corephotos

Figure 4A Figure 4B Figure 4C Figure 4D Figure 4E Figure 4FAtchleyetal

5

when more than one well exists within a single le-gal subdivision eg a 03 would identify the thirdwell drilled in a legal subdivision Core descrip-tions include documentation of the vertical distribu-tions of grain size the fraction of mud-size particles

6 EampP Note

mechanical sedimentary structures ichnogeneraand ichnofacies ichnofabric index and ichnofaunaldiversity (sensu Droser and Bottjer 1986 Bottjerand Droser 1991) sedimentary facies and cementtype and distribution Core description data were

Figure 4 Core photographs representative of facies observed within the Doe Creek at Valhalla field (compare with Table 1) Scale bar =1 cm (03 in) Burrows labeled on the photographs include Planolites (Pl) Thalassinoides (Th) Zoophycos (Zoo) Asterosoma (Ast) andOphiomorpha (Oph) (A) Facies 1a (black-laminated mudrock) within the 0214-29-74-9W60 well at 7005 m (22982 ft) (B) Facies 1b(flaser to lenticular bedded mudrock) within the 008-34-75-8W60 well at 7815 m (25639 ft) (C) Facies 1c (burrowed mudrock) withinthe 0016-29-74-9W60 well at 7105 m (2331 ft) (D) Facies 2 (intercalated burrowed mudrock and wave rippled to hummocky cross-stratified sandstone) within the 036-15-75-9W60 well at 737 m (2418 ft) (E) Facies 3 (laminated to hummocky cross-stratified sand-stone) within the 008-12-75-9W60 well at 6885 m (22588 ft) (F) Facies 4 (planar tabular to trough cross-stratified sandstone) withinthe 0010-11-75-9W60 well at 6885 m (22588 ft) Note the irregular distribution of calcite cement Well identifiers are explained in theData and Methods section

digitized and merged with digital core analysis po-rosity and permeability data and then depth shiftedto coincide with digital wireline well-log data Alldepth-reconciled borehole data types were thenexported to a computer spreadsheet for statisticalanalysis Stratigraphic correlations and preliminarymaps were computer generated and spurious com-puter contouring artifacts were hand correctedAnalysis andmapping of average daily fluid produc-tion trends are based on data compiled from the Al-berta Energy Resources Conservation Board Allwell-log analyses and predictive transforms werecompleted using GeoGraphixreg Prizmtrade log inter-pretation software

RESERVOIR CHARACTERIZATIONAND CLASSIFICATION

Facies Model

The Doe Creek Member of west-central Albertahas been extensively studied and interpreted as awave-dominated and river-influenced shelf sand-stone complex that accumulated in a shallow em-bayment where salinity varied from normalmarineto brackish (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998 Reid and Pemberton 2005 Kreitner andPlint 2006) Previous studies have identified from

Figure 5 Box and whisker plots of neutron-corrected effective porosity and maximum permeability(Kmax) versus grain size and stacked bar charts illustrating the proportion of depositional facies thatcomprise each grain-size category Abbreviations include MUD (clay plusmn silt) VFL (lower very fine sand)VFU (upper very fine sand) FL (lower fine sand) and FU (upper fine sand) Analysis is based on allcores described at Valhalla field Both porosity and increasing proportions of shallower water faciescorrespond to an increase in grain size Permeability displays a similar increase however high valueswithin the lower very fine sand and mud-size fractions are inconsistent to the trend and likely reflectpreferential sampling of thin relatively coarser grained sandstone interbeds within otherwise finergrained successions

Atchley et al 7

8 EampP Note

6 to 10 depositional facies within the Doe Creek atValhalla field (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998) We recognize six depositional facies thatare distinguished on the basis of grain sizemechani-cal sedimentary structures ichnofacies and theextent of bioturbation (ie ichnofabric index ofDroser and Bottjer 1986 and Bottjer and Droser1991 or bioturbate texture of Frey and Pemberton1990) and consistent with these previous studiesare interpreted to have been deposited within off-shore and lower and upper shoreface restricted toopen-marine environments (Table 1 Figure 4)Upper shoreface deposits are rarely observed with-in the study area Variations in ichnofabric indexand ichnofaunal constituents suggest that offshoredeposits are composed of those that accumulated un-der restricted (facies 1a) and relatively more open-marine (facies 1b and 1c) conditions (Table 1)Facies 1a and 1b are characterized by a general lackof bioturbation and almost exclusively include Pla-nolites and the Cruziana ichnofacies componentThalassinoides Conversely facies 1c is homogenizedby burrows that commonly include not only Plano-lites and Thalassinoides but also theCruziana ichno-facies constituents Zoophycos and more rarely Aster-osoma and Rhizocorallium Distal lower shorefacedeposits (facies 2) are dominated by both the Sko-lithos ichnofacies traces Ophiomorpha and Paleo-phycos and the Cruziana ichnofacies traces thatare also observed within facies 1c Proximal lowershoreface (facies 3) and upper shoreface (facies 4)deposits are characterized by Skolithos ichnofaciesforms that typically include Ophiomorpha Paleo-phycos Skolithos and Bergaureria (Table 1)

Controls on Reservoir Quality

Porosity and permeability within the Doe Creekclosely correlate with environmentally controlled

grain size and associated ichnofabric index trendsand the distribution of secondary calcite cementIncreasing grain size corresponds to both an in-crease in neutron-corrected effective porosity andfacies that accumulated in progressively shallowerwater shoreface environments (Figure 5) A similarcorrelation exists with permeability however anom-alously high permeability values within the lowervery fine sandstone andmud-size fractions likely re-flect a core analysis sampling bias introduced bythe preferential sampling of thin (10ndash30 cm [4ndash12 in]) interbeds of relatively coarser grained sand-stone within what was interval averaged duringcore description as relatively distal mud-prone de-posits (Figure 5) Note that the anomalously highpermeability values are not mimicked by neutron-corrected effective porosity because the thin coars-er grained sandstone interbeds that account for thehigh permeability values are below the 061-m (2 ft)vertical resolution of the neutron sonde (resolu-tion estimate via Alberty 1992)

A second potential influence on reservoir qual-ity is the extent of bioturbation and resultant intro-duction of interparticle silt and clay-size particles Toevaluate this possibility we compare porosity andpermeability to five categories of bioturbation in-tensity aka the ichnofabric index under two con-ditions (Figure 6) Under condition 1 the ichnofab-ric index is compared against neutron-correctedeffective porosity and core analysismaximumperme-ability (Kmax) for all core data (Figure 6A) whereascondition 2 limits the comparison to facies 2 3and 4 sandstones having a porosity of greater thanor equal to 12 (Figure 6B) ie the porosity cut-off at Valhalla field that is commonly used by localindustry producers to define reservoir-quality sand-stone (Figure 7) In condition 1 porosity and per-meability trends including the assumed perme-ability sampling bias are similar to those for grainsize and associated depositional facies (compare

Figure 6 Box and whisker plots of neutron-corrected effective porosity and maximum permeability (Kmax) versus ichnofabric indexand stacked bar charts illustrating the proportion of depositional facies that comprise each ichnofabric index category (A) Condition 1case based on analysis of all core data regardless of lithology and (B) condition 2 case based on analysis of all core data where facies 23 and 4 sandstones have a porosity of greater than or equal to 12 Regardless of condition or ichnofabric index category the highestvalues of porosity and permeability coincide with the highest proportion of the shallowest water facies This suggests that grain size anddepositional environment are the primary control on reservoir quality

Atchley et al 9

Figure 7 Scatter plot of coreporosity versus Kmax perme-ability for all Doe Creekndashcoredwells within the study area(compare with Figure 3) Brack-eted on the plot is the rangeof permeability (Kmax) acrosswellbore perforations (1ndash10 md)which as of this writing coin-cides with the lower limit of eco-nomic daily oil production(approximately lt10 bblday)observed at Valhalla field Per-meability values within this rangein turn coincide with a porosityrange of 10ndash15 and is sup-portive of the practice of Valhallafield operators to apply a 12porosity cutoff to define the lowerlimit of satisfactory Doe Creeksandstone reservoir quality

Figure 8 Box and whisker plotof neutron-corrected effectiveporosity and maximum perme-ability (Kmax) versus sandstonewhere calcite cement eitherwas (Yes) or was not (No) ob-served Analysis is based onan analysis of all core data wherefacies 2 3 and 4 sandstoneshave a porosity of greater thanor equal to 12 Both porosityand permeability are reducedin the presence of calcite cement

10 EampP Note

Figures 5 6A) In condition 2 the highest valuesof porosity and permeability similarly coincide withboth ichnofabric indices 1 2 and 5 and the highestproportion of the shallowest water sandstone de-posits (principally facies 3) (Figure 6B) These anal-yses suggest that grain-size trends and associatedenvironments of deposition instead of the ichno-fabric index are the primary sedimentologic con-trols on reservoir quality

Reservoir quality is diminished in the presenceof postdepositional calcite cement (Figures 4F 8)Calcite cement is most common near the top ofsandstone bodies and is also irregularly distributedwithin sandstone bodies Zones of calcite cementa-tion are thin (typically lt50 cm [20 in]) and likelyhave a lateral continuity of less than 4 m (13 ft)(Hogg et al 1998) Calcite cement is increasinglyabundant within shallower water deposits and mayhave resulted from the preferential flow of CaCO3-saturated fluids through their characteristically

Figure 9 Box plot of depositional facies versus the percentageof core specimens that have calcite cement Analysis is based onall cores described at Valhalla field

Figure 10 Box and whisker plots of facies versus (A) open-holedeep resistivity well-log data and (B) shale volume (Vsh) contentderived from open-hole gamma-ray well-log data (compare withTable 2)

Atchley et al 11

higher permeability relatively coarser grained sed-iments (Figure 5) Of the total core observationscalcite cement is present within 25 of the occur-rences of facies 1 (offshore) 85 of the occur-rences of facies 2 (distal lower shoreface) 16 ofthe occurrence of facies 3 (proximal lower shore-face) and 64 of the occurrences of facies 4 (up-per shoreface) (Figure 9) The particularly highpercentage of calcite cement within facies 4 is re-lated to both the limited observations of facies 4and the consistent position of facies 4 near thetop of sandstone bodies

Reservoir Facies Prediction

Comparison of core-observed depositional faciesand petrophysical attributes suggests that open-holegamma ray and deep resistivity well-log data maybe used to predict facies distributions in wells thatlack core control (Figure 10) Open-hole neutron-porosity logs were also evaluated for their poten-tial to predict facies but proved to be less reliableGuidelines for facies prediction use direct mea-surement of deep resistivity for oil-saturated reser-voir above the original oil-water contact (+46 m[151 ft]) and shale volume calculations (Vsh) de-rived from normalized gamma-ray measurementsBecause the Doe Creek study interval was oil fullduring initial field development the guidelines forthe use of deep resistivity are applicable to most (if

12 EampP Note

not all) wells with open-hole logs within the studyarea (Figure 3) Gamma-ray normalization andVsh

calculations are based on the following formulae(Larinov 1969 Asquith and Krygowski 2004 ascited by Asquith and Krygowski 2004)

IGR frac14 ethGRlog GRminTHORN=ethGRmax GRminTHORN

where the IGR is the gamma-ray index GRlog is thegamma-raymeasurement from the reservoir GRmin

is the 14 API units and GRmax is the 130 APIunits (asterisks indicate Valhalla fieldwide averageminimum and maximum values observed in open-hole wireline logs)

V sh frac14 033frac122eth2 IGRTHORN 1

where Vsh is the shale volumeTable 2 defines the guidelines for prediction

and equates the resultant well-log facies with theirdepositional facies counterparts Although well-log data cluster within discrete ranges for each de-positional facies the overlapping range of Vsh anddeep resistivity data between facies results in well-log facies predictions that reflect data blendingbetween two (or more) depositional facies (com-pare Table 2 and Figure 10) Consequently al-though predicted well-log facies are most likely as-sociated with a particular depositional facies theintroduction of error from overlapping ranges of

Table 2 Guidelines for Facies Prediction from Open-Hole Well Logs

Well-Log Facies

Depositional Facies Equivalents

Parameters for Well-Log Prediction

4

Primarily 3 and 4

Vsh le 009 (average confidence)Vsh le 009 and deep resistivity ge38 ohm m (increased confidence)

3

Primarily 3 and lesser 2

Vsh ge 009 to le 012 (average confidence)Vsh ge 009 to le 012 and deep resistivity le38 to ge 21 ohm m (increased confidence)

2

Primarily 2 and lesser 1

Vsh ge 012 to le 033 (average confidence)Vsh ge 012 to le 033 and deep resistivity le21 to ge 14 ohm m (increased confidence)

1

Primarily 1 and lesser 2

Vsh ge 033 (average confidence) Vsh ge 033 anddeep resistivity le 14 ohm m (increased confidence)

Predicted well-log facies correspond to more than one depositional facies because of the overlapping range of Vsh and deep resistivity well-log data between depositionalfacies (Figure 10) The confidence of well-log facies interpretation increases when both the Vsh and deep resistivity guidelines are satisfied

well-log data between facies categories may re-sult in misinterpretation

Calcite cement is interpreted to occur withinwell-log facies 2 3 or 4 where the well-log neu-tron porosity exceeds the density porosity bymorethan 7 Within an oil-saturated reservoir such asthe Doe Creek at Valhalla this quick-look tech-nique may be used when density-neutron well-logdata are calibrated to a sandstonematrix density val-ue of 265 gcm3 (Asquith and Krygowski 2004)

To evaluate the accuracy of the predictivemod-el a blind test was administered to two cored wellswithin the study area that had not been includedwithin the database used to develop the well-logfacies or calcite cementation predictive guidelines(Figure 11) Cores from both wells were describedand interpreted independent of knowledge regard-ing the interpreted distribution of well-log faciesIn both cases the distribution of depositional faciesobserved within cores closely matches the pre-dicted distribution (Figure 11) The predicted dis-tribution of calcite cement precisely coincides withthe observed distribution within the 0014-15-075-08W6 well but only matches the thickest of fourcalcite-cemented zones within the 0006-32-074-09W6 well (Figure 11) The inability of the density-neutron predictor to detect calcite-cemented zoneswithin the 0006-32-074-09W6 well is likely re-lated to their thin (02ndash03 m [06ndash09 ft]) discon-tinuous distribution which is below the 045ndash061-m (17ndash2-ft) vertical resolution of density-neutron well logs (resolution via Alberty 1992)

STRATIGRAPHIC FRAMEWORK

Regional Overview

TheKaskapau Formationwas deposited during theculmination of the second-order transgression as-sociated with the Zuni cratonic sequence (sensuSloss 1963) and the concomitant maximum ex-tent of the North American WIS during the LateCretaceous (Hancock and Kauffman 1979 Haqet al 1988 Caldwell et al 1993 Kauffman andCaldwell 1993 Slingerland et al 1996) Withinthe study area second-order maximum flooding

coincides with the Greenhorn eustatic transgressionjust above the CenomanianndashTuronian boundaryand the equivalent organic-rich earliest TuronianSecondWhite Speckled Shale of the Kaskapau For-mation (Figure 1) (Caldwell 1983 Haq et al 1987Wallace-Dudley and Leckie 1993 Bhattacharya1994 Hogg et al 1998 Varban and Plint 2005)A higher frequency perhaps third-order episodeof decreasing and increasing accommodation dur-ing the middle Cenomanian to lower Turonian waspossibly induced by a pulse of foreland tectonismacross the WCSB and resulted in deposition oflowstand (Dunvegan) deltaic deposits and suc-ceeding transgressive (Kaskapau) open-marinemidshelf deposits (Plint et al 1993) Because ofthe relatively rapid rates of composite accommo-dation gain that resulted from constructive inter-ference of the second- and third-order transgres-sions higher frequency fourth-order stillstands(or lowstands) of sea level are recorded as retrogra-dationally stacked alternations of deeper water off-shore mudrocks (fourth-order transgressions) andshallower water shelf sandstones (fourth-orderstillstands or regressions) (Wallace-Dudley andLeckie 1993) In ascending stratigraphic orderthese shallower water sandstones include the DoeCreek Pouce Coupe and Howard Creek membersof the Kaskapau Formation (Figure 1)

Valhalla Reservoir Interval

TheDoeCreekMember atValhalla field consists ofnumerous cyclic alternations of offshore mudrock(facies 1) and shoreface sandstone (facies 2 3 andless commonly 4) Nine of these thin (1ndash10 m [3ndash33 ft] thick) cyclic units are correlated across Val-halla and their stratigraphic designations are refer-enced sequentially relative to their position above orbelow the top of themost oil productive I sandstone(alphanumeric designation by Wallace-Dudley andLeckie 1988) Above the I sandstone these includein ascending order the I + 1 through I + 6 and be-low the I sandstone in descending order the I minus 1and I minus 2 (Figures 11 12) In addition to the I sand-stone other hydrocarbon-charged units include theoil-prone I minus 1 and I + 1 and the gas-prone I + 2The I + 1 and I + 2 coincide with the previously

Atchley et al 13

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

shoreface sandstones accumulated within a coastalembayment to theWestern Interior seaway Region-ally the Doe Creek interval thins northeast of Val-halla and is truncated beneath the K1 unconformityand shoreface sandstone bodies are encased withinoffshoremudrocks and detached from their contem-poraneous shoreline Locally at Valhalla the DoeCreek reservoir progrades toward the southwest andis extensively and commonly uniformly burrowedby a relatively diverse assemblage of trace makers

INTRODUCTION

Reserves and Objectives

Conventional in-place oil resourceswithin theWest-ern Canada sedimentary basin (WCSB) are esti-mated to total 48 billion bbl (77 million m3) andare primarily contained within reservoirs of Devo-nian and Cretaceous age (36 and 49 of in-placeoil resources respectively) (Allan and Creaney1991) Upper Cretaceous reservoirs account for23 of in-place oil resources that are highly prizedbecause of their characteristically low gravity (APIgravity greater than 30deg) and low sulfur content(generally less than 05) (Allan and Creaney1991) One such Upper Cretaceous reservoir is theDoe Creek Member of the Kaskapau Formationat Valhalla field (Figure 1) In-place oil resourcesat Valhalla are stratigraphically trapped withinshoreface sandstones of the Doe Creek and total279 million bbl (44370 103 m3) of light (APIgravity of 38deg) low-sulfur oil (Hogg et al 1998AERCB 2008) Oil was discovered within the DoeCreek at Valhalla in 1979 and widespread devel-opment drilling followed in 1982 An extensive40-ac vertical drilling program was initiated duringthe 1990s for secondary recovery of oil through apatternedwaterflood By the end of 1996more than90 of Valhalla was producing oil through sec-ondary recovery (Hogg et al 1998) To further en-hance oil recovery a fieldwide drilling program ofhorizontal production and water injection wells wasinitiated in 2004 and as of this writing is ongoing

Additional increases in recoverable reserveswithin the Doe Creek at Valhalla field will likely

2 EampP Note

require the application of tertiary recovery meth-ods The efficacy of both secondary and tertiary re-covery schemes is reliant upon the accurate depic-tion of preferred flow pathways within the reservoirinterval This study evaluates flow continuitywithintheDoeCreekMember atValhalla field by address-ing the following questions (1)What control do de-positional facies and diagenesis have on reservoirquality (2) Is it possible to predict the occurrence

Figure 1 Stratigraphic correlation chart and third-order sea lev-el history for the Late Cretaceous within west-central Alberta(lithostratigraphic designations are modified from Bhattacharya1994 age designations and sea level history are from Caldwell 1983Ogg et al 2004) Following sea level lowstand and DunveganFormation deposition the study interval was deposited duringthe ensuing third-order Greenhorn transgression (Caldwell 1983)

of depositional facies and diagenetic products inwells that lack core control (3) What is the spatialdistribution of depositional facies within a time-stratigraphic framework and does this distributioncorrespond to historic trends of fluid productionFinally do answers to these three applied questionsprovide serendipitous insight into the ongoing dis-cussions regarding the depositional and stratigraph-ic origins of the Doe Creek Member

Geologic Setting

TheValhalla field area is located within west-centralAlberta and during the LateCretaceous (Cenoma-nian) was situated along the westernmargin of theWestern Interior seaway (WIS) near its coastline

terminus against the northwest-trending Laramidedeformational front (Figures 1 2) (Varban andPlint 2005 Kreitner and Plint 2006) RegionallytheDoe CreekMember thickens to approximately115 m (377 ft) near the axis of theWCSB foredeepand thins northeastward to less than 30 m (98 ft)thick approximately 75 km (~47 mi) northeast ofValhalla field (Kreitner and Plint 2006) From itsoutcrop exposures near the Alberta-British Colum-bia border the Doe Creek Member pinches outapproximately 90 km (sim56 mi) to the southeast(Wallace-Dudley and Leckie 1993) Eastward thin-ning is related to both accommodation loss awayfrom the WCSB foredeep axis and erosional trun-cation beneath the regional K-1 intraformationalunconformity surface (Plint et al 1993 Plint

Figure 2 (A) Late Cretaceous (early Turonian) paleogeography of North America highlighting the position of the Western Interiorseaway in gray (modified from Williams and Stelck 1975 Irving et al 1993 as cited by Varban and Plint 2005) The area enlargedin panel B is indicated (B) Paleogeography of west-central Alberta and east-central British Columbia during the Late Cretaceous (upperCenomanian) Doe Creek deposition (modified from Kreitner and Plint 2006) Doe Creek sandstone bodies occur within the central partof a coastal embayment to the Western Interior seaway and are detached from their contemporaneous shoreline Panels A and B arereprinted with permission from the Bulletin of Canadian Petroleum Geology

Atchley et al 3

2000) At Valhalla field oil is produced from theDoe Creek I and N sandstones and gas and smallamounts of oil are produced from the A sandstone(Wallace-Dudley and Leckie 1993) Reservoir sand-stones range from less than 1 to 7 m (3 to 23 ft) inthickness and consist of very fine to fine-grainedmarine shoreface deposits that are detached fromtheir equivalent shoreline and coastal plain depos-its (Figure 2) As such sandstone bodies transitionboth laterally and vertically into basinal marineshales (Kreitner and Plint 2006) The isolatednature of the sandstones and the southwestwardregional structural dip account for the hydrocar-bon entrapment at Valhalla and define the fieldlimits (Figure 3)

4 EampP Note

Data and Methods

Data from Valhalla field incorporated into thestudy include wireline logs and historic fluid pro-duction data for 508 wells and descriptive and ac-companying porosity and permeability data for allavailable Doe Creek cores (approximately 1350m[4429 ft] of core from 120wells) (Figure 3) Wellsreferenced in this manuscript are designated bythe Canadian system of unique well identifiers eg0002-12-075-08W6 In this example the well islocated within legal subdivision 02 (of 16 possible)of section 12 within township 75 north range 08west of the 6th meridian The 00 designation pre-ceding 02 is the legal exception code that is used

Figure 3 Basemap for Valhalla field showing the distribution of data incorporated into the study including wells with open-hole logs(black well symbols) cased-hole logs (gray well symbols) and core from the Doe Creek sandstones (circled well symbols) Contours arefor the structural top of the Doe Creek I sandstone and the dashed line delimits the lateral extent of reservoir sandstone and the positionof the original oil-water contact (+46 m [151 ft]) within the southwestern part of the field

Table 1 Summary Table of Features Diagnostic to Sedimentary Facies Observed Within the Doe Creek Member Valhalla Field

Reservoir FaciesNo Designation 1a 1b 1c 2 3 4

Name Black-laminatedmudstone

Flaser- tolenticular-beddedmudrock

Burrowed mudrock Intercalated burrowedmudrock andwave-rippled tohummockycross-stratifiedsandstone

Laminated to hummockycross-stratified sandstone

Planar tabular totrough cross-stratifiedsandstone

Environment Offshore Offshore Offshore Distal lower shoreface Proximal lower shoreface Upper shorefaceTypical mud content gt90 60ndash80 55ndash85 10ndash50 0ndash10 0ndash5Typical grain size Silt and clay Silt and clay Silt and clay Lower very fine to lower

fine sandUpper very fine to upperfine sand

Lower fine to upperfine

Typical ichnofabricindex

1ndash2 1ndash2 4ndash5 sand = 1ndash2 mud = 2ndash4 1ndash5 1ndash2

Ichnofacies(representativeichnofauna)

Cruziana(ThallassinoidesPlanolites)

Cruziana(ThallassinoidesPlanolites)

Cruziana(ThallassinoidesPlanolitesZoophycos Scolicia)

Cruziana (ThallassinoidesPlanolites TeichichnusZoophycos ScoliciaPalaeophycus) Skolithos(Skolithos OphiomorphaPalaeophycus)

Skolithos (SkolithosOphiomorphaPalaeophycusBergaureria)

Skolithos (SkolithosOphiomorphaPalaeophycus)

Mechanicalsedimentarystructures

Millimeter-scalelamina

Millimeter-scalelamina flaserbedding

None observed Flaser bedding hummockycross-stratification waveripples

Hummocky cross-stratificationmillimeter-scale laminawave ripples

Trough cross-stratificationplanar tabularstratification

Representative corephotos

Figure 4A Figure 4B Figure 4C Figure 4D Figure 4E Figure 4FAtchleyetal

5

when more than one well exists within a single le-gal subdivision eg a 03 would identify the thirdwell drilled in a legal subdivision Core descrip-tions include documentation of the vertical distribu-tions of grain size the fraction of mud-size particles

6 EampP Note

mechanical sedimentary structures ichnogeneraand ichnofacies ichnofabric index and ichnofaunaldiversity (sensu Droser and Bottjer 1986 Bottjerand Droser 1991) sedimentary facies and cementtype and distribution Core description data were

Figure 4 Core photographs representative of facies observed within the Doe Creek at Valhalla field (compare with Table 1) Scale bar =1 cm (03 in) Burrows labeled on the photographs include Planolites (Pl) Thalassinoides (Th) Zoophycos (Zoo) Asterosoma (Ast) andOphiomorpha (Oph) (A) Facies 1a (black-laminated mudrock) within the 0214-29-74-9W60 well at 7005 m (22982 ft) (B) Facies 1b(flaser to lenticular bedded mudrock) within the 008-34-75-8W60 well at 7815 m (25639 ft) (C) Facies 1c (burrowed mudrock) withinthe 0016-29-74-9W60 well at 7105 m (2331 ft) (D) Facies 2 (intercalated burrowed mudrock and wave rippled to hummocky cross-stratified sandstone) within the 036-15-75-9W60 well at 737 m (2418 ft) (E) Facies 3 (laminated to hummocky cross-stratified sand-stone) within the 008-12-75-9W60 well at 6885 m (22588 ft) (F) Facies 4 (planar tabular to trough cross-stratified sandstone) withinthe 0010-11-75-9W60 well at 6885 m (22588 ft) Note the irregular distribution of calcite cement Well identifiers are explained in theData and Methods section

digitized and merged with digital core analysis po-rosity and permeability data and then depth shiftedto coincide with digital wireline well-log data Alldepth-reconciled borehole data types were thenexported to a computer spreadsheet for statisticalanalysis Stratigraphic correlations and preliminarymaps were computer generated and spurious com-puter contouring artifacts were hand correctedAnalysis andmapping of average daily fluid produc-tion trends are based on data compiled from the Al-berta Energy Resources Conservation Board Allwell-log analyses and predictive transforms werecompleted using GeoGraphixreg Prizmtrade log inter-pretation software

