Abstract

The Lower Cretaceous McMurray/Wabiskaw stratigraphic interval contains approximately 143 × 10⁹ m³ (902 × 10⁹ barrels) of bitumen in the Athabasca Oil Sands Area, northeastern Alberta, and of this, about 24 × 10⁹ m³ (152 × 10⁹ barrels) occurs in the surface mineable area. As the McMurray Formation and Wabiskaw Member have never been buried to significant depths, diagenesis is minimal and the quality of the resource directly reflects the depositional history of the host sediments. Therefore, oil sands at surface mineable and in situ depths represent an enormous resource in which optimal development will only be attained with a detailed geological model.

Outcrops of the McMurray Formation along the Steepbank River expose a variety of fining upward successions that have been interpreted as large estuarine channel deposits, which are commonly overlain by smaller channel and brackish bay deposits. Remnants of pre-existing fresh water sediments indicate that the estuary developed when the sea transgressed or ‘back stepped’onto a fluvial system. Outcrops along the High Hill and MacKay Rivers expose open estuarine deposits that form very high grade reservoirs where the sands are bitumen saturated. Smaller channel and brackish bay deposits are also present at these outcrops. In outcrops along the MacKay River, a different style of channel sedimentation (vertical accretion) is exposed.

The outcrop data can be synthesized into an overall estuarine depositional model that accounts for much of the reservoir variability and gives predictive power to such parameters as oil sands grade, lateral continuity and geotechnical stability. The complex reservoir architecture necessitates very close spacing for delineation wells to optimize bitumen recovery for both mining and in situ projects.