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Complex Cretaceous Shorelines Existed During Deposition of the McMurray Formation of Alberta, Canada as Evidenced by Bedrock-Controlled Estuaries and Associated Flood Tidal Deltas in the Upper McMurray


The Lower Cretaceous (Aptian) McMurray Formation is the primary reservoir in the Athabasca Oil Sands of Alberta, Canada. Sandbodies found at depth which contain heavy oil require production by Steam Assisted Gravity Drainage (SAGD). This necessitates an acute understanding of sandbody geometries and shale distribution to recognize barriers and baffles to flow. The Upper McMurray, the youngest of three informal units of the McMurray Formation, is generally interpreted as deposits of embayed coastal systems; however, this study is the first to document sandy estuarine deposits including flood tidal deltas. An under-studied area of the Athabasca Oil Sands region located 80 km northwest of Ft. McMurray, AB, Canada off the main play fairway preserves what we believe to be a 20-30 m succession of Upper McMurray estuarine and flood tidal delta deposits overlying Middle McMurray fluvial sands and the unconformity above the Devonian basement. Eight cores collected along a 20 km transect were described. Facies, sedimentary structures, surfaces, trace fossils, bitumen saturation, and lateral and vertical facies changes were documented in order to identify depositional environments and determine the geometries of sandbodies and shales. Six petrophysically distinct facies were defined and grouped into facies associations (FAs) that include flood tidal deltas, tidal flats, tidal channels, central estuary, and middle to upper shoreface deposits. Our data indicate that a flood tidal delta FA built landward of a bedrock-controlled (Devonian limestone) estuary inlet. This complex was ultimately overlain during a transgression by a marine shoreface prior to deposition of the overlying, fully marine Wabiskaw Formation. Our results also indicate that the heavily bioturbated tidal flat FA is a barrier to flow for SAGD production when encasing flood tidal delta sandbodies, as the encased sands were devoid of bitumen. The ichnological assemblage combined with sedimentary structures was key to defining paleoenvironments. Ultimately, flood tidal deltas and associated estuarine FAs of the Upper McMurray may be unique because of their association with a stationary, bedrock-controlled inlet to the east and a larger secondary opening to marine waters to the northwest. This combination of bedrock barriers and additional inlets caused the preservation of 30 m of estuarine deposits that retained a slightly more marine appearance based on the ichnological study.