RESERVOIR CHARACTERIZATIONAND CLASSIFICATION

Facies Model

The Doe Creek Member of west-central Albertahas been extensively studied and interpreted as awave-dominated and river-influenced shelf sand-stone complex that accumulated in a shallow em-bayment where salinity varied from normalmarineto brackish (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998 Reid and Pemberton 2005 Kreitner andPlint 2006) Previous studies have identified from

Figure 5 Box and whisker plots of neutron-corrected effective porosity and maximum permeability(Kmax) versus grain size and stacked bar charts illustrating the proportion of depositional facies thatcomprise each grain-size category Abbreviations include MUD (clay plusmn silt) VFL (lower very fine sand)VFU (upper very fine sand) FL (lower fine sand) and FU (upper fine sand) Analysis is based on allcores described at Valhalla field Both porosity and increasing proportions of shallower water faciescorrespond to an increase in grain size Permeability displays a similar increase however high valueswithin the lower very fine sand and mud-size fractions are inconsistent to the trend and likely reflectpreferential sampling of thin relatively coarser grained sandstone interbeds within otherwise finergrained successions

Atchley et al 7

8 EampP Note

6 to 10 depositional facies within the Doe Creek atValhalla field (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998) We recognize six depositional facies thatare distinguished on the basis of grain sizemechani-cal sedimentary structures ichnofacies and theextent of bioturbation (ie ichnofabric index ofDroser and Bottjer 1986 and Bottjer and Droser1991 or bioturbate texture of Frey and Pemberton1990) and consistent with these previous studiesare interpreted to have been deposited within off-shore and lower and upper shoreface restricted toopen-marine environments (Table 1 Figure 4)Upper shoreface deposits are rarely observed with-in the study area Variations in ichnofabric indexand ichnofaunal constituents suggest that offshoredeposits are composed of those that accumulated un-der restricted (facies 1a) and relatively more open-marine (facies 1b and 1c) conditions (Table 1)Facies 1a and 1b are characterized by a general lackof bioturbation and almost exclusively include Pla-nolites and the Cruziana ichnofacies componentThalassinoides Conversely facies 1c is homogenizedby burrows that commonly include not only Plano-lites and Thalassinoides but also theCruziana ichno-facies constituents Zoophycos and more rarely Aster-osoma and Rhizocorallium Distal lower shorefacedeposits (facies 2) are dominated by both the Sko-lithos ichnofacies traces Ophiomorpha and Paleo-phycos and the Cruziana ichnofacies traces thatare also observed within facies 1c Proximal lowershoreface (facies 3) and upper shoreface (facies 4)deposits are characterized by Skolithos ichnofaciesforms that typically include Ophiomorpha Paleo-phycos Skolithos and Bergaureria (Table 1)

Controls on Reservoir Quality

Porosity and permeability within the Doe Creekclosely correlate with environmentally controlled

grain size and associated ichnofabric index trendsand the distribution of secondary calcite cementIncreasing grain size corresponds to both an in-crease in neutron-corrected effective porosity andfacies that accumulated in progressively shallowerwater shoreface environments (Figure 5) A similarcorrelation exists with permeability however anom-alously high permeability values within the lowervery fine sandstone andmud-size fractions likely re-flect a core analysis sampling bias introduced bythe preferential sampling of thin (10ndash30 cm [4ndash12 in]) interbeds of relatively coarser grained sand-stone within what was interval averaged duringcore description as relatively distal mud-prone de-posits (Figure 5) Note that the anomalously highpermeability values are not mimicked by neutron-corrected effective porosity because the thin coars-er grained sandstone interbeds that account for thehigh permeability values are below the 061-m (2 ft)vertical resolution of the neutron sonde (resolu-tion estimate via Alberty 1992)

A second potential influence on reservoir qual-ity is the extent of bioturbation and resultant intro-duction of interparticle silt and clay-size particles Toevaluate this possibility we compare porosity andpermeability to five categories of bioturbation in-tensity aka the ichnofabric index under two con-ditions (Figure 6) Under condition 1 the ichnofab-ric index is compared against neutron-correctedeffective porosity and core analysismaximumperme-ability (Kmax) for all core data (Figure 6A) whereascondition 2 limits the comparison to facies 2 3and 4 sandstones having a porosity of greater thanor equal to 12 (Figure 6B) ie the porosity cut-off at Valhalla field that is commonly used by localindustry producers to define reservoir-quality sand-stone (Figure 7) In condition 1 porosity and per-meability trends including the assumed perme-ability sampling bias are similar to those for grainsize and associated depositional facies (compare

Figure 6 Box and whisker plots of neutron-corrected effective porosity and maximum permeability (Kmax) versus ichnofabric indexand stacked bar charts illustrating the proportion of depositional facies that comprise each ichnofabric index category (A) Condition 1case based on analysis of all core data regardless of lithology and (B) condition 2 case based on analysis of all core data where facies 23 and 4 sandstones have a porosity of greater than or equal to 12 Regardless of condition or ichnofabric index category the highestvalues of porosity and permeability coincide with the highest proportion of the shallowest water facies This suggests that grain size anddepositional environment are the primary control on reservoir quality

Atchley et al 9

Figure 7 Scatter plot of coreporosity versus Kmax perme-ability for all Doe Creekndashcoredwells within the study area(compare with Figure 3) Brack-eted on the plot is the rangeof permeability (Kmax) acrosswellbore perforations (1ndash10 md)which as of this writing coin-cides with the lower limit of eco-nomic daily oil production(approximately lt10 bblday)observed at Valhalla field Per-meability values within this rangein turn coincide with a porosityrange of 10ndash15 and is sup-portive of the practice of Valhallafield operators to apply a 12porosity cutoff to define the lowerlimit of satisfactory Doe Creeksandstone reservoir quality

Figure 8 Box and whisker plotof neutron-corrected effectiveporosity and maximum perme-ability (Kmax) versus sandstonewhere calcite cement eitherwas (Yes) or was not (No) ob-served Analysis is based onan analysis of all core data wherefacies 2 3 and 4 sandstoneshave a porosity of greater thanor equal to 12 Both porosityand permeability are reducedin the presence of calcite cement

10 EampP Note

Figures 5 6A) In condition 2 the highest valuesof porosity and permeability similarly coincide withboth ichnofabric indices 1 2 and 5 and the highestproportion of the shallowest water sandstone de-posits (principally facies 3) (Figure 6B) These anal-yses suggest that grain-size trends and associatedenvironments of deposition instead of the ichno-fabric index are the primary sedimentologic con-trols on reservoir quality

Reservoir quality is diminished in the presenceof postdepositional calcite cement (Figures 4F 8)Calcite cement is most common near the top ofsandstone bodies and is also irregularly distributedwithin sandstone bodies Zones of calcite cementa-tion are thin (typically lt50 cm [20 in]) and likelyhave a lateral continuity of less than 4 m (13 ft)(Hogg et al 1998) Calcite cement is increasinglyabundant within shallower water deposits and mayhave resulted from the preferential flow of CaCO3-saturated fluids through their characteristically

Figure 9 Box plot of depositional facies versus the percentageof core specimens that have calcite cement Analysis is based onall cores described at Valhalla field

Figure 10 Box and whisker plots of facies versus (A) open-holedeep resistivity well-log data and (B) shale volume (Vsh) contentderived from open-hole gamma-ray well-log data (compare withTable 2)

Atchley et al 11

higher permeability relatively coarser grained sed-iments (Figure 5) Of the total core observationscalcite cement is present within 25 of the occur-rences of facies 1 (offshore) 85 of the occur-rences of facies 2 (distal lower shoreface) 16 ofthe occurrence of facies 3 (proximal lower shore-face) and 64 of the occurrences of facies 4 (up-per shoreface) (Figure 9) The particularly highpercentage of calcite cement within facies 4 is re-lated to both the limited observations of facies 4and the consistent position of facies 4 near thetop of sandstone bodies

Reservoir Facies Prediction

Comparison of core-observed depositional faciesand petrophysical attributes suggests that open-holegamma ray and deep resistivity well-log data maybe used to predict facies distributions in wells thatlack core control (Figure 10) Open-hole neutron-porosity logs were also evaluated for their poten-tial to predict facies but proved to be less reliableGuidelines for facies prediction use direct mea-surement of deep resistivity for oil-saturated reser-voir above the original oil-water contact (+46 m[151 ft]) and shale volume calculations (Vsh) de-rived from normalized gamma-ray measurementsBecause the Doe Creek study interval was oil fullduring initial field development the guidelines forthe use of deep resistivity are applicable to most (if

12 EampP Note

not all) wells with open-hole logs within the studyarea (Figure 3) Gamma-ray normalization andVsh

calculations are based on the following formulae(Larinov 1969 Asquith and Krygowski 2004 ascited by Asquith and Krygowski 2004)

IGR frac14 ethGRlog GRminTHORN=ethGRmax GRminTHORN

where the IGR is the gamma-ray index GRlog is thegamma-raymeasurement from the reservoir GRmin

is the 14 API units and GRmax is the 130 APIunits (asterisks indicate Valhalla fieldwide averageminimum and maximum values observed in open-hole wireline logs)

V sh frac14 033frac122eth2 IGRTHORN 1

where Vsh is the shale volumeTable 2 defines the guidelines for prediction

and equates the resultant well-log facies with theirdepositional facies counterparts Although well-log data cluster within discrete ranges for each de-positional facies the overlapping range of Vsh anddeep resistivity data between facies results in well-log facies predictions that reflect data blendingbetween two (or more) depositional facies (com-pare Table 2 and Figure 10) Consequently al-though predicted well-log facies are most likely as-sociated with a particular depositional facies theintroduction of error from overlapping ranges of

Table 2 Guidelines for Facies Prediction from Open-Hole Well Logs

Well-Log Facies

Depositional Facies Equivalents

Parameters for Well-Log Prediction

4

Primarily 3 and 4

Vsh le 009 (average confidence)Vsh le 009 and deep resistivity ge38 ohm m (increased confidence)

3

Primarily 3 and lesser 2

Vsh ge 009 to le 012 (average confidence)Vsh ge 009 to le 012 and deep resistivity le38 to ge 21 ohm m (increased confidence)

2

Primarily 2 and lesser 1

Vsh ge 012 to le 033 (average confidence)Vsh ge 012 to le 033 and deep resistivity le21 to ge 14 ohm m (increased confidence)

1

Primarily 1 and lesser 2

Vsh ge 033 (average confidence) Vsh ge 033 anddeep resistivity le 14 ohm m (increased confidence)

Predicted well-log facies correspond to more than one depositional facies because of the overlapping range of Vsh and deep resistivity well-log data between depositionalfacies (Figure 10) The confidence of well-log facies interpretation increases when both the Vsh and deep resistivity guidelines are satisfied

well-log data between facies categories may re-sult in misinterpretation

Calcite cement is interpreted to occur withinwell-log facies 2 3 or 4 where the well-log neu-tron porosity exceeds the density porosity bymorethan 7 Within an oil-saturated reservoir such asthe Doe Creek at Valhalla this quick-look tech-nique may be used when density-neutron well-logdata are calibrated to a sandstonematrix density val-ue of 265 gcm3 (Asquith and Krygowski 2004)

To evaluate the accuracy of the predictivemod-el a blind test was administered to two cored wellswithin the study area that had not been includedwithin the database used to develop the well-logfacies or calcite cementation predictive guidelines(Figure 11) Cores from both wells were describedand interpreted independent of knowledge regard-ing the interpreted distribution of well-log faciesIn both cases the distribution of depositional faciesobserved within cores closely matches the pre-dicted distribution (Figure 11) The predicted dis-tribution of calcite cement precisely coincides withthe observed distribution within the 0014-15-075-08W6 well but only matches the thickest of fourcalcite-cemented zones within the 0006-32-074-09W6 well (Figure 11) The inability of the density-neutron predictor to detect calcite-cemented zoneswithin the 0006-32-074-09W6 well is likely re-lated to their thin (02ndash03 m [06ndash09 ft]) discon-tinuous distribution which is below the 045ndash061-m (17ndash2-ft) vertical resolution of density-neutron well logs (resolution via Alberty 1992)

STRATIGRAPHIC FRAMEWORK

Regional Overview

TheKaskapau Formationwas deposited during theculmination of the second-order transgression as-sociated with the Zuni cratonic sequence (sensuSloss 1963) and the concomitant maximum ex-tent of the North American WIS during the LateCretaceous (Hancock and Kauffman 1979 Haqet al 1988 Caldwell et al 1993 Kauffman andCaldwell 1993 Slingerland et al 1996) Withinthe study area second-order maximum flooding

coincides with the Greenhorn eustatic transgressionjust above the CenomanianndashTuronian boundaryand the equivalent organic-rich earliest TuronianSecondWhite Speckled Shale of the Kaskapau For-mation (Figure 1) (Caldwell 1983 Haq et al 1987Wallace-Dudley and Leckie 1993 Bhattacharya1994 Hogg et al 1998 Varban and Plint 2005)A higher frequency perhaps third-order episodeof decreasing and increasing accommodation dur-ing the middle Cenomanian to lower Turonian waspossibly induced by a pulse of foreland tectonismacross the WCSB and resulted in deposition oflowstand (Dunvegan) deltaic deposits and suc-ceeding transgressive (Kaskapau) open-marinemidshelf deposits (Plint et al 1993) Because ofthe relatively rapid rates of composite accommo-dation gain that resulted from constructive inter-ference of the second- and third-order transgres-sions higher frequency fourth-order stillstands(or lowstands) of sea level are recorded as retrogra-dationally stacked alternations of deeper water off-shore mudrocks (fourth-order transgressions) andshallower water shelf sandstones (fourth-orderstillstands or regressions) (Wallace-Dudley andLeckie 1993) In ascending stratigraphic orderthese shallower water sandstones include the DoeCreek Pouce Coupe and Howard Creek membersof the Kaskapau Formation (Figure 1)

Valhalla Reservoir Interval

TheDoeCreekMember atValhalla field consists ofnumerous cyclic alternations of offshore mudrock(facies 1) and shoreface sandstone (facies 2 3 andless commonly 4) Nine of these thin (1ndash10 m [3ndash33 ft] thick) cyclic units are correlated across Val-halla and their stratigraphic designations are refer-enced sequentially relative to their position above orbelow the top of themost oil productive I sandstone(alphanumeric designation by Wallace-Dudley andLeckie 1988) Above the I sandstone these includein ascending order the I + 1 through I + 6 and be-low the I sandstone in descending order the I minus 1and I minus 2 (Figures 11 12) In addition to the I sand-stone other hydrocarbon-charged units include theoil-prone I minus 1 and I + 1 and the gas-prone I + 2The I + 1 and I + 2 coincide with the previously

Atchley et al 13

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

of depositional facies and diagenetic products inwells that lack core control (3) What is the spatialdistribution of depositional facies within a time-stratigraphic framework and does this distributioncorrespond to historic trends of fluid productionFinally do answers to these three applied questionsprovide serendipitous insight into the ongoing dis-cussions regarding the depositional and stratigraph-ic origins of the Doe Creek Member

Geologic Setting

TheValhalla field area is located within west-centralAlberta and during the LateCretaceous (Cenoma-nian) was situated along the westernmargin of theWestern Interior seaway (WIS) near its coastline

terminus against the northwest-trending Laramidedeformational front (Figures 1 2) (Varban andPlint 2005 Kreitner and Plint 2006) RegionallytheDoe CreekMember thickens to approximately115 m (377 ft) near the axis of theWCSB foredeepand thins northeastward to less than 30 m (98 ft)thick approximately 75 km (~47 mi) northeast ofValhalla field (Kreitner and Plint 2006) From itsoutcrop exposures near the Alberta-British Colum-bia border the Doe Creek Member pinches outapproximately 90 km (sim56 mi) to the southeast(Wallace-Dudley and Leckie 1993) Eastward thin-ning is related to both accommodation loss awayfrom the WCSB foredeep axis and erosional trun-cation beneath the regional K-1 intraformationalunconformity surface (Plint et al 1993 Plint

Figure 2 (A) Late Cretaceous (early Turonian) paleogeography of North America highlighting the position of the Western Interiorseaway in gray (modified from Williams and Stelck 1975 Irving et al 1993 as cited by Varban and Plint 2005) The area enlargedin panel B is indicated (B) Paleogeography of west-central Alberta and east-central British Columbia during the Late Cretaceous (upperCenomanian) Doe Creek deposition (modified from Kreitner and Plint 2006) Doe Creek sandstone bodies occur within the central partof a coastal embayment to the Western Interior seaway and are detached from their contemporaneous shoreline Panels A and B arereprinted with permission from the Bulletin of Canadian Petroleum Geology

Atchley et al 3

2000) At Valhalla field oil is produced from theDoe Creek I and N sandstones and gas and smallamounts of oil are produced from the A sandstone(Wallace-Dudley and Leckie 1993) Reservoir sand-stones range from less than 1 to 7 m (3 to 23 ft) inthickness and consist of very fine to fine-grainedmarine shoreface deposits that are detached fromtheir equivalent shoreline and coastal plain depos-its (Figure 2) As such sandstone bodies transitionboth laterally and vertically into basinal marineshales (Kreitner and Plint 2006) The isolatednature of the sandstones and the southwestwardregional structural dip account for the hydrocar-bon entrapment at Valhalla and define the fieldlimits (Figure 3)

4 EampP Note

Data and Methods

Data from Valhalla field incorporated into thestudy include wireline logs and historic fluid pro-duction data for 508 wells and descriptive and ac-companying porosity and permeability data for allavailable Doe Creek cores (approximately 1350m[4429 ft] of core from 120wells) (Figure 3) Wellsreferenced in this manuscript are designated bythe Canadian system of unique well identifiers eg0002-12-075-08W6 In this example the well islocated within legal subdivision 02 (of 16 possible)of section 12 within township 75 north range 08west of the 6th meridian The 00 designation pre-ceding 02 is the legal exception code that is used

Figure 3 Basemap for Valhalla field showing the distribution of data incorporated into the study including wells with open-hole logs(black well symbols) cased-hole logs (gray well symbols) and core from the Doe Creek sandstones (circled well symbols) Contours arefor the structural top of the Doe Creek I sandstone and the dashed line delimits the lateral extent of reservoir sandstone and the positionof the original oil-water contact (+46 m [151 ft]) within the southwestern part of the field

Table 1 Summary Table of Features Diagnostic to Sedimentary Facies Observed Within the Doe Creek Member Valhalla Field

Reservoir FaciesNo Designation 1a 1b 1c 2 3 4

Name Black-laminatedmudstone

Flaser- tolenticular-beddedmudrock

Burrowed mudrock Intercalated burrowedmudrock andwave-rippled tohummockycross-stratifiedsandstone

Laminated to hummockycross-stratified sandstone

Planar tabular totrough cross-stratifiedsandstone

Environment Offshore Offshore Offshore Distal lower shoreface Proximal lower shoreface Upper shorefaceTypical mud content gt90 60ndash80 55ndash85 10ndash50 0ndash10 0ndash5Typical grain size Silt and clay Silt and clay Silt and clay Lower very fine to lower

fine sandUpper very fine to upperfine sand

Lower fine to upperfine

Typical ichnofabricindex

1ndash2 1ndash2 4ndash5 sand = 1ndash2 mud = 2ndash4 1ndash5 1ndash2

Ichnofacies(representativeichnofauna)

Cruziana(ThallassinoidesPlanolites)

Cruziana(ThallassinoidesPlanolites)

Cruziana(ThallassinoidesPlanolitesZoophycos Scolicia)

Cruziana (ThallassinoidesPlanolites TeichichnusZoophycos ScoliciaPalaeophycus) Skolithos(Skolithos OphiomorphaPalaeophycus)

Skolithos (SkolithosOphiomorphaPalaeophycusBergaureria)

Skolithos (SkolithosOphiomorphaPalaeophycus)

Mechanicalsedimentarystructures

Millimeter-scalelamina

Millimeter-scalelamina flaserbedding

None observed Flaser bedding hummockycross-stratification waveripples

Hummocky cross-stratificationmillimeter-scale laminawave ripples

Trough cross-stratificationplanar tabularstratification

Representative corephotos

Figure 4A Figure 4B Figure 4C Figure 4D Figure 4E Figure 4FAtchleyetal

5

when more than one well exists within a single le-gal subdivision eg a 03 would identify the thirdwell drilled in a legal subdivision Core descrip-tions include documentation of the vertical distribu-tions of grain size the fraction of mud-size particles

6 EampP Note

mechanical sedimentary structures ichnogeneraand ichnofacies ichnofabric index and ichnofaunaldiversity (sensu Droser and Bottjer 1986 Bottjerand Droser 1991) sedimentary facies and cementtype and distribution Core description data were

Figure 4 Core photographs representative of facies observed within the Doe Creek at Valhalla field (compare with Table 1) Scale bar =1 cm (03 in) Burrows labeled on the photographs include Planolites (Pl) Thalassinoides (Th) Zoophycos (Zoo) Asterosoma (Ast) andOphiomorpha (Oph) (A) Facies 1a (black-laminated mudrock) within the 0214-29-74-9W60 well at 7005 m (22982 ft) (B) Facies 1b(flaser to lenticular bedded mudrock) within the 008-34-75-8W60 well at 7815 m (25639 ft) (C) Facies 1c (burrowed mudrock) withinthe 0016-29-74-9W60 well at 7105 m (2331 ft) (D) Facies 2 (intercalated burrowed mudrock and wave rippled to hummocky cross-stratified sandstone) within the 036-15-75-9W60 well at 737 m (2418 ft) (E) Facies 3 (laminated to hummocky cross-stratified sand-stone) within the 008-12-75-9W60 well at 6885 m (22588 ft) (F) Facies 4 (planar tabular to trough cross-stratified sandstone) withinthe 0010-11-75-9W60 well at 6885 m (22588 ft) Note the irregular distribution of calcite cement Well identifiers are explained in theData and Methods section

digitized and merged with digital core analysis po-rosity and permeability data and then depth shiftedto coincide with digital wireline well-log data Alldepth-reconciled borehole data types were thenexported to a computer spreadsheet for statisticalanalysis Stratigraphic correlations and preliminarymaps were computer generated and spurious com-puter contouring artifacts were hand correctedAnalysis andmapping of average daily fluid produc-tion trends are based on data compiled from the Al-berta Energy Resources Conservation Board Allwell-log analyses and predictive transforms werecompleted using GeoGraphixreg Prizmtrade log inter-pretation software

RESERVOIR CHARACTERIZATIONAND CLASSIFICATION

Facies Model

The Doe Creek Member of west-central Albertahas been extensively studied and interpreted as awave-dominated and river-influenced shelf sand-stone complex that accumulated in a shallow em-bayment where salinity varied from normalmarineto brackish (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998 Reid and Pemberton 2005 Kreitner andPlint 2006) Previous studies have identified from

Figure 5 Box and whisker plots of neutron-corrected effective porosity and maximum permeability(Kmax) versus grain size and stacked bar charts illustrating the proportion of depositional facies thatcomprise each grain-size category Abbreviations include MUD (clay plusmn silt) VFL (lower very fine sand)VFU (upper very fine sand) FL (lower fine sand) and FU (upper fine sand) Analysis is based on allcores described at Valhalla field Both porosity and increasing proportions of shallower water faciescorrespond to an increase in grain size Permeability displays a similar increase however high valueswithin the lower very fine sand and mud-size fractions are inconsistent to the trend and likely reflectpreferential sampling of thin relatively coarser grained sandstone interbeds within otherwise finergrained successions

Atchley et al 7

8 EampP Note

6 to 10 depositional facies within the Doe Creek atValhalla field (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998) We recognize six depositional facies thatare distinguished on the basis of grain sizemechani-cal sedimentary structures ichnofacies and theextent of bioturbation (ie ichnofabric index ofDroser and Bottjer 1986 and Bottjer and Droser1991 or bioturbate texture of Frey and Pemberton1990) and consistent with these previous studiesare interpreted to have been deposited within off-shore and lower and upper shoreface restricted toopen-marine environments (Table 1 Figure 4)Upper shoreface deposits are rarely observed with-in the study area Variations in ichnofabric indexand ichnofaunal constituents suggest that offshoredeposits are composed of those that accumulated un-der restricted (facies 1a) and relatively more open-marine (facies 1b and 1c) conditions (Table 1)Facies 1a and 1b are characterized by a general lackof bioturbation and almost exclusively include Pla-nolites and the Cruziana ichnofacies componentThalassinoides Conversely facies 1c is homogenizedby burrows that commonly include not only Plano-lites and Thalassinoides but also theCruziana ichno-facies constituents Zoophycos and more rarely Aster-osoma and Rhizocorallium Distal lower shorefacedeposits (facies 2) are dominated by both the Sko-lithos ichnofacies traces Ophiomorpha and Paleo-phycos and the Cruziana ichnofacies traces thatare also observed within facies 1c Proximal lowershoreface (facies 3) and upper shoreface (facies 4)deposits are characterized by Skolithos ichnofaciesforms that typically include Ophiomorpha Paleo-phycos Skolithos and Bergaureria (Table 1)

Controls on Reservoir Quality

Porosity and permeability within the Doe Creekclosely correlate with environmentally controlled

grain size and associated ichnofabric index trendsand the distribution of secondary calcite cementIncreasing grain size corresponds to both an in-crease in neutron-corrected effective porosity andfacies that accumulated in progressively shallowerwater shoreface environments (Figure 5) A similarcorrelation exists with permeability however anom-alously high permeability values within the lowervery fine sandstone andmud-size fractions likely re-flect a core analysis sampling bias introduced bythe preferential sampling of thin (10ndash30 cm [4ndash12 in]) interbeds of relatively coarser grained sand-stone within what was interval averaged duringcore description as relatively distal mud-prone de-posits (Figure 5) Note that the anomalously highpermeability values are not mimicked by neutron-corrected effective porosity because the thin coars-er grained sandstone interbeds that account for thehigh permeability values are below the 061-m (2 ft)vertical resolution of the neutron sonde (resolu-tion estimate via Alberty 1992)

A second potential influence on reservoir qual-ity is the extent of bioturbation and resultant intro-duction of interparticle silt and clay-size particles Toevaluate this possibility we compare porosity andpermeability to five categories of bioturbation in-tensity aka the ichnofabric index under two con-ditions (Figure 6) Under condition 1 the ichnofab-ric index is compared against neutron-correctedeffective porosity and core analysismaximumperme-ability (Kmax) for all core data (Figure 6A) whereascondition 2 limits the comparison to facies 2 3and 4 sandstones having a porosity of greater thanor equal to 12 (Figure 6B) ie the porosity cut-off at Valhalla field that is commonly used by localindustry producers to define reservoir-quality sand-stone (Figure 7) In condition 1 porosity and per-meability trends including the assumed perme-ability sampling bias are similar to those for grainsize and associated depositional facies (compare

