delineation of a sandstone-filled incised valley in the ... · study area location and context. a)...

ABSTRACT

Major heavy oil deposits are present in Lower Cretaceous strata of west-central Saskatchewan. The Winter Heavy Oil Pool(approximately 566 044 mmbl) consists of bitumen-rich sands from the Aptian–Albian Dina and Cummings members ofthe Mannville Group. This succession unconformably overlies Paleozoic carbonates and is conformably overlain by theLloydminster Member.

Lower Cretaceous deposition in the Winter area was influenced by topography on the regional sub-Cretaceous uncon-formity, including the Unity uplands south of the study area and major paleovalleys opening to the northwest. The overalldepositional framework consists of a lowstand and transgressive fluvial sandstone deposit (Dina Member) that evolved intoa brackish embayment system (Cummings Member). Upon sea level fall, a valley was incised into the mudstone-dominated marginal marine deposits, which filled with sandstone during a subsequent sea-level rise (Cummings Member).Exploitable heavy oil reservoirs are contained within these incised valley sandstone beds. Further transgression led to a rising water table with widespread deposition of an organic-rich shale and coal followed by marine shale of theLloydminster Member. The new depositional model for the area should lead to more optimal placement of horizontal wellsin the reservoir, which is vital to continued bitumen extraction from the Winter Pool.

RÉSUMÉ

D’importants gisements de pétrole lourds existent dans les strates du Crétacé inférieur du centre-ouest de la Saskatchewan.Le gisement de pétrole lourd de Winter (environ 566 044 Mb) se compose de sable riche en bitume de l’Aptien-Albien desmembres de Dina et de Cummings du groupe de Manville. Cette succession repose en discordance sur les carbonates duPaléozoïque et le membre de Lloydminster repose en concordance sur celle-ci.

La topographie de la discordance sous-crétacée régionale, y compris les hautes terres de Unity au sud de la régionétudiée et les paléovallées importantes s’ouvrant au nord-ouest ont influencé les dépôts du Crétacé inférieur dans la régionde Winter. La structure sédimentaire globale est formée de grès fluviatile transgressifs et de bas niveau (membre de Dina)ayant évolué en un système d’échancrure saumâtre (membre de Cummings). Au cours de la chute du niveau de la mer, unevallée s’est entaillée dans les sédiments marins marginaux à prédominance de mudstone pour se remplir de grès au coursde l’élévation ultérieure du niveau de la mer (membre de Cummings). Les lits de grès de ces vallées entaillées contiennentdes roches-réservoirs de pétrole lourd exploitables. Une transgression additionnelle entraîna une montée de la nappe phréatique avec un dépôt d’envergure de shale et de charbon riche en matière organique suivie de shale marin du membre

409

BULLETIN OF CANADIAN PETROLEUM GEOLOGYVOL. 57, NO. 4 (DECEMBER, 2009), P. 409–429

Delineation of a sandstone-filled incised valley in the Lower Cretaceous Dina–Cummings interval: implications for development of the Winter Pool, west-central Saskatchewan

DUSTIN B. BAUER

University of CalgaryDepartment of Geoscience

Calgary, AB T2N [email protected]

STEPHEN M. HUBBARD

University of CalgaryDepartment of Geoscience

Calgary, AB T2N [email protected]

DALE A. LECKIE

Nexen Inc.801–7 Avenue SW

Calgary, AB T2P [email protected]

GRAHAM DOLBY

6719 Leaside Drive NWCalgary, AB T3E 6H6

[email protected]

INTRODUCTION

The Winter Heavy Oil Pool of west-central Saskatchewan(Townships 42 and 43, Ranges 25 and 26 W3M; Fig. 1) is esti-mated to originally have contained 566,044 mbbl of 11º to 13ºAPI oil within the Lower Cretaceous Dina and Cummingsmembers of the Mannville Group (Vigrass et al., 1994). Thepool was discovered in 1980, with initial oil recovery achievedthrough vertical well production. Horizontal well developmentwas initiated in 1988 in conjunction with enhanced recoverytechniques involving fluid injection (Vigrass et al., 1994).Optimizing oil recovery requires strategic horizontal wellplacement, which demands a detailed understanding of thereservoir geology (Vigrass et al., 1994; Catania and Wilson,1999). Despite extensive oil reserves and significant economicimplications, only a few detailed reservoir investigations havebeen undertaken in the Lower Cretaceous interval of west-cen-tral Saskatchewan (e.g. Hayes et al., 1994; Catania and Wilson,1999). Reservoir mapping and geological analysis of suchintervals is vital to the continued development of these reser-voirs (Catania and Wilson, 1999).

The main objectives of this investigation were to: 1) build apredictive depositional model for the Winter area throughdetailed core and wire-line log evaluation; and 2) integrate thepool-scale reservoir geology into the established regional geo-logical setting in order to reconstruct the depositional history of

Lower Cretaceous heavy oil-bearing rocks. The results of thisinvestigation indicate that exploitable bitumen at Winter is con-tained within an incised valley fill, likely associated with lateAptian to early Albian base level fluctuations that caused inci-sion into Barremian and early Aptian deposits (cf. Gross, 1980;Hradsky and Griffin, 1984; Smith, 1994; Terzuoli and Walker,1997). The revised depositional model will facilitate more effi-cient well placement within the Winter Pool and provideinsight into adjacent strata, optimizing future reservoir delin-eation and development in the region.

