And High-Relief Leduc Formation Reefs: a Seismic Analysis

Missouri University of Science and Technology Scholars' Mine

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01 Nov 1989

Low- and High-Relief Leduc Formation Reefs: A Seismic Analysis

Neil Lennart Anderson Missouri University of Science and Technology, [email protected]

Robert James Sidford Brown

Ronald C. Hinds

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Recommended Citation N. L. Anderson et al., "Low- and High-Relief Leduc Formation Reefs: A Seismic Analysis," Geophysics, vol. 54, no. 11, pp. 1410-1419, Society of Exploration Geophysicists, Nov 1989. The definitive version is available at https://doi.org/10.1190/1.1442604

This Article - Journal is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in Geosciences and Geological and Petroleum Engineering Faculty Research & Creative Works by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. GEOPHYSICS, VOL. 54, NO. 11 (NOVEMBER 1989), P. 1410-1419, 13 FIGS., 2 TABLES.

Low- and high-relief Leduc formation reefs: A seismic analysis

N. L. Anderson*, R. J. Brownt, and R. C. Hinds§

ABSTRACT pullup at the Beaverhill Lake level beneath the Leduc- Leduc reefs have grown to widely varying heights Woodbend atoll, 15 ms for the Morinville reef. Also, it and areal extents along the Rimbey-Meadowbrook is very difficult to differentiate the Leduc reflection trend of central Alberta, resulting in significantly dif­ from the Duvernay reflection, with which it merges, on ferent seismic signatures. Three examples considered the Morinville (basal-reef) section. In contrast, the in this paper include two high-relief or full reefs from Leduc reflection can be correlated readily on the the Leduc-Woodbend field, an atoll and a pinnacle, Leduc-Woodbend atoll section; and reflections from each around 200 m in height but differing greatly in the offreef shales (Duvernay and Ireton formations) areal extent, about 100 km2 for the atoll and 1 km2 for terminate abruptly against the reef flank. the pinnacle. The third example, a low-relief or basal In addition, the amplitude of the underlying Cooking reef from the Morinville field, is about 100 m high and Lake platform reflection varies laterally, depending on 1 km2 in areal extent. the velocity of the overlying formation (Duvernay The Leduc-Woodbend and Morinville reefs exhibit shale or Leduc reef) and, to a lesser extent, the quite different seismic signatures. For example, 25 ms thickness of the overlying reef. This variation is not as of time-structural drape along the top of the Devonian useful in distinguishing between low-relief and high- is observed across the Leduc-Woodbend atoll but only relief reefs as it is in indicating the presence or absence 15 ms across the Morinville reef. There is 30 ms of of reef.

INTRODUCTION ically drape across Leduc buildups. This drape is a function of the thickness of the overlying sedimentary section and of The Leduc formation of Alberta developed as fringing reef the compactabilities of these reef and offreef facies through complexes, linear chains of reefs, isolated atolls, and iso­ all postdepositional volume changes, whether chemical or lated pinnacles (Klovan, 1964; Mountjoy, 1980; Stoakes and physical in origin. Wendte, 1987). In central Alberta, these carbonate buildups The Leduc-Woodbend and Morinville fields (Figure 1), overlie a regional platform facies, the Cooking Lake forma­ which provide the seismic examples for this paper, lie along tion, while to the northwest, in the Sturgeon Lake area the Rimbey-Meadowbrook trend which separates the west (Figure 1), they rest on the Beaverhill Lake group (Figure 2). and east Ireton shale basins (Figure 3). The Leduc carbonate The Leduc formation in central Alberta is typically encased buildups represented are a large atoll (D-3A pool) and in the impermeable shales of the Ireton and Duvernay adjacent pinnacle (D-3F pool) at the Leduc-Woodbend field formations. To the north and northeast, full reefs are capped (Figure 4) and the basal reef at the Morinville field. The by and are in communication with the Grosmont formation. Morinville reef stands about 100 m above the Cooking Lake Typically, Leduc reservoirs have developed within the updip formation platform and encompasses a basal area of less edges of the fringing reef complexes and the larger atolls and than 1 km2. The Leduc-Woodbend atoll reefs, in contrast, throughout the smaller atolls and pinnacles. rise some 200 m above the platform and have areal extents in As a result of differential compaction of reef and offreef excess of 100 km2 (atoll) and 1 km2 (pinnacle). The variation facies (O’Connor and Gretener, 1974), overlying strata typ­ in the heights of such closely spaced buildups indicates that Manuscript received by the Editor December 12, 1988; revised manuscript received May 30, 1989. *Department of Geological Sciences, Ohio University, Athens, OH 45701-2979. ^Department of Geology and Geophysics, University of Calgary, 2500 University Drive, N.W., Calgary, Alta., Canada T2N 1N4. IDepartment of Geology, University of Pretoria, Pretoria, Republic of South Africa 0001. © 1989 Society of Exploration Geophysicists. All rights reserved.