Figure 6 Box and whisker plots of neutron-corrected effective porosity and maximum permeability (Kmax) versus ichnofabric indexand stacked bar charts illustrating the proportion of depositional facies that comprise each ichnofabric index category (A) Condition 1case based on analysis of all core data regardless of lithology and (B) condition 2 case based on analysis of all core data where facies 23 and 4 sandstones have a porosity of greater than or equal to 12 Regardless of condition or ichnofabric index category the highestvalues of porosity and permeability coincide with the highest proportion of the shallowest water facies This suggests that grain size anddepositional environment are the primary control on reservoir quality

Atchley et al 9

Figure 7 Scatter plot of coreporosity versus Kmax perme-ability for all Doe Creekndashcoredwells within the study area(compare with Figure 3) Brack-eted on the plot is the rangeof permeability (Kmax) acrosswellbore perforations (1ndash10 md)which as of this writing coin-cides with the lower limit of eco-nomic daily oil production(approximately lt10 bblday)observed at Valhalla field Per-meability values within this rangein turn coincide with a porosityrange of 10ndash15 and is sup-portive of the practice of Valhallafield operators to apply a 12porosity cutoff to define the lowerlimit of satisfactory Doe Creeksandstone reservoir quality

Figure 8 Box and whisker plotof neutron-corrected effectiveporosity and maximum perme-ability (Kmax) versus sandstonewhere calcite cement eitherwas (Yes) or was not (No) ob-served Analysis is based onan analysis of all core data wherefacies 2 3 and 4 sandstoneshave a porosity of greater thanor equal to 12 Both porosityand permeability are reducedin the presence of calcite cement

10 EampP Note

Figures 5 6A) In condition 2 the highest valuesof porosity and permeability similarly coincide withboth ichnofabric indices 1 2 and 5 and the highestproportion of the shallowest water sandstone de-posits (principally facies 3) (Figure 6B) These anal-yses suggest that grain-size trends and associatedenvironments of deposition instead of the ichno-fabric index are the primary sedimentologic con-trols on reservoir quality

Reservoir quality is diminished in the presenceof postdepositional calcite cement (Figures 4F 8)Calcite cement is most common near the top ofsandstone bodies and is also irregularly distributedwithin sandstone bodies Zones of calcite cementa-tion are thin (typically lt50 cm [20 in]) and likelyhave a lateral continuity of less than 4 m (13 ft)(Hogg et al 1998) Calcite cement is increasinglyabundant within shallower water deposits and mayhave resulted from the preferential flow of CaCO3-saturated fluids through their characteristically

Figure 9 Box plot of depositional facies versus the percentageof core specimens that have calcite cement Analysis is based onall cores described at Valhalla field

Figure 10 Box and whisker plots of facies versus (A) open-holedeep resistivity well-log data and (B) shale volume (Vsh) contentderived from open-hole gamma-ray well-log data (compare withTable 2)

Atchley et al 11

higher permeability relatively coarser grained sed-iments (Figure 5) Of the total core observationscalcite cement is present within 25 of the occur-rences of facies 1 (offshore) 85 of the occur-rences of facies 2 (distal lower shoreface) 16 ofthe occurrence of facies 3 (proximal lower shore-face) and 64 of the occurrences of facies 4 (up-per shoreface) (Figure 9) The particularly highpercentage of calcite cement within facies 4 is re-lated to both the limited observations of facies 4and the consistent position of facies 4 near thetop of sandstone bodies

Reservoir Facies Prediction

Comparison of core-observed depositional faciesand petrophysical attributes suggests that open-holegamma ray and deep resistivity well-log data maybe used to predict facies distributions in wells thatlack core control (Figure 10) Open-hole neutron-porosity logs were also evaluated for their poten-tial to predict facies but proved to be less reliableGuidelines for facies prediction use direct mea-surement of deep resistivity for oil-saturated reser-voir above the original oil-water contact (+46 m[151 ft]) and shale volume calculations (Vsh) de-rived from normalized gamma-ray measurementsBecause the Doe Creek study interval was oil fullduring initial field development the guidelines forthe use of deep resistivity are applicable to most (if

12 EampP Note

not all) wells with open-hole logs within the studyarea (Figure 3) Gamma-ray normalization andVsh

calculations are based on the following formulae(Larinov 1969 Asquith and Krygowski 2004 ascited by Asquith and Krygowski 2004)

IGR frac14 ethGRlog GRminTHORN=ethGRmax GRminTHORN

where the IGR is the gamma-ray index GRlog is thegamma-raymeasurement from the reservoir GRmin

is the 14 API units and GRmax is the 130 APIunits (asterisks indicate Valhalla fieldwide averageminimum and maximum values observed in open-hole wireline logs)

V sh frac14 033frac122eth2 IGRTHORN 1

where Vsh is the shale volumeTable 2 defines the guidelines for prediction

and equates the resultant well-log facies with theirdepositional facies counterparts Although well-log data cluster within discrete ranges for each de-positional facies the overlapping range of Vsh anddeep resistivity data between facies results in well-log facies predictions that reflect data blendingbetween two (or more) depositional facies (com-pare Table 2 and Figure 10) Consequently al-though predicted well-log facies are most likely as-sociated with a particular depositional facies theintroduction of error from overlapping ranges of

Table 2 Guidelines for Facies Prediction from Open-Hole Well Logs

Well-Log Facies

Depositional Facies Equivalents

Parameters for Well-Log Prediction

4

Primarily 3 and 4

Vsh le 009 (average confidence)Vsh le 009 and deep resistivity ge38 ohm m (increased confidence)

3

Primarily 3 and lesser 2

Vsh ge 009 to le 012 (average confidence)Vsh ge 009 to le 012 and deep resistivity le38 to ge 21 ohm m (increased confidence)

2

Primarily 2 and lesser 1

Vsh ge 012 to le 033 (average confidence)Vsh ge 012 to le 033 and deep resistivity le21 to ge 14 ohm m (increased confidence)

1

Primarily 1 and lesser 2

Vsh ge 033 (average confidence) Vsh ge 033 anddeep resistivity le 14 ohm m (increased confidence)

Predicted well-log facies correspond to more than one depositional facies because of the overlapping range of Vsh and deep resistivity well-log data between depositionalfacies (Figure 10) The confidence of well-log facies interpretation increases when both the Vsh and deep resistivity guidelines are satisfied

well-log data between facies categories may re-sult in misinterpretation

Calcite cement is interpreted to occur withinwell-log facies 2 3 or 4 where the well-log neu-tron porosity exceeds the density porosity bymorethan 7 Within an oil-saturated reservoir such asthe Doe Creek at Valhalla this quick-look tech-nique may be used when density-neutron well-logdata are calibrated to a sandstonematrix density val-ue of 265 gcm3 (Asquith and Krygowski 2004)

To evaluate the accuracy of the predictivemod-el a blind test was administered to two cored wellswithin the study area that had not been includedwithin the database used to develop the well-logfacies or calcite cementation predictive guidelines(Figure 11) Cores from both wells were describedand interpreted independent of knowledge regard-ing the interpreted distribution of well-log faciesIn both cases the distribution of depositional faciesobserved within cores closely matches the pre-dicted distribution (Figure 11) The predicted dis-tribution of calcite cement precisely coincides withthe observed distribution within the 0014-15-075-08W6 well but only matches the thickest of fourcalcite-cemented zones within the 0006-32-074-09W6 well (Figure 11) The inability of the density-neutron predictor to detect calcite-cemented zoneswithin the 0006-32-074-09W6 well is likely re-lated to their thin (02ndash03 m [06ndash09 ft]) discon-tinuous distribution which is below the 045ndash061-m (17ndash2-ft) vertical resolution of density-neutron well logs (resolution via Alberty 1992)

STRATIGRAPHIC FRAMEWORK

Regional Overview

TheKaskapau Formationwas deposited during theculmination of the second-order transgression as-sociated with the Zuni cratonic sequence (sensuSloss 1963) and the concomitant maximum ex-tent of the North American WIS during the LateCretaceous (Hancock and Kauffman 1979 Haqet al 1988 Caldwell et al 1993 Kauffman andCaldwell 1993 Slingerland et al 1996) Withinthe study area second-order maximum flooding

coincides with the Greenhorn eustatic transgressionjust above the CenomanianndashTuronian boundaryand the equivalent organic-rich earliest TuronianSecondWhite Speckled Shale of the Kaskapau For-mation (Figure 1) (Caldwell 1983 Haq et al 1987Wallace-Dudley and Leckie 1993 Bhattacharya1994 Hogg et al 1998 Varban and Plint 2005)A higher frequency perhaps third-order episodeof decreasing and increasing accommodation dur-ing the middle Cenomanian to lower Turonian waspossibly induced by a pulse of foreland tectonismacross the WCSB and resulted in deposition oflowstand (Dunvegan) deltaic deposits and suc-ceeding transgressive (Kaskapau) open-marinemidshelf deposits (Plint et al 1993) Because ofthe relatively rapid rates of composite accommo-dation gain that resulted from constructive inter-ference of the second- and third-order transgres-sions higher frequency fourth-order stillstands(or lowstands) of sea level are recorded as retrogra-dationally stacked alternations of deeper water off-shore mudrocks (fourth-order transgressions) andshallower water shelf sandstones (fourth-orderstillstands or regressions) (Wallace-Dudley andLeckie 1993) In ascending stratigraphic orderthese shallower water sandstones include the DoeCreek Pouce Coupe and Howard Creek membersof the Kaskapau Formation (Figure 1)

Valhalla Reservoir Interval

TheDoeCreekMember atValhalla field consists ofnumerous cyclic alternations of offshore mudrock(facies 1) and shoreface sandstone (facies 2 3 andless commonly 4) Nine of these thin (1ndash10 m [3ndash33 ft] thick) cyclic units are correlated across Val-halla and their stratigraphic designations are refer-enced sequentially relative to their position above orbelow the top of themost oil productive I sandstone(alphanumeric designation by Wallace-Dudley andLeckie 1988) Above the I sandstone these includein ascending order the I + 1 through I + 6 and be-low the I sandstone in descending order the I minus 1and I minus 2 (Figures 11 12) In addition to the I sand-stone other hydrocarbon-charged units include theoil-prone I minus 1 and I + 1 and the gas-prone I + 2The I + 1 and I + 2 coincide with the previously

Atchley et al 13

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

2000) At Valhalla field oil is produced from theDoe Creek I and N sandstones and gas and smallamounts of oil are produced from the A sandstone(Wallace-Dudley and Leckie 1993) Reservoir sand-stones range from less than 1 to 7 m (3 to 23 ft) inthickness and consist of very fine to fine-grainedmarine shoreface deposits that are detached fromtheir equivalent shoreline and coastal plain depos-its (Figure 2) As such sandstone bodies transitionboth laterally and vertically into basinal marineshales (Kreitner and Plint 2006) The isolatednature of the sandstones and the southwestwardregional structural dip account for the hydrocar-bon entrapment at Valhalla and define the fieldlimits (Figure 3)

4 EampP Note

Data and Methods

Data from Valhalla field incorporated into thestudy include wireline logs and historic fluid pro-duction data for 508 wells and descriptive and ac-companying porosity and permeability data for allavailable Doe Creek cores (approximately 1350m[4429 ft] of core from 120wells) (Figure 3) Wellsreferenced in this manuscript are designated bythe Canadian system of unique well identifiers eg0002-12-075-08W6 In this example the well islocated within legal subdivision 02 (of 16 possible)of section 12 within township 75 north range 08west of the 6th meridian The 00 designation pre-ceding 02 is the legal exception code that is used

Figure 3 Basemap for Valhalla field showing the distribution of data incorporated into the study including wells with open-hole logs(black well symbols) cased-hole logs (gray well symbols) and core from the Doe Creek sandstones (circled well symbols) Contours arefor the structural top of the Doe Creek I sandstone and the dashed line delimits the lateral extent of reservoir sandstone and the positionof the original oil-water contact (+46 m [151 ft]) within the southwestern part of the field

Table 1 Summary Table of Features Diagnostic to Sedimentary Facies Observed Within the Doe Creek Member Valhalla Field

Reservoir FaciesNo Designation 1a 1b 1c 2 3 4

Name Black-laminatedmudstone

Flaser- tolenticular-beddedmudrock

Burrowed mudrock Intercalated burrowedmudrock andwave-rippled tohummockycross-stratifiedsandstone

Laminated to hummockycross-stratified sandstone

Planar tabular totrough cross-stratifiedsandstone

Environment Offshore Offshore Offshore Distal lower shoreface Proximal lower shoreface Upper shorefaceTypical mud content gt90 60ndash80 55ndash85 10ndash50 0ndash10 0ndash5Typical grain size Silt and clay Silt and clay Silt and clay Lower very fine to lower

fine sandUpper very fine to upperfine sand

Lower fine to upperfine

Typical ichnofabricindex

1ndash2 1ndash2 4ndash5 sand = 1ndash2 mud = 2ndash4 1ndash5 1ndash2

Ichnofacies(representativeichnofauna)

Cruziana(ThallassinoidesPlanolites)

Cruziana(ThallassinoidesPlanolites)

Cruziana(ThallassinoidesPlanolitesZoophycos Scolicia)

Cruziana (ThallassinoidesPlanolites TeichichnusZoophycos ScoliciaPalaeophycus) Skolithos(Skolithos OphiomorphaPalaeophycus)

Skolithos (SkolithosOphiomorphaPalaeophycusBergaureria)

Skolithos (SkolithosOphiomorphaPalaeophycus)

Mechanicalsedimentarystructures

Millimeter-scalelamina

Millimeter-scalelamina flaserbedding

None observed Flaser bedding hummockycross-stratification waveripples

Hummocky cross-stratificationmillimeter-scale laminawave ripples

Trough cross-stratificationplanar tabularstratification

Representative corephotos

Figure 4A Figure 4B Figure 4C Figure 4D Figure 4E Figure 4FAtchleyetal

5

when more than one well exists within a single le-gal subdivision eg a 03 would identify the thirdwell drilled in a legal subdivision Core descrip-tions include documentation of the vertical distribu-tions of grain size the fraction of mud-size particles

6 EampP Note

mechanical sedimentary structures ichnogeneraand ichnofacies ichnofabric index and ichnofaunaldiversity (sensu Droser and Bottjer 1986 Bottjerand Droser 1991) sedimentary facies and cementtype and distribution Core description data were

Figure 4 Core photographs representative of facies observed within the Doe Creek at Valhalla field (compare with Table 1) Scale bar =1 cm (03 in) Burrows labeled on the photographs include Planolites (Pl) Thalassinoides (Th) Zoophycos (Zoo) Asterosoma (Ast) andOphiomorpha (Oph) (A) Facies 1a (black-laminated mudrock) within the 0214-29-74-9W60 well at 7005 m (22982 ft) (B) Facies 1b(flaser to lenticular bedded mudrock) within the 008-34-75-8W60 well at 7815 m (25639 ft) (C) Facies 1c (burrowed mudrock) withinthe 0016-29-74-9W60 well at 7105 m (2331 ft) (D) Facies 2 (intercalated burrowed mudrock and wave rippled to hummocky cross-stratified sandstone) within the 036-15-75-9W60 well at 737 m (2418 ft) (E) Facies 3 (laminated to hummocky cross-stratified sand-stone) within the 008-12-75-9W60 well at 6885 m (22588 ft) (F) Facies 4 (planar tabular to trough cross-stratified sandstone) withinthe 0010-11-75-9W60 well at 6885 m (22588 ft) Note the irregular distribution of calcite cement Well identifiers are explained in theData and Methods section

digitized and merged with digital core analysis po-rosity and permeability data and then depth shiftedto coincide with digital wireline well-log data Alldepth-reconciled borehole data types were thenexported to a computer spreadsheet for statisticalanalysis Stratigraphic correlations and preliminarymaps were computer generated and spurious com-puter contouring artifacts were hand correctedAnalysis andmapping of average daily fluid produc-tion trends are based on data compiled from the Al-berta Energy Resources Conservation Board Allwell-log analyses and predictive transforms werecompleted using GeoGraphixreg Prizmtrade log inter-pretation software

RESERVOIR CHARACTERIZATIONAND CLASSIFICATION

Facies Model

The Doe Creek Member of west-central Albertahas been extensively studied and interpreted as awave-dominated and river-influenced shelf sand-stone complex that accumulated in a shallow em-bayment where salinity varied from normalmarineto brackish (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998 Reid and Pemberton 2005 Kreitner andPlint 2006) Previous studies have identified from

Figure 5 Box and whisker plots of neutron-corrected effective porosity and maximum permeability(Kmax) versus grain size and stacked bar charts illustrating the proportion of depositional facies thatcomprise each grain-size category Abbreviations include MUD (clay plusmn silt) VFL (lower very fine sand)VFU (upper very fine sand) FL (lower fine sand) and FU (upper fine sand) Analysis is based on allcores described at Valhalla field Both porosity and increasing proportions of shallower water faciescorrespond to an increase in grain size Permeability displays a similar increase however high valueswithin the lower very fine sand and mud-size fractions are inconsistent to the trend and likely reflectpreferential sampling of thin relatively coarser grained sandstone interbeds within otherwise finergrained successions

Atchley et al 7

8 EampP Note

6 to 10 depositional facies within the Doe Creek atValhalla field (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998) We recognize six depositional facies thatare distinguished on the basis of grain sizemechani-cal sedimentary structures ichnofacies and theextent of bioturbation (ie ichnofabric index ofDroser and Bottjer 1986 and Bottjer and Droser1991 or bioturbate texture of Frey and Pemberton1990) and consistent with these previous studiesare interpreted to have been deposited within off-shore and lower and upper shoreface restricted toopen-marine environments (Table 1 Figure 4)Upper shoreface deposits are rarely observed with-in the study area Variations in ichnofabric indexand ichnofaunal constituents suggest that offshoredeposits are composed of those that accumulated un-der restricted (facies 1a) and relatively more open-marine (facies 1b and 1c) conditions (Table 1)Facies 1a and 1b are characterized by a general lackof bioturbation and almost exclusively include Pla-nolites and the Cruziana ichnofacies componentThalassinoides Conversely facies 1c is homogenizedby burrows that commonly include not only Plano-lites and Thalassinoides but also theCruziana ichno-facies constituents Zoophycos and more rarely Aster-osoma and Rhizocorallium Distal lower shorefacedeposits (facies 2) are dominated by both the Sko-lithos ichnofacies traces Ophiomorpha and Paleo-phycos and the Cruziana ichnofacies traces thatare also observed within facies 1c Proximal lowershoreface (facies 3) and upper shoreface (facies 4)deposits are characterized by Skolithos ichnofaciesforms that typically include Ophiomorpha Paleo-phycos Skolithos and Bergaureria (Table 1)

Controls on Reservoir Quality

Porosity and permeability within the Doe Creekclosely correlate with environmentally controlled

grain size and associated ichnofabric index trendsand the distribution of secondary calcite cementIncreasing grain size corresponds to both an in-crease in neutron-corrected effective porosity andfacies that accumulated in progressively shallowerwater shoreface environments (Figure 5) A similarcorrelation exists with permeability however anom-alously high permeability values within the lowervery fine sandstone andmud-size fractions likely re-flect a core analysis sampling bias introduced bythe preferential sampling of thin (10ndash30 cm [4ndash12 in]) interbeds of relatively coarser grained sand-stone within what was interval averaged duringcore description as relatively distal mud-prone de-posits (Figure 5) Note that the anomalously highpermeability values are not mimicked by neutron-corrected effective porosity because the thin coars-er grained sandstone interbeds that account for thehigh permeability values are below the 061-m (2 ft)vertical resolution of the neutron sonde (resolu-tion estimate via Alberty 1992)

A second potential influence on reservoir qual-ity is the extent of bioturbation and resultant intro-duction of interparticle silt and clay-size particles Toevaluate this possibility we compare porosity andpermeability to five categories of bioturbation in-tensity aka the ichnofabric index under two con-ditions (Figure 6) Under condition 1 the ichnofab-ric index is compared against neutron-correctedeffective porosity and core analysismaximumperme-ability (Kmax) for all core data (Figure 6A) whereascondition 2 limits the comparison to facies 2 3and 4 sandstones having a porosity of greater thanor equal to 12 (Figure 6B) ie the porosity cut-off at Valhalla field that is commonly used by localindustry producers to define reservoir-quality sand-stone (Figure 7) In condition 1 porosity and per-meability trends including the assumed perme-ability sampling bias are similar to those for grainsize and associated depositional facies (compare

Figure 6 Box and whisker plots of neutron-corrected effective porosity and maximum permeability (Kmax) versus ichnofabric indexand stacked bar charts illustrating the proportion of depositional facies that comprise each ichnofabric index category (A) Condition 1case based on analysis of all core data regardless of lithology and (B) condition 2 case based on analysis of all core data where facies 23 and 4 sandstones have a porosity of greater than or equal to 12 Regardless of condition or ichnofabric index category the highestvalues of porosity and permeability coincide with the highest proportion of the shallowest water facies This suggests that grain size anddepositional environment are the primary control on reservoir quality

Atchley et al 9

Figure 7 Scatter plot of coreporosity versus Kmax perme-ability for all Doe Creekndashcoredwells within the study area(compare with Figure 3) Brack-eted on the plot is the rangeof permeability (Kmax) acrosswellbore perforations (1ndash10 md)which as of this writing coin-cides with the lower limit of eco-nomic daily oil production(approximately lt10 bblday)observed at Valhalla field Per-meability values within this rangein turn coincide with a porosityrange of 10ndash15 and is sup-portive of the practice of Valhallafield operators to apply a 12porosity cutoff to define the lowerlimit of satisfactory Doe Creeksandstone reservoir quality

Figure 8 Box and whisker plotof neutron-corrected effectiveporosity and maximum perme-ability (Kmax) versus sandstonewhere calcite cement eitherwas (Yes) or was not (No) ob-served Analysis is based onan analysis of all core data wherefacies 2 3 and 4 sandstoneshave a porosity of greater thanor equal to 12 Both porosityand permeability are reducedin the presence of calcite cement

10 EampP Note

Figures 5 6A) In condition 2 the highest valuesof porosity and permeability similarly coincide withboth ichnofabric indices 1 2 and 5 and the highestproportion of the shallowest water sandstone de-posits (principally facies 3) (Figure 6B) These anal-yses suggest that grain-size trends and associatedenvironments of deposition instead of the ichno-fabric index are the primary sedimentologic con-trols on reservoir quality

Reservoir quality is diminished in the presenceof postdepositional calcite cement (Figures 4F 8)Calcite cement is most common near the top ofsandstone bodies and is also irregularly distributedwithin sandstone bodies Zones of calcite cementa-tion are thin (typically lt50 cm [20 in]) and likelyhave a lateral continuity of less than 4 m (13 ft)(Hogg et al 1998) Calcite cement is increasinglyabundant within shallower water deposits and mayhave resulted from the preferential flow of CaCO3-saturated fluids through their characteristically

Figure 9 Box plot of depositional facies versus the percentageof core specimens that have calcite cement Analysis is based onall cores described at Valhalla field

Figure 10 Box and whisker plots of facies versus (A) open-holedeep resistivity well-log data and (B) shale volume (Vsh) contentderived from open-hole gamma-ray well-log data (compare withTable 2)

Atchley et al 11

higher permeability relatively coarser grained sed-iments (Figure 5) Of the total core observationscalcite cement is present within 25 of the occur-rences of facies 1 (offshore) 85 of the occur-rences of facies 2 (distal lower shoreface) 16 ofthe occurrence of facies 3 (proximal lower shore-face) and 64 of the occurrences of facies 4 (up-per shoreface) (Figure 9) The particularly highpercentage of calcite cement within facies 4 is re-lated to both the limited observations of facies 4and the consistent position of facies 4 near thetop of sandstone bodies

Reservoir Facies Prediction

Comparison of core-observed depositional faciesand petrophysical attributes suggests that open-holegamma ray and deep resistivity well-log data maybe used to predict facies distributions in wells thatlack core control (Figure 10) Open-hole neutron-porosity logs were also evaluated for their poten-tial to predict facies but proved to be less reliableGuidelines for facies prediction use direct mea-surement of deep resistivity for oil-saturated reser-voir above the original oil-water contact (+46 m[151 ft]) and shale volume calculations (Vsh) de-rived from normalized gamma-ray measurementsBecause the Doe Creek study interval was oil fullduring initial field development the guidelines forthe use of deep resistivity are applicable to most (if

12 EampP Note

not all) wells with open-hole logs within the studyarea (Figure 3) Gamma-ray normalization andVsh

calculations are based on the following formulae(Larinov 1969 Asquith and Krygowski 2004 ascited by Asquith and Krygowski 2004)

IGR frac14 ethGRlog GRminTHORN=ethGRmax GRminTHORN

where the IGR is the gamma-ray index GRlog is thegamma-raymeasurement from the reservoir GRmin

is the 14 API units and GRmax is the 130 APIunits (asterisks indicate Valhalla fieldwide averageminimum and maximum values observed in open-hole wireline logs)

V sh frac14 033frac122eth2 IGRTHORN 1

where Vsh is the shale volumeTable 2 defines the guidelines for prediction

and equates the resultant well-log facies with theirdepositional facies counterparts Although well-log data cluster within discrete ranges for each de-positional facies the overlapping range of Vsh anddeep resistivity data between facies results in well-log facies predictions that reflect data blendingbetween two (or more) depositional facies (com-pare Table 2 and Figure 10) Consequently al-though predicted well-log facies are most likely as-sociated with a particular depositional facies theintroduction of error from overlapping ranges of

Table 2 Guidelines for Facies Prediction from Open-Hole Well Logs

Well-Log Facies

Depositional Facies Equivalents

Parameters for Well-Log Prediction

4

Primarily 3 and 4

Vsh le 009 (average confidence)Vsh le 009 and deep resistivity ge38 ohm m (increased confidence)

3

Primarily 3 and lesser 2

Vsh ge 009 to le 012 (average confidence)Vsh ge 009 to le 012 and deep resistivity le38 to ge 21 ohm m (increased confidence)

2

Primarily 2 and lesser 1

Vsh ge 012 to le 033 (average confidence)Vsh ge 012 to le 033 and deep resistivity le21 to ge 14 ohm m (increased confidence)

1

Primarily 1 and lesser 2

Vsh ge 033 (average confidence) Vsh ge 033 anddeep resistivity le 14 ohm m (increased confidence)

Predicted well-log facies correspond to more than one depositional facies because of the overlapping range of Vsh and deep resistivity well-log data between depositionalfacies (Figure 10) The confidence of well-log facies interpretation increases when both the Vsh and deep resistivity guidelines are satisfied

well-log data between facies categories may re-sult in misinterpretation

Calcite cement is interpreted to occur withinwell-log facies 2 3 or 4 where the well-log neu-tron porosity exceeds the density porosity bymorethan 7 Within an oil-saturated reservoir such asthe Doe Creek at Valhalla this quick-look tech-nique may be used when density-neutron well-logdata are calibrated to a sandstonematrix density val-ue of 265 gcm3 (Asquith and Krygowski 2004)