REGIONAL DEPOSITIONAL SETTING

Lower Mannville strata span the entire Western CanadianSedimentary Basin (WCSB) and resulted from transgression of the Boreal Sea into the foreland setting during LateAptian–Early Albian time (Fig. 1; Cant and Abrahamson,1996). Mannville Group deposition occurred over a widespreadunconformity surface, underlain by Devonian to Mississippiancarbonates along the eastern margin of the WCSB including thearea studied (Fig. 1; Christopher, 1980). It has been postulatedthat differential erosion of southwest dipping Paleozoic car-bonate units, along with other tectonic factors (e.g. position offorebulge), influenced the development of topography on theunconformity across the plains of the WCSB (Christopher,1984, 1997; Ranger and Pemberton, 1988, 1997; Cant and

410 D. BAUER, S. HUBBARD, D. LECKIE and G. DOLBY

de Lloydminster. Le nouveau modèle sédimentaire pour la région devrait conduire à la mise en place plus optimale de puitshorizontaux dans la roche-réservoir, ce qui se révèle vital pour l’extraction continue du bitume dans le gisement pétrolifèrede Winter.

Michel Ory

Fig. 1. Study area location and context. A) Paleogeographic framework for the Dina Member and lithostratigraphic equivalents in the WesternCanadian Sedimentary Basin (modified from Smith, 1994). B) Detailed map showing wells present, including horizontal development wells; locations denoted by stars represent wells with cores examined in the study.

Stockmal, 1989; Leckie and Smith, 1992). Cretaceous deposi-tion in the study area, recorded by the Dina, Cummings andLloydminster members of the Lower Mannville Group (Fig. 2),was influenced by this paleotopography including the Unityuplands south of the study area, major paleovalleys opening tothe northwest, and a series of Paleozoic highs west of the studyarea (Figs. 1A, 3 and 4A; Christopher, 1997). Local topo-graphic relief consists of incised valleys, terraces and Paleozoichighs (Fig. 4B; Christopher, 1997).

Incision and subsequent infill of valleys on or above thesub-Cretaceous unconformity surface occurred throughoutMannville Group deposition. These paleovalleys range in scalefrom major conduits (10’s of km wide and >40 m deep), tosmaller features (5 km wide and 25 m deep; e.g. Zaitlin andShultz, 1984) associated with local topographic relief and rela-tive sea level fluctuations (Smith, 1994). In the context of thisstudy, it is important to differentiate large-scale paleovalleysthat incised into Paleozoic carbonates (Figs. 1A and 4A) fromthe smaller incised valley that contains the reservoir sandstonein the Winter area (Figs. 3 and 4B, C).

The Dina Member consists of sandstone regionally inter-preted as a primarily fluvial succession that filled major paleo-valleys on the sub-Cretaceous unconformity surface (Smith,1994; Christopher, 2003). The overlying Cummings Memberhas been interpreted as a tidally-influenced brackish baydeposit that accumulated on top of the Dina Member duringtransgression (Smith, 1994). The reservoir sandstone at Winteris considered herein to be part of the Cummings Member,although it is younger than the underlying mudstone- and siltstone-dominated bay deposits. The Lloydminster Member

represents a major transgression recorded by the transition of ashoreline sandstone to shallow marine shelf mudstone (Smith,1994). Far to the north of the study area, the lithostratigraphi-cally equivalent units consist of shoreline sediments associatedwith heavy oil deposits in northeastern Alberta and northwest-ern Saskatchewan (Hayes et al., 1994).

METHODOLOGY

The Winter Heavy Oil Pool study area contains 200 verticalwells from which data were obtained, including 10 cores exam-ined in detail for this study (Fig. 1; Table 1). Sedimentologicaland stratigraphic characteristics were documented from analy-sis of grain size, sedimentary structures, biogenic structures,palynology, and wire-line log data. The amount of biogenicreworking of deposits was classified based on the bioturbationindex (BI) scheme of MacEachern and Bann (2008). In thisapproach a non-bioturbated unit has a BI of 0 and a completelyreworked unit a BI of 6.

Gross isopach mapping of Lower Mannville strata pro-vides local insight into the topographic surface on whichCretaceous clastics accumulated (Ranger and Pemberton,1988, 1997; Christopher, 1997). The gross thickness of thecombined Dina–Cummings interval was mapped (Fig. 4B);the top of the isopach interval is represented by a distinctivelog response that is interpreted to reflect a major flooding sur-face directly overlying the Cummings Member. This surfaceis associated with the regionally correlated Lloydminster/Clearwater–Wabasca/Wilrich surface (McLean and Wall,1981; Cant and Abrahamson, 1996; Christopher, 1997). Thick

DELINEATION OF A SANDSTONE-FILLED INCISED VALLEY, WINTER POOL, SK 411

Fig. 2. Cretaceous stratigraphic nomenclature for the Mannville Group (modified from Smith,1994). Note that G.P. is “General Petroleum” and IVF is “Incised Valley Fill”.


Fig

. 3.

Sem

i-reg

iona

l cr

oss-

sect

ions

dem

onst

ratin

g th

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sitio

n of

the

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dy a

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with

res

pect

to

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the

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ity s

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isopach areas represent paleolowlands, and regions of little tono deposition represent the paleohighlands.