1410 Seismic Study of Low and High Leduc Reefs 1411 reef growth stopped at different times, with more than one Interpretation of the geologic cross-section sequence having been involved. See Mitchum et al. (1977a, b) and Mitchum and Uliana (1985) for a discussion of Since the Leduc-Woodbend field was developed in the carbonate sequence stratigraphy. Not surprisingly, the seis­ early 1950s, only electric logs were run for many of the wells mic signatures of these morphologically diverse reefs also in the study area. Thus, in order to allow the geologic differ significantly. cross-section to show sonic or neutron logs, some offline In this paper we discuss these three reef examples with wells of more recent vintage have been incorporated (Fig­ regard to features such as time-structural drape, velocity ures 4 and 5). pullup, and the contrast between the seismic images of the The deepest horizon labeled on the cross-section (Figure onreef and offreef shales; these features are those that are 5), the Cooking Lake formation, is the platform facies for characteristically associated with the seismic signatures of Leduc formation reefs. Only one of the four wells used the Leduc formation. Only a few other studies of the (6-7-50-25W4) penetrated the top of the Cooking Lake. seismological aspects of Leduc reefs are available in the However, this horizon has been drawn (Figure 5) as planar literature. These include studies by Pallister (1965), Davis and with the regional dip by inference from its seismic image (1972), Bubb and Hatlelid (1977), Anderson (1986), Ander­ (Figure 6), which is discussed further in the next section. son and Brown (1987), Anderson et al. (1988a, b), and The Leduc formation in this area (Figures 4 and 5) can be Anderson et al. (1989). differentiated into full reefs and a basal reef. The term full reef is applied to both the Leduc-Woodbend atoll and the LEDUC-WOODBEND FIELD adjacent pinnacles which attain thicknesses (excluding the The Leduc-Woodbend field was selected as an example for two reasons. First, the Leduc-Woodbend atoll (D-3A pool) is a significant reef reservoir from an economic per­ spective. Production to date is on the order of 38 x 106 m3 of oil and 800 x 106 m3 of gas. Second, the example seismic line (dashed line in Figure 4) is exceptionally well oriented for illustrative purposes: from west to east, it crosses the Leduc-Woodbend atoll (D-3A pool), the adjacent pinnacle reef (around the 2-14 well), the low-relief Leduc buildup or basal reef (to the east of well 14-12), and extends into the east Ireton shale basin (east of the -1050 m contour).

118° 116° 114° 112°

Fig. 2. Stratigraphic chart for the Upper Devonian of the central plains of Alberta (modified after AGAT Laborato­ ries, 1988). Approximate ages (Ma) are shown on the left.

W EST EAST SHALE BASIN SHALE BASIN

h r— — ------[-— NISKU FORMATION , 1 , I , i~ ~ U ^ ...... ■ " —______■ MfeMBEfr— —

IRETON 0 D U C \ FORMATION/^

FRASNIAN | .— 1 - ,-_X - DUVERNAY FM.