To evaluate the accuracy of the predictivemod-el a blind test was administered to two cored wellswithin the study area that had not been includedwithin the database used to develop the well-logfacies or calcite cementation predictive guidelines(Figure 11) Cores from both wells were describedand interpreted independent of knowledge regard-ing the interpreted distribution of well-log faciesIn both cases the distribution of depositional faciesobserved within cores closely matches the pre-dicted distribution (Figure 11) The predicted dis-tribution of calcite cement precisely coincides withthe observed distribution within the 0014-15-075-08W6 well but only matches the thickest of fourcalcite-cemented zones within the 0006-32-074-09W6 well (Figure 11) The inability of the density-neutron predictor to detect calcite-cemented zoneswithin the 0006-32-074-09W6 well is likely re-lated to their thin (02ndash03 m [06ndash09 ft]) discon-tinuous distribution which is below the 045ndash061-m (17ndash2-ft) vertical resolution of density-neutron well logs (resolution via Alberty 1992)

STRATIGRAPHIC FRAMEWORK

Regional Overview

TheKaskapau Formationwas deposited during theculmination of the second-order transgression as-sociated with the Zuni cratonic sequence (sensuSloss 1963) and the concomitant maximum ex-tent of the North American WIS during the LateCretaceous (Hancock and Kauffman 1979 Haqet al 1988 Caldwell et al 1993 Kauffman andCaldwell 1993 Slingerland et al 1996) Withinthe study area second-order maximum flooding

coincides with the Greenhorn eustatic transgressionjust above the CenomanianndashTuronian boundaryand the equivalent organic-rich earliest TuronianSecondWhite Speckled Shale of the Kaskapau For-mation (Figure 1) (Caldwell 1983 Haq et al 1987Wallace-Dudley and Leckie 1993 Bhattacharya1994 Hogg et al 1998 Varban and Plint 2005)A higher frequency perhaps third-order episodeof decreasing and increasing accommodation dur-ing the middle Cenomanian to lower Turonian waspossibly induced by a pulse of foreland tectonismacross the WCSB and resulted in deposition oflowstand (Dunvegan) deltaic deposits and suc-ceeding transgressive (Kaskapau) open-marinemidshelf deposits (Plint et al 1993) Because ofthe relatively rapid rates of composite accommo-dation gain that resulted from constructive inter-ference of the second- and third-order transgres-sions higher frequency fourth-order stillstands(or lowstands) of sea level are recorded as retrogra-dationally stacked alternations of deeper water off-shore mudrocks (fourth-order transgressions) andshallower water shelf sandstones (fourth-orderstillstands or regressions) (Wallace-Dudley andLeckie 1993) In ascending stratigraphic orderthese shallower water sandstones include the DoeCreek Pouce Coupe and Howard Creek membersof the Kaskapau Formation (Figure 1)

Valhalla Reservoir Interval

TheDoeCreekMember atValhalla field consists ofnumerous cyclic alternations of offshore mudrock(facies 1) and shoreface sandstone (facies 2 3 andless commonly 4) Nine of these thin (1ndash10 m [3ndash33 ft] thick) cyclic units are correlated across Val-halla and their stratigraphic designations are refer-enced sequentially relative to their position above orbelow the top of themost oil productive I sandstone(alphanumeric designation by Wallace-Dudley andLeckie 1988) Above the I sandstone these includein ascending order the I + 1 through I + 6 and be-low the I sandstone in descending order the I minus 1and I minus 2 (Figures 11 12) In addition to the I sand-stone other hydrocarbon-charged units include theoil-prone I minus 1 and I + 1 and the gas-prone I + 2The I + 1 and I + 2 coincide with the previously

Atchley et al 13

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

Table 1 Summary Table of Features Diagnostic to Sedimentary Facies Observed Within the Doe Creek Member Valhalla Field

Reservoir FaciesNo Designation 1a 1b 1c 2 3 4

Name Black-laminatedmudstone

Flaser- tolenticular-beddedmudrock

Burrowed mudrock Intercalated burrowedmudrock andwave-rippled tohummockycross-stratifiedsandstone

Laminated to hummockycross-stratified sandstone

Planar tabular totrough cross-stratifiedsandstone

Environment Offshore Offshore Offshore Distal lower shoreface Proximal lower shoreface Upper shorefaceTypical mud content gt90 60ndash80 55ndash85 10ndash50 0ndash10 0ndash5Typical grain size Silt and clay Silt and clay Silt and clay Lower very fine to lower

fine sandUpper very fine to upperfine sand

Lower fine to upperfine

Typical ichnofabricindex

1ndash2 1ndash2 4ndash5 sand = 1ndash2 mud = 2ndash4 1ndash5 1ndash2

Ichnofacies(representativeichnofauna)

Cruziana(ThallassinoidesPlanolites)

Cruziana(ThallassinoidesPlanolites)

Cruziana(ThallassinoidesPlanolitesZoophycos Scolicia)

Cruziana (ThallassinoidesPlanolites TeichichnusZoophycos ScoliciaPalaeophycus) Skolithos(Skolithos OphiomorphaPalaeophycus)

Skolithos (SkolithosOphiomorphaPalaeophycusBergaureria)

Skolithos (SkolithosOphiomorphaPalaeophycus)

Mechanicalsedimentarystructures

Millimeter-scalelamina

Millimeter-scalelamina flaserbedding

None observed Flaser bedding hummockycross-stratification waveripples

Hummocky cross-stratificationmillimeter-scale laminawave ripples

Trough cross-stratificationplanar tabularstratification

Representative corephotos

Figure 4A Figure 4B Figure 4C Figure 4D Figure 4E Figure 4FAtchleyetal

5

when more than one well exists within a single le-gal subdivision eg a 03 would identify the thirdwell drilled in a legal subdivision Core descrip-tions include documentation of the vertical distribu-tions of grain size the fraction of mud-size particles

6 EampP Note

mechanical sedimentary structures ichnogeneraand ichnofacies ichnofabric index and ichnofaunaldiversity (sensu Droser and Bottjer 1986 Bottjerand Droser 1991) sedimentary facies and cementtype and distribution Core description data were

Figure 4 Core photographs representative of facies observed within the Doe Creek at Valhalla field (compare with Table 1) Scale bar =1 cm (03 in) Burrows labeled on the photographs include Planolites (Pl) Thalassinoides (Th) Zoophycos (Zoo) Asterosoma (Ast) andOphiomorpha (Oph) (A) Facies 1a (black-laminated mudrock) within the 0214-29-74-9W60 well at 7005 m (22982 ft) (B) Facies 1b(flaser to lenticular bedded mudrock) within the 008-34-75-8W60 well at 7815 m (25639 ft) (C) Facies 1c (burrowed mudrock) withinthe 0016-29-74-9W60 well at 7105 m (2331 ft) (D) Facies 2 (intercalated burrowed mudrock and wave rippled to hummocky cross-stratified sandstone) within the 036-15-75-9W60 well at 737 m (2418 ft) (E) Facies 3 (laminated to hummocky cross-stratified sand-stone) within the 008-12-75-9W60 well at 6885 m (22588 ft) (F) Facies 4 (planar tabular to trough cross-stratified sandstone) withinthe 0010-11-75-9W60 well at 6885 m (22588 ft) Note the irregular distribution of calcite cement Well identifiers are explained in theData and Methods section

digitized and merged with digital core analysis po-rosity and permeability data and then depth shiftedto coincide with digital wireline well-log data Alldepth-reconciled borehole data types were thenexported to a computer spreadsheet for statisticalanalysis Stratigraphic correlations and preliminarymaps were computer generated and spurious com-puter contouring artifacts were hand correctedAnalysis andmapping of average daily fluid produc-tion trends are based on data compiled from the Al-berta Energy Resources Conservation Board Allwell-log analyses and predictive transforms werecompleted using GeoGraphixreg Prizmtrade log inter-pretation software

RESERVOIR CHARACTERIZATIONAND CLASSIFICATION

Facies Model

The Doe Creek Member of west-central Albertahas been extensively studied and interpreted as awave-dominated and river-influenced shelf sand-stone complex that accumulated in a shallow em-bayment where salinity varied from normalmarineto brackish (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998 Reid and Pemberton 2005 Kreitner andPlint 2006) Previous studies have identified from

Figure 5 Box and whisker plots of neutron-corrected effective porosity and maximum permeability(Kmax) versus grain size and stacked bar charts illustrating the proportion of depositional facies thatcomprise each grain-size category Abbreviations include MUD (clay plusmn silt) VFL (lower very fine sand)VFU (upper very fine sand) FL (lower fine sand) and FU (upper fine sand) Analysis is based on allcores described at Valhalla field Both porosity and increasing proportions of shallower water faciescorrespond to an increase in grain size Permeability displays a similar increase however high valueswithin the lower very fine sand and mud-size fractions are inconsistent to the trend and likely reflectpreferential sampling of thin relatively coarser grained sandstone interbeds within otherwise finergrained successions

Atchley et al 7

8 EampP Note

6 to 10 depositional facies within the Doe Creek atValhalla field (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998) We recognize six depositional facies thatare distinguished on the basis of grain sizemechani-cal sedimentary structures ichnofacies and theextent of bioturbation (ie ichnofabric index ofDroser and Bottjer 1986 and Bottjer and Droser1991 or bioturbate texture of Frey and Pemberton1990) and consistent with these previous studiesare interpreted to have been deposited within off-shore and lower and upper shoreface restricted toopen-marine environments (Table 1 Figure 4)Upper shoreface deposits are rarely observed with-in the study area Variations in ichnofabric indexand ichnofaunal constituents suggest that offshoredeposits are composed of those that accumulated un-der restricted (facies 1a) and relatively more open-marine (facies 1b and 1c) conditions (Table 1)Facies 1a and 1b are characterized by a general lackof bioturbation and almost exclusively include Pla-nolites and the Cruziana ichnofacies componentThalassinoides Conversely facies 1c is homogenizedby burrows that commonly include not only Plano-lites and Thalassinoides but also theCruziana ichno-facies constituents Zoophycos and more rarely Aster-osoma and Rhizocorallium Distal lower shorefacedeposits (facies 2) are dominated by both the Sko-lithos ichnofacies traces Ophiomorpha and Paleo-phycos and the Cruziana ichnofacies traces thatare also observed within facies 1c Proximal lowershoreface (facies 3) and upper shoreface (facies 4)deposits are characterized by Skolithos ichnofaciesforms that typically include Ophiomorpha Paleo-phycos Skolithos and Bergaureria (Table 1)

Controls on Reservoir Quality

Porosity and permeability within the Doe Creekclosely correlate with environmentally controlled

grain size and associated ichnofabric index trendsand the distribution of secondary calcite cementIncreasing grain size corresponds to both an in-crease in neutron-corrected effective porosity andfacies that accumulated in progressively shallowerwater shoreface environments (Figure 5) A similarcorrelation exists with permeability however anom-alously high permeability values within the lowervery fine sandstone andmud-size fractions likely re-flect a core analysis sampling bias introduced bythe preferential sampling of thin (10ndash30 cm [4ndash12 in]) interbeds of relatively coarser grained sand-stone within what was interval averaged duringcore description as relatively distal mud-prone de-posits (Figure 5) Note that the anomalously highpermeability values are not mimicked by neutron-corrected effective porosity because the thin coars-er grained sandstone interbeds that account for thehigh permeability values are below the 061-m (2 ft)vertical resolution of the neutron sonde (resolu-tion estimate via Alberty 1992)

A second potential influence on reservoir qual-ity is the extent of bioturbation and resultant intro-duction of interparticle silt and clay-size particles Toevaluate this possibility we compare porosity andpermeability to five categories of bioturbation in-tensity aka the ichnofabric index under two con-ditions (Figure 6) Under condition 1 the ichnofab-ric index is compared against neutron-correctedeffective porosity and core analysismaximumperme-ability (Kmax) for all core data (Figure 6A) whereascondition 2 limits the comparison to facies 2 3and 4 sandstones having a porosity of greater thanor equal to 12 (Figure 6B) ie the porosity cut-off at Valhalla field that is commonly used by localindustry producers to define reservoir-quality sand-stone (Figure 7) In condition 1 porosity and per-meability trends including the assumed perme-ability sampling bias are similar to those for grainsize and associated depositional facies (compare

Figure 6 Box and whisker plots of neutron-corrected effective porosity and maximum permeability (Kmax) versus ichnofabric indexand stacked bar charts illustrating the proportion of depositional facies that comprise each ichnofabric index category (A) Condition 1case based on analysis of all core data regardless of lithology and (B) condition 2 case based on analysis of all core data where facies 23 and 4 sandstones have a porosity of greater than or equal to 12 Regardless of condition or ichnofabric index category the highestvalues of porosity and permeability coincide with the highest proportion of the shallowest water facies This suggests that grain size anddepositional environment are the primary control on reservoir quality

Atchley et al 9

Figure 7 Scatter plot of coreporosity versus Kmax perme-ability for all Doe Creekndashcoredwells within the study area(compare with Figure 3) Brack-eted on the plot is the rangeof permeability (Kmax) acrosswellbore perforations (1ndash10 md)which as of this writing coin-cides with the lower limit of eco-nomic daily oil production(approximately lt10 bblday)observed at Valhalla field Per-meability values within this rangein turn coincide with a porosityrange of 10ndash15 and is sup-portive of the practice of Valhallafield operators to apply a 12porosity cutoff to define the lowerlimit of satisfactory Doe Creeksandstone reservoir quality

Figure 8 Box and whisker plotof neutron-corrected effectiveporosity and maximum perme-ability (Kmax) versus sandstonewhere calcite cement eitherwas (Yes) or was not (No) ob-served Analysis is based onan analysis of all core data wherefacies 2 3 and 4 sandstoneshave a porosity of greater thanor equal to 12 Both porosityand permeability are reducedin the presence of calcite cement

10 EampP Note

Figures 5 6A) In condition 2 the highest valuesof porosity and permeability similarly coincide withboth ichnofabric indices 1 2 and 5 and the highestproportion of the shallowest water sandstone de-posits (principally facies 3) (Figure 6B) These anal-yses suggest that grain-size trends and associatedenvironments of deposition instead of the ichno-fabric index are the primary sedimentologic con-trols on reservoir quality

Reservoir quality is diminished in the presenceof postdepositional calcite cement (Figures 4F 8)Calcite cement is most common near the top ofsandstone bodies and is also irregularly distributedwithin sandstone bodies Zones of calcite cementa-tion are thin (typically lt50 cm [20 in]) and likelyhave a lateral continuity of less than 4 m (13 ft)(Hogg et al 1998) Calcite cement is increasinglyabundant within shallower water deposits and mayhave resulted from the preferential flow of CaCO3-saturated fluids through their characteristically

Figure 9 Box plot of depositional facies versus the percentageof core specimens that have calcite cement Analysis is based onall cores described at Valhalla field

Figure 10 Box and whisker plots of facies versus (A) open-holedeep resistivity well-log data and (B) shale volume (Vsh) contentderived from open-hole gamma-ray well-log data (compare withTable 2)

Atchley et al 11

higher permeability relatively coarser grained sed-iments (Figure 5) Of the total core observationscalcite cement is present within 25 of the occur-rences of facies 1 (offshore) 85 of the occur-rences of facies 2 (distal lower shoreface) 16 ofthe occurrence of facies 3 (proximal lower shore-face) and 64 of the occurrences of facies 4 (up-per shoreface) (Figure 9) The particularly highpercentage of calcite cement within facies 4 is re-lated to both the limited observations of facies 4and the consistent position of facies 4 near thetop of sandstone bodies

Reservoir Facies Prediction

Comparison of core-observed depositional faciesand petrophysical attributes suggests that open-holegamma ray and deep resistivity well-log data maybe used to predict facies distributions in wells thatlack core control (Figure 10) Open-hole neutron-porosity logs were also evaluated for their poten-tial to predict facies but proved to be less reliableGuidelines for facies prediction use direct mea-surement of deep resistivity for oil-saturated reser-voir above the original oil-water contact (+46 m[151 ft]) and shale volume calculations (Vsh) de-rived from normalized gamma-ray measurementsBecause the Doe Creek study interval was oil fullduring initial field development the guidelines forthe use of deep resistivity are applicable to most (if

12 EampP Note

not all) wells with open-hole logs within the studyarea (Figure 3) Gamma-ray normalization andVsh

calculations are based on the following formulae(Larinov 1969 Asquith and Krygowski 2004 ascited by Asquith and Krygowski 2004)

IGR frac14 ethGRlog GRminTHORN=ethGRmax GRminTHORN

where the IGR is the gamma-ray index GRlog is thegamma-raymeasurement from the reservoir GRmin

is the 14 API units and GRmax is the 130 APIunits (asterisks indicate Valhalla fieldwide averageminimum and maximum values observed in open-hole wireline logs)

V sh frac14 033frac122eth2 IGRTHORN 1

where Vsh is the shale volumeTable 2 defines the guidelines for prediction

and equates the resultant well-log facies with theirdepositional facies counterparts Although well-log data cluster within discrete ranges for each de-positional facies the overlapping range of Vsh anddeep resistivity data between facies results in well-log facies predictions that reflect data blendingbetween two (or more) depositional facies (com-pare Table 2 and Figure 10) Consequently al-though predicted well-log facies are most likely as-sociated with a particular depositional facies theintroduction of error from overlapping ranges of

Table 2 Guidelines for Facies Prediction from Open-Hole Well Logs

Well-Log Facies

Depositional Facies Equivalents

Parameters for Well-Log Prediction

4

Primarily 3 and 4

Vsh le 009 (average confidence)Vsh le 009 and deep resistivity ge38 ohm m (increased confidence)

3

Primarily 3 and lesser 2

Vsh ge 009 to le 012 (average confidence)Vsh ge 009 to le 012 and deep resistivity le38 to ge 21 ohm m (increased confidence)

2

Primarily 2 and lesser 1

Vsh ge 012 to le 033 (average confidence)Vsh ge 012 to le 033 and deep resistivity le21 to ge 14 ohm m (increased confidence)

1

Primarily 1 and lesser 2

Vsh ge 033 (average confidence) Vsh ge 033 anddeep resistivity le 14 ohm m (increased confidence)

Predicted well-log facies correspond to more than one depositional facies because of the overlapping range of Vsh and deep resistivity well-log data between depositionalfacies (Figure 10) The confidence of well-log facies interpretation increases when both the Vsh and deep resistivity guidelines are satisfied

well-log data between facies categories may re-sult in misinterpretation

Calcite cement is interpreted to occur withinwell-log facies 2 3 or 4 where the well-log neu-tron porosity exceeds the density porosity bymorethan 7 Within an oil-saturated reservoir such asthe Doe Creek at Valhalla this quick-look tech-nique may be used when density-neutron well-logdata are calibrated to a sandstonematrix density val-ue of 265 gcm3 (Asquith and Krygowski 2004)

To evaluate the accuracy of the predictivemod-el a blind test was administered to two cored wellswithin the study area that had not been includedwithin the database used to develop the well-logfacies or calcite cementation predictive guidelines(Figure 11) Cores from both wells were describedand interpreted independent of knowledge regard-ing the interpreted distribution of well-log faciesIn both cases the distribution of depositional faciesobserved within cores closely matches the pre-dicted distribution (Figure 11) The predicted dis-tribution of calcite cement precisely coincides withthe observed distribution within the 0014-15-075-08W6 well but only matches the thickest of fourcalcite-cemented zones within the 0006-32-074-09W6 well (Figure 11) The inability of the density-neutron predictor to detect calcite-cemented zoneswithin the 0006-32-074-09W6 well is likely re-lated to their thin (02ndash03 m [06ndash09 ft]) discon-tinuous distribution which is below the 045ndash061-m (17ndash2-ft) vertical resolution of density-neutron well logs (resolution via Alberty 1992)

STRATIGRAPHIC FRAMEWORK

Regional Overview

TheKaskapau Formationwas deposited during theculmination of the second-order transgression as-sociated with the Zuni cratonic sequence (sensuSloss 1963) and the concomitant maximum ex-tent of the North American WIS during the LateCretaceous (Hancock and Kauffman 1979 Haqet al 1988 Caldwell et al 1993 Kauffman andCaldwell 1993 Slingerland et al 1996) Withinthe study area second-order maximum flooding

coincides with the Greenhorn eustatic transgressionjust above the CenomanianndashTuronian boundaryand the equivalent organic-rich earliest TuronianSecondWhite Speckled Shale of the Kaskapau For-mation (Figure 1) (Caldwell 1983 Haq et al 1987Wallace-Dudley and Leckie 1993 Bhattacharya1994 Hogg et al 1998 Varban and Plint 2005)A higher frequency perhaps third-order episodeof decreasing and increasing accommodation dur-ing the middle Cenomanian to lower Turonian waspossibly induced by a pulse of foreland tectonismacross the WCSB and resulted in deposition oflowstand (Dunvegan) deltaic deposits and suc-ceeding transgressive (Kaskapau) open-marinemidshelf deposits (Plint et al 1993) Because ofthe relatively rapid rates of composite accommo-dation gain that resulted from constructive inter-ference of the second- and third-order transgres-sions higher frequency fourth-order stillstands(or lowstands) of sea level are recorded as retrogra-dationally stacked alternations of deeper water off-shore mudrocks (fourth-order transgressions) andshallower water shelf sandstones (fourth-orderstillstands or regressions) (Wallace-Dudley andLeckie 1993) In ascending stratigraphic orderthese shallower water sandstones include the DoeCreek Pouce Coupe and Howard Creek membersof the Kaskapau Formation (Figure 1)

Valhalla Reservoir Interval

TheDoeCreekMember atValhalla field consists ofnumerous cyclic alternations of offshore mudrock(facies 1) and shoreface sandstone (facies 2 3 andless commonly 4) Nine of these thin (1ndash10 m [3ndash33 ft] thick) cyclic units are correlated across Val-halla and their stratigraphic designations are refer-enced sequentially relative to their position above orbelow the top of themost oil productive I sandstone(alphanumeric designation by Wallace-Dudley andLeckie 1988) Above the I sandstone these includein ascending order the I + 1 through I + 6 and be-low the I sandstone in descending order the I minus 1and I minus 2 (Figures 11 12) In addition to the I sand-stone other hydrocarbon-charged units include theoil-prone I minus 1 and I + 1 and the gas-prone I + 2The I + 1 and I + 2 coincide with the previously

Atchley et al 13

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

when more than one well exists within a single le-gal subdivision eg a 03 would identify the thirdwell drilled in a legal subdivision Core descrip-tions include documentation of the vertical distribu-tions of grain size the fraction of mud-size particles

6 EampP Note

mechanical sedimentary structures ichnogeneraand ichnofacies ichnofabric index and ichnofaunaldiversity (sensu Droser and Bottjer 1986 Bottjerand Droser 1991) sedimentary facies and cementtype and distribution Core description data were

Figure 4 Core photographs representative of facies observed within the Doe Creek at Valhalla field (compare with Table 1) Scale bar =1 cm (03 in) Burrows labeled on the photographs include Planolites (Pl) Thalassinoides (Th) Zoophycos (Zoo) Asterosoma (Ast) andOphiomorpha (Oph) (A) Facies 1a (black-laminated mudrock) within the 0214-29-74-9W60 well at 7005 m (22982 ft) (B) Facies 1b(flaser to lenticular bedded mudrock) within the 008-34-75-8W60 well at 7815 m (25639 ft) (C) Facies 1c (burrowed mudrock) withinthe 0016-29-74-9W60 well at 7105 m (2331 ft) (D) Facies 2 (intercalated burrowed mudrock and wave rippled to hummocky cross-stratified sandstone) within the 036-15-75-9W60 well at 737 m (2418 ft) (E) Facies 3 (laminated to hummocky cross-stratified sand-stone) within the 008-12-75-9W60 well at 6885 m (22588 ft) (F) Facies 4 (planar tabular to trough cross-stratified sandstone) withinthe 0010-11-75-9W60 well at 6885 m (22588 ft) Note the irregular distribution of calcite cement Well identifiers are explained in theData and Methods section

digitized and merged with digital core analysis po-rosity and permeability data and then depth shiftedto coincide with digital wireline well-log data Alldepth-reconciled borehole data types were thenexported to a computer spreadsheet for statisticalanalysis Stratigraphic correlations and preliminarymaps were computer generated and spurious com-puter contouring artifacts were hand correctedAnalysis andmapping of average daily fluid produc-tion trends are based on data compiled from the Al-berta Energy Resources Conservation Board Allwell-log analyses and predictive transforms werecompleted using GeoGraphixreg Prizmtrade log inter-pretation software

RESERVOIR CHARACTERIZATIONAND CLASSIFICATION

Facies Model

The Doe Creek Member of west-central Albertahas been extensively studied and interpreted as awave-dominated and river-influenced shelf sand-stone complex that accumulated in a shallow em-bayment where salinity varied from normalmarineto brackish (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998 Reid and Pemberton 2005 Kreitner andPlint 2006) Previous studies have identified from

Figure 5 Box and whisker plots of neutron-corrected effective porosity and maximum permeability(Kmax) versus grain size and stacked bar charts illustrating the proportion of depositional facies thatcomprise each grain-size category Abbreviations include MUD (clay plusmn silt) VFL (lower very fine sand)VFU (upper very fine sand) FL (lower fine sand) and FU (upper fine sand) Analysis is based on allcores described at Valhalla field Both porosity and increasing proportions of shallower water faciescorrespond to an increase in grain size Permeability displays a similar increase however high valueswithin the lower very fine sand and mud-size fractions are inconsistent to the trend and likely reflectpreferential sampling of thin relatively coarser grained sandstone interbeds within otherwise finergrained successions

Atchley et al 7

8 EampP Note

6 to 10 depositional facies within the Doe Creek atValhalla field (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998) We recognize six depositional facies thatare distinguished on the basis of grain sizemechani-cal sedimentary structures ichnofacies and theextent of bioturbation (ie ichnofabric index ofDroser and Bottjer 1986 and Bottjer and Droser1991 or bioturbate texture of Frey and Pemberton1990) and consistent with these previous studiesare interpreted to have been deposited within off-shore and lower and upper shoreface restricted toopen-marine environments (Table 1 Figure 4)Upper shoreface deposits are rarely observed with-in the study area Variations in ichnofabric indexand ichnofaunal constituents suggest that offshoredeposits are composed of those that accumulated un-der restricted (facies 1a) and relatively more open-marine (facies 1b and 1c) conditions (Table 1)Facies 1a and 1b are characterized by a general lackof bioturbation and almost exclusively include Pla-nolites and the Cruziana ichnofacies componentThalassinoides Conversely facies 1c is homogenizedby burrows that commonly include not only Plano-lites and Thalassinoides but also theCruziana ichno-facies constituents Zoophycos and more rarely Aster-osoma and Rhizocorallium Distal lower shorefacedeposits (facies 2) are dominated by both the Sko-lithos ichnofacies traces Ophiomorpha and Paleo-phycos and the Cruziana ichnofacies traces thatare also observed within facies 1c Proximal lowershoreface (facies 3) and upper shoreface (facies 4)deposits are characterized by Skolithos ichnofaciesforms that typically include Ophiomorpha Paleo-phycos Skolithos and Bergaureria (Table 1)