Net/gross ratio mapping was completed using a clean sand-stone gamma ray cutoff of <60 API, determined through coreevaluations (Fig. 4C). A series of reservoir horizon slice mapswere constructed (Fig. 5). Each map represents a five-meterinterval measured from the maximum gamma radiation read-ing at the top of the stratigraphic succession above theCummings Member. The dominant lithology was determinedfrom analysis of successive 5 m thick intervals in each wellthrough gamma-ray log analysis (Fig. 5A). Fundamental tothis approach is the assumption that the datum is well pre-served and represented as a relatively flat surface followingdeposition. This assumption is considered valid, because the datum used is the regionally correlated Lloydminster/Clearwater–Wabasca/Wilrich surface (McLean and Wall,1981; Cant and Abrahamson, 1996; Christopher, 1997). Thereservoir horizon slice maps were used to aid interpretationsby objectively revealing three-dimensional facies distributionand sandstone body geometries (cf. Hubbard et al., 1999;Crerar and Arnott, 2007).

Eight palynological samples were collected from fourwells (Table 2). Each of the facies associations defined in thestudy were sampled and analyzed in order to integrate resultswith ichnological and sedimentological observations. A count

of 200 specimens was made in each sample to determine theproportions of the principal palynological groups. A system-atic logging of each slide was then made in order to note anygroups not observed in the count. In many samples, dinocystswere not recorded in the count of 200 due to the overwhelm-ing dominance of terrestrial spores and pollen, yet they werepresent in the slide (Table 2). The numbers and types ofmicroplankton were assessed in order to determine paleo-salinity levels using a modified version of the classificationpresented by Leckie et al. (1990).

FACIES ANALYSIS

Eight sedimentary facies were defined and four facies associa-tions interpreted based on genetic characteristics and lithogicalrelationships assessed during core analysis (Table 1; Figs. 6–8).Facies association 1 (FA1) consists of interbedded fine-grainedsandstone (Lf3), siltstone and mudstone (Lf1 and Lf2). Faciesassociation 2 (FA2) consists mainly of medium-grained sand-stone (Lf4), facies association 3 (FA3) consists of organic-richshale (Lf5) and coal (Lf6), and facies association 4 (FA4) is com-posed of fine- to medium-grained sandstone (Lf7) overlain bymudstone (Lf8). FA1–FA3 are associated with the CummingsMember and FA4 with the Lloydminster Member (Table 1). Nocores through the Dina Member were available in the study area.


Fig. 4. A) Regional isopach map of Lower Mannville strata, including the Dina and Cummings members and the lithostrati-graphically equivalent McCloud Member (modified from Christopher, 1997). B) Isopach and C) net/gross ratio maps of Dina andCummings members in the Winter area. Yellow double-headed open arrows indicate trend of linear sandstone thick. Horizontaldevelopment well trajectories shown, roughly aligned with this trend.


Tab

le 1

.F

acie

s ta

ble,

incl

udin

g th

e su

mm

ary

and

desc

riptio

ns o

f th

e ei

ght

faci

es a

nd f

our

faci

es a

ssoc

iatio

ns id

entif

ied

in t

he W

inte

r P

ool a

rea.

Tra

ce f

ossi

l na

me

abbr

evia

tions

: A

reni

colit

es(A

r),

Ast

eros

oma

(As)

, C

hond

rites

(Ch)

, C

osm

orph

aphe

(Cos

), C

ylin

dric

hnus

(Cy)

, G

yrol

ithes

(Gy)

, O

phio

mor

pha

(Op)

, P

alae

ophy

cus

(Pa)

, P

hyco

siph

on(P

h),

Pla

nolit

es(P

l), R

osse

lia(R

o),

Sko

litho

s(S

k),

Teic

hich

nus

(Te)

, Tr

ichi

chnu

s(T

r).


Fig. 5. Reservoir horizon slice maps. A) Gamma-ray log illustration of how intervals were averaged using API units and appliedto mapping. Corresponding well location (31/12-36-042-26W3) is indicated on the maps (star). B) Interval 1, positioned 45–50 mbelow regional datum. C) Interval 2, 40–45 m. D) Interval 3, 35–40 m. E) Interval 4, 30–35 m. F) Interval 5, 25–30 m. G) Interval 6,20–25 m. H) Interval 7, 15–20 m. I) Interval 8, 10–15 m. J) Interval 9, 5–10 m. K) Interval 10, 0–5 m. Township grid labels for allmaps shown in Part K.


Tab

le 2

.P

alyn

ofac

ies

anal

ysis

, in

clud

ing

sam

ple

over

view

, as

soci

ated

fac

ies

asso

ciat

ions

and

rel

ativ

e sa

linity

inte

rpre

tatio

ns.

The

num

ber

of s

peci

men

cou

nts

for

each

co

mpo

nent

in t

he s

ampl

es is

not

ed,

as a

re im

port

ant

cons

titue

nts

that

wer

e pr

esen

t in

the

sam

ples

but

not

in t

he c

ount

s.


Fig. 6. Core description through Cummings Member strata from 41/03-36-042-26W3, a typicalwell from the west side of the thick isopach trend (Fig. 4B). A sequence boundary on the valleymargin capping brackish embayment deposits (FA1) and overlain by organic shale and coal (FA3)is indicated. See Figure 7 for a legend to the symbols used. See Figure 1 for well location.

CROSS-SECTIONS

Two stratigraphic cross-sections are shown to illustrate theregional characteristics of sediment distribution beyond thelimits of the study area, including the paleotopographic reliefon the pre-Cretaceous unconformity (Fig. 3). A detailed cross-section of the reservoir perpendicular to the thick isopach trend(Fig. 9) depicts the stratigraphic distribution of the facies asso-ciations observed in core.