___ _— i ' i 1 ’ l ! i i ; | GROUP

UPPER DEVONIAN BAJEAU COOKING LAKE FORMATION : : ■ WOODBEND MBR/~n ' 1 r- — | r -T- T ^

[ [ r | Fig. 1. Distribution of Upper Devonian carbonate complexes (Leduc formation and equivalents) and intervening shale Fig. 3. Stratigraphic terminology of the Woodbend group in basins (after Belyea, 1964). the subsurface of central Alberta (after Stoakes, 1980). 1412 Anderson et al. platform facies) of about 200 m. We show below that these at the 2-14 well (D-3F-pool; Figure 4). The pinnacle does not can be distinguished visually from the offreef Ireton and exhibit a raised rim and the basal reef, as contoured, has an Duvernay shales on the basis of seismic image, drape, and abrupt eastern edge (Figure 4). Relief at the Leduc level velocity pullup. The term basal reef is applied to lower-relief between the atoll and the pinnacle (Figures 4 and 5) is Leduc carbonates (on the order of 50 m thick, excluding the estimated from the seismic data (Figure 6), since the wells in platform facies) which lie basinward of the atoll and around this area were abandoned in the Ireton formation. the periphery of the pinnacles. The origin of the basal reef is The Leduc formation is overlain by impermeable shales of uncertain; it could represent either an early sequence of reef the Duvernay and Ireton formations, which are primary growth or, alternatively, detritus from the full reefs. How­ source rocks for the reefal reservoirs. Presumably, at the ever, it appears to have an abrupt eastern edge on the end of Woodbend time, the Ireton formation top was more or example seismic line (Figure 6), a feature more characteristic less flat within the study area. Now, however, as a result of of earlier reef growth than of detritus. differential compaction, the top of the Ireton is up to 40 m With respect to the full reefs, note (Figures 5 and 6) that higher above full-reef positions than above offreef positions the atoll exhibits a raised peripheral rim (Mossop, 1972) of (Figure 5). about 20 m relief, its upper surface being highest just east of Strata overlying the Ireton formation are similarly draped the 8-17 well, dropping down toward the reef center to the across the full reefs. From the well-log tops and using the west, and appears to be separated from the adjacent pinnacle seismic lines as control, we have found the Calmar, the

F ig . 4. Map of the Leduc-Woodbend study area and contour map (in meters relative to mean sea level) of the Leduc formation top (or, in its absence, the Cooking Lake formation top), showing the seismic line (dashed) and wells used in the geologic cross-section (solid lines). (For an explanation of well symbols, see Sheriff, 1984, p. 299.)

KILOMETERS

8-17 2-14 14-12 6-7

F ig . 5. Geologic cross-sectional interpretation, Leduc-Woodbend study area. Seismic Study of Low and High Leduc Reefs 1413 Wabamun, and Mannville horizons to be up to 40, 25, and 15 the 1-D synthetics enable confident identification of events, m regionally higher, respectively, above full buildups than the 2-D model illustrates the contrasting seismic signatures elsewhere. The Devonian Calmar formation and Wabamun of full-reef, basal-reef, and offreef facies. group are shown in Figure 2; the Mannville group (not The deepest reflections identified in Figure 6, the Beaver- shown) is in the Lower Cretaceous. hill Lake and Cooking Lake events, are up to 30 ms higher beneath the full Leduc formation reef than elsewhere and yet Interpretation of the example seismic line are more or less time-structurally parallel to underlying 48-channel seismic data were acquired in 1983 using a reflections. Relief is therefore attributed to velocity pullup. 12-85 Hz Vibroseis source, a symmetric 1675 m maximum The observed magnitude of this velocity pullup (=30 ms) is offset split spread, a 134 m source interval, and a 33.5 m accounted for by the considerable lateral velocity difference group interval. Figure 6a shows the 12-fold nonmigrated, between the Leduc reef (Vp - 5500 m/s) and the offreef normal-polarity seismic section. [reton shale (Vp = 3800 m/s) for about 200 m of reef relief. Figure 7 shows one-dimensional (1-D) synthetic seismo­ A minor component is contributed to the pullup by the grams computed from the sonic logs for the 14-11-50-26W4 overdraped Ireton-to-Cretaceous units, whose velocities and 14-12-50-26W4 wells. Density logs were not available. both increase and decrease with depth (Figure 8, Table 1). These 1-D synthetics were correlated with the example We see no evidence of significant lateral velocity variations seismic line in order to confirm polarity and identify the within units overlying this reef, as has been discussed for more prominent reflections. other Leduc reefs (Davis, 1972; Anderson et al., 1988a). The two-dimensional (2-D) synthetic section (Figure 8, The reflection from the top of the Cooking Lake formation Table 1), and the corresponding acoustic cross-sectional (Vp — 5900 m/s) is typically of low amplitude where it is model (Figure 5), were developed interactively, using the overlain by the Leduc formation (Vp = 5500 m/s), though well-log and seismic data as control, through repeated mod­ this amplitude varies, perhaps as a result of interference with ifications until reasonable agreement was obtained. Whereas the Leduc reflection. The Cooking Lake reflection is of w KILOMETERS (a)

