the seismic signatures of some western...
TRANSCRIPT
JOURNAL OF THE CANlOlAN SOClETI OF ExPLORAnoN OEDP”YSlClSTS VOL. 23. NO 1 (“EC. 1987,. P. 7-26
THE SEISMIC SIGNATURES OF SOME WESTERN CANADIAN DEVONIAN REEFS
NEIL L. ANDERSON’,’ AND Ft. JAMES BROWN’
ABSTRACT
Western Canadian Devonian reefs generate diagnostic seis- mic signatures which in fact are geophysical representations of geological features. Analysis of these seismic signatures can elucidate the morphological and stratigraphical relationships between reefs and the adjacent sedimentary sections, particu- larly in areas where we,, control is sparse.
Seismic signatures may contain significant elements of two basic components: later.4 character “tiations and time-structural relief. Time-structural relief may be velocity-generated and/or due to real structure. The analysis of these components estab- lishes further the nature of the generating geological features and places constraints upon such phenomena as differential compaction, reef-focused ~a,, dissolution, paleotopography, lateral and verrical facies variations, regional dip and morphology.
In this paper a summary is given of an integrated geophy- sical/geological analysis which has been carried out for some examples of the following six types of western Canadian Devonian reefs: I) Rainbow Mbr; 2) Upper Keg River Reef Mbr; 3) Winnipeg”& Fro; 4) Swan Hills Fm; 5) Leduc Fm, and 61 Zeta Lake Mbr. Seismic sections crossing examples of known reefs are presented and discussed and the seismic signaturesofthesixtypesofreefsarecomparedandcontrasted. The authors feel that this analysis of seismic signatures for some western Canadian Devonian reefs will establish further therelationshipsbetweenthesereefsandtheadjacenlsedimen- tary sections and will demonstrate the usefulness of integrated geophysical/geological study of reefs.
INTRODUCTION
The seismic signatures, or distinguishing seismogeol- ogical features, of subsurface western Canadian Devonian reefs generally exhibit two basic components: lateral character variations and time-structural relief. Ideally both constituents are geophysical representations of geological features and are primarily functions of the
morphology of the reef and the nature of the basin in which the reef developed. In this paper the seismic signatures of 21 reef examples representing six reef types (Figures 1 and 2) are summarized. The seismic sections for these 21 examples are cross-referenced repeatedly throughout this paper, so that the following orderingoffigures is considered to be optimal: Rainbow Mbr, Figures 4 to 7; Upper Keg River Reef Mbr, Fig- ures 8 to 11; Winnipegosis Fm, Figures I2 to 14; Swan Hills Fm, Figures 15 to 17; Leduc Fm, Figures I8 to 21; and Zeta Lake Mbr, Figures 22 to 24. Three of these reef types, Rainbow Mbr, Upper Keg River Reef Mbr and Winnipegosis Fm, developed in evaporitic basins: the others, Swan Hills Fm, Leduc Fm and Zeta Lake Mbr, developed in shale basins. In the following discussion, emphasis isplacedon therelationshipbetween each seismic signature and the nature of the correspond- ing basin of origin.
Historically, studies of western Canadian Devonian reefs have been primarily either geologically orgeophy- sically oriented. For example, Hriskevich (1966, 1967, 1970), Langton and Chin (1968) and Barss ef al. (1970) published geologically oriented studies ofRainbow Mbr reefs, while Klovan (1964), McCamis and GrifEth( 1967), Fischbuch (1968) and Perrin (1982) did likewise for Leduc Fm, Upper Keg River Reef Mbr, Swan Hills Fm, and Winnipegosis Fm reefs, respectively. Similarly, Gendzwill (1978) and McQuillin et al. (1984) wrote geophysically oriented papers on Winnipegosis Fm and Rainbow Mbr reefs, respectively, while Bubb and H&lid (1977) discuss five western Canadian Devonian reefs of three different types: Leduc Fm(3), Upper Keg River ReefMbr(1) and Swan Hills Fm(1). Very few integrated studies have been published and individual reports have generally been restricted to a single reef type. Two exceptions are Pallister’s (1965) geophysically ori&ted monograph on the application of seismic techniques to
Manuscript received by the Editor May 15, ,987; revised manuscript received October 1, ,987. ‘Department of Geology and Geophysics, The University of Calgary, Calgary, Alberta T2N IN4 “Present address: Petrel Robertson Ltd., 565, 800 6 Avenue S.W., Calgary, A,be”a TZP X3 The authors wish to extend sincere thanks to Ihe many donors of the seismic data, a list of whom is given by Anderson (1986). The efforts of Ms. Bonnie Williams (typing) and Mr. Bill Matheson (drafting) are gratefully acknowledged. Finally, we thank Dr. Rob Stewart, Past Editor of the Journal, and Associate Editor BrianRussc,,, foragreeingto handle the review process forthispaper, and thethreereviewersfortheirconstructive criticisms.
7
8
0
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0
N
0
W
-1
a
a
N.L. ANDERSON and R.J. BROWN
1 N.W. ALBERTA 1 CENTRAL ALBERTA 1 S.SASKATCHEWAN 1
WABAU”N
ClLMAR - GRAMfNlb
4 I i FC
‘SILT’
BLUE RIDGE MEMBER
IRMATION
WOLFLAKEMEMBER
CYNTHIA
/
ZETA
MEMBER LAKE
MEMBER
LOBSTICK
MEMBER
Fig. 1. Correlation chart showing the stratigraphy of the relevant areas: N.W. Alberta, Central Alberta and S. Saskatchewan (after E.R.C.B., 1965): Rainbow Lake (after Hriskevich. 1966); Zama (after McCamis and Griffith, 1967): Swan Hills (atter Fischbuch. 1966) and West Pambina (after Chevron Standard Ltd., 1979).
reef traps and the discussion of the geology and the geophysics of Zeta Lake Mhr reefs by Chevron Stan- dard Ltd. (1979).