FACIES ASSOCIATION 1 – BRACKISH EMBAYMENT

Description

FA1 consists of poorly consolidated interbedded fine-grainedsandstone (Lf3), siltstone and mudstone (Lf1 and Lf2) (Table 1;

Fig. 10); associated units are generally located east and west ofthe main north-south linear trend of thick isopach and highnet/gross ratio, present in the south-central portion of the studyarea (Fig. 4B, C). Fine-grained sandstone layers (Lf3) are<0.5 m thick and interbedded siltstone and mudstone-dominated units (Lf1 and Lf2) are typically <3 m in thickness.No rhythmicity or regularity with regards to sandstone andmudstone layer frequency and thickness is discernable.

Sandstone layers (Lf3) consist of lower to upper fine-grained quartzose sediments. Physical structures includetrough-cross bedding and current ripples (Fig. 10A–C). Thesandstone is characterized by moderate to low oil staining withsubtle fining upward sequences (0.2–1 m thick) within the oth-erwise blocky layers. Lf3 also contains siltstone and mudstonelaminae, commonly present in pairs (Fig. 10D). Soft sedimentdeformation, slumping features and flame structures are inter-mittently present (Fig. 10E–F). The only physical structuresobserved in finer-grained layers (Lf1 and Lf2) are pinstripelaminae of silt (Fig. 10D–E).

Sandstone of facies association 1 (Lf3) contains rare anddiminutive trace fossils with a bioturbation index (BI) = 0–2and a diversity range of 0–2 ichnotaxa including Arenicolites(Fig. 10B) and Palaeophycus. Mudstone and siltstone of Lf1characteristically has 1–3 ichnotaxa, including diminutivePalaeophycus, Planolites, and Teichichnus with a BI = 0–1;trace fossils in Lf2 include Arenicolites, Chondrites,Cylindrichnus, Gyrothlithes, Palaeophycus, Phycosiphon,Planolites, and Teichichnus (Fig. 10E–F). Lf2 preserves 2–5ichnotaxa and a BI = 2–5. Trace fossil size and abundance isvariable within FA1. Higher ichnogeneric diversity is associ-ated with an increase in the abundance, and commonly the sizeof trace fossils.

Palynological analyses yielded abundant inaperturatepollen, Classopollis, fresh water algae (Zygnemataceae), andrare freshwater and brackish dinocysts (Table 2).

Interpretation

The variability of lithology and sedimentary structures in thisfacies association indicates a depositional setting with fluctuat-ing sediment input and current energy levels. Trough-cross bed-ding records local current reworking of sand in a settingotherwise dominated by quiescent conditions associated withsedimentation of silt and mud. Mudstone laminae, includingdouble mud drapes, are suggestive of tidal influence (Visser,1980). Trace fossils are diminutive, make up a low to moderatelydiverse assemblage, and represent a variety of simple life modesand feeding behaviors. These observations are consistent with astressed, brackish-water depositional setting (e.g. Pemberton etal., 1982; Gingras et al., 1999; Buatois et al., 2008). Similar ich-nofossil assemblages have been reported in numerous units ofthe Western Canada Sedimentary Basin, commonly attributed tolow-energy sub-environments of estuaries (e.g. Beynon et al.,1988; Pattison, 1992; MacEachern and Pemberton, 1994). Thepresence of Classopollis in significant numbers, and freshwateralgae, is suggestive of sediment derivation from local point


Fig. 7. Legend of symbols used in core descriptions (Figs. 6 and 8).


Fig. 8. Core description from 01/07-32-042-25W3 characterized by a complete sandstone package(FA2) from within the linear thick isopach trend (Fig. 4B) and the valley floor sequence boundary. The section is capped by transgressive brackish water deposits (FA1) and an organic rich shale (FA3). SeeFigure 7 for a legend to the symbols used. See Figure 1 for well location.


Fig

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sources off the adjacent paleohighlands (Table 2; Van Geel andGrenfell, 1996). Facies association 1 is interpreted to be a mixed-energy brackish water embayment or back-barrier deposit (cf.,Howard and Frey, 1985; Dalrymple et al., 1992).

FACIES ASSOCIATION 2 – VALLEY FILL SANDSTONE

Description

FA2 consists primarily of Lf4, quartzose sandstone that ranges ingrain size from upper fine to upper medium, with a dominanceof lower medium sized grains (Fig. 11). The facies association is

found within the north-south trending isopach thick through 42-25W3 and 43-25W3 (Fig. 4B). The facies association con-tains rare, thin layers (<2 cm) of organic-rich interbedded mud-stone. Organic-rich debris is also distributed in some intervalswithin the sandstone. Physical structures include trough-crossbedding and current ripple cross lamination, which are typi-cally accentuated by the presence of organic-rich laminae (e.g.Fig. 11). The medium-grained sandstone also commonlyexhibits low angle to planar stratification (Fig. 11A). Trace fos-sils are largely absent (BI = 0) with the exception of rare,