LEA PARK 10 2o SECOND SPECKS o VIKING uio MANNVILLE to WABAMUN IRETON COOKING LAKE BEAVERHILL LAKE

8-17 2-14 14-12 (b) 6-7

LEDUC

F ig . 6. (a) Normal-polarity seismic section, Leduc-Woodbend study area, (b) Blow-up of a portion of (a) with the Leduc formation shaded black. Horizon names are indicated with first letters [cf., (a)]. 1414 Anderson et al. slightly higher (moderate) amplitude where it is overlain by Table 1. Names of the units shown in the acoustic model of the offreef Duvernay formation (V = 5300 m/s). The Leduc Figure 8, together with the respective velocities and densities event, in contrast, is generally of high amplitude (Figures 6 used. Reasonable inferred values were used for density since and 8). Exceptions occur as a result of defocusing and/or density logs were not available diffractions across the flank of the full reef and between the P . atoll and the adjacent pinnacle. Between these reefs (near (mfs) (kg/m3 trace 325 on Figure 6 and around 4000 m distance on Figure 8), the Leduc event seems to exhibit a slight “bow-tie” (1) Cooking Lake 5900 2750 (2) Leduc 5500 2750 effect; relief here (Figure 5) is estimated based on analysis of (3) middle Ireton 3500-3900 2650 time-structure along the Cooking Lake event. (The thick­ (4) Ireton (upper) 3900 2700 nesses of the Ireton and Leduc formations were estimated (5) Nisku 5800 2750 relative to the observed pullup in full-reef locations.) Ac­ (6) Calmar 4900 2650 (7) Wabamun 5700 2750 cording to our interpretation (Figures 5 and 6), the basal-reef (8) Cretaceous 3800 2650 reflection terminates near trace 101 on Figure 6a or around (9) lower Ireton 3700 2650 (10) lower Ireton 3700 2650 (ID Duvernay 5300 2700

8000 m on Figure 8, suggesting that this basal unit has an Ricker O Phase REFL. DEPTH VELOCITY 28 ms, 28 Hz COEF. KB km /s abrupt eastern edge (Figure 5), consistent with the premise Revr Norm that it represents an early sequence of reef growth. The seismic images of the Ireton and Duvernay shales (Figures 6 and 8) show moderate- to high-amplitude laterally Belly River continuous reflections bounded by the high-amplitude Ireton o 2 event above and the low- to moderate-amplitude Cooking Lea Park Lake event below. This pattern reflects the lithology of these units, for they consist of a sequence of laterally continuous Colorado argillaceous [Vp — 3800 m/s (Ireton)] to limy [Vp — 5300 m/s (Duvernay)] shales (Figure 8). The seismic image of these Second Specks shales is easily distinguished from that of the full Leduc reef Viking but less easily differentiated from that of the basal reef. Mannville The full reef, being relatively homogeneous on the scale of Wabamun seismic wavelengths, is characterized by laterally discontin­ Nisku uous low-amplitude reflections bounded above by the high- ireton Leduc amplitude Leduc event and below by the moderate-ampli­

(a) tude Cooking Lake event. In contrast, the image of the basal reef (Figures 6 and 8) is dominated by the high-amplitude reflection from its top, which effectively masks any internal