The geologically oriented studies have largely been restricted to the analysis of the basinal distribution of a single type of reef and/or to the analysis of the facie and fauna within a single reef type. Some more recent exceptions are the works of Klovan (1974) on western Canadian Devonian reefs generally, Mountjoy (1980) on Upper Devonian reefs of western Canada and Wil- liams (1984) on the Devonian Elk Point basin.
There is a relative paucity of off-reef wells in the vicinity of tested reefs which often prevents a determi- nation of the relationship between a reef and the adja- cent sedimentary section. In addition, most reef and off-reef wells do not penetrate the paleosurface on which the reefs developed. Consequently, the relationship between reefs and platform topography is generally poorly understood. However, by studying a suite of several reefs from the same area and environment, and as similar as possible in terms of age, size, geophysical image, known geology, etc., one can construct a work-
SEISMIC SIGNATURES OF DEVONIAN REEFS 9
NORTHWEST TERRITORIES
ALBERTA
*SWAN HILLS
WINNIPEGOSIS \
CAINADA ~-~-----~~~~~~I~~ (
USA \N~RTH
IDAHO DAKOT MONTANA
Fig. 2. Geographical locations of the Devonian reels selected as examples.
ing composite or typical model for that reef type which, intheahsenceofcertainspecific knowledgeforaparticu- lar reef, can be applied as a best estimate.
The geophysically oriented studies of western Cana- dian Devonian reefs and their associated features have been similarly restricted, but principally due to eco- nomic considerations. Seismic data are expensive to acquire, process and interpret and can have appreciable resale value. Understandably, the proprietary owners of such data are often reluctant to release them for publication. When they do so, only a cursory documen- tation of the more anomalous components of the seis- mic signature is usually presented; the subtlecomponents are often overlooked. Emphasis is placed on document- ing the visually anomalous components as opposed to analyzing in detail the seismic signatures of the reefs. Thus, a significant void has existed within the published literature.
In a recent thesis by Anderson (1986), an attempt was made to elucidatefurtherthe relationship between reefs and the adjacent sedimentary sections. This paper pres- ents in condensed form the results of that study and further-details on the examples discussed here are avail- able therein. In-depth analyses of individual reef types are also being prepared for publication (see e.g., Ander- son et al.. 1986b, c; Brown ef al., 1986).
SEI~MI~ SIGNATURES
Lateral character variations
Lateral character variations can he classified as phase and/or amplitude changes along seismic events and as changes in the pattern of a sequence of events (the pattern of the sequence of events originating from a
stratigraphic unit is henceforth referred to as the seis- mic image). Such variations, or seismic representations ofstratigraphicfeatures, mayincludevariationsinacous- tic impedance contrast, which may arise from gradational or abrupt facies changes, focusing and defocusing of reflections from irregular surfaces, constructive or destructive interference as a function of varying inter- val thickness, and amplitude variations which may be the result of differential attenuation. Following Sheriff (1973). attenuation is defined as any reduction in the amplitude of seismic waves, such as produced by divergence, reflection and scattering, and absorption. The term differential attenuation is herein defined as any lateral change in the amplitude of a seismic event which is attributable to lateral variations in the attenuative properties of the relevant rock formations. Character variations whichareindependentofreefsarenotconsid- ered to be components of the seismic signatures.
There are many modelling studies in the literature which demonstrate the seismic effects of the strati- graphic features mentioned: Harms and Tackenberg (1972), Meckel and Nath (1977), Neidell and Poggia- gliolmi (1977), Gelfand and Lamer (1984), and Jain (1986), to mention just a few. Modelling results for the reef examples of this paper are presented by Anderson (1986) and most of these will soon he published in a series of papers by Anderson and others.
Character variadons associated with prominent seismic
reflections - In Table I, observed changes in phase and/or amplitude along single seismic events for the six reef types are summarized and classified as: prereef platform, reefplatform, reeffoff-reefcontact, and postreef. An illustration of these terms is provided in Figure 3.
Amplitude variations along prereef platform events arecharacteristicofthe threeevaporitic-hasinreeftypes. For example, the amplitude of the Cold Lake Fm and other prereef platform events is generally lowest where Black Creek Mbr salts are present, higher beneath the Keg River Fm reefs (Figures 4 to I I) and highest offreef where Black Creek Mbr salts are absent. This relation- ship ofobserved amplitude to salt thickness and to reef presence cannot be ascribed to lateral variations in acoustic-impedance contrast or to interference; it is therefore attributed to differential attenuation, perhaps accentuated by the defocusing effect of the reefs. More specifically, transmission losses appear to be lowest off reef where salts are not present, higher through reef- bearing strata and highest off reef where Black Creek Mbr salts remain. Amplitude changes are similarly observed along prereef platform events immediately beneath Winnipeg&s Fm reefs (Figures 12 to 14). These reflections typically are of significantly lower ampli- tude beneath the reef rims than elsewhere. Evidently, transmission losses through the rim facies significantly accentuate attenuation through the salts.