Fig. 10. Sedimentary characteristics of facies association 1. A) Fine-grained sandstone (Lf3); 41/07-33-042-25W3, 695.50m. B) Fine-grained sandstone (Lf 3) with Arenicolites (Ar); 41/07-33-042-25W3, 697.00 m. C) Fine-grained current rippled sand-stone (Lf 3); 41/07-33-042-25W3, 696.70 m. D) Slumped fine-grained sandstone (Lf 3) and pinstripe laminated siltstone andmudstone (Lf 1) with diminutive Planolites (Pl) and Gyrolithes (Gy); 41/07-33-042-25W3, 695.40 m. Double mud drapes are alsointerpreted (md). E) Laminated siltstone and mudstone (Lf 1) with flame structures (fl) and rare simple trace fossils includingPlanolites (Pl); 41/07-33-042-25W3, 6694.60 m. F) Moderately to highly bioturbated mudstone and siltstone (Lf 2) with smallCylindrichnus (Cy), Palaeophycus (Pa) Planolites (Pl), and Teichichnus (Te); 41/07-33-042-25W3, 684.00 m. Note that eachcore is approximately 8 cm in diameter.

diminutive Palaeophycus. The facies association is almostubiquitously heavily oil stained and bitumen impregnated,which may mask physical or biological features. Palynologicalsamples exhibited numerous to abundant Classopollis, alongwith freshwater algae and rare dinocysts, both freshwater andbrackish (Table 2).

Interpretation

The medium grained sandstone is interpreted to represent a valley fill deposit due in large part to its mapped morphologicalshape (Figs. 4B and 5) and bed-scale physical characteristics.Physical sedimentary structures, including trough-cross bed-ding and current ripples, as well as the overall lack of evidencefor infaunal reworking, are consistent with fluvially-dominatedvalley-filling processes (Li et al., 2006). Palynological analysissuggests a fresh to slightly brackish depositional setting with adry hinterland component (Table 2; Van Geel and Grenfell,1996). The linear, north-south trend of the FA2 sandstone bodyfollows a major paleotopographic low on the uncomformitysurface (Figs. 3 and 4B), and this low probably controlled sed-iment distribution (cf., Zaitlin et al., 1994; Ranger andPemberton, 1997).

FACIES ASSOCIATION 3 – COASTAL

ORGANIC-RICH SHALE AND COAL

Description

FA3 (Fig. 12) was deposited throughout the study area (Fig. 5J)overlying FA1 or FA2, and is primarily composed of organic-rich shale (Lf5) overlain by coal (Lf6). The facies associationcontains abundant wood debris and root material. Grain sizevaries from mud to lower very fine-grained sandstone. Thefiner intervals contain organic-rich layers.

Lf5 is characterized by root traces along with diminutivePalaeophycus, Planolites, and Teichichnus with a BI = 0–1. Lf6contains root traces and is sharply overlain by Lf7 (Fig. 12B).Palynological analysis of a silty sample at the base of FA3 contains rare dinocysts and freshwater dinocysts andalgae, abundant bisaccates, and rare inaperturate pollen(41/03-36-42-26W3; Table 2). A second sample from thisfacies association is characterized by a high number ofschizaceous spores and rare freshwater and brackishdinocysts (21/13-06-43-25W3; Table 2).


Fig. 11. Sedimentary characteristics of facies association 2 (Lf 4). A) Medium-grained sandstone (Lf 4) with low-angle cross stratification;41/03-29-042-25W3, 677.40 m. B) Medium-grained sandstone (Lf 4) characterized by trough-cross bedding; 21/10-28-042-25W3, 668.80 m. C) Mudstone clasts within medium-grained sandstone (Lf 4); 41/09-31-042-25W3, 678.50 m. Note that each core is approximately 8cm in diameter.

Fig. 12. Sedimentary characteristics of facies association 3. A)Organic-rich shale (Lf 5) overlain by coal (Lf 6) with contact demarcatedby white arrow; 41/07-33-042-25W3, 679.00 m. B) Coal (Lf 6) sharplyoverlain by fine-grained sandstone (Lf 7) with roots (r) apparentlydescending from contact; 41/07-33-042-25W3, 678.50 m. Note thateach core is approximately 8 cm in diameter.

Interpretation

FA3 can be correlated through all wells in the study area andprevious investigations of the strata at Winter by Groeneveldand Stasiuk (1984) and Groeneveld (1990) attributed the unitsto a low-energy, marine influenced coastal grass marsh.Palynological data indicate that the depositional setting wasprone to flood events (Table 2; Van Geel and Grenfell, 1996).The transition through time from deposition of FA2 to FA3(Fig. 9) records the change from a relatively higher energyenvironment characterized by current-reworked sand (Table 1)into a low energy setting prone to the development of organic-rich shale and coal (Fig 12).

FACIES ASSOCIATION 4 – TRANSGRESSIVE

SANDSTONE AND MUDSTONE

Description

FA4 (Fig. 13) contains two lithological units; a fine- tomedium-grained sandstone (Lf7) and an overlying mudstone(Lf8). These units overlie FA3 and were deposited across theentire study area (Fig. 5K). The base of the mudstone unit isdemarcated by the maximum gamma radiation log response onwire-line logs and as previously discussed, was used as thedatum for stratigraphic correlations and mapping.

Sandstone (Lf7) ranges in grain size from fine to mediumand shows coarsening upwards over its lower 1–2 m, then fin-ing upwards into Lf8. Physical structures within the sandstoneinclude current ripples, trough-cross bedding, and planar lami-nation (Fig. 13B, E). The top of Lf7 is recognized on logs by asharp decrease in gamma radiation. Facies of this associationare characterized by variable thicknesses.

