Deltaic Sedimentation in Athabasca Tar Sands

Abstract

The Athabasca tar sands of northern Alberta underlie an area greater than 26,000 sq km and contain more than 700 billion bbl of heavy oil. Excellent outcrops of the oil-impregnated Lower Cretaceous strata (McMurray and Clearwater Formations) are present in the valley of the Athabasca River and its tributaries near Fort McMurray. The middle and upper members of the reservoir consist of many small deltas of limited thickness (less than 30 m) built by rivers flowing northward into a lake or lagoon. The water occupied an elongated depression formed by extensive salt removal from evaporite beds beneath the Devonian limestone which underlies the tar sands. Lithofacies associated with the changing environments of deposition during the infilling of this depression include, succes ively, fluvial sand; delta-front sand and silt; deltaic-platform sand, silt, and clay; and nearshore marine sand. The distribution of oil within the tar-sands reservoir is strictly controlled by the original porosity and permeability of the sediments. Thus, the maximum amount of oil (18-20 percent by weight) is present in well-sorted, clean sands that are most commonly of fluvial origin. No oil is present in the poorly sorted, argillaceous parts of the nearshore marine sands, or in the silt beds within the deltaic complex.