Abstract

The Athabasca tar sands of northern Alberta underlie an area greater than 26,000 sq km and contain in excess of 700 × 109 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 in the vicinity of Fort McMurray. The middle and upper members of the reservoir consist of many small deltas of modest thickness (less than 30 m) built by rivers flowing north into a lake or lagoon which occupied an elongated depression formed by extensive salt removal from evaporite beds beneath the Devonian limestone that underlies the tar sands. Lithofacies associated with the changing environments of deposition during the infilling of this depression include successively fluvial sand; delta-front sand and silt; delta-plalform sand; and silt, clay, and nearshore marine sand.

Fluvial sand.—This medium- to coarse-grained, well-sorted sand, composed of quartz, k-feldspar, and muscovite, is present in the basal part of the McMurray Formation where it forms lenticular sand bodies (2-7 m thick) containing sets of medium-scale (25 cm), high-angle, cross-beds. Included in these sand bodies are shale chips, mummified logs, and abundant comminuted carbon. Grain size is uniform throughout and the upper and the lower contacts are sharp. There is little mineral cement, and there is little evidence of animal activity.

Delta-front sand.—This fine-grained, well-sorted, sand, composed of quartz, k-feldspar, and muscovite, is present in the middle McMurray Formation as a large, low-angle (5-7°), cross-stratified unit 20-30 m thick. Individual beds 10-20 cm thick are separated by discontinuous, thin (5-100-mm) silt layers. Internal structures include micro-cross-laminations, burrows, and “castings.” No megafossils are present but silt beds contain an abundant microflora of pollen and spores. The lower contact is usually sharp and the upper contact is transitional. Within this unit there is a gradual and uniform increase in “shaliness” from base to top and mineral cement is rare.

Delta-platform sand.—This fine-grained sand and silt unit, composed of quartz, muscovite, and k-feldspar, is present in the upper part of the McMurray Formation forming a widespread unit, 7-10 m thick, composed of horizontally bedded silt and sand laminae. The lower contact is transitional with the underlying delta-front unit but the upper contact with the nearshore marine sand of the Clearwater Formation is sharp. Siderite and calcite-cemented beds are common within this unit which also contains lenticular beds of brackish-water mollusks. Also present in this unit is a sparse fauna of agglutinated foraminifers together with fish teeth, and a microflora of spores, pollen, and microplankton.

Nearshore marine sand.—This medium-grained sand is clean in places but mainly is a poorly-sorted mixture of sand, silt, and clay composed of quartz, chert, and glauconite. This sand forms a widespread marker bed, 5-10 m thick at the base of the Clearwater Formation, and is known as the Wabiskaw Member. Bedding in this unit has been virtually destroyed by animal activity and the most conspicuous internal features are burrowing structures filled with clean sand. A rich microfauna is found in this unit consisting inter alia of calcareous Foraminifera, Radiolaria, sponge spicules, and hystrichospherids. Lower and upper contacts are sharp with a uniform upward increase in the sand to matrix ratio.

The distribution of oil within the tar-sands reservoir is controlled by the original porosity and permeability of the sediments. Thus the maximum amount of oil (18-20% by weight) is present in well-sorted, clean sands which are most commonly of fluvial origin. No oil is present in the poorly-sorted, argillaceous parts of the nearshore marine sand, or in the silt beds within the deltaic complex.

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