Trace fossils within FA4 are robust and represented in moderately-high diversity. Individual sandstone beds (Lf7)with a BI = 0–3 typically contain 3–4 ichnotaxa includingArenicolites, Asterosoma, Ophiomorphia, Palaeophycus,Phycosiphon, Rosselia, Skolithos, Teichichnus, and Trichichnus(Fig. 13). The overlying mudstone unit (Lf8) contains bedswith a higher diversity of trace fossils (4–7 ichnotaxa/bed),including Asterosoma, Chondrites, Cosmorhaphe, Ophiomorpha,Palaeophycus, Phycosiphon, Planolites, Rosselia, Skolithos, andTeichichnus with a BI = 2–4 (Figure 13). The ichnogenerawithin FA4 have a higher diversity and are relatively largerthan the assemblages observed in FA1-3. Abundant brackishand stressed-marine dinocyst species were observed in palyno-logical samples (Table 2).

Interpretation

FA4 is interpreted to represent continued transgression fromthe marine influenced deposit of FA3 (Groeneveld and Stasiuk,1984), to a marine mudstone layer, which marks the firstwidely correlatable Cretaceous flooding surface in the studyarea (Fig. 3). The transgressive sandstone unconformably over-lies FA3 (i.e. a transgressive surface of erosion), and gradesinto the overlying mudstone. High current energy associatedwith transgression is recorded by the presence of ripples,

trough-cross bedding and high-energy planar lamination. Thestressed-marine dinocyst assemblage in the palynology samplesupports a restricted marine environment interpretation associ-ated with the early stages of the marine transgression (Van Geeland Grenfell, 1996). Ichnological observations including anincrease in size and diversity of trace fossils through this inter-val also support the overall transgressive interpretation(MacEachern and Pemberton, 1994). Trace fossils preservingcomplex organism behaviours are also supportive of the inter-pretation of more marine conditions, including Asterosoma,Cosmorhaphe and Ophiomorpha, not observed in underlyingsalinity-stressed deposits (FA1–FA3). FA4 is interpreted to bea part of the Lloydminster Member, associated with a basin-wide marine flooding event (McLean and Wall, 1981; Cant andAbrahamson, 1996; Christopher, 2003).

DEPOSITIONAL HISTORY

SEDIMENTATION ON THE UNCONFORMITY: DINA MEMBER

FLUVIAL AND CUMMINGS MEMBER ESTUARINE DEPOSITS

Cretaceous deposition took place above the undulating sub-Cretaceous unconformity in the study area. A major rise in rela-tive sea-level during the Late Barremian corresponded with acessation of fluviatile deposition associated with the DinaMember in lowland valleys on this unconformity surface(Fig. 14A; Smith, 1994; Christopher, 2003). Although these unitswere penetrated by most wells in the study area they were notcored (Figs. 3 and 9). Continued transgression resulted in thedevelopment of a barrier-island complex, inferred from the pres-ence of extensive fine-grained brackish bay deposits in the studyarea (FA1; Fig. 14B) and an uncored east-west trending (ie.,shore-parallel) sandstone body in the northern-northeastern por-tion of the study area (along the northern boundary of 43-25W3;Figs. 4C and 5F–I). Modern coastlines of Georgia are interpretedto be analogous to the depositional setting (Fig. 14C). In this ana-logue, silty and muddy sediments are widely deposited in theback-barrier setting, representing tidal flat, lagoon, and marshsub-environments (Dorjes and Howard, 1975; Letzch and Frey,1980; Howard and Frey, 1985). Zones of sand deposition in theback-barrier setting are largely restricted to fluvial channels andto tidal flats and channels in close proximity to tidal inlets (e.g. Dorjes and Howard, 1975; Howard and Frey, 1985). Tidalchannels filled with sandstone oriented southwest-northeast and north-south are interpreted in reservoir horizon maps in 42-26W3 (Figs. 5E–5H). Extensive sand deposited by marineprocesses is commonly associated with tidal inlets in wave-influ-enced or wave-dominated estuarine settings (Roy et al., 1980;Dalrymple et al., 1992). Fine-grained back-barrier deposits aretypical of a mixed energy zone in the central part of the estuar-ine setting, where mixing fresh and marine water induces sedi-mentation of flocculated mud (Kranck, 1981; Rahmani, 1988).Zaitlin and Shultz (1984, 1990) recorded a similar distribution ofestuarine sediments in the Lloydminster Member in the Senlacarea to the south of the Winter Heavy Oil Pool.



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WINTER RESERVOIR ORIGIN: CUMMINGS MEMBER

VALLEY INCISION AND FILLING

Following the transgressive filling of the estuary (FA1), base-level fall resulted in local fluvial incision of the estuarine strata(Fig. 14A). The incised valley generated, trending north-southalong the centre of the study area, filled with a thick sandstonesuccession during late lowstand and subsequent transgression(FA2; Fig. 14D). The incised valley was focused over the pre-viously formed valley-system developed on the sub-Cretaceousunconformity (Fig. 4B). The incised valley interpretation forWinter Pool sandstones is supported by numerous stratigraphicand sedimentological observations that have been deemed critical for the recognition of incised valleys in the rock record(cf. Zaitlin et al. 1994; Boyd et al. 2006). Compared to mean-dering channel complexes that are commonly associated withwidespread point-bar deposits (Thomas et al., 1987), FA2 sand-stones are limited to the distinctive linear trend mapped, con-sistent with confinement within a valley and therefore a lack ofchannel migration (Figs. 4B, 14A and D). There is a discon-formable relationship between valley fill and adjacent strata.The incised valley cuts out regionally-correlated stratigraphicsurfaces outside the trend and internal marker beds within thefill on-lap the valley walls (Figs. 3 and 8). Sub-aerial exposurealong the valley margins, defined by a rooted horizon, isnotable (e.g. 41/03-36-042-26W3; Fig. 6). The valley basesequence boundary is characterized by coal development over-lying a rooted horizon (e.g. 01/07-32-042-25W3; Fig. 8).Where channels within the incised valley deposited sandstone(FA2) on top of older fluvial sandstones of the Dina Member,the sequence boundary is difficult to discern in core, a problemexacerbated by extensive bitumen staining.