Distance (m) 0 2500 5000 7500 X0000

Fig. 7. 1-D synthetic seismograms from the Leduc-Wood- Fig. 8. Acoustic model and corresponding 2-D normal- bend study area, (a) The 14-11-50-26W4 onreef well, (b) The incidence synthetic seismic section, Leduc-Woodbend study 14-12-50-26W4 offreef (basal-reef) well. area. Seismic Study of Low and High Leduc Reefs 1415 pattern. The basal-reef event merges visually with the reflec­ The Morinville field was selected as an example for two tion from the flank of the full-reef buildups. reasons. First, although the Morinville D-3B-pool reef and Time-drape on the Ireton event across the full reef the neighboring St. Albert-Big Lake D-3B-pool reef (Figure amounts to as much as 35 ms, while on the Wabamun and 10) are closely spaced Leduc buildups, they attained start­ Mannville events, it is as much as 30 and 15 ms, respec­ lingly dissimilar heights, implying that cessation of reef tively. This time-structural relief is attributed to both sub­ growth occurred at different times. Second, the example surface structure and to the pullup-pushdown effects of seismic line (dashed line in Figure 9) is exceptionally well drape (generally a result of differential compaction). No oriented across the Morinville reef and illustrates clearly the significant lateral amplitude variations are observed along seismic signature of this basal reef. post-Ireton events. Interpretation of the geologic cross-section MORINVILLE FIELD The deepest horizon identified on the geologic cross- The Leduc formation in the Morinville area is again section (Figure 10) is the top of the platform facies (Cooking differentiated into full reef and basal reef. The full reef, Lake). The offreef carbonates penetrated by the 2-11-54- typified by the nearby St. Albert-Big Lake reef (Figure 9), 26W4 and 12-13-54-26W4 wells are interpreted as the Cook­ rises some 250 m above the Cooking Lake platform. In ing Lake formation; elsewhere, structure at this level was contrast, the basal reef, typified by the Morinville buildup determined by interpolation using these two control points. (Figures 9 and 10), stands only 100 m above the platform. Local structural relief along the platform, which could have Both buildups are productive. initiated reef growth, can be neither discerned on the seismic section (Figure 11) nor estimated from well control (due to the difficulty in distinguishing the Leduc and Cooking Lake formations on well logs). R.26 W 4 The Cooking Lake formation is overlain by the Leduc formation, where present, and elsewhere by the Duvernay formation. A map of the Leduc-Cooking Lake structure (Figure 9) shows that the northern, western, and eastern edges of the Morinville reef are abrupt, thinning from 100 m to near zero over a distance of about 200 m. In contrast, the southeastern margin appears to thin from 100 m (1-14- 54-26W4) to 50 m (12-12-54-26W4) to near zero over a distance of about 1 km. This southward thinning could be gradational, step-like, or indicative of a second isolated buildup. A wedge-shaped slope would be consistent with a sheltered leeward shelf edge or leeward detrital edge; a step-like thinning could be indicative of episodic backstep- ping phases of reef growth or, perhaps, wave-cut terraces. An alternative explanation is that a second isolated buildup lies to the southeast of the wells in the Morinville D-3B pool (Figure 9). The abrupt northern, eastern, and western edges are possibly representative of reef growth in a high-energy environment. From the data incorporated, the Morinville reef cannot be contoured in such a way as to exhibit a raised rim (Figure 9). Offreef, the Cooking Lake formation is overlain by the relatively high-velocity shales of the Duvernay formation which terminate against the flank of the Morinville reef (Figure 10). The Ireton formation also drapes across the Morinville reef (Figure 10). The top of the Ireton formation is about 20 m high over the reef apex relative to the apparent regional dip, which is determined by interpolation between the two offreef wells, 2-11 and 12-13. In contrast, the sub-Cretaceous unconformity (the Wabamun top) was found to drape across the Morinville reef by only about 10 m. Interpretation of the example seismic section