Amplitude variations along prereef platform events are also associated with two of the three shale-basin reef types, Swan Hills Fm and Leduc Fm. The varia-
10 N.L. ANDERSON and R.J. BROWN
tions beneath the Leduc Fm are attributed to differen- tial attenuation; the changes below the Swan Hills Fm STIIUCIUTldI DBASE (restricted to the Gilwood Mbr event) are considered as “,K,N(I - CrlETAIEOUS duetoconstructiveinterference. Forexample, theampli- PRECRETACEOUS “NCDNiDRMlT” tudes of prereef platform events are generally lower beneath the Leduc Fm (Figures 18 to 21) than off reef, suggesting that attenuation through these reefs exceeds that through off-reef sediments. The absence of similar anomalies beneaththeZetaLakeMbrexamples (Figures 22 to 24) may be due to the relatively small height and areal extent of the reefs. In contrast, amplitude varia- tions along pre-Swan Hills Fm platform reflections (Figures 15 to 17) are restricted to the Gilwood Mbr event. This reflection, off reef, has a visibly higher
PRL.REEF PLATFOEM
Fig. 3. Schematic diagram illustrating the reef terminology employed as well as the overall eflects of differential compaction using a Leduc reef example.
0 METRES 2000 w SHOTPOINT NUMBER , 1 E
z i 2 ; z : ; z i:: 0
2 :: : 0 : o D D ,c.N”r.~,,,a
Fig. 4. Rainbow Mbr example 1
Rainbow Member winnipegmis Formation
Lower amplitude benedth the pwi- ohera, reef rim.
NO significant “a~ldtlmi.
swan Hills Formation
GilWOd Mb? has high amplitude Off mei d”d low amplitude beneath the reef.
iifectively masked by Beaverhill Lake Group e”ellt.
Effectively masked by high-ampiitwde “eaverhill Lake 61O”P event.
NO significant YdrlatlmS,
84SE CRETACEOUS
TOP WAB/\M”N FORM.4IION
TOP SLAVE POlNi FORMAIION TOP BLACK CREEK MBR TOP UPPER KEG RlVER Mm TOP LOWER KEG RiVER Mm TOP CHINCHAGA FORMAT,ON TOP COLD LAKE FM BASE COLD LAKE FM
kd”C Formation zeta Lake Member
lower amplitude NO significant beneath the reefs “anations. than “ff reef. Yost pranourced in atoll examples.
Signifirnnt uari- NO significant aticns only across variations. Steeply dipping iones a”erlyi”g reei fldrlki.
Table 1. Character variations associated with prominent seismic reflectors.
0.5
1.0
1.5
SEISMIC SIGNATURES OF DEVONIAN REEFS
SHOTPOINT NVMBER 0 METRES 100~
II
” r
WABAMUN FORMATiON
SIAYE POiNi FORM*iiON CYLNCHAGA FM
COLD LAKE FORMATION
COLD LAKE FORMAIIION
Fig. 5. Rainbow Mbr example 2.
SHOTPOINT NUMBER 0 METRES 200”
w I I E
Fig. 6. Rainbow Mbr example 3,
I2 N.L.. ANDERSON and R.I. HROWN
SHOTPOINT NUMBER o METRES 2000 I I
E
LOWER CRETACEOUS REFLECTOR
‘TOP WABAMUN FORMATiON
TOP SLAVE POINT FM
TOP LOWER KEG Ri”ER MBR
-TOP CHiNCHAGA FORMATION
.TOP COLD
LAKE FM BASE COLD LAKE FM
Fig. 7. Rainbow Mbr example 4.
SHOTPOINT NUMBER 0 METRES ,000
T”F WAa4M”N IDRMAIION
iOP CAhBONATE M8RhEH !i”Ri SiMPSON .%iALE!
TO/l SL”“L POlN, i”iMAiiDN
,OP Low<* *co RIVER MEMBL’I
TOP CO‘0 LAKE FOFMAIION
Fig. 8. Upper Keg River Reef Mbr example 1
SEISMIC SIGNATURES OF DEVONIAN REEFS 13
SHOTPOINT NUMBER ” METRES loo0
.TOP COLD LAKE FOIIM”TlON
Fig. 9. Upper Keg River Reef Mbr example 2.
SHOTPOINT NUMBER 0 METRES 2000 I I
TOP WBAMUN FORMAilON
TOP CARBONATE MARKER
TOP SLAVE POlNT FORMAilON
, TOP CHINCHAGA FM TOP COLD LeKE
FORMAivJN
Fig. 10. Upper Keg River Reel Mbr example 3.
N.I.. ANDERSON and R.J. BROWN
SHOTPOINT NUMBER 0 METRES 1000
s N
25 23 21 19 17 15 13 11 9 7 5 3
’ MlSS,SS,PP,,4N
Fig. 11. Upper Keg River Reef Mbr example 4
SEISMIC SIGNATURES OF DEVONIAN REEFS 15
Fig. 12. Winnipegosis Fm example 1.
SHOlPOINT NUMBER 0 METRES 1000
I I
WY
: - r I .o
w
5 p: I-
2 3
9 +
1.5
TOP
TOP
P&l/ME F~,WT,ON WlNNlPEGOSlS FORM.4 TlON
E
CREJACEOUS REFLECTOR
MISSISSIPPIAN REFLECTOR
TOP PRAlRlE FORMATION
TOP WINNIPEG~SIS FORMATION
TOP WINNIPEG FORMATION
Fig. 13. Winnipegosis Fm example 2.
16 N.L. ANDERSON and K.J. BROWN
SHOTPOINT NUMBER 0 METRES ,000
Flg. 14. Winnipegosis Fm example 3.