REGIONAL TRANSGRESSION: CUMMINGS MEMBER

TO LLOYDMINSTER MEMBER TRANSITION

Relative sea-level rise continued and resulted in back-steppingcoastal environments, characterized by deposition of wide-spread organic shale and coal across the study area (FA3). Amarine-influenced coastal grass marsh origin for the coals hasbeen demonstrated by Groeneveld and Stasiuk (1984) andGroeneveld (1990). Continued transgression resulted in thewidespread deposition of sandstone and marine mudstone of


Fig. 14. Interpreted depositional environments for the strata studied.A) Schematic study area cross-section showing stages of depositionalhistory including: 1) Dina Member fluvial sandstone deposition onunconformity surface; 2) transgression resulting in barrier-bar complex(back-barrier facies prominent) across map area; 3) Sea-level fall andvalley incision; 4) fill of valley primarily with sandstone on transgression;5) deposition of shoreline and open marine units on continued transgression. B) Map-view interpretation of brackish-water back-barrierdepositional environment of study area based on facies mapping(Fig. 5), net/gross mapping (Fig. 4C) and sedimentological analysis. C) Modern analogue of Ogeechee River, Georgia Coast, U.S.A. (e.g.Dorjes and Howard, 1975; Howard and Frey, 1985) demonstrating asimilar facies architecture and distribution as Part B. D) Incised valleyinterpretation of study area for reservoir sandstone that eroded throughthe brackish-water back-barrier deposits (Part B).

FA4, with maximum transgression recorded in the mudstoneinterval (Lf8) of the Lloydminster Member (cf. Christopher,1997, 2003).

DISCUSSION

REGIONAL SEDIMENTOLOGICAL CONSIDERATIONS

Lower Cretaceous units that drape the eastern portion of thebasin-wide sub-Cretaceous unconformity of the WCSB, includ-ing the lithostratigraphically equivalent Dina–Cummings,McMurray, and Bluesky–Gething intervals (Fig. 2), collectivelycontain one of the largest hydrocarbon accumulations in theworld. Numerous factors contributed to the development oflithologic heterogeneity in these strata. Due to the broad, shallow physiography of the basin, complex marginal marine,brackish-water prone environments (e.g. estuaries, incised valleys, deltas, embayments) were prevalent (Pemberton et al.,1982; Flach and Mossop, 1985; Wightman et al., 1987; Smith,1988; Zaitlin et al., 1995; Hubbard et al., 2004). The deposi-tional surface of the basin had a low topographic gradient thatwas significantly influenced by sea-level fluctuations, with thepotential for shoreline position to move >100 km during a ver-tical sea-level shift of a few meters (cf. Catuneanu et al., 1997;Plint and Kreitner, 2007). The effect of tides is also notable inthe strata studied at Winter, with variable tidal ranges speculatedfor the shallow seaway (e.g. Smith, 1988). A complicated strati-graphic architecture is expected, and is well-documented for theDina-Cummings deposits, the McMurray Formation (e.g. Stroblet al., 1997a; 1997b; Wightman and Pemberton, 1997), as wellas the Bluesky-Gething interval in west-central Alberta (e.g.Hubbard et al., 2002; Caplan et al., 2007; MacKay andDalymple, 2008).

Depositional models for the McMurray Formation haveshown that sediment distribution was also strongly controlledby topography on the sub-Cretaceous unconformity (Stewart,1963; Ranger and Pemberton, 1988, 1997), consistent withinterpretations made in this study (Fig. 4). With a similardepositional history to that interpreted at Winter, fluvial andbrackish embayment sediments were deposited on the sub-Cretaceous unconformity in the Peace River–Cadotte region,with a broad estuary-incised valley developed at the top of the stratigraphic succession directly prior to widespreadmaximum flooding (Hubbard et al., 1999; Caplan et al., 2007; Mackay and Dalrymple, 2008). Despite the depositionalheterogeneity preserved in the Lower Cretaceous section,paleotopographic trends mapped on the sub-Cretaceousunconformity provide first-order predictive models for reservoir exploration and delineation (e.g. Ranger andPemberton, 1988). Notably, the Winter pool, the main zone ofreservoir development in the Bluesky–Gething interval in thePeace River Oil Sands area, and the main depositional fair-ways for McMurray Formation sedimentation and thus reservoir distribution are all focused on paleotopographic lowson this unconformity (e.g. Ranger and Pemberton, 1988;Hubbard et al., 1999).