Controls on Reservoir Quality

Porosity and permeability within the Doe Creekclosely correlate with environmentally controlled

grain size and associated ichnofabric index trendsand the distribution of secondary calcite cementIncreasing grain size corresponds to both an in-crease in neutron-corrected effective porosity andfacies that accumulated in progressively shallowerwater shoreface environments (Figure 5) A similarcorrelation exists with permeability however anom-alously high permeability values within the lowervery fine sandstone andmud-size fractions likely re-flect a core analysis sampling bias introduced bythe preferential sampling of thin (10ndash30 cm [4ndash12 in]) interbeds of relatively coarser grained sand-stone within what was interval averaged duringcore description as relatively distal mud-prone de-posits (Figure 5) Note that the anomalously highpermeability values are not mimicked by neutron-corrected effective porosity because the thin coars-er grained sandstone interbeds that account for thehigh permeability values are below the 061-m (2 ft)vertical resolution of the neutron sonde (resolu-tion estimate via Alberty 1992)

A second potential influence on reservoir qual-ity is the extent of bioturbation and resultant intro-duction of interparticle silt and clay-size particles Toevaluate this possibility we compare porosity andpermeability to five categories of bioturbation in-tensity aka the ichnofabric index under two con-ditions (Figure 6) Under condition 1 the ichnofab-ric index is compared against neutron-correctedeffective porosity and core analysismaximumperme-ability (Kmax) for all core data (Figure 6A) whereascondition 2 limits the comparison to facies 2 3and 4 sandstones having a porosity of greater thanor equal to 12 (Figure 6B) ie the porosity cut-off at Valhalla field that is commonly used by localindustry producers to define reservoir-quality sand-stone (Figure 7) In condition 1 porosity and per-meability trends including the assumed perme-ability sampling bias are similar to those for grainsize and associated depositional facies (compare

Figure 6 Box and whisker plots of neutron-corrected effective porosity and maximum permeability (Kmax) versus ichnofabric indexand stacked bar charts illustrating the proportion of depositional facies that comprise each ichnofabric index category (A) Condition 1case based on analysis of all core data regardless of lithology and (B) condition 2 case based on analysis of all core data where facies 23 and 4 sandstones have a porosity of greater than or equal to 12 Regardless of condition or ichnofabric index category the highestvalues of porosity and permeability coincide with the highest proportion of the shallowest water facies This suggests that grain size anddepositional environment are the primary control on reservoir quality

Atchley et al 9

Figure 7 Scatter plot of coreporosity versus Kmax perme-ability for all Doe Creekndashcoredwells within the study area(compare with Figure 3) Brack-eted on the plot is the rangeof permeability (Kmax) acrosswellbore perforations (1ndash10 md)which as of this writing coin-cides with the lower limit of eco-nomic daily oil production(approximately lt10 bblday)observed at Valhalla field Per-meability values within this rangein turn coincide with a porosityrange of 10ndash15 and is sup-portive of the practice of Valhallafield operators to apply a 12porosity cutoff to define the lowerlimit of satisfactory Doe Creeksandstone reservoir quality

Figure 8 Box and whisker plotof neutron-corrected effectiveporosity and maximum perme-ability (Kmax) versus sandstonewhere calcite cement eitherwas (Yes) or was not (No) ob-served Analysis is based onan analysis of all core data wherefacies 2 3 and 4 sandstoneshave a porosity of greater thanor equal to 12 Both porosityand permeability are reducedin the presence of calcite cement

10 EampP Note

Figures 5 6A) In condition 2 the highest valuesof porosity and permeability similarly coincide withboth ichnofabric indices 1 2 and 5 and the highestproportion of the shallowest water sandstone de-posits (principally facies 3) (Figure 6B) These anal-yses suggest that grain-size trends and associatedenvironments of deposition instead of the ichno-fabric index are the primary sedimentologic con-trols on reservoir quality

Reservoir quality is diminished in the presenceof postdepositional calcite cement (Figures 4F 8)Calcite cement is most common near the top ofsandstone bodies and is also irregularly distributedwithin sandstone bodies Zones of calcite cementa-tion are thin (typically lt50 cm [20 in]) and likelyhave a lateral continuity of less than 4 m (13 ft)(Hogg et al 1998) Calcite cement is increasinglyabundant within shallower water deposits and mayhave resulted from the preferential flow of CaCO3-saturated fluids through their characteristically

Figure 9 Box plot of depositional facies versus the percentageof core specimens that have calcite cement Analysis is based onall cores described at Valhalla field

Figure 10 Box and whisker plots of facies versus (A) open-holedeep resistivity well-log data and (B) shale volume (Vsh) contentderived from open-hole gamma-ray well-log data (compare withTable 2)

Atchley et al 11

higher permeability relatively coarser grained sed-iments (Figure 5) Of the total core observationscalcite cement is present within 25 of the occur-rences of facies 1 (offshore) 85 of the occur-rences of facies 2 (distal lower shoreface) 16 ofthe occurrence of facies 3 (proximal lower shore-face) and 64 of the occurrences of facies 4 (up-per shoreface) (Figure 9) The particularly highpercentage of calcite cement within facies 4 is re-lated to both the limited observations of facies 4and the consistent position of facies 4 near thetop of sandstone bodies

Reservoir Facies Prediction

Comparison of core-observed depositional faciesand petrophysical attributes suggests that open-holegamma ray and deep resistivity well-log data maybe used to predict facies distributions in wells thatlack core control (Figure 10) Open-hole neutron-porosity logs were also evaluated for their poten-tial to predict facies but proved to be less reliableGuidelines for facies prediction use direct mea-surement of deep resistivity for oil-saturated reser-voir above the original oil-water contact (+46 m[151 ft]) and shale volume calculations (Vsh) de-rived from normalized gamma-ray measurementsBecause the Doe Creek study interval was oil fullduring initial field development the guidelines forthe use of deep resistivity are applicable to most (if

12 EampP Note

not all) wells with open-hole logs within the studyarea (Figure 3) Gamma-ray normalization andVsh

calculations are based on the following formulae(Larinov 1969 Asquith and Krygowski 2004 ascited by Asquith and Krygowski 2004)

IGR frac14 ethGRlog GRminTHORN=ethGRmax GRminTHORN

where the IGR is the gamma-ray index GRlog is thegamma-raymeasurement from the reservoir GRmin

is the 14 API units and GRmax is the 130 APIunits (asterisks indicate Valhalla fieldwide averageminimum and maximum values observed in open-hole wireline logs)

V sh frac14 033frac122eth2 IGRTHORN 1

where Vsh is the shale volumeTable 2 defines the guidelines for prediction

and equates the resultant well-log facies with theirdepositional facies counterparts Although well-log data cluster within discrete ranges for each de-positional facies the overlapping range of Vsh anddeep resistivity data between facies results in well-log facies predictions that reflect data blendingbetween two (or more) depositional facies (com-pare Table 2 and Figure 10) Consequently al-though predicted well-log facies are most likely as-sociated with a particular depositional facies theintroduction of error from overlapping ranges of

Table 2 Guidelines for Facies Prediction from Open-Hole Well Logs

Well-Log Facies

Depositional Facies Equivalents

Parameters for Well-Log Prediction

4

Primarily 3 and 4

Vsh le 009 (average confidence)Vsh le 009 and deep resistivity ge38 ohm m (increased confidence)

3

Primarily 3 and lesser 2

Vsh ge 009 to le 012 (average confidence)Vsh ge 009 to le 012 and deep resistivity le38 to ge 21 ohm m (increased confidence)

2

Primarily 2 and lesser 1

Vsh ge 012 to le 033 (average confidence)Vsh ge 012 to le 033 and deep resistivity le21 to ge 14 ohm m (increased confidence)

1

Primarily 1 and lesser 2

Vsh ge 033 (average confidence) Vsh ge 033 anddeep resistivity le 14 ohm m (increased confidence)

Predicted well-log facies correspond to more than one depositional facies because of the overlapping range of Vsh and deep resistivity well-log data between depositionalfacies (Figure 10) The confidence of well-log facies interpretation increases when both the Vsh and deep resistivity guidelines are satisfied

well-log data between facies categories may re-sult in misinterpretation

Calcite cement is interpreted to occur withinwell-log facies 2 3 or 4 where the well-log neu-tron porosity exceeds the density porosity bymorethan 7 Within an oil-saturated reservoir such asthe Doe Creek at Valhalla this quick-look tech-nique may be used when density-neutron well-logdata are calibrated to a sandstonematrix density val-ue of 265 gcm3 (Asquith and Krygowski 2004)

To evaluate the accuracy of the predictivemod-el a blind test was administered to two cored wellswithin the study area that had not been includedwithin the database used to develop the well-logfacies or calcite cementation predictive guidelines(Figure 11) Cores from both wells were describedand interpreted independent of knowledge regard-ing the interpreted distribution of well-log faciesIn both cases the distribution of depositional faciesobserved within cores closely matches the pre-dicted distribution (Figure 11) The predicted dis-tribution of calcite cement precisely coincides withthe observed distribution within the 0014-15-075-08W6 well but only matches the thickest of fourcalcite-cemented zones within the 0006-32-074-09W6 well (Figure 11) The inability of the density-neutron predictor to detect calcite-cemented zoneswithin the 0006-32-074-09W6 well is likely re-lated to their thin (02ndash03 m [06ndash09 ft]) discon-tinuous distribution which is below the 045ndash061-m (17ndash2-ft) vertical resolution of density-neutron well logs (resolution via Alberty 1992)

STRATIGRAPHIC FRAMEWORK

Regional Overview

TheKaskapau Formationwas deposited during theculmination of the second-order transgression as-sociated with the Zuni cratonic sequence (sensuSloss 1963) and the concomitant maximum ex-tent of the North American WIS during the LateCretaceous (Hancock and Kauffman 1979 Haqet al 1988 Caldwell et al 1993 Kauffman andCaldwell 1993 Slingerland et al 1996) Withinthe study area second-order maximum flooding

coincides with the Greenhorn eustatic transgressionjust above the CenomanianndashTuronian boundaryand the equivalent organic-rich earliest TuronianSecondWhite Speckled Shale of the Kaskapau For-mation (Figure 1) (Caldwell 1983 Haq et al 1987Wallace-Dudley and Leckie 1993 Bhattacharya1994 Hogg et al 1998 Varban and Plint 2005)A higher frequency perhaps third-order episodeof decreasing and increasing accommodation dur-ing the middle Cenomanian to lower Turonian waspossibly induced by a pulse of foreland tectonismacross the WCSB and resulted in deposition oflowstand (Dunvegan) deltaic deposits and suc-ceeding transgressive (Kaskapau) open-marinemidshelf deposits (Plint et al 1993) Because ofthe relatively rapid rates of composite accommo-dation gain that resulted from constructive inter-ference of the second- and third-order transgres-sions higher frequency fourth-order stillstands(or lowstands) of sea level are recorded as retrogra-dationally stacked alternations of deeper water off-shore mudrocks (fourth-order transgressions) andshallower water shelf sandstones (fourth-orderstillstands or regressions) (Wallace-Dudley andLeckie 1993) In ascending stratigraphic orderthese shallower water sandstones include the DoeCreek Pouce Coupe and Howard Creek membersof the Kaskapau Formation (Figure 1)

Valhalla Reservoir Interval

TheDoeCreekMember atValhalla field consists ofnumerous cyclic alternations of offshore mudrock(facies 1) and shoreface sandstone (facies 2 3 andless commonly 4) Nine of these thin (1ndash10 m [3ndash33 ft] thick) cyclic units are correlated across Val-halla and their stratigraphic designations are refer-enced sequentially relative to their position above orbelow the top of themost oil productive I sandstone(alphanumeric designation by Wallace-Dudley andLeckie 1988) Above the I sandstone these includein ascending order the I + 1 through I + 6 and be-low the I sandstone in descending order the I minus 1and I minus 2 (Figures 11 12) In addition to the I sand-stone other hydrocarbon-charged units include theoil-prone I minus 1 and I + 1 and the gas-prone I + 2The I + 1 and I + 2 coincide with the previously

Atchley et al 13

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

digitized and merged with digital core analysis po-rosity and permeability data and then depth shiftedto coincide with digital wireline well-log data Alldepth-reconciled borehole data types were thenexported to a computer spreadsheet for statisticalanalysis Stratigraphic correlations and preliminarymaps were computer generated and spurious com-puter contouring artifacts were hand correctedAnalysis andmapping of average daily fluid produc-tion trends are based on data compiled from the Al-berta Energy Resources Conservation Board Allwell-log analyses and predictive transforms werecompleted using GeoGraphixreg Prizmtrade log inter-pretation software

RESERVOIR CHARACTERIZATIONAND CLASSIFICATION

Facies Model

The Doe Creek Member of west-central Albertahas been extensively studied and interpreted as awave-dominated and river-influenced shelf sand-stone complex that accumulated in a shallow em-bayment where salinity varied from normalmarineto brackish (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998 Reid and Pemberton 2005 Kreitner andPlint 2006) Previous studies have identified from

Figure 5 Box and whisker plots of neutron-corrected effective porosity and maximum permeability(Kmax) versus grain size and stacked bar charts illustrating the proportion of depositional facies thatcomprise each grain-size category Abbreviations include MUD (clay plusmn silt) VFL (lower very fine sand)VFU (upper very fine sand) FL (lower fine sand) and FU (upper fine sand) Analysis is based on allcores described at Valhalla field Both porosity and increasing proportions of shallower water faciescorrespond to an increase in grain size Permeability displays a similar increase however high valueswithin the lower very fine sand and mud-size fractions are inconsistent to the trend and likely reflectpreferential sampling of thin relatively coarser grained sandstone interbeds within otherwise finergrained successions

Atchley et al 7

8 EampP Note

6 to 10 depositional facies within the Doe Creek atValhalla field (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998) We recognize six depositional facies thatare distinguished on the basis of grain sizemechani-cal sedimentary structures ichnofacies and theextent of bioturbation (ie ichnofabric index ofDroser and Bottjer 1986 and Bottjer and Droser1991 or bioturbate texture of Frey and Pemberton1990) and consistent with these previous studiesare interpreted to have been deposited within off-shore and lower and upper shoreface restricted toopen-marine environments (Table 1 Figure 4)Upper shoreface deposits are rarely observed with-in the study area Variations in ichnofabric indexand ichnofaunal constituents suggest that offshoredeposits are composed of those that accumulated un-der restricted (facies 1a) and relatively more open-marine (facies 1b and 1c) conditions (Table 1)Facies 1a and 1b are characterized by a general lackof bioturbation and almost exclusively include Pla-nolites and the Cruziana ichnofacies componentThalassinoides Conversely facies 1c is homogenizedby burrows that commonly include not only Plano-lites and Thalassinoides but also theCruziana ichno-facies constituents Zoophycos and more rarely Aster-osoma and Rhizocorallium Distal lower shorefacedeposits (facies 2) are dominated by both the Sko-lithos ichnofacies traces Ophiomorpha and Paleo-phycos and the Cruziana ichnofacies traces thatare also observed within facies 1c Proximal lowershoreface (facies 3) and upper shoreface (facies 4)deposits are characterized by Skolithos ichnofaciesforms that typically include Ophiomorpha Paleo-phycos Skolithos and Bergaureria (Table 1)

Controls on Reservoir Quality

Porosity and permeability within the Doe Creekclosely correlate with environmentally controlled

grain size and associated ichnofabric index trendsand the distribution of secondary calcite cementIncreasing grain size corresponds to both an in-crease in neutron-corrected effective porosity andfacies that accumulated in progressively shallowerwater shoreface environments (Figure 5) A similarcorrelation exists with permeability however anom-alously high permeability values within the lowervery fine sandstone andmud-size fractions likely re-flect a core analysis sampling bias introduced bythe preferential sampling of thin (10ndash30 cm [4ndash12 in]) interbeds of relatively coarser grained sand-stone within what was interval averaged duringcore description as relatively distal mud-prone de-posits (Figure 5) Note that the anomalously highpermeability values are not mimicked by neutron-corrected effective porosity because the thin coars-er grained sandstone interbeds that account for thehigh permeability values are below the 061-m (2 ft)vertical resolution of the neutron sonde (resolu-tion estimate via Alberty 1992)

A second potential influence on reservoir qual-ity is the extent of bioturbation and resultant intro-duction of interparticle silt and clay-size particles Toevaluate this possibility we compare porosity andpermeability to five categories of bioturbation in-tensity aka the ichnofabric index under two con-ditions (Figure 6) Under condition 1 the ichnofab-ric index is compared against neutron-correctedeffective porosity and core analysismaximumperme-ability (Kmax) for all core data (Figure 6A) whereascondition 2 limits the comparison to facies 2 3and 4 sandstones having a porosity of greater thanor equal to 12 (Figure 6B) ie the porosity cut-off at Valhalla field that is commonly used by localindustry producers to define reservoir-quality sand-stone (Figure 7) In condition 1 porosity and per-meability trends including the assumed perme-ability sampling bias are similar to those for grainsize and associated depositional facies (compare

Figure 6 Box and whisker plots of neutron-corrected effective porosity and maximum permeability (Kmax) versus ichnofabric indexand stacked bar charts illustrating the proportion of depositional facies that comprise each ichnofabric index category (A) Condition 1case based on analysis of all core data regardless of lithology and (B) condition 2 case based on analysis of all core data where facies 23 and 4 sandstones have a porosity of greater than or equal to 12 Regardless of condition or ichnofabric index category the highestvalues of porosity and permeability coincide with the highest proportion of the shallowest water facies This suggests that grain size anddepositional environment are the primary control on reservoir quality

Atchley et al 9

Figure 7 Scatter plot of coreporosity versus Kmax perme-ability for all Doe Creekndashcoredwells within the study area(compare with Figure 3) Brack-eted on the plot is the rangeof permeability (Kmax) acrosswellbore perforations (1ndash10 md)which as of this writing coin-cides with the lower limit of eco-nomic daily oil production(approximately lt10 bblday)observed at Valhalla field Per-meability values within this rangein turn coincide with a porosityrange of 10ndash15 and is sup-portive of the practice of Valhallafield operators to apply a 12porosity cutoff to define the lowerlimit of satisfactory Doe Creeksandstone reservoir quality

Figure 8 Box and whisker plotof neutron-corrected effectiveporosity and maximum perme-ability (Kmax) versus sandstonewhere calcite cement eitherwas (Yes) or was not (No) ob-served Analysis is based onan analysis of all core data wherefacies 2 3 and 4 sandstoneshave a porosity of greater thanor equal to 12 Both porosityand permeability are reducedin the presence of calcite cement

10 EampP Note

Figures 5 6A) In condition 2 the highest valuesof porosity and permeability similarly coincide withboth ichnofabric indices 1 2 and 5 and the highestproportion of the shallowest water sandstone de-posits (principally facies 3) (Figure 6B) These anal-yses suggest that grain-size trends and associatedenvironments of deposition instead of the ichno-fabric index are the primary sedimentologic con-trols on reservoir quality

Reservoir quality is diminished in the presenceof postdepositional calcite cement (Figures 4F 8)Calcite cement is most common near the top ofsandstone bodies and is also irregularly distributedwithin sandstone bodies Zones of calcite cementa-tion are thin (typically lt50 cm [20 in]) and likelyhave a lateral continuity of less than 4 m (13 ft)(Hogg et al 1998) Calcite cement is increasinglyabundant within shallower water deposits and mayhave resulted from the preferential flow of CaCO3-saturated fluids through their characteristically

Figure 9 Box plot of depositional facies versus the percentageof core specimens that have calcite cement Analysis is based onall cores described at Valhalla field

Figure 10 Box and whisker plots of facies versus (A) open-holedeep resistivity well-log data and (B) shale volume (Vsh) contentderived from open-hole gamma-ray well-log data (compare withTable 2)

Atchley et al 11

higher permeability relatively coarser grained sed-iments (Figure 5) Of the total core observationscalcite cement is present within 25 of the occur-rences of facies 1 (offshore) 85 of the occur-rences of facies 2 (distal lower shoreface) 16 ofthe occurrence of facies 3 (proximal lower shore-face) and 64 of the occurrences of facies 4 (up-per shoreface) (Figure 9) The particularly highpercentage of calcite cement within facies 4 is re-lated to both the limited observations of facies 4and the consistent position of facies 4 near thetop of sandstone bodies

Reservoir Facies Prediction

Comparison of core-observed depositional faciesand petrophysical attributes suggests that open-holegamma ray and deep resistivity well-log data maybe used to predict facies distributions in wells thatlack core control (Figure 10) Open-hole neutron-porosity logs were also evaluated for their poten-tial to predict facies but proved to be less reliableGuidelines for facies prediction use direct mea-surement of deep resistivity for oil-saturated reser-voir above the original oil-water contact (+46 m[151 ft]) and shale volume calculations (Vsh) de-rived from normalized gamma-ray measurementsBecause the Doe Creek study interval was oil fullduring initial field development the guidelines forthe use of deep resistivity are applicable to most (if

12 EampP Note

not all) wells with open-hole logs within the studyarea (Figure 3) Gamma-ray normalization andVsh

calculations are based on the following formulae(Larinov 1969 Asquith and Krygowski 2004 ascited by Asquith and Krygowski 2004)

IGR frac14 ethGRlog GRminTHORN=ethGRmax GRminTHORN

where the IGR is the gamma-ray index GRlog is thegamma-raymeasurement from the reservoir GRmin

is the 14 API units and GRmax is the 130 APIunits (asterisks indicate Valhalla fieldwide averageminimum and maximum values observed in open-hole wireline logs)

V sh frac14 033frac122eth2 IGRTHORN 1

where Vsh is the shale volumeTable 2 defines the guidelines for prediction

and equates the resultant well-log facies with theirdepositional facies counterparts Although well-log data cluster within discrete ranges for each de-positional facies the overlapping range of Vsh anddeep resistivity data between facies results in well-log facies predictions that reflect data blendingbetween two (or more) depositional facies (com-pare Table 2 and Figure 10) Consequently al-though predicted well-log facies are most likely as-sociated with a particular depositional facies theintroduction of error from overlapping ranges of

Table 2 Guidelines for Facies Prediction from Open-Hole Well Logs

Well-Log Facies

Depositional Facies Equivalents

Parameters for Well-Log Prediction

4

Primarily 3 and 4

Vsh le 009 (average confidence)Vsh le 009 and deep resistivity ge38 ohm m (increased confidence)

3

Primarily 3 and lesser 2

Vsh ge 009 to le 012 (average confidence)Vsh ge 009 to le 012 and deep resistivity le38 to ge 21 ohm m (increased confidence)

2

Primarily 2 and lesser 1

Vsh ge 012 to le 033 (average confidence)Vsh ge 012 to le 033 and deep resistivity le21 to ge 14 ohm m (increased confidence)

1

Primarily 1 and lesser 2

Vsh ge 033 (average confidence) Vsh ge 033 anddeep resistivity le 14 ohm m (increased confidence)

Predicted well-log facies correspond to more than one depositional facies because of the overlapping range of Vsh and deep resistivity well-log data between depositionalfacies (Figure 10) The confidence of well-log facies interpretation increases when both the Vsh and deep resistivity guidelines are satisfied

well-log data between facies categories may re-sult in misinterpretation

Calcite cement is interpreted to occur withinwell-log facies 2 3 or 4 where the well-log neu-tron porosity exceeds the density porosity bymorethan 7 Within an oil-saturated reservoir such asthe Doe Creek at Valhalla this quick-look tech-nique may be used when density-neutron well-logdata are calibrated to a sandstonematrix density val-ue of 265 gcm3 (Asquith and Krygowski 2004)

To evaluate the accuracy of the predictivemod-el a blind test was administered to two cored wellswithin the study area that had not been includedwithin the database used to develop the well-logfacies or calcite cementation predictive guidelines(Figure 11) Cores from both wells were describedand interpreted independent of knowledge regard-ing the interpreted distribution of well-log faciesIn both cases the distribution of depositional faciesobserved within cores closely matches the pre-dicted distribution (Figure 11) The predicted dis-tribution of calcite cement precisely coincides withthe observed distribution within the 0014-15-075-08W6 well but only matches the thickest of fourcalcite-cemented zones within the 0006-32-074-09W6 well (Figure 11) The inability of the density-neutron predictor to detect calcite-cemented zoneswithin the 0006-32-074-09W6 well is likely re-lated to their thin (02ndash03 m [06ndash09 ft]) discon-tinuous distribution which is below the 045ndash061-m (17ndash2-ft) vertical resolution of density-neutron well logs (resolution via Alberty 1992)

STRATIGRAPHIC FRAMEWORK

Regional Overview

TheKaskapau Formationwas deposited during theculmination of the second-order transgression as-sociated with the Zuni cratonic sequence (sensuSloss 1963) and the concomitant maximum ex-tent of the North American WIS during the LateCretaceous (Hancock and Kauffman 1979 Haqet al 1988 Caldwell et al 1993 Kauffman andCaldwell 1993 Slingerland et al 1996) Withinthe study area second-order maximum flooding

coincides with the Greenhorn eustatic transgressionjust above the CenomanianndashTuronian boundaryand the equivalent organic-rich earliest TuronianSecondWhite Speckled Shale of the Kaskapau For-mation (Figure 1) (Caldwell 1983 Haq et al 1987Wallace-Dudley and Leckie 1993 Bhattacharya1994 Hogg et al 1998 Varban and Plint 2005)A higher frequency perhaps third-order episodeof decreasing and increasing accommodation dur-ing the middle Cenomanian to lower Turonian waspossibly induced by a pulse of foreland tectonismacross the WCSB and resulted in deposition oflowstand (Dunvegan) deltaic deposits and suc-ceeding transgressive (Kaskapau) open-marinemidshelf deposits (Plint et al 1993) Because ofthe relatively rapid rates of composite accommo-dation gain that resulted from constructive inter-ference of the second- and third-order transgres-sions higher frequency fourth-order stillstands(or lowstands) of sea level are recorded as retrogra-dationally stacked alternations of deeper water off-shore mudrocks (fourth-order transgressions) andshallower water shelf sandstones (fourth-orderstillstands or regressions) (Wallace-Dudley andLeckie 1993) In ascending stratigraphic orderthese shallower water sandstones include the DoeCreek Pouce Coupe and Howard Creek membersof the Kaskapau Formation (Figure 1)

Valhalla Reservoir Interval

TheDoeCreekMember atValhalla field consists ofnumerous cyclic alternations of offshore mudrock(facies 1) and shoreface sandstone (facies 2 3 andless commonly 4) Nine of these thin (1ndash10 m [3ndash33 ft] thick) cyclic units are correlated across Val-halla and their stratigraphic designations are refer-enced sequentially relative to their position above orbelow the top of themost oil productive I sandstone(alphanumeric designation by Wallace-Dudley andLeckie 1988) Above the I sandstone these includein ascending order the I + 1 through I + 6 and be-low the I sandstone in descending order the I minus 1and I minus 2 (Figures 11 12) In addition to the I sand-stone other hydrocarbon-charged units include theoil-prone I minus 1 and I + 1 and the gas-prone I + 2The I + 1 and I + 2 coincide with the previously