SHOTPOINT NUMBER 0 METRES 2000
E
CRETACEOUS REFLECTOR
M,SS,SS,PP,AN REFLECTOR
TOP PRrnlRiE FORMAilON
TOP WiNNlPEGOSlS FORMAliiON
TOP WlNN,PEG FDRMAiiON
TOP
TOP
TOP
TOP
NEAR
WABAMUN FORMATiON
LOWER iRETON FM
BEAYERHIII LilKE GROUP GLLWOOD Mm SANDSTONE
BASEMEN7 EVENT
Fig. 15. Swan Hills Fm example 1
SEISMIC SIGNATURES OF DEVONIAN REEFS
SHOTPOINT NUMBER 0 METRES 1000 1 t
17
TOP LOWER iRETON FM
TOP BEAVERIIIL‘ LAKE GROUP
TOP GiLWOOD MBR SPNDSiONC
NEAR 8ASEMENi EYENT
Fig. 15. Swan Hills Fm example 2.
Sti0TP0iN~ NUMBER 0 METRES 2000
TOP WABAMUN FORMATlON
TOP LOWER IRE
TOP BE*YERH,LL GROUP
TOP GlLWOOD M1 SANDSTONE
NEAR EdSEMENT
Fig. 17. Swan Hills Fm example 3.
SHOTPOINT NUMBER 0 METRES 4000
‘Eli
SAS,
TOP
TOP
TOP
TOP
TOP
‘TON FM
LAKE
38
EVENT
PARK FORM41ION
41 CREIACEOUS REFLECTOR
NORDEGG FORMATiON
WABAMUN ymANTmN
FORMAilON COOXLNG LAKE FORMATLON ELK POiNi GROUP
Fig. 18. Leduc Fm example 1
18 N.L. ANDERSON and R.J. BROWN
SHOTPOINT NUMBER 0 METRES 2000 L ,
TOP CARDIUM FORMAI,ON TOP “KlNG FcJRMAI,ON
TOP WABAMUN FORMAi,ON TOP lREiON FOFM4mm TOP COOKlNG LAKE FM
Fig. 19. Leduc Fm example 2,
SHOTPOINT NUMBER 0 METRES ,ooo I I
CRETACEOUS REFLECTOR
,OP CARDIUM FORMATlON
TOP “iKING FORM*T,DN
TOP BANFF FORMATION
TOP W*IBAMUN FcJRM.4ITION
TOP lREiON FORMAIION
TOP COOKlNG ‘AKE FORMA\IION
E
Fig. 20. Leduc Fm example 3.
SELSMIC SIGNATURES OF DEVONIAN REEFS 19
Flg. 21. Leduc Fm example 4.
amplitude due to constructive interference with the Swan Hills Fm event. Such amplitude enhancement does not occur beneath the reef.
Significant amplitude variations along reef-platform events are characteristic of four of the six reef types. In the vicinity of the other two (Winnipegosis Fm and Swan Hills Fm) the platform events cannot everywhere be confidently resolved. For example, the Ashen Fm event off reef is effectively masked by the high-amplitude Winnipegosis Fm event (Figures 12 to 14). Similarly, the Swan Hills Fm platform event is masked by the BeaverhillLakeGroupevent(Figures 15 to 17). Ingeneral, the Lower Keg River Mbr, Cooking Lake Fm and lreton Fm events are variable-amplitude events beneath the Keg River Fm (Figures 4 to 1 l), Leduc Fm (Figures 18 to Zl), and Zeta Lake Mbr (Figures 22 to 24) reefs, respectively, and uniform-and higher-amplitudeoff reef. These variations are attributed to differential attenua- tion above the platform, the heterogeneous n;(ture of the reef/platform contact and the homogeneous nature of the off-reef/platform contact. In the case of the Leduc Fm reef examples (Figures 18 to 21), the acoustic- impedance contrast off reef is significantly higher than below the reef. Although the polarity of the platform
events. on reef and off reef, does not appear to be reversed on any of the examples, such reversals can occur, particularly within evaporitic basins. For instance, if the platform sediment had a higher acoustic imped- ance than the overlying reef and a lower acoustic imped- ance than the basal off-reef sediment (Figure 3) then the polarity of the platform event, off reef and on reef, would reversejust under the reefedge. The misinterpre- tation of such reversals could result in anomalous pseudostructure along the reef platform on an inverted seismic section.
The amplitude of the reflection from the reef/off-reef contact (Figure 3) is primarily a function of the nature of the basin, more specifically the lithology of the off-reef sediment. The acoustic-impedance contrast between the carbonates and off-reef shales varies significantly. (See Anderson ( 1986) for geologic models derived from well-log information.) For example, the Leduc Fm generally has a significantly higher velocity than the off-reef Ireton Fm (typical ratio 1.2:1); the Swan Hills Fm has a velocity slightly higher than the off-reef Water- ways Fm (typical ratio I. I: 1). The reflections from the top of the Leduc Fm and Zeta Lake Mbr are generally prominent events (Figures 18 to 24); the Swan Hills Fm
20 N.L. ANDERSON and R.I. BROWN
SHOTPOINT NUMBER 0 METRES 1000 I I
E
0.5
2.5
Fig. 22. Zeta Lake Mbr examt~le 1.
event is effectively masked by the Beaverhill Lake Group event (Figures 15 to 17). The flanks of the Leduc Fm and Zeta Lake Mbr reefs are not seismically visible due to their steep dips. Similarly, the acoustic-impedance contrast between the evaporitic-basin reef types and theoff-reefevaporitesvariessignificantly. Forexample, the Winnipegosis Fm has a significantly higher velocity than the off-reef Prairie Fm salts (typical ratio I .3: 1). while the Keg River Fm reefs may or may not have a higher velocity than the overlying sediment, depending upon the lithology and porosity. Due to the loweracoustic- impedance contrast, the Keg River FmiMuskeg Fm contact is generally a low-amplitude event across the reef (Figures 4 to 11); the contact is not seismically visible in reef-flank positions due to the steep dip. In contrast, the Winnipegosis FmiPrairie Fm contact is prominent across both reef top and flank positions (Figures 12 to 14).