F ig . 9. Map of the Morinville/St. Albert-Big Lake area and The 48-channel seismic data were recorded in 1983 using a contour map (in meters relative to mean sea level) of the Leduc formation top (or, in its absence, the Cooking Lake P-shooter (60-drop) source (Sheriff, 1984, p. 193), a 1530 m formation top), showing the seismic line (dashed) and the split spread, a 60 m shot spacing, and a 30 m group interval. wells used in the geologic cross-section (solid lines). (For an Figure 11a shows the 12-fold nonmigrated, normal-polarity explanation of well symbols, see Sheriff, 1984, p. 299.) seismic section. In Figure 12, synthetic seismograms for the 1416 SECONDS METERS -1050 -500 formation shaded black. Horizon names are indicated with first letters [cf., (a)] with the exception that L denotes denotes L that exception the with (a)] [cf., letters first with indicated are names Horizon black. shaded formation Leduc and Duvernay, whose reflections merge. reflections whose Duvernay, and Leduc F ig . 11. (a) Normal-polarity seismic section, Morinville study area; (b) blow-up of a portion of (a) with the Leduc Leduc the with (a) of portion a of blow-up (b) area; study Morinville section, seismic Normal-polarity (a)11. 2-11 2-11 F ig . 10. Geologic cross-sectional interpretation, Morinville study area. study Morinville interpretation, cross-sectional Geologic 10. 12-12 12-12 KILOMETERS KILOMETERS Anderson et al et Anderson 1-14 (b) 1-14 1-14 12-13 12-13 12-13 T ‘ IIG SPECKS VIKING :‘ r DUVERNAY . ; OKN LAKE COOKING .; BEAVERHILLLAKE - N EOD WHITE SECOND NISKU CALMAR WABAMUN COOKING LAKE COOKING IRETON DUVERNAY Seismic Study of Low and High Leduc Reefs 1417 1-14-54-26W4 onreef and the 16-11-54-26W4 offreef wells constructive interference beneath the reef as one might (Figure 9) are shown. The 2-D synthetic seismic section expect from the 2-D synthetic (Figure 13). The Duvernay (Figure 13, Table 2) illustrates the contrasting seismic signa­ and Leduc formations have relatively high P-wave veloci­ tures of the basal-reef and offreef facies. ties, about 5300 m/s and 5500 m/s, respectively. The acous­ The deepest reflections labeled on the seismic section tic-impedance contrast between these sediments and the (Figure 11) are the Beaverhill Lake and Cooking Lake underlying Cooking Lake carbonates (Vp - 5900 m/s) is events. The Cooking Lake reflection has relatively low relatively low. Both the Beaverhill Lake and Cooking Lake amplitude both onreef and offreef, and one does not see any events are pulled up by about 15 ms beneath the Morinville

R>ck»r O Phasfl REFL DEPTH VELOCITY 28 ms. 28 Hz COEF KB kft/S Richer O P lu s . REFL DEPTH VELOCITY Revr Norm 28 ms. 28 Hz COEF KB km l% Revr Norm

<•> (b)

Fig. 12. 1-D synthetic seismograms for the Morinville study area, (a) 1-14-54-26W4 onreef (basal-reef) well; (b) 16-11-54-26W4 offreef well.

Illlllllllll [I T h i ITT 111 w . 11

Vi HiUiiiii M > ittM > I Duvornay— Cooking: l aka ii lllililliiilllllli 11! ilillllllll lllilllllllll 111 III Hill llllllhi