Atchley et al 13

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

8 EampP Note

6 to 10 depositional facies within the Doe Creek atValhalla field (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998) We recognize six depositional facies thatare distinguished on the basis of grain sizemechani-cal sedimentary structures ichnofacies and theextent of bioturbation (ie ichnofabric index ofDroser and Bottjer 1986 and Bottjer and Droser1991 or bioturbate texture of Frey and Pemberton1990) and consistent with these previous studiesare interpreted to have been deposited within off-shore and lower and upper shoreface restricted toopen-marine environments (Table 1 Figure 4)Upper shoreface deposits are rarely observed with-in the study area Variations in ichnofabric indexand ichnofaunal constituents suggest that offshoredeposits are composed of those that accumulated un-der restricted (facies 1a) and relatively more open-marine (facies 1b and 1c) conditions (Table 1)Facies 1a and 1b are characterized by a general lackof bioturbation and almost exclusively include Pla-nolites and the Cruziana ichnofacies componentThalassinoides Conversely facies 1c is homogenizedby burrows that commonly include not only Plano-lites and Thalassinoides but also theCruziana ichno-facies constituents Zoophycos and more rarely Aster-osoma and Rhizocorallium Distal lower shorefacedeposits (facies 2) are dominated by both the Sko-lithos ichnofacies traces Ophiomorpha and Paleo-phycos and the Cruziana ichnofacies traces thatare also observed within facies 1c Proximal lowershoreface (facies 3) and upper shoreface (facies 4)deposits are characterized by Skolithos ichnofaciesforms that typically include Ophiomorpha Paleo-phycos Skolithos and Bergaureria (Table 1)

Controls on Reservoir Quality

Porosity and permeability within the Doe Creekclosely correlate with environmentally controlled

grain size and associated ichnofabric index trendsand the distribution of secondary calcite cementIncreasing grain size corresponds to both an in-crease in neutron-corrected effective porosity andfacies that accumulated in progressively shallowerwater shoreface environments (Figure 5) A similarcorrelation exists with permeability however anom-alously high permeability values within the lowervery fine sandstone andmud-size fractions likely re-flect a core analysis sampling bias introduced bythe preferential sampling of thin (10ndash30 cm [4ndash12 in]) interbeds of relatively coarser grained sand-stone within what was interval averaged duringcore description as relatively distal mud-prone de-posits (Figure 5) Note that the anomalously highpermeability values are not mimicked by neutron-corrected effective porosity because the thin coars-er grained sandstone interbeds that account for thehigh permeability values are below the 061-m (2 ft)vertical resolution of the neutron sonde (resolu-tion estimate via Alberty 1992)

A second potential influence on reservoir qual-ity is the extent of bioturbation and resultant intro-duction of interparticle silt and clay-size particles Toevaluate this possibility we compare porosity andpermeability to five categories of bioturbation in-tensity aka the ichnofabric index under two con-ditions (Figure 6) Under condition 1 the ichnofab-ric index is compared against neutron-correctedeffective porosity and core analysismaximumperme-ability (Kmax) for all core data (Figure 6A) whereascondition 2 limits the comparison to facies 2 3and 4 sandstones having a porosity of greater thanor equal to 12 (Figure 6B) ie the porosity cut-off at Valhalla field that is commonly used by localindustry producers to define reservoir-quality sand-stone (Figure 7) In condition 1 porosity and per-meability trends including the assumed perme-ability sampling bias are similar to those for grainsize and associated depositional facies (compare

Figure 6 Box and whisker plots of neutron-corrected effective porosity and maximum permeability (Kmax) versus ichnofabric indexand stacked bar charts illustrating the proportion of depositional facies that comprise each ichnofabric index category (A) Condition 1case based on analysis of all core data regardless of lithology and (B) condition 2 case based on analysis of all core data where facies 23 and 4 sandstones have a porosity of greater than or equal to 12 Regardless of condition or ichnofabric index category the highestvalues of porosity and permeability coincide with the highest proportion of the shallowest water facies This suggests that grain size anddepositional environment are the primary control on reservoir quality

Atchley et al 9

Figure 7 Scatter plot of coreporosity versus Kmax perme-ability for all Doe Creekndashcoredwells within the study area(compare with Figure 3) Brack-eted on the plot is the rangeof permeability (Kmax) acrosswellbore perforations (1ndash10 md)which as of this writing coin-cides with the lower limit of eco-nomic daily oil production(approximately lt10 bblday)observed at Valhalla field Per-meability values within this rangein turn coincide with a porosityrange of 10ndash15 and is sup-portive of the practice of Valhallafield operators to apply a 12porosity cutoff to define the lowerlimit of satisfactory Doe Creeksandstone reservoir quality

Figure 8 Box and whisker plotof neutron-corrected effectiveporosity and maximum perme-ability (Kmax) versus sandstonewhere calcite cement eitherwas (Yes) or was not (No) ob-served Analysis is based onan analysis of all core data wherefacies 2 3 and 4 sandstoneshave a porosity of greater thanor equal to 12 Both porosityand permeability are reducedin the presence of calcite cement

10 EampP Note

Figures 5 6A) In condition 2 the highest valuesof porosity and permeability similarly coincide withboth ichnofabric indices 1 2 and 5 and the highestproportion of the shallowest water sandstone de-posits (principally facies 3) (Figure 6B) These anal-yses suggest that grain-size trends and associatedenvironments of deposition instead of the ichno-fabric index are the primary sedimentologic con-trols on reservoir quality

Reservoir quality is diminished in the presenceof postdepositional calcite cement (Figures 4F 8)Calcite cement is most common near the top ofsandstone bodies and is also irregularly distributedwithin sandstone bodies Zones of calcite cementa-tion are thin (typically lt50 cm [20 in]) and likelyhave a lateral continuity of less than 4 m (13 ft)(Hogg et al 1998) Calcite cement is increasinglyabundant within shallower water deposits and mayhave resulted from the preferential flow of CaCO3-saturated fluids through their characteristically

Figure 9 Box plot of depositional facies versus the percentageof core specimens that have calcite cement Analysis is based onall cores described at Valhalla field

Figure 10 Box and whisker plots of facies versus (A) open-holedeep resistivity well-log data and (B) shale volume (Vsh) contentderived from open-hole gamma-ray well-log data (compare withTable 2)

Atchley et al 11

higher permeability relatively coarser grained sed-iments (Figure 5) Of the total core observationscalcite cement is present within 25 of the occur-rences of facies 1 (offshore) 85 of the occur-rences of facies 2 (distal lower shoreface) 16 ofthe occurrence of facies 3 (proximal lower shore-face) and 64 of the occurrences of facies 4 (up-per shoreface) (Figure 9) The particularly highpercentage of calcite cement within facies 4 is re-lated to both the limited observations of facies 4and the consistent position of facies 4 near thetop of sandstone bodies

Reservoir Facies Prediction

Comparison of core-observed depositional faciesand petrophysical attributes suggests that open-holegamma ray and deep resistivity well-log data maybe used to predict facies distributions in wells thatlack core control (Figure 10) Open-hole neutron-porosity logs were also evaluated for their poten-tial to predict facies but proved to be less reliableGuidelines for facies prediction use direct mea-surement of deep resistivity for oil-saturated reser-voir above the original oil-water contact (+46 m[151 ft]) and shale volume calculations (Vsh) de-rived from normalized gamma-ray measurementsBecause the Doe Creek study interval was oil fullduring initial field development the guidelines forthe use of deep resistivity are applicable to most (if

12 EampP Note

not all) wells with open-hole logs within the studyarea (Figure 3) Gamma-ray normalization andVsh

calculations are based on the following formulae(Larinov 1969 Asquith and Krygowski 2004 ascited by Asquith and Krygowski 2004)

IGR frac14 ethGRlog GRminTHORN=ethGRmax GRminTHORN

where the IGR is the gamma-ray index GRlog is thegamma-raymeasurement from the reservoir GRmin

is the 14 API units and GRmax is the 130 APIunits (asterisks indicate Valhalla fieldwide averageminimum and maximum values observed in open-hole wireline logs)

V sh frac14 033frac122eth2 IGRTHORN 1

where Vsh is the shale volumeTable 2 defines the guidelines for prediction

and equates the resultant well-log facies with theirdepositional facies counterparts Although well-log data cluster within discrete ranges for each de-positional facies the overlapping range of Vsh anddeep resistivity data between facies results in well-log facies predictions that reflect data blendingbetween two (or more) depositional facies (com-pare Table 2 and Figure 10) Consequently al-though predicted well-log facies are most likely as-sociated with a particular depositional facies theintroduction of error from overlapping ranges of

Table 2 Guidelines for Facies Prediction from Open-Hole Well Logs

Well-Log Facies

Depositional Facies Equivalents

Parameters for Well-Log Prediction

4

Primarily 3 and 4

Vsh le 009 (average confidence)Vsh le 009 and deep resistivity ge38 ohm m (increased confidence)

3

Primarily 3 and lesser 2

Vsh ge 009 to le 012 (average confidence)Vsh ge 009 to le 012 and deep resistivity le38 to ge 21 ohm m (increased confidence)

2

Primarily 2 and lesser 1

Vsh ge 012 to le 033 (average confidence)Vsh ge 012 to le 033 and deep resistivity le21 to ge 14 ohm m (increased confidence)

1

Primarily 1 and lesser 2

Vsh ge 033 (average confidence) Vsh ge 033 anddeep resistivity le 14 ohm m (increased confidence)

Predicted well-log facies correspond to more than one depositional facies because of the overlapping range of Vsh and deep resistivity well-log data between depositionalfacies (Figure 10) The confidence of well-log facies interpretation increases when both the Vsh and deep resistivity guidelines are satisfied

well-log data between facies categories may re-sult in misinterpretation

Calcite cement is interpreted to occur withinwell-log facies 2 3 or 4 where the well-log neu-tron porosity exceeds the density porosity bymorethan 7 Within an oil-saturated reservoir such asthe Doe Creek at Valhalla this quick-look tech-nique may be used when density-neutron well-logdata are calibrated to a sandstonematrix density val-ue of 265 gcm3 (Asquith and Krygowski 2004)

To evaluate the accuracy of the predictivemod-el a blind test was administered to two cored wellswithin the study area that had not been includedwithin the database used to develop the well-logfacies or calcite cementation predictive guidelines(Figure 11) Cores from both wells were describedand interpreted independent of knowledge regard-ing the interpreted distribution of well-log faciesIn both cases the distribution of depositional faciesobserved within cores closely matches the pre-dicted distribution (Figure 11) The predicted dis-tribution of calcite cement precisely coincides withthe observed distribution within the 0014-15-075-08W6 well but only matches the thickest of fourcalcite-cemented zones within the 0006-32-074-09W6 well (Figure 11) The inability of the density-neutron predictor to detect calcite-cemented zoneswithin the 0006-32-074-09W6 well is likely re-lated to their thin (02ndash03 m [06ndash09 ft]) discon-tinuous distribution which is below the 045ndash061-m (17ndash2-ft) vertical resolution of density-neutron well logs (resolution via Alberty 1992)

STRATIGRAPHIC FRAMEWORK

Regional Overview

TheKaskapau Formationwas deposited during theculmination of the second-order transgression as-sociated with the Zuni cratonic sequence (sensuSloss 1963) and the concomitant maximum ex-tent of the North American WIS during the LateCretaceous (Hancock and Kauffman 1979 Haqet al 1988 Caldwell et al 1993 Kauffman andCaldwell 1993 Slingerland et al 1996) Withinthe study area second-order maximum flooding

coincides with the Greenhorn eustatic transgressionjust above the CenomanianndashTuronian boundaryand the equivalent organic-rich earliest TuronianSecondWhite Speckled Shale of the Kaskapau For-mation (Figure 1) (Caldwell 1983 Haq et al 1987Wallace-Dudley and Leckie 1993 Bhattacharya1994 Hogg et al 1998 Varban and Plint 2005)A higher frequency perhaps third-order episodeof decreasing and increasing accommodation dur-ing the middle Cenomanian to lower Turonian waspossibly induced by a pulse of foreland tectonismacross the WCSB and resulted in deposition oflowstand (Dunvegan) deltaic deposits and suc-ceeding transgressive (Kaskapau) open-marinemidshelf deposits (Plint et al 1993) Because ofthe relatively rapid rates of composite accommo-dation gain that resulted from constructive inter-ference of the second- and third-order transgres-sions higher frequency fourth-order stillstands(or lowstands) of sea level are recorded as retrogra-dationally stacked alternations of deeper water off-shore mudrocks (fourth-order transgressions) andshallower water shelf sandstones (fourth-orderstillstands or regressions) (Wallace-Dudley andLeckie 1993) In ascending stratigraphic orderthese shallower water sandstones include the DoeCreek Pouce Coupe and Howard Creek membersof the Kaskapau Formation (Figure 1)

Valhalla Reservoir Interval

TheDoeCreekMember atValhalla field consists ofnumerous cyclic alternations of offshore mudrock(facies 1) and shoreface sandstone (facies 2 3 andless commonly 4) Nine of these thin (1ndash10 m [3ndash33 ft] thick) cyclic units are correlated across Val-halla and their stratigraphic designations are refer-enced sequentially relative to their position above orbelow the top of themost oil productive I sandstone(alphanumeric designation by Wallace-Dudley andLeckie 1988) Above the I sandstone these includein ascending order the I + 1 through I + 6 and be-low the I sandstone in descending order the I minus 1and I minus 2 (Figures 11 12) In addition to the I sand-stone other hydrocarbon-charged units include theoil-prone I minus 1 and I + 1 and the gas-prone I + 2The I + 1 and I + 2 coincide with the previously

Atchley et al 13

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

6 to 10 depositional facies within the Doe Creek atValhalla field (Wallace-Dudley and Leckie 1988Wallace-Dudley and Leckie 1993 Hogg et al1998) We recognize six depositional facies thatare distinguished on the basis of grain sizemechani-cal sedimentary structures ichnofacies and theextent of bioturbation (ie ichnofabric index ofDroser and Bottjer 1986 and Bottjer and Droser1991 or bioturbate texture of Frey and Pemberton1990) and consistent with these previous studiesare interpreted to have been deposited within off-shore and lower and upper shoreface restricted toopen-marine environments (Table 1 Figure 4)Upper shoreface deposits are rarely observed with-in the study area Variations in ichnofabric indexand ichnofaunal constituents suggest that offshoredeposits are composed of those that accumulated un-der restricted (facies 1a) and relatively more open-marine (facies 1b and 1c) conditions (Table 1)Facies 1a and 1b are characterized by a general lackof bioturbation and almost exclusively include Pla-nolites and the Cruziana ichnofacies componentThalassinoides Conversely facies 1c is homogenizedby burrows that commonly include not only Plano-lites and Thalassinoides but also theCruziana ichno-facies constituents Zoophycos and more rarely Aster-osoma and Rhizocorallium Distal lower shorefacedeposits (facies 2) are dominated by both the Sko-lithos ichnofacies traces Ophiomorpha and Paleo-phycos and the Cruziana ichnofacies traces thatare also observed within facies 1c Proximal lowershoreface (facies 3) and upper shoreface (facies 4)deposits are characterized by Skolithos ichnofaciesforms that typically include Ophiomorpha Paleo-phycos Skolithos and Bergaureria (Table 1)

Controls on Reservoir Quality

Porosity and permeability within the Doe Creekclosely correlate with environmentally controlled

grain size and associated ichnofabric index trendsand the distribution of secondary calcite cementIncreasing grain size corresponds to both an in-crease in neutron-corrected effective porosity andfacies that accumulated in progressively shallowerwater shoreface environments (Figure 5) A similarcorrelation exists with permeability however anom-alously high permeability values within the lowervery fine sandstone andmud-size fractions likely re-flect a core analysis sampling bias introduced bythe preferential sampling of thin (10ndash30 cm [4ndash12 in]) interbeds of relatively coarser grained sand-stone within what was interval averaged duringcore description as relatively distal mud-prone de-posits (Figure 5) Note that the anomalously highpermeability values are not mimicked by neutron-corrected effective porosity because the thin coars-er grained sandstone interbeds that account for thehigh permeability values are below the 061-m (2 ft)vertical resolution of the neutron sonde (resolu-tion estimate via Alberty 1992)

A second potential influence on reservoir qual-ity is the extent of bioturbation and resultant intro-duction of interparticle silt and clay-size particles Toevaluate this possibility we compare porosity andpermeability to five categories of bioturbation in-tensity aka the ichnofabric index under two con-ditions (Figure 6) Under condition 1 the ichnofab-ric index is compared against neutron-correctedeffective porosity and core analysismaximumperme-ability (Kmax) for all core data (Figure 6A) whereascondition 2 limits the comparison to facies 2 3and 4 sandstones having a porosity of greater thanor equal to 12 (Figure 6B) ie the porosity cut-off at Valhalla field that is commonly used by localindustry producers to define reservoir-quality sand-stone (Figure 7) In condition 1 porosity and per-meability trends including the assumed perme-ability sampling bias are similar to those for grainsize and associated depositional facies (compare

Figure 6 Box and whisker plots of neutron-corrected effective porosity and maximum permeability (Kmax) versus ichnofabric indexand stacked bar charts illustrating the proportion of depositional facies that comprise each ichnofabric index category (A) Condition 1case based on analysis of all core data regardless of lithology and (B) condition 2 case based on analysis of all core data where facies 23 and 4 sandstones have a porosity of greater than or equal to 12 Regardless of condition or ichnofabric index category the highestvalues of porosity and permeability coincide with the highest proportion of the shallowest water facies This suggests that grain size anddepositional environment are the primary control on reservoir quality

Atchley et al 9

Figure 7 Scatter plot of coreporosity versus Kmax perme-ability for all Doe Creekndashcoredwells within the study area(compare with Figure 3) Brack-eted on the plot is the rangeof permeability (Kmax) acrosswellbore perforations (1ndash10 md)which as of this writing coin-cides with the lower limit of eco-nomic daily oil production(approximately lt10 bblday)observed at Valhalla field Per-meability values within this rangein turn coincide with a porosityrange of 10ndash15 and is sup-portive of the practice of Valhallafield operators to apply a 12porosity cutoff to define the lowerlimit of satisfactory Doe Creeksandstone reservoir quality

Figure 8 Box and whisker plotof neutron-corrected effectiveporosity and maximum perme-ability (Kmax) versus sandstonewhere calcite cement eitherwas (Yes) or was not (No) ob-served Analysis is based onan analysis of all core data wherefacies 2 3 and 4 sandstoneshave a porosity of greater thanor equal to 12 Both porosityand permeability are reducedin the presence of calcite cement

10 EampP Note

Figures 5 6A) In condition 2 the highest valuesof porosity and permeability similarly coincide withboth ichnofabric indices 1 2 and 5 and the highestproportion of the shallowest water sandstone de-posits (principally facies 3) (Figure 6B) These anal-yses suggest that grain-size trends and associatedenvironments of deposition instead of the ichno-fabric index are the primary sedimentologic con-trols on reservoir quality

Reservoir quality is diminished in the presenceof postdepositional calcite cement (Figures 4F 8)Calcite cement is most common near the top ofsandstone bodies and is also irregularly distributedwithin sandstone bodies Zones of calcite cementa-tion are thin (typically lt50 cm [20 in]) and likelyhave a lateral continuity of less than 4 m (13 ft)(Hogg et al 1998) Calcite cement is increasinglyabundant within shallower water deposits and mayhave resulted from the preferential flow of CaCO3-saturated fluids through their characteristically

Figure 9 Box plot of depositional facies versus the percentageof core specimens that have calcite cement Analysis is based onall cores described at Valhalla field

Figure 10 Box and whisker plots of facies versus (A) open-holedeep resistivity well-log data and (B) shale volume (Vsh) contentderived from open-hole gamma-ray well-log data (compare withTable 2)

Atchley et al 11

higher permeability relatively coarser grained sed-iments (Figure 5) Of the total core observationscalcite cement is present within 25 of the occur-rences of facies 1 (offshore) 85 of the occur-rences of facies 2 (distal lower shoreface) 16 ofthe occurrence of facies 3 (proximal lower shore-face) and 64 of the occurrences of facies 4 (up-per shoreface) (Figure 9) The particularly highpercentage of calcite cement within facies 4 is re-lated to both the limited observations of facies 4and the consistent position of facies 4 near thetop of sandstone bodies

Reservoir Facies Prediction

Comparison of core-observed depositional faciesand petrophysical attributes suggests that open-holegamma ray and deep resistivity well-log data maybe used to predict facies distributions in wells thatlack core control (Figure 10) Open-hole neutron-porosity logs were also evaluated for their poten-tial to predict facies but proved to be less reliableGuidelines for facies prediction use direct mea-surement of deep resistivity for oil-saturated reser-voir above the original oil-water contact (+46 m[151 ft]) and shale volume calculations (Vsh) de-rived from normalized gamma-ray measurementsBecause the Doe Creek study interval was oil fullduring initial field development the guidelines forthe use of deep resistivity are applicable to most (if

12 EampP Note

not all) wells with open-hole logs within the studyarea (Figure 3) Gamma-ray normalization andVsh

calculations are based on the following formulae(Larinov 1969 Asquith and Krygowski 2004 ascited by Asquith and Krygowski 2004)

IGR frac14 ethGRlog GRminTHORN=ethGRmax GRminTHORN

where the IGR is the gamma-ray index GRlog is thegamma-raymeasurement from the reservoir GRmin

is the 14 API units and GRmax is the 130 APIunits (asterisks indicate Valhalla fieldwide averageminimum and maximum values observed in open-hole wireline logs)

V sh frac14 033frac122eth2 IGRTHORN 1

where Vsh is the shale volumeTable 2 defines the guidelines for prediction

and equates the resultant well-log facies with theirdepositional facies counterparts Although well-log data cluster within discrete ranges for each de-positional facies the overlapping range of Vsh anddeep resistivity data between facies results in well-log facies predictions that reflect data blendingbetween two (or more) depositional facies (com-pare Table 2 and Figure 10) Consequently al-though predicted well-log facies are most likely as-sociated with a particular depositional facies theintroduction of error from overlapping ranges of

Table 2 Guidelines for Facies Prediction from Open-Hole Well Logs

Well-Log Facies

Depositional Facies Equivalents

Parameters for Well-Log Prediction

4

Primarily 3 and 4

Vsh le 009 (average confidence)Vsh le 009 and deep resistivity ge38 ohm m (increased confidence)

3

Primarily 3 and lesser 2

Vsh ge 009 to le 012 (average confidence)Vsh ge 009 to le 012 and deep resistivity le38 to ge 21 ohm m (increased confidence)

2

Primarily 2 and lesser 1

Vsh ge 012 to le 033 (average confidence)Vsh ge 012 to le 033 and deep resistivity le21 to ge 14 ohm m (increased confidence)

1

Primarily 1 and lesser 2

Vsh ge 033 (average confidence) Vsh ge 033 anddeep resistivity le 14 ohm m (increased confidence)

Predicted well-log facies correspond to more than one depositional facies because of the overlapping range of Vsh and deep resistivity well-log data between depositionalfacies (Figure 10) The confidence of well-log facies interpretation increases when both the Vsh and deep resistivity guidelines are satisfied

well-log data between facies categories may re-sult in misinterpretation

Calcite cement is interpreted to occur withinwell-log facies 2 3 or 4 where the well-log neu-tron porosity exceeds the density porosity bymorethan 7 Within an oil-saturated reservoir such asthe Doe Creek at Valhalla this quick-look tech-nique may be used when density-neutron well-logdata are calibrated to a sandstonematrix density val-ue of 265 gcm3 (Asquith and Krygowski 2004)

To evaluate the accuracy of the predictivemod-el a blind test was administered to two cored wellswithin the study area that had not been includedwithin the database used to develop the well-logfacies or calcite cementation predictive guidelines(Figure 11) Cores from both wells were describedand interpreted independent of knowledge regard-ing the interpreted distribution of well-log faciesIn both cases the distribution of depositional faciesobserved within cores closely matches the pre-dicted distribution (Figure 11) The predicted dis-tribution of calcite cement precisely coincides withthe observed distribution within the 0014-15-075-08W6 well but only matches the thickest of fourcalcite-cemented zones within the 0006-32-074-09W6 well (Figure 11) The inability of the density-neutron predictor to detect calcite-cemented zoneswithin the 0006-32-074-09W6 well is likely re-lated to their thin (02ndash03 m [06ndash09 ft]) discon-tinuous distribution which is below the 045ndash061-m (17ndash2-ft) vertical resolution of density-neutron well logs (resolution via Alberty 1992)

STRATIGRAPHIC FRAMEWORK

Regional Overview

TheKaskapau Formationwas deposited during theculmination of the second-order transgression as-sociated with the Zuni cratonic sequence (sensuSloss 1963) and the concomitant maximum ex-tent of the North American WIS during the LateCretaceous (Hancock and Kauffman 1979 Haqet al 1988 Caldwell et al 1993 Kauffman andCaldwell 1993 Slingerland et al 1996) Withinthe study area second-order maximum flooding

coincides with the Greenhorn eustatic transgressionjust above the CenomanianndashTuronian boundaryand the equivalent organic-rich earliest TuronianSecondWhite Speckled Shale of the Kaskapau For-mation (Figure 1) (Caldwell 1983 Haq et al 1987Wallace-Dudley and Leckie 1993 Bhattacharya1994 Hogg et al 1998 Varban and Plint 2005)A higher frequency perhaps third-order episodeof decreasing and increasing accommodation dur-ing the middle Cenomanian to lower Turonian waspossibly induced by a pulse of foreland tectonismacross the WCSB and resulted in deposition oflowstand (Dunvegan) deltaic deposits and suc-ceeding transgressive (Kaskapau) open-marinemidshelf deposits (Plint et al 1993) Because ofthe relatively rapid rates of composite accommo-dation gain that resulted from constructive inter-ference of the second- and third-order transgres-sions higher frequency fourth-order stillstands(or lowstands) of sea level are recorded as retrogra-dationally stacked alternations of deeper water off-shore mudrocks (fourth-order transgressions) andshallower water shelf sandstones (fourth-orderstillstands or regressions) (Wallace-Dudley andLeckie 1993) In ascending stratigraphic orderthese shallower water sandstones include the DoeCreek Pouce Coupe and Howard Creek membersof the Kaskapau Formation (Figure 1)

Valhalla Reservoir Interval

TheDoeCreekMember atValhalla field consists ofnumerous cyclic alternations of offshore mudrock(facies 1) and shoreface sandstone (facies 2 3 andless commonly 4) Nine of these thin (1ndash10 m [3ndash33 ft] thick) cyclic units are correlated across Val-halla and their stratigraphic designations are refer-enced sequentially relative to their position above orbelow the top of themost oil productive I sandstone(alphanumeric designation by Wallace-Dudley andLeckie 1988) Above the I sandstone these includein ascending order the I + 1 through I + 6 and be-low the I sandstone in descending order the I minus 1and I minus 2 (Figures 11 12) In addition to the I sand-stone other hydrocarbon-charged units include theoil-prone I minus 1 and I + 1 and the gas-prone I + 2The I + 1 and I + 2 coincide with the previously