Significant amplitude variations along prominent postreef events are restricted to those highly dipping reflections overlying reef flanks. For example, the Ireton Fm event is a lower-amplitude event above the flanks of the large Leduc Fm atoll example I (Figure 18) than elsewhere.
Character variations associared with the seismic images
ojreejandofS-reejj~cies- In Table 2, the seismic images
TOP “iKiNG FORMATiON
TOP M,SSi.SS,PPiAN
TOP CYNTHIA MEMBER
TOP IRE TON FORMA TiON
TOP D”“EFlNAY FORMAilON
of the six reef types and their corresponding off-reef facies are summarized. The reef images of the Rainbow Mbr (Figurer 4 to 7) and Upper Keg River Reef Mbr (Figures 8 to II) consist of low-amplitude discontinu- ous events bounded at the top and base by the reflec- tionsfrom the reef/Muskeg Fm contact and the reef/Lower Keg River Mbr contact, respectively. These seismic images are bounded laterally by the image of the Mus- keg Fm which varies significantly within the Black Creek basin.
In areas where a thick (>I5 m) Black Creek Mbr is present, the off-reef image of the Muskeg Fm is domi- nated by the high-amplitude reflections from the top and base of these salts (e.g.. Figures 4 and 6). The reflections from the salt are overlain by the lower- amplitude events from the interlayered dolomites and anhydrites of the upper Muskeg Fm. In areas where extensivepost-MuskegFmsaltdissolution hasoccurred and any residual Black Creek Mbr is not seismically visible (<I5 m thick), the image is comprised of low- amplitude, discontinuous reflections which are virtu- ally indistinguishable from the image of the Keg River Fm reefs (e.g., Figures 5 and 7). In areas where the Black Creek Mbr was not deposited or was removed by an early phase of dissolution, the image consists of low- tomedium-amplitude, relativelycontinuousreflec- tions (e.g., Figures 8 to 11).
SHOTPOINT NUMBER o METRES 2000
21
Fig. 23. Zeta Lake Mbr example 2
Reef facier
Off-reef facier
Rainbow Member
Dominated by high- amplitude reflec- tions from tc’p and base of these Salts. Reiatively tow amplitude of internal eYentS is reflection of homogeneity Of these salts.
swan Hills Fonnatlon
EffeCriYely masked by high-amplitude BeaYerhill Lake Group event.
Beaverhill Lake Group went effec- tiw1y masks any reflections origi- nating within or at base of Water- ways Frn.
CRETACEOUS REFLECTOR
TOP VfKlNG FORMATION
TOP MlSSlSSlPPlAN
TOP CYNTHIA MBR
TOP IRETON FORMATION
TOP DUVERNAY
FORMATION
Leduc FOrnw.iO”
Low-amplitude dis- continuaus eYe”tS bounded by Ireton Frn,kdK Fm and Cooking Lake Fnl, k&C Fm events.
zeta Lake blber
Dominated by reflections origi- nating at top and base Of zeta Lake Mr. Internal refiections effec- tiw1y masked by these high-ampli- tude events.
Dominated by cm- this Mbr and Ire- ton Frn,NiSk” Fill events. *“ternal Peflections effec- tive1y marked by these high-amp,i- tude events.
Table 2. Character variations associated with the seismic images of reef and off-reef facies
The seismic image of the Leduc Fm (Figures 18 to 21) In contrast to the Rainbow Mbr, Upper Keg River similarly consists of low-amplitude, discontinuous Reef Mbr and the Leduc Fm, the seismic images of events, bounded at the top and base by the reefilreton Winnipegosis Fm (Figures 12 to 14) and Zeta Lake Mbr Fm contact and the reef/Cooking Lake Fm interface, (Figures 22 to 24) reefs are dominated by the relatively respectively. The flanks of the image of the Leduc Fm high-amplitude reflections from their tops and bases. are bounded by the image of the laterally adjacent shales. These events mask any reflections from within these The image of these off-reef shales is comprised of later- relatively low-relief reefs. The seismic image of the ally continuous, low- to high-amplitude events which Swan Hills Fm-Snipe Lake reef (Figures 15 to 17) is contrast with and generally terminate against the image masked by the high-amplitude Beaverhill Lake Group of the Leduc Fm. event.
N.L. ANDERSON and R.J. BROWN
Fig. 24. Zeta Lake Mbr example 3.
Time-structural relief
Time-structuralreliefiscomprisedofvelocity-generated relief and structural relief. Velocity-generated relief is primarily due to lateral variations in average velocity (associated with facies changes) and local thickness variations. Structural relief can be syndepositional or postdepositional in origin. Postdepositional relief results from geological phenomena such as erosion, reef-focused salt dissolution and diffcrcntial compaction. Timc- structural relief which is independent of a particular reef is not considered to be a component of its seismic signature.