Fig. 13. Acoustic model and corresponding 2-D synthetic seismic section, Morinville study area. 1418 Anderson et al. reef (Figure lib). This pullup is due mainly to the lateral associated with the seismic signatures of the example reefs. velocity differences between the Leduc reef (Vp — 5500 m/s) The lower amplitude of the Cooking Lake event beneath the and the ofifreef shales [Vp — 3500 to 5300 m/s (Figure 13, Leduc relative to ofifreef (beneath the Duvernay) is attrib­ Table 2)] over 100 m or more of reef relief and partly to the uted to a corresponding decrease in acoustic-impedance lateral velocity differences resulting from the overdraped contrast along the Cooking Lake horizon (see velocities in Ireton-to-Cretaceous units. Since the Cooking Lake event Tables 1 and 2). Other amplitude variations occur when the approximately parallels the underlying reflections, it is un­ Leduc event interferes with reflections originating within the likely that a significant component of this time-structural overlying shales. Specifically, the tops of the full reefs relief is attributable to real structure because, for example, generate a high-amplitude reflection which constructively the Upper Cretaceous Lea Park event is relatively flat across interferes with the Ireton event, whereas the flanks of the the seismic section (Figure 11a). basal reef at Morinville are effectively masked by the Du­ In our interpretation (Figure 10), the Leduc formation vernay event. protrudes through the Duvernay to the lower Ireton from a The seismic image of the Leduc is also a function of reef point between the 12-12 and 1-14 wells to a point between the height. In the full-reef case, laterally continuous reflections 1-14 and 12-13 wells, corresponding to traces 75 to 105 on characterizing the ofifreef shales terminate abruptly against Figure 11, where the Leduc event is a high-amplitude peak. the reef-flank reflection, the seismic image of the full reef The Leduc in flank positions, where it is overlain by the consisting of low-amplitude laterally discontinuous reflec­ Duvernay, is effectively masked by the high-amplitude Du­ tions bounded by the high-amplitude Leduc and moderate- vernay event. As illustrated in Figures 10 and lib, the amplitude Cooking Lake events. The seismic image of the Duvernay wedges out near the apex of the Morinville reef. basal reef is dominated by the closely spaced Leduc and As a consequence, its reflection merges with the Leduc Cooking Lake events, the former of which could be easily event near the crest of the reef; and here these two events misidentified as a reflection originating within the basal cannot visually be distinguished. shales. As illustrated on the 2-D synthetic (Figure 13) and the field Structural relief relative to the Cooking Lake platform is section (Figure 11), the seismic image of the Morinville reef about 50 to 100 m for the basal reef examples and about 200 is dominated by the high-amplitude Leduc and low-ampli­ m for the full reefs. As a result of the compaction of reef and tude Cooking Lake events, two closely spaced reflections ofifreef sediment, postreef strata drape across the Leduc which mask any internal reflection pattern. Similarly, the formation. The tops of the Ireton formation and Wabamun Duvernay-Cooking Lake interval is dominated by the closely group, for example, drape by about 40 m and 25 m, respec­ spaced high-amplitude Duvernay and low-amplitude Cook­ tively, across the full reef and by about 20 m and 10 m, ing lake events; and any internal reflection pattern is also respectively, across the basal reef (Morinville). This struc­ effectively masked. Consequently, the flank of the reef tural relief is transformed into time-structural relief on the cannot be mapped confidently on the basis of reflection seismic data. Clearly the full reefs with their greater associ­ pattern alone; and, therefore, by necessity, it is estimated ated relief are more confidently interpreted on the seismic based on well control, drape, velocity pullup, and lateral data. changes in the seismic image of the Ireton formation as Greater velocity-generated time-structural relief is simi­ described below. larly associated with the full reefs. For example, the Cooking In Figure 10, the Ireton is seen to thin by about 30 m Lake event is pulled up beneath the full and basal (Morin­ across the Morinville reef. As a result of such thinning, the ville) reefs by 30 ms and 15 ms, respectively. As discussed seismic image (Figure 11) varies laterally, having lower- by Anderson et al. (1988a), the magnitude of velocity­ amplitude reflections onreef than ofifreef, possibly as a result generated relief is a function primarily of the height of the of changes in the overall interference pattern. The Ireton event is time-structurally highest across the apex of the reef, lower across the reef flank, and lowest ofifreef. This relief, Table 2. Names of the units shown in the acoustic model of primarily due to the aforementioned drape, is accentuated by Figure 13, together with the respective velocities and densities used. Reasonable inferred values were used for density since velocity pullup. density logs were not available CONCLUDING DISCUSSION (m/s) (kg/m3) Each of these three reef examples (Leduc-Woodbend atoll and pinnacle and the Morinville basal reef) generates a (1) Cooking Lake 5900 2750 (2) Duvernay 5300 