Stratigraphy of the Middle Devonian Keg River and Prairie Evaporite formations, northeast Alberta, Canada

Abstract

Northeast Alberta is the location of one of the world’s largest petroleum deposits, the Athabasca Oil Sands, hosted in Lower Cretaceous McMurray Formation sandstones. The McMurray Formation was deposited on a deeply eroded Upper Devonian unconformity surface that was strongly influenced by underlying carbonate and evaporite units. Development of the oil sands has resulted in a significant increase in new data related to the underlying Devonian of northeast Alberta. In particular, the requirement for deep disposal of water has resulted in a number of wells being drilled into and through the porous dolomites of the Middle Devonian Keg River Formation that is a few hundred metres below the McMurray bitumen deposits. In addition, the acquisition of large areas of high-resolution 3D seismic data to assist the delineation and development of the oil sands reserves has provided powerful insights into the depositional patterns and later dissolutive geometries of the underlying carbonate and evaporite deposits. The Keg River and overlying Prairie Evaporite formations that are the focus of this paper were deposited in the intracratonic Elk Point Basin that covered most of the area now delineated by the Western Canada Sedimentary Basin. These units and their lateral equivalents have been extensively studied in Saskatchewan, northwest Alberta, southern Northwest Territories and northeast British Columbia. In contrast, the two formations have been lightly studied and rarely published upon in northeast Alberta. The current detailed study of several areas with abundant new data, combined with a review of the regional dataset in northeast Alberta has provided new insights into the geology of these units. A revised stratigraphy for the two formations is presented which includes the formal definition of the proposed Aurora Member, a significant anhydrite unit within the Prairie Evaporite Formation that was previously included as a sub-unit of the Whitkow Member. The Keg River Formation is divided into the Lower and Upper Keg River members. The Lower Keg River Member is a thin, regionally consistent unit composed of an upward deepening transgressive systems tract. In most places, the Lower Keg River is overlain by the Upper Keg River Member which has two distinct facies. At the base is a widespread shallowing upwards microbial oncolitic facies, here referred to as the Keg River Ramp, which provided the foundation for more aerially restricted overlying deposits of shallow water carbonate. On the edge of the basin, a Basin Margin Platform formed and prograded out over the Keg River Ramp. In the centre of the basin, biogenic build-ups developed. Integrated interpretation of the seismic with the wells shows that the build-ups often have well established but aerially restricted reef margins of frame-building organisms. However, the bulk of these build-ups consist of loosely consolidated peloidal grainstones and packstones. A drop in sea level in conjunction with the development of a continuous barrier reef trend, the Presqu’ile Barrier, across the mouth of the Elk Point Basin caused restriction of the basin resulting in widespread laminites forming at the transitional contact between the Keg River and overlying Prairie Evaporite formations. As sea-level continued to drop, evaporitic drawdown converted the Elk Point Basin into a vast inland saline sea with an exposed marginal platform and emergent carbonate build-ups. The Prairie Evaporite Formation deposits of dolomite, anhydrite, gypsum, halite and potash were deposited in and around this sea. During this evaporitic phase almost all of the underlying Keg River limestone was converted to dolomite. The dolomitization resulted in the development of excellent porosity in most of the Keg River strata. The Prairie Evaporite is divided into two major brining upward cycles of deposition. The lower Prairie Evaporite cycle began with anhydrite and gypsum deposits forming adjacent to the exposed Keg River build-ups. These anhydrite units have been recognized and described in other parts of the Elk Point Basin but have never been formally defined stratigraphically. The paper provides detailed description and stratigraphic definition for this unit as the proposed Aurora Member. In the deeper parts of the basins between the reefs, evaporitic laminites accumulated. Eventually, the brines became supersaline and the remaining topography between the reefs was filled up with the Whitkow Member which consists mainly of halite. The upper Prairie Evaporite brining upward cycle began when a renewed rise in sea-level breached the Presqu’ile Barrier and allowed mesohaline brine to flood back across the basin. Continuing sea-level rise allowed reef growth to re-establish on top of the barrier which again led to restricted marine water flow into the basin. The vertical aggradation of the Presqu’ile Barrier as sea-level rose provided the accommodation space for the upper Prairie Evaporite cycle. The anhydrite portion of the cycle is represented by the widespread Shell Lake Member which transitions upward into the Leofnard Member which is composed predominantly of halite. In northeast Alberta, the Prairie Evaporite is capped by a thin clastic unit and the cycle of deposition is terminated by a significant unconformity at the base of the Watt Mountain Formation.