Velocity-generatedrimesrrucrure-Velocity-generated time structure associated with reefs is caused primarily in two ways: I) lateral variations in the thickness of reef andcontemporaneousorenvelopingoff-reefsediments, and 2) lateral thickness variations in formations postdat- ing the reef. In Table 3, the velocity~generated time structure observed to be associated with each reef type (Anderson, 1986) is summarired. Velocity-generated time structure results primarily from reef-related, lat-
eral variations in the thicknesses of relatively homoge- neous (with respect to velocity) layers in the sedimentary section. The net effect of lateral variation in the thick- nesses of these layers (r.g., reef, off-reef and overlying lithological units) is lateral variation in theaverage veloc- ity of the sedimentary section, which generates a com- ponent of time-structural relief. Reef-related variations of thickness within shale basins are either primary (deposition of reef and off-reef sediments) or secondary (differential compaction and compensation) features. Variations within evaporite basins also result from these phenomena but may be accentuated by an additional secondaryfeature,namely,reef-focusedsaltdissolution.
Consider the Rainbow Mbr reef examples (Figures 4 to 7). The thicknesses of the sedimentary layers vary significantly as a result of both primary and secondary phenomena. For example, the Slave Point FmiCold Lake Fm interval was relatively uniform or uniformly divergent in the Rainbow basin at the end of Slave Point Fm time. However, this interval is now thickest across the reef, thinner across thick (>lS m) sections of Black Creek Mbr and thinnest above zones of extensive post-
SEISMIC SIGNATURES OF DEVONIAN REEFS 23
WinnipegOSiS Formation
Table 3. Velocity-generated time structure; measures of absolute relief are understood to be prxitive unless specifically given a negatiVe sign or qualifier.
Slave Point Fm salt dissolution (e.g., Figures 4 and 7). Conversely, the Fort Simpson FmiSlave Point Fm inter- val is thinnest above the reef, thicker above thick salt and thickest above zones of extensive post-Slave Point Fm dissolution. The net result of these and similar thickness variations (such as the variations in the thick- nesses of the Rainbow Mbr and Muskeg Fm) is the lateral variation in average velocity over a particular depth interval rather than lithologic interval which cre- ates a component of time-structural relief. While time- structural relief is present throughout the sedimentary section, it generally attains the highest magnitude, and is most easily distinguished from real structure, in the case of near-basement events. Up to 10 ms of velocity- generated time-structural relief is observed along the Cold Lake event beneath the Rainbow Mbr reefs (Figures 4 to 7). This is believed to be primarily due to the thickening of the high-velocity Slave Point FmiCold Lake Fm interval and the corresponding thinning of the low-velocity Fort SimpsoniSlave Point Fm interval across the reef. Similarly, the velocity-generated time-structural relief (up to 10 ms) observed beneath the Upper Keg River Reef Mbr is attributed primarily to the thickening and thinning of these two intervals across these reefs (Figures 8 to 11).
The velocity-generated time-structural relief (up to 10 ms) observed beneath the Winnipegosis Fm reef examples (Figures 12 to 14) is also attributed both to primarydepositionalfeaturesand tosecondarydifferen- tial compaction and reef-focused salt dissolution. Relief is primarily ascribed to the velocity contrast between Winnipegosis Fm reefs and equivalent thicknesses of off-reef Prairie Fm. A component, estimated to be less than 5 ms, is also attributed to the thinning of the Prairie Fm above the reef as a result of differential compaction and/or postdepositional dissolution. Gendzwill (1978) supports the thesis that the low along the Prairie Fm horizon above the reefs primarily results from post- depositionalreef-focuseddissolution. Theselowsappear to be compensated by carbonates of higher velocity than the salts, which effectively pulls up the underlying events.
In contrast, the less than 5 ms of velocity-generated time-structural relief statistically observed along the Gilwood Mbr event beneath the Swan Hills Fm reef (Figures 15 to 17) is attributed to lateral variations in the thickness and velocity of the lithological units within the overlying Beaverhill Lake Group. As significant structural drape is not observed along the Beaverhill Lake Group horizon across the edge of the Swan Hills Fm reef, lateral changes in either the thickness or the average velocity within the overlying sedimentary sec- tion are probably not reef-related. The observed veloc- ity pull-up beneath the Snipe Lake reef is principally a function of the relative thickness and velocity of the reef and off-reef facies (Figures 15 to 17).
Significant drape, however, does occur across the Leduc Fm reef examples (Figures 18 to 21). As a result, postreef stratigraphic units are generally thicker off reef. These variations in the thicknesses of overlying stratigraphic units would generate up to one half or more of the observed 20 to 25 ms of pull-up along prereef events; the other component is believed due to the velocity contrast between the reefs and an equiva- lentthicknessofoff-reefshales(Anderson, 1986, §6.5D). Analysis of integrated sonic-log data from the vicinity of those Leduc Fm reefs which are included in this overall study (Anderson, 1986) indicates that signifi- cant lateral velocity variations in the Blairmore Group/ Ireton Fm interval as described by Davis (1971, 1972) are not present and therefore do not generate a signifi- cant component of the time-structural relief observed along prereef events (Anderson et al., 1986a).
Significantly less drape is observed across the lower- relief Zeta Lake Mbr reefs (Figures 22 to 24). Local velocity pull-up beneath these reefs is about IO ms. This reliefis principally a function of the relative thicknesses and velocities of the Zeta Lake Mbr and off-reef Nisku facies.
Structural relief- In Table 4, structural relief asso- ciated with the six reef types is summarized. Structural relief, for the purposes of this paper, is restricted to that component of relief which is directly attributed to the presence of reef and consists of structures which either
24 N.L. ANDERSON
focusedreefgrowthorresultedfrom thepresenceofthe reef. Structural relief, as shown in Table 4, is catego- rized as relief along: a) prominent prereef platform reflectors; b) the reef platform; c) the reef/off-recffacies contact; d) prominent Devonian reflectors: e) the base of Cretaceous, and f) prominent Cretaceous events. Relief along postreef horizons in shale basins is primar- ily ascribed to differential compaction of reef and off- reef facies. See Figure 3 for an illustration of such effects. In evaporitic basins, structure due to differen- tial compaction is often accentuated by reef-focused salt dissolution.