Atchley et al 13

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

Figure 7 Scatter plot of coreporosity versus Kmax perme-ability for all Doe Creekndashcoredwells within the study area(compare with Figure 3) Brack-eted on the plot is the rangeof permeability (Kmax) acrosswellbore perforations (1ndash10 md)which as of this writing coin-cides with the lower limit of eco-nomic daily oil production(approximately lt10 bblday)observed at Valhalla field Per-meability values within this rangein turn coincide with a porosityrange of 10ndash15 and is sup-portive of the practice of Valhallafield operators to apply a 12porosity cutoff to define the lowerlimit of satisfactory Doe Creeksandstone reservoir quality

Figure 8 Box and whisker plotof neutron-corrected effectiveporosity and maximum perme-ability (Kmax) versus sandstonewhere calcite cement eitherwas (Yes) or was not (No) ob-served Analysis is based onan analysis of all core data wherefacies 2 3 and 4 sandstoneshave a porosity of greater thanor equal to 12 Both porosityand permeability are reducedin the presence of calcite cement

10 EampP Note

Figures 5 6A) In condition 2 the highest valuesof porosity and permeability similarly coincide withboth ichnofabric indices 1 2 and 5 and the highestproportion of the shallowest water sandstone de-posits (principally facies 3) (Figure 6B) These anal-yses suggest that grain-size trends and associatedenvironments of deposition instead of the ichno-fabric index are the primary sedimentologic con-trols on reservoir quality

Reservoir quality is diminished in the presenceof postdepositional calcite cement (Figures 4F 8)Calcite cement is most common near the top ofsandstone bodies and is also irregularly distributedwithin sandstone bodies Zones of calcite cementa-tion are thin (typically lt50 cm [20 in]) and likelyhave a lateral continuity of less than 4 m (13 ft)(Hogg et al 1998) Calcite cement is increasinglyabundant within shallower water deposits and mayhave resulted from the preferential flow of CaCO3-saturated fluids through their characteristically

Figure 9 Box plot of depositional facies versus the percentageof core specimens that have calcite cement Analysis is based onall cores described at Valhalla field

Figure 10 Box and whisker plots of facies versus (A) open-holedeep resistivity well-log data and (B) shale volume (Vsh) contentderived from open-hole gamma-ray well-log data (compare withTable 2)

Atchley et al 11

higher permeability relatively coarser grained sed-iments (Figure 5) Of the total core observationscalcite cement is present within 25 of the occur-rences of facies 1 (offshore) 85 of the occur-rences of facies 2 (distal lower shoreface) 16 ofthe occurrence of facies 3 (proximal lower shore-face) and 64 of the occurrences of facies 4 (up-per shoreface) (Figure 9) The particularly highpercentage of calcite cement within facies 4 is re-lated to both the limited observations of facies 4and the consistent position of facies 4 near thetop of sandstone bodies

Reservoir Facies Prediction

Comparison of core-observed depositional faciesand petrophysical attributes suggests that open-holegamma ray and deep resistivity well-log data maybe used to predict facies distributions in wells thatlack core control (Figure 10) Open-hole neutron-porosity logs were also evaluated for their poten-tial to predict facies but proved to be less reliableGuidelines for facies prediction use direct mea-surement of deep resistivity for oil-saturated reser-voir above the original oil-water contact (+46 m[151 ft]) and shale volume calculations (Vsh) de-rived from normalized gamma-ray measurementsBecause the Doe Creek study interval was oil fullduring initial field development the guidelines forthe use of deep resistivity are applicable to most (if

12 EampP Note

not all) wells with open-hole logs within the studyarea (Figure 3) Gamma-ray normalization andVsh

calculations are based on the following formulae(Larinov 1969 Asquith and Krygowski 2004 ascited by Asquith and Krygowski 2004)

IGR frac14 ethGRlog GRminTHORN=ethGRmax GRminTHORN

where the IGR is the gamma-ray index GRlog is thegamma-raymeasurement from the reservoir GRmin

is the 14 API units and GRmax is the 130 APIunits (asterisks indicate Valhalla fieldwide averageminimum and maximum values observed in open-hole wireline logs)

V sh frac14 033frac122eth2 IGRTHORN 1

where Vsh is the shale volumeTable 2 defines the guidelines for prediction

and equates the resultant well-log facies with theirdepositional facies counterparts Although well-log data cluster within discrete ranges for each de-positional facies the overlapping range of Vsh anddeep resistivity data between facies results in well-log facies predictions that reflect data blendingbetween two (or more) depositional facies (com-pare Table 2 and Figure 10) Consequently al-though predicted well-log facies are most likely as-sociated with a particular depositional facies theintroduction of error from overlapping ranges of

Table 2 Guidelines for Facies Prediction from Open-Hole Well Logs

Well-Log Facies

Depositional Facies Equivalents

Parameters for Well-Log Prediction

4

Primarily 3 and 4

Vsh le 009 (average confidence)Vsh le 009 and deep resistivity ge38 ohm m (increased confidence)

3

Primarily 3 and lesser 2

Vsh ge 009 to le 012 (average confidence)Vsh ge 009 to le 012 and deep resistivity le38 to ge 21 ohm m (increased confidence)

2

Primarily 2 and lesser 1

Vsh ge 012 to le 033 (average confidence)Vsh ge 012 to le 033 and deep resistivity le21 to ge 14 ohm m (increased confidence)

1

Primarily 1 and lesser 2

Vsh ge 033 (average confidence) Vsh ge 033 anddeep resistivity le 14 ohm m (increased confidence)

Predicted well-log facies correspond to more than one depositional facies because of the overlapping range of Vsh and deep resistivity well-log data between depositionalfacies (Figure 10) The confidence of well-log facies interpretation increases when both the Vsh and deep resistivity guidelines are satisfied

well-log data between facies categories may re-sult in misinterpretation

Calcite cement is interpreted to occur withinwell-log facies 2 3 or 4 where the well-log neu-tron porosity exceeds the density porosity bymorethan 7 Within an oil-saturated reservoir such asthe Doe Creek at Valhalla this quick-look tech-nique may be used when density-neutron well-logdata are calibrated to a sandstonematrix density val-ue of 265 gcm3 (Asquith and Krygowski 2004)

To evaluate the accuracy of the predictivemod-el a blind test was administered to two cored wellswithin the study area that had not been includedwithin the database used to develop the well-logfacies or calcite cementation predictive guidelines(Figure 11) Cores from both wells were describedand interpreted independent of knowledge regard-ing the interpreted distribution of well-log faciesIn both cases the distribution of depositional faciesobserved within cores closely matches the pre-dicted distribution (Figure 11) The predicted dis-tribution of calcite cement precisely coincides withthe observed distribution within the 0014-15-075-08W6 well but only matches the thickest of fourcalcite-cemented zones within the 0006-32-074-09W6 well (Figure 11) The inability of the density-neutron predictor to detect calcite-cemented zoneswithin the 0006-32-074-09W6 well is likely re-lated to their thin (02ndash03 m [06ndash09 ft]) discon-tinuous distribution which is below the 045ndash061-m (17ndash2-ft) vertical resolution of density-neutron well logs (resolution via Alberty 1992)

STRATIGRAPHIC FRAMEWORK

Regional Overview

TheKaskapau Formationwas deposited during theculmination of the second-order transgression as-sociated with the Zuni cratonic sequence (sensuSloss 1963) and the concomitant maximum ex-tent of the North American WIS during the LateCretaceous (Hancock and Kauffman 1979 Haqet al 1988 Caldwell et al 1993 Kauffman andCaldwell 1993 Slingerland et al 1996) Withinthe study area second-order maximum flooding

coincides with the Greenhorn eustatic transgressionjust above the CenomanianndashTuronian boundaryand the equivalent organic-rich earliest TuronianSecondWhite Speckled Shale of the Kaskapau For-mation (Figure 1) (Caldwell 1983 Haq et al 1987Wallace-Dudley and Leckie 1993 Bhattacharya1994 Hogg et al 1998 Varban and Plint 2005)A higher frequency perhaps third-order episodeof decreasing and increasing accommodation dur-ing the middle Cenomanian to lower Turonian waspossibly induced by a pulse of foreland tectonismacross the WCSB and resulted in deposition oflowstand (Dunvegan) deltaic deposits and suc-ceeding transgressive (Kaskapau) open-marinemidshelf deposits (Plint et al 1993) Because ofthe relatively rapid rates of composite accommo-dation gain that resulted from constructive inter-ference of the second- and third-order transgres-sions higher frequency fourth-order stillstands(or lowstands) of sea level are recorded as retrogra-dationally stacked alternations of deeper water off-shore mudrocks (fourth-order transgressions) andshallower water shelf sandstones (fourth-orderstillstands or regressions) (Wallace-Dudley andLeckie 1993) In ascending stratigraphic orderthese shallower water sandstones include the DoeCreek Pouce Coupe and Howard Creek membersof the Kaskapau Formation (Figure 1)

Valhalla Reservoir Interval

TheDoeCreekMember atValhalla field consists ofnumerous cyclic alternations of offshore mudrock(facies 1) and shoreface sandstone (facies 2 3 andless commonly 4) Nine of these thin (1ndash10 m [3ndash33 ft] thick) cyclic units are correlated across Val-halla and their stratigraphic designations are refer-enced sequentially relative to their position above orbelow the top of themost oil productive I sandstone(alphanumeric designation by Wallace-Dudley andLeckie 1988) Above the I sandstone these includein ascending order the I + 1 through I + 6 and be-low the I sandstone in descending order the I minus 1and I minus 2 (Figures 11 12) In addition to the I sand-stone other hydrocarbon-charged units include theoil-prone I minus 1 and I + 1 and the gas-prone I + 2The I + 1 and I + 2 coincide with the previously

Atchley et al 13

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

Figures 5 6A) In condition 2 the highest valuesof porosity and permeability similarly coincide withboth ichnofabric indices 1 2 and 5 and the highestproportion of the shallowest water sandstone de-posits (principally facies 3) (Figure 6B) These anal-yses suggest that grain-size trends and associatedenvironments of deposition instead of the ichno-fabric index are the primary sedimentologic con-trols on reservoir quality

Reservoir quality is diminished in the presenceof postdepositional calcite cement (Figures 4F 8)Calcite cement is most common near the top ofsandstone bodies and is also irregularly distributedwithin sandstone bodies Zones of calcite cementa-tion are thin (typically lt50 cm [20 in]) and likelyhave a lateral continuity of less than 4 m (13 ft)(Hogg et al 1998) Calcite cement is increasinglyabundant within shallower water deposits and mayhave resulted from the preferential flow of CaCO3-saturated fluids through their characteristically

Figure 9 Box plot of depositional facies versus the percentageof core specimens that have calcite cement Analysis is based onall cores described at Valhalla field

Figure 10 Box and whisker plots of facies versus (A) open-holedeep resistivity well-log data and (B) shale volume (Vsh) contentderived from open-hole gamma-ray well-log data (compare withTable 2)

Atchley et al 11

higher permeability relatively coarser grained sed-iments (Figure 5) Of the total core observationscalcite cement is present within 25 of the occur-rences of facies 1 (offshore) 85 of the occur-rences of facies 2 (distal lower shoreface) 16 ofthe occurrence of facies 3 (proximal lower shore-face) and 64 of the occurrences of facies 4 (up-per shoreface) (Figure 9) The particularly highpercentage of calcite cement within facies 4 is re-lated to both the limited observations of facies 4and the consistent position of facies 4 near thetop of sandstone bodies

Reservoir Facies Prediction

Comparison of core-observed depositional faciesand petrophysical attributes suggests that open-holegamma ray and deep resistivity well-log data maybe used to predict facies distributions in wells thatlack core control (Figure 10) Open-hole neutron-porosity logs were also evaluated for their poten-tial to predict facies but proved to be less reliableGuidelines for facies prediction use direct mea-surement of deep resistivity for oil-saturated reser-voir above the original oil-water contact (+46 m[151 ft]) and shale volume calculations (Vsh) de-rived from normalized gamma-ray measurementsBecause the Doe Creek study interval was oil fullduring initial field development the guidelines forthe use of deep resistivity are applicable to most (if

12 EampP Note

not all) wells with open-hole logs within the studyarea (Figure 3) Gamma-ray normalization andVsh

calculations are based on the following formulae(Larinov 1969 Asquith and Krygowski 2004 ascited by Asquith and Krygowski 2004)

IGR frac14 ethGRlog GRminTHORN=ethGRmax GRminTHORN

where the IGR is the gamma-ray index GRlog is thegamma-raymeasurement from the reservoir GRmin

is the 14 API units and GRmax is the 130 APIunits (asterisks indicate Valhalla fieldwide averageminimum and maximum values observed in open-hole wireline logs)

V sh frac14 033frac122eth2 IGRTHORN 1

where Vsh is the shale volumeTable 2 defines the guidelines for prediction

and equates the resultant well-log facies with theirdepositional facies counterparts Although well-log data cluster within discrete ranges for each de-positional facies the overlapping range of Vsh anddeep resistivity data between facies results in well-log facies predictions that reflect data blendingbetween two (or more) depositional facies (com-pare Table 2 and Figure 10) Consequently al-though predicted well-log facies are most likely as-sociated with a particular depositional facies theintroduction of error from overlapping ranges of

Table 2 Guidelines for Facies Prediction from Open-Hole Well Logs

Well-Log Facies

Depositional Facies Equivalents

Parameters for Well-Log Prediction

4

Primarily 3 and 4

Vsh le 009 (average confidence)Vsh le 009 and deep resistivity ge38 ohm m (increased confidence)

3

Primarily 3 and lesser 2

Vsh ge 009 to le 012 (average confidence)Vsh ge 009 to le 012 and deep resistivity le38 to ge 21 ohm m (increased confidence)

2

Primarily 2 and lesser 1

Vsh ge 012 to le 033 (average confidence)Vsh ge 012 to le 033 and deep resistivity le21 to ge 14 ohm m (increased confidence)

1

Primarily 1 and lesser 2

Vsh ge 033 (average confidence) Vsh ge 033 anddeep resistivity le 14 ohm m (increased confidence)

Predicted well-log facies correspond to more than one depositional facies because of the overlapping range of Vsh and deep resistivity well-log data between depositionalfacies (Figure 10) The confidence of well-log facies interpretation increases when both the Vsh and deep resistivity guidelines are satisfied

well-log data between facies categories may re-sult in misinterpretation

Calcite cement is interpreted to occur withinwell-log facies 2 3 or 4 where the well-log neu-tron porosity exceeds the density porosity bymorethan 7 Within an oil-saturated reservoir such asthe Doe Creek at Valhalla this quick-look tech-nique may be used when density-neutron well-logdata are calibrated to a sandstonematrix density val-ue of 265 gcm3 (Asquith and Krygowski 2004)

To evaluate the accuracy of the predictivemod-el a blind test was administered to two cored wellswithin the study area that had not been includedwithin the database used to develop the well-logfacies or calcite cementation predictive guidelines(Figure 11) Cores from both wells were describedand interpreted independent of knowledge regard-ing the interpreted distribution of well-log faciesIn both cases the distribution of depositional faciesobserved within cores closely matches the pre-dicted distribution (Figure 11) The predicted dis-tribution of calcite cement precisely coincides withthe observed distribution within the 0014-15-075-08W6 well but only matches the thickest of fourcalcite-cemented zones within the 0006-32-074-09W6 well (Figure 11) The inability of the density-neutron predictor to detect calcite-cemented zoneswithin the 0006-32-074-09W6 well is likely re-lated to their thin (02ndash03 m [06ndash09 ft]) discon-tinuous distribution which is below the 045ndash061-m (17ndash2-ft) vertical resolution of density-neutron well logs (resolution via Alberty 1992)

STRATIGRAPHIC FRAMEWORK

Regional Overview

TheKaskapau Formationwas deposited during theculmination of the second-order transgression as-sociated with the Zuni cratonic sequence (sensuSloss 1963) and the concomitant maximum ex-tent of the North American WIS during the LateCretaceous (Hancock and Kauffman 1979 Haqet al 1988 Caldwell et al 1993 Kauffman andCaldwell 1993 Slingerland et al 1996) Withinthe study area second-order maximum flooding

coincides with the Greenhorn eustatic transgressionjust above the CenomanianndashTuronian boundaryand the equivalent organic-rich earliest TuronianSecondWhite Speckled Shale of the Kaskapau For-mation (Figure 1) (Caldwell 1983 Haq et al 1987Wallace-Dudley and Leckie 1993 Bhattacharya1994 Hogg et al 1998 Varban and Plint 2005)A higher frequency perhaps third-order episodeof decreasing and increasing accommodation dur-ing the middle Cenomanian to lower Turonian waspossibly induced by a pulse of foreland tectonismacross the WCSB and resulted in deposition oflowstand (Dunvegan) deltaic deposits and suc-ceeding transgressive (Kaskapau) open-marinemidshelf deposits (Plint et al 1993) Because ofthe relatively rapid rates of composite accommo-dation gain that resulted from constructive inter-ference of the second- and third-order transgres-sions higher frequency fourth-order stillstands(or lowstands) of sea level are recorded as retrogra-dationally stacked alternations of deeper water off-shore mudrocks (fourth-order transgressions) andshallower water shelf sandstones (fourth-orderstillstands or regressions) (Wallace-Dudley andLeckie 1993) In ascending stratigraphic orderthese shallower water sandstones include the DoeCreek Pouce Coupe and Howard Creek membersof the Kaskapau Formation (Figure 1)

Valhalla Reservoir Interval

TheDoeCreekMember atValhalla field consists ofnumerous cyclic alternations of offshore mudrock(facies 1) and shoreface sandstone (facies 2 3 andless commonly 4) Nine of these thin (1ndash10 m [3ndash33 ft] thick) cyclic units are correlated across Val-halla and their stratigraphic designations are refer-enced sequentially relative to their position above orbelow the top of themost oil productive I sandstone(alphanumeric designation by Wallace-Dudley andLeckie 1988) Above the I sandstone these includein ascending order the I + 1 through I + 6 and be-low the I sandstone in descending order the I minus 1and I minus 2 (Figures 11 12) In addition to the I sand-stone other hydrocarbon-charged units include theoil-prone I minus 1 and I + 1 and the gas-prone I + 2The I + 1 and I + 2 coincide with the previously

Atchley et al 13

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

higher permeability relatively coarser grained sed-iments (Figure 5) Of the total core observationscalcite cement is present within 25 of the occur-rences of facies 1 (offshore) 85 of the occur-rences of facies 2 (distal lower shoreface) 16 ofthe occurrence of facies 3 (proximal lower shore-face) and 64 of the occurrences of facies 4 (up-per shoreface) (Figure 9) The particularly highpercentage of calcite cement within facies 4 is re-lated to both the limited observations of facies 4and the consistent position of facies 4 near thetop of sandstone bodies

Reservoir Facies Prediction

Comparison of core-observed depositional faciesand petrophysical attributes suggests that open-holegamma ray and deep resistivity well-log data maybe used to predict facies distributions in wells thatlack core control (Figure 10) Open-hole neutron-porosity logs were also evaluated for their poten-tial to predict facies but proved to be less reliableGuidelines for facies prediction use direct mea-surement of deep resistivity for oil-saturated reser-voir above the original oil-water contact (+46 m[151 ft]) and shale volume calculations (Vsh) de-rived from normalized gamma-ray measurementsBecause the Doe Creek study interval was oil fullduring initial field development the guidelines forthe use of deep resistivity are applicable to most (if

12 EampP Note

not all) wells with open-hole logs within the studyarea (Figure 3) Gamma-ray normalization andVsh

calculations are based on the following formulae(Larinov 1969 Asquith and Krygowski 2004 ascited by Asquith and Krygowski 2004)

IGR frac14 ethGRlog GRminTHORN=ethGRmax GRminTHORN

where the IGR is the gamma-ray index GRlog is thegamma-raymeasurement from the reservoir GRmin

is the 14 API units and GRmax is the 130 APIunits (asterisks indicate Valhalla fieldwide averageminimum and maximum values observed in open-hole wireline logs)

V sh frac14 033frac122eth2 IGRTHORN 1

where Vsh is the shale volumeTable 2 defines the guidelines for prediction

and equates the resultant well-log facies with theirdepositional facies counterparts Although well-log data cluster within discrete ranges for each de-positional facies the overlapping range of Vsh anddeep resistivity data between facies results in well-log facies predictions that reflect data blendingbetween two (or more) depositional facies (com-pare Table 2 and Figure 10) Consequently al-though predicted well-log facies are most likely as-sociated with a particular depositional facies theintroduction of error from overlapping ranges of

Table 2 Guidelines for Facies Prediction from Open-Hole Well Logs

Well-Log Facies

Depositional Facies Equivalents

Parameters for Well-Log Prediction

4

Primarily 3 and 4

Vsh le 009 (average confidence)Vsh le 009 and deep resistivity ge38 ohm m (increased confidence)

3

Primarily 3 and lesser 2

Vsh ge 009 to le 012 (average confidence)Vsh ge 009 to le 012 and deep resistivity le38 to ge 21 ohm m (increased confidence)

2

Primarily 2 and lesser 1

Vsh ge 012 to le 033 (average confidence)Vsh ge 012 to le 033 and deep resistivity le21 to ge 14 ohm m (increased confidence)

1

Primarily 1 and lesser 2

Vsh ge 033 (average confidence) Vsh ge 033 anddeep resistivity le 14 ohm m (increased confidence)

Predicted well-log facies correspond to more than one depositional facies because of the overlapping range of Vsh and deep resistivity well-log data between depositionalfacies (Figure 10) The confidence of well-log facies interpretation increases when both the Vsh and deep resistivity guidelines are satisfied

well-log data between facies categories may re-sult in misinterpretation

Calcite cement is interpreted to occur withinwell-log facies 2 3 or 4 where the well-log neu-tron porosity exceeds the density porosity bymorethan 7 Within an oil-saturated reservoir such asthe Doe Creek at Valhalla this quick-look tech-nique may be used when density-neutron well-logdata are calibrated to a sandstonematrix density val-ue of 265 gcm3 (Asquith and Krygowski 2004)

To evaluate the accuracy of the predictivemod-el a blind test was administered to two cored wellswithin the study area that had not been includedwithin the database used to develop the well-logfacies or calcite cementation predictive guidelines(Figure 11) Cores from both wells were describedand interpreted independent of knowledge regard-ing the interpreted distribution of well-log faciesIn both cases the distribution of depositional faciesobserved within cores closely matches the pre-dicted distribution (Figure 11) The predicted dis-tribution of calcite cement precisely coincides withthe observed distribution within the 0014-15-075-08W6 well but only matches the thickest of fourcalcite-cemented zones within the 0006-32-074-09W6 well (Figure 11) The inability of the density-neutron predictor to detect calcite-cemented zoneswithin the 0006-32-074-09W6 well is likely re-lated to their thin (02ndash03 m [06ndash09 ft]) discon-tinuous distribution which is below the 045ndash061-m (17ndash2-ft) vertical resolution of density-neutron well logs (resolution via Alberty 1992)

STRATIGRAPHIC FRAMEWORK

Regional Overview

TheKaskapau Formationwas deposited during theculmination of the second-order transgression as-sociated with the Zuni cratonic sequence (sensuSloss 1963) and the concomitant maximum ex-tent of the North American WIS during the LateCretaceous (Hancock and Kauffman 1979 Haqet al 1988 Caldwell et al 1993 Kauffman andCaldwell 1993 Slingerland et al 1996) Withinthe study area second-order maximum flooding

coincides with the Greenhorn eustatic transgressionjust above the CenomanianndashTuronian boundaryand the equivalent organic-rich earliest TuronianSecondWhite Speckled Shale of the Kaskapau For-mation (Figure 1) (Caldwell 1983 Haq et al 1987Wallace-Dudley and Leckie 1993 Bhattacharya1994 Hogg et al 1998 Varban and Plint 2005)A higher frequency perhaps third-order episodeof decreasing and increasing accommodation dur-ing the middle Cenomanian to lower Turonian waspossibly induced by a pulse of foreland tectonismacross the WCSB and resulted in deposition oflowstand (Dunvegan) deltaic deposits and suc-ceeding transgressive (Kaskapau) open-marinemidshelf deposits (Plint et al 1993) Because ofthe relatively rapid rates of composite accommo-dation gain that resulted from constructive inter-ference of the second- and third-order transgres-sions higher frequency fourth-order stillstands(or lowstands) of sea level are recorded as retrogra-dationally stacked alternations of deeper water off-shore mudrocks (fourth-order transgressions) andshallower water shelf sandstones (fourth-orderstillstands or regressions) (Wallace-Dudley andLeckie 1993) In ascending stratigraphic orderthese shallower water sandstones include the DoeCreek Pouce Coupe and Howard Creek membersof the Kaskapau Formation (Figure 1)

Valhalla Reservoir Interval

TheDoeCreekMember atValhalla field consists ofnumerous cyclic alternations of offshore mudrock(facies 1) and shoreface sandstone (facies 2 3 andless commonly 4) Nine of these thin (1ndash10 m [3ndash33 ft] thick) cyclic units are correlated across Val-halla and their stratigraphic designations are refer-enced sequentially relative to their position above orbelow the top of themost oil productive I sandstone(alphanumeric designation by Wallace-Dudley andLeckie 1988) Above the I sandstone these includein ascending order the I + 1 through I + 6 and be-low the I sandstone in descending order the I minus 1and I minus 2 (Figures 11 12) In addition to the I sand-stone other hydrocarbon-charged units include theoil-prone I minus 1 and I + 1 and the gas-prone I + 2The I + 1 and I + 2 coincide with the previously

Atchley et al 13

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

well-log data between facies categories may re-sult in misinterpretation

Calcite cement is interpreted to occur withinwell-log facies 2 3 or 4 where the well-log neu-tron porosity exceeds the density porosity bymorethan 7 Within an oil-saturated reservoir such asthe Doe Creek at Valhalla this quick-look tech-nique may be used when density-neutron well-logdata are calibrated to a sandstonematrix density val-ue of 265 gcm3 (Asquith and Krygowski 2004)

To evaluate the accuracy of the predictivemod-el a blind test was administered to two cored wellswithin the study area that had not been includedwithin the database used to develop the well-logfacies or calcite cementation predictive guidelines(Figure 11) Cores from both wells were describedand interpreted independent of knowledge regard-ing the interpreted distribution of well-log faciesIn both cases the distribution of depositional faciesobserved within cores closely matches the pre-dicted distribution (Figure 11) The predicted dis-tribution of calcite cement precisely coincides withthe observed distribution within the 0014-15-075-08W6 well but only matches the thickest of fourcalcite-cemented zones within the 0006-32-074-09W6 well (Figure 11) The inability of the density-neutron predictor to detect calcite-cemented zoneswithin the 0006-32-074-09W6 well is likely re-lated to their thin (02ndash03 m [06ndash09 ft]) discon-tinuous distribution which is below the 045ndash061-m (17ndash2-ft) vertical resolution of density-neutron well logs (resolution via Alberty 1992)