2700 different seismic signature, the characteristics of which can (3) lower Ireton 3700 2650 be related both to the morphology of the buildup and to its (4) middle Ireton 4100 2700 relationship to the surrounding sedimentary units. These (5) upper Ireton 3800 2650 signatures consist of four basic components: lateral phase (6) Ireton 3800-5000 2700 (uppermost) and/or amplitude variations, seismic image, structural relief, (7) Nisku 5800 2750 and velocity-generated relief (Anderson and Brown, 1987). (8) Calmar 4800 2650 Below, the main components are shown to be primarily a (9) Wabamun 5700 2750 function of the height of the reef and, to a lesser degree, its (10) Cretaceous 2700-3800 2650 areal extent. (ID Leduc 5500 2750 Consider, for example, phase and/or amplitude variations (12) Duvemary 5300 2700 Seismic Study of Low and High Leduc Reefs 1419 reef (given the velocity contrast between Leduc carbonates look at the question of lateral velocity variations over Leduc formation and Rainbow member reefs, in Bloy, G. R., and and Ireton shales) and, to a lesser extent, of the magnitude of Charest, M., Eds., Principles and concepts for the exploration of structural drape. reefs in the western Canada basin: Can. Soc. Petr. Geol., Contin. Each of these seismic signature components for the Leduc Educ., Short Course Notes. ------1988b, Geophysical aspects of Wabamun salt distribution in formation is principally a function of the reef height: the southern Alberta: Can. J. Expl. Geophys., 24, 166-178. greater the relief, the more anomalous the seismic signature. Anderson, N. L., White, D., and Hinds, R., 1989, Woodbend group In the present study, the extent of the relief affects (1) the reservoirs, in Anderson, N. L., Hills, L. V., and Cederwall, D. A., Eds., Geophysical atlas of western Canadian hydrocarbon degree of drape, (2) the amount of subreef velocity pullup, pools: Can. Soc. Expl. Geophys./Can. Soc. Petr. Geol., 101-132. (3) the degree of continuity through the reef of reflections Belyea, H. R., 1964, Woodbend, Winterbum, and Wabamun from offreef units and, to some extent, (4) differences in the groups, Part II, chap. 6, in McCrossan, R. G., and Glaister, R. P., Eds., Geological history of western Canada: Alta. Soc. Petr. character of lateral amplitude variations. Since reefs can Geol., 66-88. vary appreciably in height within a localized area, the Bubb, J. N., and Hatlelid, W. G., 1977, Seismic recognition of interpreter should evaluate critically any and all seismic carbonate buildups, in Payton, C. E., Ed., Seismic stratigraphy— applications to hydrocarbon exploration: Am. Assn. Petr. Geol., anomalies present at the reef-target level with a range of Mem. 26, 185-204. carbonate buildups in mind. Davis, T. L., 1972, Velocity variations around Leduc reefs, Alberta: Geophysics, 37, 584-604. ACKNOWLEDGMENTS Klovan, J. E., 1964, Facies analysis of the Redwater reef complex, Alberta, Canada: Bull. Can. Petr. Geol., 12, 1-100. Mitchum, R. M., Jr., Vail, P. R., and Sangree, J. B., 1977a, The two-dimensional seismic models were generated using Stratigraphic interpretation of seismic reflection patterns in dep- Geophysical Micro Computer Applications (International) ositional sequences, in Payton, C. E., Ed., Seismic stratigraphy— Ltd. (GMA) software. The seismic lines from the Leduc- applications to hydrocarbon exploration: Am. Assn. Petr. Geol., Mem. 26, 117-133. Woodbend area and from the Morinville area were provided Mitchum, R. M., Jr., Vail, P. R., and Thompson, S., Ill, 1977b, The by Pulse 87 (Calgary) and CGG Geophysics (Calgary), depositional sequence as a basic unit for stratigraphic analysis, in Payton, C. E., Ed., Seismic stratigraphy—applications to hydro­ respectively. These data were donated originally for use in carbon exploration: Am. Assn. Petr. Geol., Mem. 26, 53—62. the Geophysical Atlas of Western Canadian Hydrocarbon Mitchum, R. M., Jr., and Uliana, M. A., 1985, Seismic stratigraphy Pools and were processed by Veritas Seismic (Calgary) and of carbonate depositional sequences, Upper Jurassic-Lower Cre­ taceous, Neuquen basin, Argentina, in Berg, O. R., and Woolv- CGG Geophysics (Calgary), respectively. Well logs were erton, D. G., Eds., Seismic stratigraphy II: An integrated ap­ obtained from the microfiche well-log library at the Univer­ proach to hydrocarbon exploration: Am. Assn. Petr. Geol., Mem. sity of Calgary, Department of Geology and Geophysics, 39, 255-274. Mossop, G. D., 1972, Origin of the peripheral rim, Redwater reef, with the assistance of Ms. Doris Johnston. We would also Alberta: Bull. Can. Soc. Petr. Geol., 20, 238-280. like to thank Mr. C. M. Crous of BP Canada Ltd. for Mountjoy, E., 1980, Some questions about the development of Upper Devonian carbonate buildups (reefs), western Canada: reviewing the text and providing valuable suggestions. Sup­ Bull. Can. Soc. Petr. Geol., 28, 315-344. port from the Natural Science and Engineering Research O’Connor, M. J., and Gretener, P. E., 1974, Differential compaction Council of Canada was provided through an operating grant within the Woodbend Group of central Alberta: Bull. Can. Soc. Petr. Geol., 22, 269-304. to R. J. Brown. Pallister, A. E., 1965, Application of seismic techniques to reef traps: Univ. of Alberta, Calgary (now Univ. of Calgary), Div. REFERENCES Contin. Educ. Sheriff, R. 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