RESERVOIR IMPLICATIONS

Hydrocarbon pools of the Lloydminster Heavy Oil area havetypically been associated with stratigraphic traps (e.g. Orr etal., 1977; Vigrass, 1977; Putnam, 1980, 1982; McCallum,1984; Smith, 1984; Smith et al., 1984; van Hulten, 1984; vanHulten and Smith, 1984; MacEachern, 1989). As a result, sed-imentological characterization has a direct impact on reservoirdelineation and the development of predictive depositionalmodels can help to optimize hydrocarbon recovery techniques(e.g. Strobl et al., 1997b). The deposits of marginal marine set-tings, like those of the studied Cummings–Dina interval, arenotably lithologically heterogeneous and it is well establishedthat this can have a deleterious affect on reservoir performance(e.g. Weber, 1986; Jordan and Pryor, 1992; Pemberton andGingras, 2005).

In the McMurray Formation of Alberta, steam injectivityusing Steam Assisted Gravity Drainage (SAGD) is better intrough-cross bedded sandstone beds than in beds characterizedby inclined heterolithic stratification (IHS) due to the loweredvertical permeability of IHS-dominated units (Strobl et al.,1997b; Wightman and Pemberton, 1997). In the Dina–Cummings interval within the study area, IHS is not common,however, pool-scale heterogeneity amongst muddy baydeposits (FA1) and sandy incised valley fill deposits (FA2) ismappable (Fig. 5), providing insight into reservoir continuityprediction and contributing to optimal placement of horizontalproduction wells.

Deposition of the Winter Heavy Oil Pool valley fill sedi-ments occurred in an overall low accommodation settingwithin the distal eastern reaches of the Western Canadianforeland basin. The incised valley is 1.5–3.5 km wide with anapproximate maximum depth of 40 m (valley fill aspect ratio= 38–88). These dimensions are consistent with numerousincised valleys studied previously (e.g. Harms, 1966; Palmeret al., 1979; Pryer and Potter, 1979; Zaitlin and Shultz, 1984;Sonnenberg, 1985; Kraft et al., 1987; Rhamani, 1988; Thorne,1994). Due to lack of well control and contrast betweenCummings Member incised valley sandstone deposits andsandstone of the underlying Dina Member, the valley base isdifficult to delineate. Determination of the cross-sectionalvalley shape has economic implications associated with volu-metric calculations and horizontal well placement. Tuttle etal. (1966) and Schumm and Ethridge (1994) identified geological characteristics that, despite limited well control,allow an interpretation of valley base shape. Tuttle et al.(1966) observed that the erosional resistance of the substrateexerts a strong control on valley size and shape. Poorly con-solidated deposits are commonly associated with the develop-ment of a broad rectangular shape as opposed to a moretriangular morphology, typical of incisions into competentsubstrates (Tuttle et al., 1966; Schumm and Ethridge, 1994).It is likely that brackish embayment (FA1) and fluvial chan-nel deposits (Dina Member) were poorly consolidated at thetime of incision (e.g. sandstone is weakly cemented, primarilywith bitumen). From these observations it can be deduced that


the Winter Pool valley floor likely has a broad and rectangu-lar shape, justifying the calculation of an optimistic estima-tion of reservoir sandstone volume.

CONCLUSIONS

The depositional history of the Lower Cretaceous section in theWinter area can be simplified into five main stages as shown inFigure 14A. The linear trend of incised valley fill (FA2) thatmakes up the Winter reservoir is characterized by a distinctlydifferent depositional pattern and architecture relative to theunderlying brackish embayment-estuarine deposit (FA1).Recognition and mapping of the brackish embayment back-barrier system (Stage 2, Fig. 14A) and the subsequent valleyincision and fill (Stages 3 and 4, Fig. 14A) within the strati-graphic interval of interest is vital for interpreting the distribu-tion of reservoir sandstones and heterolithic non-reservoirdeposits in the prolific Cummings–Dina interval of west-cen-tral Saskatchewan.

Numerous sedimentological similarities exist betweenCummings–Dina strata in the Lloydminster area and lithos-tratigraphically equivalent heavy oil-bearing units in Alberta,including the McMurray Formation and the Bluesky–Gethinginterval. Firstly, like the more studied lithostratigraphic equiv-alents in Alberta, the Cummings–Dina deposits are character-ized by brackish-water dominated units. A complexstratigraphic architecture typical of these marginal marine set-tings is mapped at Winter and therefore continued detailed sed-imentological and stratigraphical analysis is essential for theidentification of future development opportunities in theregion. Furthermore, an underlying topographic control associ-ated with the pre-Cretaceous unconformity influenced sedi-ment distribution in the Winter area, consistent with thatrecognized in the Athabasca and Peace River oil sands regions.In particular, significant reservoir sandstone bodies in all threeareas are commonly aligned with underlying mapped paleoto-pographic lows on the pre-Cretaceous unconformity. The link-age between paleotopography and sandstone distributionprovides further insight into future exploration and reservoirdelineation in the Lloydminster region.

ACKNOWLEDGMENTS

This paper stems from the undergraduate honours thesis of thesenior author. We are extremely grateful for the financial, tech-nical and logistical support of this project from Nexen Inc.Additional funding for the project was provided from a NaturalSciences and Engineering Research Council of Canada(NSERC) Discovery Grant to SMH. The manuscript benefitedfrom discussions with Drs. Andrew Leier and Per Pedersen(University of Calgary). Bulletin reviews by Drs. Guy Plint andJ-P Zonneveld, as well as input from Editor RobertMacNaughton, significantly improved the clarity and focus ofthe manuscript.

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Manuscript received: November 11, 2008

Date accepted: December 2, 2009

Associate Editor: John-Paul Zonneveld


delineation of a sandstone-filled incised valley in the ... · study area location and context. a)...

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