STRATIGRAPHIC FRAMEWORK

Regional Overview

TheKaskapau Formationwas deposited during theculmination of the second-order transgression as-sociated with the Zuni cratonic sequence (sensuSloss 1963) and the concomitant maximum ex-tent of the North American WIS during the LateCretaceous (Hancock and Kauffman 1979 Haqet al 1988 Caldwell et al 1993 Kauffman andCaldwell 1993 Slingerland et al 1996) Withinthe study area second-order maximum flooding

coincides with the Greenhorn eustatic transgressionjust above the CenomanianndashTuronian boundaryand the equivalent organic-rich earliest TuronianSecondWhite Speckled Shale of the Kaskapau For-mation (Figure 1) (Caldwell 1983 Haq et al 1987Wallace-Dudley and Leckie 1993 Bhattacharya1994 Hogg et al 1998 Varban and Plint 2005)A higher frequency perhaps third-order episodeof decreasing and increasing accommodation dur-ing the middle Cenomanian to lower Turonian waspossibly induced by a pulse of foreland tectonismacross the WCSB and resulted in deposition oflowstand (Dunvegan) deltaic deposits and suc-ceeding transgressive (Kaskapau) open-marinemidshelf deposits (Plint et al 1993) Because ofthe relatively rapid rates of composite accommo-dation gain that resulted from constructive inter-ference of the second- and third-order transgres-sions higher frequency fourth-order stillstands(or lowstands) of sea level are recorded as retrogra-dationally stacked alternations of deeper water off-shore mudrocks (fourth-order transgressions) andshallower water shelf sandstones (fourth-orderstillstands or regressions) (Wallace-Dudley andLeckie 1993) In ascending stratigraphic orderthese shallower water sandstones include the DoeCreek Pouce Coupe and Howard Creek membersof the Kaskapau Formation (Figure 1)

Valhalla Reservoir Interval

TheDoeCreekMember atValhalla field consists ofnumerous cyclic alternations of offshore mudrock(facies 1) and shoreface sandstone (facies 2 3 andless commonly 4) Nine of these thin (1ndash10 m [3ndash33 ft] thick) cyclic units are correlated across Val-halla and their stratigraphic designations are refer-enced sequentially relative to their position above orbelow the top of themost oil productive I sandstone(alphanumeric designation by Wallace-Dudley andLeckie 1988) Above the I sandstone these includein ascending order the I + 1 through I + 6 and be-low the I sandstone in descending order the I minus 1and I minus 2 (Figures 11 12) In addition to the I sand-stone other hydrocarbon-charged units include theoil-prone I minus 1 and I + 1 and the gas-prone I + 2The I + 1 and I + 2 coincide with the previously

Atchley et al 13

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

Figure 11 Gamma-ray density neutron-porosity (calibrated to sandstone matrix) and deep induction logs for wells (A) 0006-32-074-09W6 and (B) 0014-15-075-08W6 In addition both wells include core description logs of the ichnofabric index and ichnodiversity andthe distribution of the most common trace fossils observed (O = Ophiomorpha Pl = Planolites Sc = Scolicia T = Teichichnus Th =Thalassinoides Zoo = Zoophycos) Annotated as dashed lines within the gamma-ray track are the tops of the I minus 2 through I + 6 intervalscorrelated throughout Valhalla field The I I + 1 and I + 2 intervals coincide with the I N and A sandstones of Wallace-Dudley and Leckie(1988) Both wells were used as a blind test of the guidelines for the prediction of well-log facies and calcite cement occurrence withinreservoir sandstone (see the text for discussion and Table 2) Note the close correspondence between core observed and log-predicteddistributions presented within the depth track

14 EampP Note

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

Figure 12 Stratigraphic cross section AAprime illustrating the distribution of Doe Creek stratigraphic surfaces I minus 1 through I + 6 The cross section stratigraphic datum is the gamma-raymaximum (shale) that occurs 2ndash3 m (6ndash10 ft) above the I + 6 surface The map location of cross section AAprime is shown in Figure 13 The depth track of each well log highlights thedistribution of facies observed in cores and predicted from well logs Facies distributions are interpolated across the cross section Stratal geometries and facies distributions within the I minus 1through I + 2 reservoir interval suggest southwestward progradation

Atchleyetal

15

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

named N and A sandstones of Wallace-Dudleyand Leckie (1988)

Shoreface potentially reservoir-quality sand-stone facies 2 3 and 4 only occur within the I minus 1I I + 1 and I + 2 and therefore attribute mapping

16 EampP Note

is limited to these intervals Based on core obser-vations and interpretation of facies distributionsfrom open-hole well logs maps of facies distri-bution and gross reservoir sandstone thicknesswere produced for the I minus 1 through I + 2 intervals

Figure 13 Facies distri-bution and gross reservoirsandstone thickness mapsfor the (A) Iminus1 (B) I (C) I + 1and (D) I + 2 intervalsThe location of cross sec-tion AAprime presented inFigure 12 is highlightedFacies distributions arebased on both core obser-vation and application ofthe guidelines for open-hole well-log prediction(Table 2) Only wells withcore (circled) andor open-hole well logs (well sym-bols) are used in mappingGross reservoir sandstone isdefined as a borehole in-terval that has a porositygreater than 12 andsatisfies the shale volume(Vsh) predictive guidelinesfor facies 2 3 and 4(Table 2) To emphasizethe location of relativelythick sandstone bodiesonly contour values greaterthan zero are presentedSandstone bodies progradesouthwestward throughtime and the I interval(B) contains the thickestand most extensive sand-stone of the highest reser-voir quality The I sandstoneextends as a northeast-trending linear ridge thatbifurcates within the cen-tral part of T75N R8W6 TheI sandstone is thickest andhas the greatest proportionof the highest reservoir-quality facies (facies 3 and 4)near the boundary of T74-75N R9W6

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

Figure 13 Continued

(Figures 12 13) From base to top within thissuccession the reservoir sandstone is restricted tothe northeasternmost part of Valhalla within theI minus 1 interval extends across the length of the field

within the I interval and is restricted to succes-sively more southwestern locations within the I +1 and I + 2 intervals (Figure 13) Thin sandstonesof low reservoir quality (facies 2) occur within the

Atchley et al 17

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

18 EampP Note

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

I minus 1 interval and the thickest sandstones of highreservoir quality (facies 3 and 4) and continuity co-incide with the I interval and extend across thelength of Valhalla field as an axial ridge that bi-furcates within the central part of T75N R8W6(Figure 13A B) Although the I + 1 and I + 2 areboth dominated by relatively low-reservoir-qualitysandstone (facies 2) the I + 2 reservoir sandstone isof greater thickness and extent than observed withinthe I + 1 and also includes facies 4 sandstone of thehighest reservoir quality (Figure 13C D)

Analysis of ichnofaunal diversity and ichno-fabric index trends within the reservoir interval pro-vides additional insight into depositional conditionsDuring core description the ichnofaunal diversitywas recorded as a bed-scale continuous log of thenumber of taxon observed and the ichnofabricindex as the proportion of primary sedimentary struc-tures disturbed by burrows (Droser and Bottjer1986 Bottjer and Droser 1991) (Figure 11) SeeFigure 6 for an explanation of how ichnofabric in-dex categories are defined Throughout the I minus 1 toI + 2 interval ichnofabric index values for offshoremudrocks typically range from 4 to 5 and ichnofau-nal diversity values from 3 to 4 Taxa commonly in-clude the Cruziana forms Thalassinoides PlanolitesTeichichnus and Zoophycos and less commonly Rhi-zocorallium and Asterosoma (Figure 11) Althoughthe core is rarely available below the I minus 2 the fewopportunities to observe this offshore (facies 1a 1b)mudrock-dominated interval suggest ichnofabricindices of 1 to 2 and ichnogenera limited to Thalas-sinoides and Planolites (Figure 4A B) Sandstoneswithin the I minus 1 to I + 2 interval have an ichnofabricindex that ranges from 1 to 4 and a correspondinglyvariable range of ichnofaunal diversity (Figure 11)Low ichnofaunal diversity values occur within sand-stones that contain Skolithos forms that are mostcommonly limited to Ophiomorpha Palaeophycosandor Bergauria High ichnofaunal diversity val-ues are related to sandstones that include not onlythese Skolithos forms but alsoCruziana forms that

penetrated and reworked the sandstone during syn-depositional bioturbation of the overlying offshoremudrocks Within the I interval trends of highichnofabric index generally coincide with the thick-est shoreface (facies 3 and 4) sandstones (compareFigures 13B 14A) Conversely trends of highestichnodiversity are more variable and coincide withboth offshore mudrocks and shoreface sandstones(compare Figures 13B 14B)

IMPLICATIONS

Reservoir Performance

Although fluid production is comingled betweenhydrocarbon-charged sandstones within the I minus 1through I + 2 interval most production is providedby the laterally extensive I sandstone As such amap of average daily total fluid production essen-tially reflects the historic performance of the Isandstone (Figure 15A) The predicted distribu-tion and thickness of depositional facies corre-spond closely to reservoir-quality sandstone grosspore volume and fluid production trends (compareFigures 13B 15) Overall distributions of the shal-lowest water facies 3 and 4 coincide with regionsof the thickest gross pore volume and highest av-erage daily total fluid production Interestinglythis trend appears to be maintained regardless ofthe presence of calcite cement Fairways of calcitecement within reservoir-quality I sandstone ap-pear not to correlate with reduced production rates(compare Figures 15A 16) An exception to thefacies-dependent control on production trends isobserved within the southernmost part of T75NR9W6 Here average daily total fluid productionrates are relatively low although production coin-cides with the thickest andmost extensive accumu-lation of reservoir-quality facies 3 and 4 observedat Valhalla field (Figures 13B 15) Reduced pro-duction rates are most likely related to high water

Figure 14 Maps of the I interval (A) ichnofabric index and (B) ichnofaunal diversity Cored wells used in mapping are highlighted witha black circle around well symbols Gray circles around well symbols indicate wells with core that does not include the I interval Values ofhigh ichnofabric index correspond to the thickest I interval shoreface sandstones whereas trends of high ichnofaunal diversity coincidewith both offshore mudrocks and shoreface sandstones (compare with Figure 13B)

Atchley et al 19

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

20 EampP Note

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

recovery within well completions near or within theoil-water transition that occurs above the originaloil-water contact (+46 m [151 ft]) located withinthe northernmost part of T74N R9W6 (compare

Figures 3 13ndash15) Preferential water productionin this area may be the result of irregular lateralwater invasion within high-permeability sandstonebeds induced by reservoir pressure drawdown

Figure 15 Maps of the (A) Doe Creek sandstone average daily total fluid production and (B) I interval sandstone gross pore volumeThe map of daily total fluid production is based on all wellbores within the study area whereas the I interval sandstone gross porevolume map is only based on wellbores with open-hole well logs The sandstone gross pore volume is calculated for the I interval that hasa porosity greater than 12 and satisfies the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) Because mostproduction is provided by the I sandstone the average daily total fluid production attributed to the entire Doe Creek sandstone reservoirinterval by the Alberta Energy Resources Conservation Board (AERCB) and compiled within AccuMapreg primarily reflects I sandstonereservoir properties Trends of fluid production correspond closely to gross pore volume (B) and facies distribution and gross reservoirsandstone thickness (Figure 13B) Low fluid recovery within thick shoreface sandstones (facies 3 and 4) near the border of T74-75NR9W6 may reflect lateral water invasion associated with reservoir pressure drawdown near the original oil-water contact (+46 m [151 ft])(compare Figures 3 13B 15A)

Figure 16Map of calcite cement fraction within the I interval gross reservoir sandstone Gross reservoir sandstone is defined as havinga porosity greater than 12 and satisfying the shale volume (Vsh) predictive guidelines for facies 2 3 and 4 (Table 2) and associatedcalcite cement is interpreted to occur where the well-log neutron porosity exceeds the density porosity by more than 7 Only the open-hole wellbores used in mapping are shown Because of the relatively wide spacing of well control points the map likely exaggerates thecontinuity of calcite-cemented fairways

Atchley et al 21

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

Depositional and Stratigraphic Model

Both the Doe Creek and Pouce Coupe are distrib-uted as enigmatic isolated sandstone bodies withina coastal embayment to theWIS and are detachedfrom their equivalent shoreline deposits and encasedin offshore mudrock (Figure 3) (eg Wallace-Dudley and Leckie 1988 Plint et al 1993 Kreitnerand Plint 2006) The isolated nature of the sand-stone bodies has been variously attributed to (1) theformation of sand shoals on the lee side of age-equivalent lobes to the Dunvegan delta (Wallace-Dudley and Leckie 1988) (2) the progradation ofdeltas during sea level lowstands and subsequentpreservation of delta front deposits as discontinuouserosional remnants following transgressive ravine-ment (Wallace-Dudley and Leckie 1993 Kreitnerand Plint 2006) and (3) the erosion of a forebulgeuplift and subsequent transport of shelf sand to-ward a subsiding foredeep (Plint et al 1993 Plint2000) The first two explanations presume an east-ward and southeastward deltaic sediment transportaway from the WIS coastline (Kreitner and Plint2006) whereas the third explanation suggests a ra-vinement reworking of older Kaskapau sediments byshoreface shelf processes and southwestward trans-port toward the WIS shoreline (Plint et al 1993)

Sedimentologic observations and stratigraphicrelationships within the Doe Creek sandstone atValhalla are most consistent with the forebulgemodel of Plint et al (1993) and Plint (2000) Thismodel suggests that the ravinement reworking ofsand across a rising forebulge located northeast ofValhalla provided sand that was presumably trans-ported by shelf currents southwestward toward asubsiding foredeep Accordingly Doe Creek sand-stones thin to the northeastwhere they are truncatedbeneath the regional K1 intraformational forebulgeunconformity (sensuPlint et al 1993) and transitioninto offshore mudrocks to the southwest Becausethe sandstones are inferred to have accumulatedacross the shelf in a location seaward of and there-fore detached from the equivalent western shore-line of theWIS the sandswould likely be influencedmore by wave than river processes This interpreta-tion is supported by the southwestward prograda-tion of the I minus 1 to I + 2 interval (Figures 12 13)

22 EampP Note

and the dominance of wave-generated hummockycross strata and wave ripples within Doe Creeksandstones across Valhalla (eg Figure 4D E) Ifthe I minus 1 to I + 2 interval had been deposited inassociation with deltas extending from the WISshoreline located to the west or northwest an east-ward or southeastward direction of progradationwould more likely be present within the Doe Creekreservoir interval Additionally bedforms common-ly associated with delta-front sediment gravity-flowprocesses ie turbidites are not observed withinthe Doe Creek at Valhalla The occurrence of theDoe Creek sandstone interval as an isolated south-westward-elongate distribution may reflect sanddeposition as a southwestward-accreting lee-sideshoal complex (sensuWallace-Dudley and Leckie1988) that formed as westerly or southwesterlyunidirectional shelf currents dissipated upon entryinto the protected coastal embayment to the WIS(Figure 2B) The relatively high and uniform ichno-fabric index and ichnofaunal diversity values ob-served within both offshore mudrocks and shore-face sandstones suggest that the salinity andorsediment stress was relatively low duringDoe Creekdeposition (Figure 14) (sensu Gani et al 2008)This observation is inconsistent with the reduced sa-linity and increased turbidity that wouldmore likelyaccompany a lowstand delta complex The abun-dance of Zoophycos traces observed within offshoremudrocks across Valhalla may indicate either a lackof delta-front sediment gravity processes (Seilacher1967) or reduced bottom-water oxygen levels (Freyand Seilacher 1980) In general the ichnofaunal di-versity atValhalla is lower than documented for com-parable Cretaceous fully marine wave-dominatedsuccessions observed elsewhere within the WIS (egPemberton et al 1992) This reduction in relativediversity may reflect Doe Creek deposition within acoastal embayment that would bemore prone to sa-linity andor turbidity fluctuations than a coastalsetting outside of an embayment

CONCLUSIONS

1 TheDoeCreekMember at Valhalla field is com-posed of six depositional facies that accumulated

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

within offshore distal andproximal lower shore-face and upper shoreface environments

2 Reservoir quality correlates closely with grainsize and depositional facies An increase in effec-tive porosity andKmax permeability correspondsto the coarser grain texture observed in progres-sively shallower water shoreface deposits Thecorrelation of the ichnofabric index with effec-tive porosity andKmax permeability similarly re-flects depositional grain-size trends instead ofthe introduction of mudwithin interparticle porespace during bioturbation

3 Reservoir quality is diminished by postdeposi-tional calcite cement that occurs in thin (typicallylt50 cm [20 in]) discontinuous zones Calcitecement is most common at the top of sandstonebodies and is increasingly abundant within pro-gressively shallower water shoreface sandstones

4 Guidelines developed from open-hole well logsmay be used to predict depositional facies andcalcite cement inwells lacking core control Val-ue ranges for deep resistivity andor shale vol-ume calculations (Vsh) derived from naturalgamma radiation measurements may be usedto predict facies distributions Decreasing Vsh

values and increasing deep resistivity values cor-respond to progressively shallower water envi-ronments Zones of calcite cement greater thanapproximately 05 m (16 ft) thick may be pre-dicted within shoreface sandstones where theneutron porosity exceeds the density porosityby greater than or equal to 7

5 At Valhalla field the Doe Creek sandstone issubdivided into nine thin (1ndash10 m [3ndash33 ft]thick) cyclic alternations of offshore mudrockand shoreface sandstone that are designatedthe I minus 2 through I + 6 units Sandstones withinthe I minus 1 I I + 1 and I + 2 are hydrocarboncharged The I sandstone is the primary reser-voir at Valhalla field

6 During deposition sandstones within the I minus 1through I+2units are accreted as a southwestward-prograding succession across Valhalla field TheI minus 1 sandstone is limited to the northeasternmostpart of Valhalla the I sandstone extends acrossthe length ofValhalla and the I + 1 and I + 2 sand-stones are restricted to the successively more

western parts of the field The thickest sand-stones of the highest reservoir quality and conti-nuity (facies 3 and 4) coincide with the I interval

7 Within the I sandstone thepredicteddistributionof the shallowest water and highest reservoir-quality facies (facies 3 and 4) corresponds closelyto trends of sandstone gross pore volume andaverage daily total fluid production The only ex-ception occurs in the southwesternmost part ofValhalla within completions just above the orig-inal oil-water contact (+46m [151 ft]) Reducedproduction rates in this area are most likely re-lated to irregular lateral water invasion withinhigh-permeability sandstone beds induced byreservoir pressure drawdown

8 Regional and local observations aremost consis-tent with the model of Plint et al (1993) andPlint (2000) which suggest that the Doe Creeksandstone was derived from ravinement erosionof a rising forebulge located northeast of Valhal-la and southwestward sediment transport byregional shelf currents toward a subsiding fore-deep Thinning and truncation of theDoeCreekMember beneath the regional K1 intraforma-tional unconformity and southwestward prograda-tion of the wave-rippled and hummocky cross-stratified Iminus1 through I + 2 sandstones atValhallaindicate an easterly source area and depositionwithin a wave-dominated setting Relatively highand uniform ichnofabric index values withinoffshore mudrocks and shoreface sandstones ofthe Doe Creek reservoir interval are also sugges-tive of awave-dominated relatively open-marinesetting however ichnofaunal diversity is lowerthan that observed within comparable wave-dominated successions observed elsewhere with-in theWIS and is consistentwith depositionwith-in a coastal embayment prone to salinity andorturbidity variations

REFERENCES CITED

AERCB (Alberta Energy Resources Conservation Board)2008 Oil pool reserves file Calgary Alberta AlbertaEnergy Resources Conservation Board

Alberty M W 1992 Wireline methods in D Morton-Thompson and AMWoods eds Development geology

Atchley et al 23

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

reference manual AAPG Methods in Exploration Se-ries 10 p 141ndash194

Allan J and S Creaney 1991 Oil families of the WesternCanada Basin Bulletin of Canadian Petroleum Geologyv 39 p 107ndash122

Asquith G and D Krygowski 2004 Basic well log analy-sis 2d ed AAPG Methods in Exploration Series 16244 p

Bhattacharya J 1994 Cretaceous Dunvegan in theWesternCanada sedimentary basin inG Mossop and I Shetsencomps Geological atlas of the Western Canada sedi-mentary basin Canadian Society of Petroleum Geolo-gists and the Alberta Research Council Calgary Al-berta Canada p 365ndash374

Bottjer D J and M L Droser 1991 Ichnofabric and basinanalysis Palaios v 6 p 199ndash205

Caldwell W G E 1983 Early Cretaceous transgressionsand regressions in the southern Interior Plains in D FStott and D J Glass eds The Mesozoic of middleNorth America Canadian Society of Petroleum Geolo-gists Memoir 9 p 179ndash203

Caldwell W G E R Diner D L Eicher S P Fowler B RNorth C R Stelck and L von Holdt Wilhelm 1993Foraminiferal biostratigraphy of Cretaceous marine cy-clothems in W G E Caldwell and E G Kauffmaneds Evolution of the Western Interior basin Geologi-cal Association of Canada Special Paper 39 p 477ndash520

Droser M L and D J Bottjer 1986 A semiquantitativefield classification of ichnofabric Journal of SedimentaryPetrology v 56 p 558ndash569

Frey R W and S G Pemberton 1990 Bioturbate textureor ichnofabric Palaio v 5 p 385ndash386 doi1023073514896

Frey R W and A Seilacher 1980 Uniformity in marine in-vertebrate ichnology Lethia v 13 p 183ndash207 doi101111j1502-39311980tb00632x

Gani M R J P Bhattacharya and J A MacEachern 2008Using ichnology to determine the relative influence ofwaves storms tides and rivers in deltaic deposits Ex-amples from Cretaceous Western Interior seaway USAin J MacEachern K L Bann M K Gingras and S GPemberton eds Applied ichnology SEPM Short CourseNotes 52 p 209ndash225

Hancock J M and E G Kauffman 1979 The great trans-gressions of the Late Cretaceous Journal of GeologicalSociety (London) v 136 p 175ndash186 doi101144gsjgs13620175

Haq B U J Hardenbol and P R Vail 1987 Chronology offluctuating sea levels since the Triassic Science v 235p 1156ndash1166 doi101126science23547931156

Haq B U J Hardenbol and P R Vail 1988 Mesozoic andCenozoic chronostratigraphy and eustatic cycles inC K Wilgus B S Hastings C G St C KendallH W Posamentier C A Ross and J C Van Wagonereds Sea-level changes An integrated approach SEPMSpecial Publication 42 p 71ndash108

Hogg J R DWDearborn KA Lapointe andA L Sacheli1998 Exploration and exploitation history of the ValhallaDoe Creek ldquoIrdquo Pool northwestern Alberta in J R Hogg

24 EampP Note

ed Oil and gas pools of the Western Canada sedimen-tary basin Canadian Society of Petroleum GeologistsSpecial Publication S-51 p 77ndash88

Irving E P J Wynne and B R Globerman 1993 Creta-ceous paleolatitudes and overprints of North Americancraton in W G E Caldwell and E G Kauffmanneds Evolution of the Western Interior basin GeologicalAssociation of Canada Special Paper 39 p 91ndash96

Kauffman E G and W G E Caldwell 1993 The WesternInterior basin in space and time in W G E Caldwelland E G Kauffman eds Evolution of the Western In-terior basin Geological Association of Canada SpecialPaper 39 p 1ndash30

Kreitner M A and A G Plint 2006 Allostratigraphy andpaleogeography of the upper Cenomanian lower Kas-kapau Formation in subsurface and outcrop Alberta andBritish Columbia Bulletin of Canadian Petroleum Geol-ogy v 54 p 110ndash137 doi102113gscpgbull542110

Larinov V V 1969 Borehole radiometry Moscow NedraOgg J G F P Agterberg and F M Gradstein 2004 The

Cretaceous period in F Gradstein J Ogg and A Smitheds A geologic time scale Cambridge United KingdomCambridge University Press p 344ndash383

Pemberton S G J C VanWagoner andGDWach 1992Ichnofacies of a wave-dominated shoreline in S GPemberton ed Applications of ichnology to petroleumexploration SEPM Short Course Notes 17 p 339ndash382

Plint AG 2000 Sequence stratigraphy and paleogeographyof a Cenomanian deltaic complex The Dunvegan andlower Kaskapau formations in subsurface and outcropAlberta and British Columbia Canada Bulletin of Cana-dian Petroleum Geology v 48 p 43ndash79 doi10211348143

Plint A G B S Hart and W S Donaldson 1993 Litho-spheric flexure as a control on stratal geometry and faciesdistribution inUpper Cretaceous rocks of theAlberta fore-land basin Basin Research v 5 p 69ndash77 doi101111j1365-21171993tb00058x

Reid S A and S G Pemberton 2005 Beyond the shore-face Recognizing the unique ichnological character ofsubaqueous delta deposits in the Doe Creek Membernorthwest Alberta (abs) 2005 AAPG Annual Conven-tion Abstracts with Programs Calgary Alberta httpwwwsearchanddiscoverynetdocumentsabstracts2005annualcalgaryabstractsreidhtm (accessed No-vember 5 2009)

Seilacher A 1967 Bathymetry of trace fossils Marine Ge-ology v 5 p 413ndash428 doi1010160025-3227(67)90051-5

Slingerland R L R KumpM A Arthur P J Fawcett B BSageman and E J Barron 1996 Estuarine circulation inthe TuronianWestern Interior seaway of NorthAmericaGeological Society of America Bulletin v 108 p 941ndash952 doi1011300016-7606(1996)108lt0941ECITTWgt23CO2

Sloss L L 1963 Sequences in the cratonic interior of NorthAmerica Bulletin of the Geological Society of Americav 74 p 93ndash114 doi1011300016-7606(1963)74[93SITCIO]20CO2

Varban B L and A G Plint 2005 Allostratigraphy of theKaskapau Formation (CenomanianndashTuronian) in thesubsurface and outcrop NE British Columbia and NWAlberta Western Canada foreland basin Bulletin ofCanadian Petroleum Geology v 53 p 357ndash389doi102113534357

Wallace-Dudley K E and D A Leckie 1988 Preliminaryobservations on the sedimentology of the Doe CreekMember Kaskapau Formation in the Valhalla fieldnorthwestern Alberta in D P James and D A Leckieeds Sequences stratigraphy sedimentology Surface and

subsurface Canadian Society of Petroleum GeologistsMemoir 15 p 485ndash496

Wallace-Dudley K E andD A Leckie 1993 The lower Kas-kapau Formation (Cenomanian) A multiple-frequencyretrogradational shelf system Alberta Canada AAPGBulletin v 77 p 414ndash435

Williams G D and C R Stelck 1975 Speculations on theCretaceous paleogeography of North America inW G ECaldwell ed The Cretaceous system in theWestern In-terior of North America Geological Association of Can-ada Special Paper 13 p 1ndash20

Atchley et al 25