The estimates of structural relief discussed below have been determined from an analysis of inverted seis- mic data. The inversion process (Figure 25) involves the conversion of time structure to depth structure through the use ofaveraged well-log interval velocities. Each of the example seismic lines has been inverted and displayed by Anderson (1986) who gives a full discus- sion of the methodology employed, as well as of the uncertainty in localized relief estimates (about IO m and 5 m on prereef and postreef horizons, respectively). Below, only a summary of the observations is given.
Positive structural relief, up to 30 m in magnitude, is observed along the Cold Lake Fm and Chinchaga Fm horizons beneath some Rainbow Mbr reefs (Figures 4 to 7). This relief is possibly due to post-Chinchaga Fmipre-Upper Keg River Fm faulting and may have focusedreefgrowth. Similarstructureshavc bccnconti-
and R.J. BROWN
dently identified on the basis of well-log control in the Rainbow basin (Hriskevich, 1970). Similar apparent structural relief along preplatform horizons below the Leduc Fm example 1 (Figure 18) and Zeta Lake Mbr examples (Figures 22 to 24) cannot be attributed with the same degree of confidence to real structural relief. These apparent structures may be caused by the use of inappropriate reef and/or off-reef interval velocities.
STEP 1 MEASUREMENT OF ISOCHRON “ALUES BE,WEEN
PROMINENT REFLECTORS ON THE mm st,w,c
SEillON~
STEP 2~ TRANSFORMATION OF ISOIHRON “AIUES NT0
ISOPACH VALUES “SING WELL-LOG YELOClTY
STEP 3 SEo”tNTlA~ STACKING OF ISOPACH “ALUES
CONTlN”O”S DEPTH -“ELOCIIY MODELS
DEPTH IS RELATIVE TO A NEAR-SURFAct
SE1SMlC DATUM
Fig. 25. Sequence followed in the process of inverSion from time Section to c0ntinwus depth-velocity model (Anderson. 1986)
*wan HiITS kd”C Formation
NO sigrliiicant relief confiderrly identified.
NO significant relief C”lifid?lltly identified.
zeta Lake Member Rainbow Men&r Upper Keg River
NO apparer,t sig- niiiunt reliefs.
Prereef platform
Platform
Reef
Postreef (De”“ni.3”)
up to 30 m appar- en+. relief alOng Cold illk Frn be- low these reef*.
NO dppdrrrl‘ sig- nificant reiicf.
No iignificarlt relief confidently identified.
“0 to 20 m aooar- NO apparent sig- nificant relief.
Ashem Fill not identifiable Off reef: relief Cd” not be esti.atLxi.
sva,, Hilli Frn platform eYent ““t confidently identifiable.
ii” significant relief cunfidrntly identified.
ent relief below the reefs. stmr- t”re d”“edrS t” be pAppet- keg River in orisin.
Rainbow Mbr eYent not identifiable with i”fficient confidence t” es- m;,“‘“““‘“’
Reflection from top <If swan Iii,,* in, reef not i”“+i”E”tl~ identifiable.
Roei lil,lS up t” 4s m higher than inirli”i 1agoon.
TOP Of zeta ILake M’r porosity up tn 15 m higher timn tap of adjacent Cynthia mr.
Wabanlun F”, up to 10 n, l”caily high aCrOSs Leta Lake her.
lllssissippia” top up t” 10 “1 locally high abnve zeta Lake Mbr.
NO apparent sig- niiicant local WliBf.
NO apparent rig- “ifiCdi,T relief.
NO CretaCe”“S event confidentlj cnrrelnted across section*.
NO dpparent sig- nifirant relief.
li” d”“,irent sig- niiicant relief.
Table 4. Structural relief; absolute quantities as in Table 3
SEISMlC SKiNATURES OF DEVONIAN REEFS 25
For example, structural relief of up to 30 m is estimated along the preplatform events beneath the Leduc Fm example 1 (Figure 18); similarly, structural relief of up to IO m is estimated along the prereefplatform horizons beneath the Zeta Lake Mbr examples (Figures 22 to 24). These structures may be real and could have focused reef growth; however, since the uncertainty in prereef relief estimates is also about 10 m (Anderson, 1986, p. 332) these may only be pseudostructures. Sufficient well control is not available to determine the reliability of these structural estimates.
The Lower Keg River Mbr horizon (reef platform facies) beneath the Rainbow Mbr, is not often structur- ally parallel to the Cold Lake Fm and Chinchaga Fm horizons (Figures 4 to 7), supporting the premise that such structural relief is real. Conversely, the Cooking Lake Fm and Ireton Fm horizons beneath the Leduc Fm (Figures 18 to 21) and Zeta Lake Mbr(Figures 22 to 24) reefs, respectively, are structurally parallel to the respective prereef platform horizons, suggesting that apparent structure may be psuedorelief generated by the use of inappropriate interval velocities. No signifi- cant relief is present along the reef platform or prereef platform horizons below the Upper Keg River Mbr (Figures 8 to 1 I), Winnipegosis Fm (Figures I2 to 14). 01 Swan Hills Fm (Figures 15 to 17) reef examples.
