Facies Analysis of Upper Devonian Wabamun Group in West-Central Alberta, Canada

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 part of the Rocky Mountains of Alberta. East of Leduc and north of Stettler, the Wabamun has been irregularly truncated by pre-Cretaceous erosion. For purposes of facies analysis the Wabamun is subdivided into four units: (1) basal unit including the lowermost evaporitic interval and lateral marine carbonate equivalents; (2) lower middle Wabamun representing the overlying interval to the top of the Crossfield dolomite in the Calgary area; (3) upper middle Wabamun including post-Crossfield and pre-Big Valley beds; and (4) upper Wabamun representing the Big Valley formation and approximate lateral equivalents. The evaporites and dolomites are dominant 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. Evaporites and associated extremely fine-textured dolomites exhibit most widespread distribution in the basal Wabamun. The main evaporites extend northwest through Stettler to Leduc whereas the extremely fine-textured dolomites also occur farther northwest. In the middle Wabamun, the evaporites terminate a relatively short distance northwest and north of Stettler because of probable removal by solution and pre-Cretaceous erosion. Presumably middle Wabamun evaporites originally extended north beyond their present limit. 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. Limestones range from calcilutites to calcarenites and exhibit incipient dolomitization that commonly imparts a mottled appearance to the rock. The calcarenites are generally of the pseudo-oolitic or pelletoid type, although bioclastic limestone is characteristic of the uppermost, and locally, of lower beds. The pelletoid limestones and calcilutites were deposited on a widespread bank on which slight restriction of the shallow waters promoted precipitation of calcium carbonate. Anhydrites and cryptocrystalline dolomite were precipitated in the Stettler-Drumheller area near the bank margin at the east, and the associated calcareous muds were diagenetically dolomitized to microsucrosic dolomite. Coarser (very fine to medium crystalline) dolomites are characteristic of a belt between the limestone province at the northwest and evaporites at the southeast. Local vuggy dolomite sections in this belt are interpreted to be possible reefs. 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 regional dip (mainly eastward) is responsible for gas entrapment in the Okotoks-East Calgary-Olds trend of fields. In the Edmonton area, oil or gas has been localized in erosional hills made up of porous Wabamun dolomites at the pre-Cretaceous unconformity.