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The AAPG/Datapages Combined Publications Database

CSPG Bulletin

Abstract


Journal of the Alberta Society of Petroleum Geologists
Vol. 7 (1959), No. 8. (August), Pages 188-189

Ninth Annual Field Conference Abstracts

Facies Analysis of Upper Devonian Wabamun Group in West Central Alberta [Abstract]

John M. Andrichuk

ABSTRACT

The Wabamun group, consisting mainly of carbonate rocks and anhydrite, increases in thickness westward from less than 500 feet in the Leduc and Stettler areas to approximately 1,800 feet in parts of the Rocky Mountains of Alberta where it is known as the Palliser formation. East of Leduc and north of Stettler the Wabamun has been partially or completely removed by pre-Cretaceous erosion.

For purpose of facies analysis the Wabamun is subdivided into four units that are believed to represent distinct rock sequences in parts of the area. These are basal, lower-middle, upper-middle and upper Wabamun. Because satisfactory marker beds occur only at the base and top, and not within the group, an operational subdivision is employed based on the assumption that thickness variations are primarily the result of differential subsidence during deposition (excluding effects of later solution of evaporites). Thus, within any given time interval the thickness of rocks deposited should exhibit constant proportion to the overall Wabamun thickness. The basal Wabamun includes the lowermost evaporitic interval and lateral marine carbonate equivalents. The lower-middle Wabamun represents the overlying interval to the top of the "Crossfield member" in the Calgary area. The upper middle Wabamun includes post-"Crossfield" and pre-Big Valley beds; the upper Wabamun represents the Big Valley formation and approximate lateral equivalents.

Three main facies provinces are recognized, namely, evaporites and dolomites in the southeastern part of the map-area, limestones in the northwestern part (and locally at the southwest), and fine to coarse crystalline dolomite in the intervening area. The microcrystalline and earthy dolomites are interpreted to be of secondary or diagenetic origin whereas the cryptocrystalline translucent dolomites are considered to be of probable primary origin. The limestones range from calcilutites to calcarenites and exhibit incipient dolomitization that commonly imparts a mottled appearance to the

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rock. The carcarenites are generally of the pseudo-oolitic or pelletoid type in most of the Wabamun; bioclastic limestone is characteristic of the uppermost unit and locally of the lower beds.

Evaporites and associated extremely fine-textured dolomites represent the most widespread distribution in the basal Wabamun. The main evaporites extend northwest through Stettler to Leduc whereas the extremely fine-textured dolomites occur farther northwest. In the middle Wabamun, the evaporites terminate a relatively short distance northwest and north of Stettler. Originally these evaporites may have extended north beyond their present limit but were probably removed by solution and pre-Cretaceous erosion. During deposition of the basal and upper Wabamun units, a subsidiary evaporite province occurred in the Foothills belt west of Calgary and may have extended north as far as the Clearwater River.

Beales (1956) originally noted the similarity of Palliser limestones of southwestern Alberta to those presently being deposited on the Bahama Banks. Pelletoid limestones are also characteristic of the Wabamun to the northwest of the Conference area, and although dolomitization has destroyed or obscured the original limestone texture in other sections, relicts of calcarenite texture in some dolomites suggest that widespread bank conditions existed in the Wabamun sea. Sutterlin (1958) described the mechanism by which these rapidly growing banks restricted free circulation of marine waters eastward, causing evaporite precipitation in the Stettler-Drumheller area.

Petroleum has accumulated in two main types of stratigraphically controlled traps within the Wabamun group. Facies change from porous secondary dolomites to relatively non-permeable equivalents approximately up-dip regionally (mainly eastward) is responsible for gas entrapment in the Okotoks-East Calgary-Olds trend of fields. In the Edmonton area, oil and gas are localized in erosional hills made up of porous Wabamun dolomites at the pre-Cretaceous unconformity.

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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES

Andrichuk and Edie, Consultants, Calgary

Copyright © 2004 by The Society of Canadian Petroleum Geologists. All Rights Reserved.