While structural relief along the reef/off-reef contact is associated with all six reef types, the magnitude is confidently estimated for only three. The reflections from the Rainbow Mbr/Muskeg Fm contact and Upper Keg River Reef Mbr/Muskeg Fm contact are not confi- dently identified due to the similarity of reef and Mus- keg Fm velocities (Figures 4 to 1 I). Similarly, the Swan Hills Fm reef/off-reef contact is not confidently identified, as this reflection is masked by the Beaverhill Lake Group event (Figures 15 to 17). In contrast, the tops of the Winnipegosis Fm, Leduc Fm, and Zeta Lake Mbr are confidently identified (Figures 12 to 14 and 18 to 24). The Winnipegosis Fm reefs are 35 to 60 m structurally higher than the adjacent interreef sediments. Differen- tial compaction appears to have occurred between the reef rim and interior lagoonal facies. The rim appears to be up to 15 m structurally higher than the interior lagoon of the Winnipegosis Fm reef examples 1 and 2 (Figures 12 and 13) suggesting that the lows along the top of the Prairie Fm above these reefs may be partially due to differential compaction as well as to postdepositional reef-focused salt dissolution. Similarly, the rim of the large Leduc Fm atoll example 1 (Figure 18) is as much as 45 m structurally higher than the interior lagoonal facies. The apexes of the pinnacle reef examples 3 and 4 are shown to be as high structurally as the respective rims of the adjacent atolls (Figures 20 and 21), indicat- ing that the pinnacle and the organic rims of the atolls were compacted to a similar degree. The top of the porosity within the Zeta Lake Mbr (Figures 22 to 24) is 35 m to 55 m structurally higher than the top of the laterally adjacent Cynthia Mbr shale. Structurally low lagoonal sections are not observed.
Structural relief along prominent postreef Devonian horizons is associated with five reef types. No apprecia- ble drape is associated with any prominent post-Swan Hills Fm horizon (Figures IS to 17). Relief, however, is observed along the Slave Point Fm horizon across the Rainbow Mbr and Upper Keg River Reef Mbr examples (Figures 4 to I I). The Slave Point Fm is up to 70 m structurally higher above the apex of the Rainbow Mbr reefs than off-reef. Similarly, this horizon is up to 60 m locally high above the apex of the Upper Keg River Reef Mbr. Relief along the Slave Point Fm horizon above Keg River Fm reefs is attributed to differential compaction of reef and off-reef sediment and to post- Slave Point Fm dissolution of Black Creek Mbr salt which necessarily only occurred off reef. In contrast, the top of the Prairie Fm above the Winnipegosis Fm reefs (Figures 12 to 14) is locally up to 30 m structurally low. These lows are attributed to two phenomena: dif- ferential compaction of reef and off-reef facies, and postdepositional dissolution of the evaporites of the Prairie Fm (Gendzwill, 1978). Structural drape is also observed along the Ireton Fm horizon across the Leduc Fm reef examples (Figures 18 to 21). This horizon is locally up to 70 m structurally high above these reefs, primarily due to the differential compaction of the reef and off-reef facies.
Successively smaller structural-drape and salt- dissolution effects are observed across these reefs along shallower horizons. The magnitude of structural relief along shallower horizons, as shown in Table 4, is princi- pally a function of the following: the relative compact- ability of reef and off-reef sediments; the initial deposi- tional thicknesses of reef and off-reef sediments, and, where applicable, the timing and extent of reef-focused salt dissolution. The magnitude is also affected by the volumeofsediments removed by erosion. Forexample, it is typically the case that appreciably more relief is observedbeneath thepre-Cretaceousunconformitythan at or above this horizon. The effect of depositional hiatus on structural drape is discussed by Labute and Gretener (1969). Utilizing well-log tops, they were able to study structural drape with a degree of resolution not attainable with a seismic data base. The seismic data suggest, however, that while the well-log tops are rela- tively accurate, the interpretation of the well-log infor- mationmight be slightlymisleadingifthickness variations in the sedimentary section overlying a reef are attrib- uted only to vertical strain, induced by differential com- paction and/or salt dissolution. In fact, an analysis of the seismic data suggests that differential compaction or reef-focused salt dissolution may generate lateral as well as vertical strain in the overlying sediments. For example (Figure 4). extensive salt dissolution occurred in the vicinity of the large Rainbow Mbr pinnacle reef. Dissolution occurred primarily about the periphery of the reef within a zone about 5 km wide. The pronounced structural low along the Wabamun Fm horizon, attrib- uted principally to post-Devonian dissolution of the Black Creek Mbr, occurs across the reef within a zone
26 N.L. ANDERSON and R.J. BROWN
about 6 km wide. This change cannot be ascribed to Fresnel-zone size increase because these generally increase with depth. Over the -1200 m from the Waba- mum top to the Black Creek top the Fresnel-zone radii would increase from -100 m to -200 m (Neidell and Poggiagliolmi, 1977). The implication is that lateral as well as vertical strain is associated with differential compaction and salt dissolution and that thickness vari- ations are not necessarily an accurate estimate of the rate or extent of either salt dissolution or differential compaction, especially in the shallower portion of the sedimentary section.
CONCLUDING REMARKS
In this paper, an appropriate classification scheme and analytical methodology have been introduced whereby the seismic signatures of six different types of westernCanadianDevonian reefs have been documented in terms oflateral character variations and time-structural relief. It is recognized that the observations reported in this paper and in the study of Anderson (1986) are based on theanalysisofrelativelyfew reefexamples. However, it is hoped that these are reasonably representative examples, that a basis for more extensive integrated geophysical/geological studies of reefs has been esta- blished, and that the necessary data will be made avail- able for such future publications.
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