ABSTRACT

Analyses of cores and wireline logs from 28 wells within an eight-township area demonstrate that the Lower Cretaceous McMurray Formation consists of three fundamental progradational depositional units bounded by time-stratigraphic markers. Within this framework the sequences of sedimentary textures and structures indicate the gross deltaic facies change laterally southwestward in a progressive fashion from a fluvially dominated distributary megafacies consisting of distributary channel, stream-mouth bar and estuarine-type sands, to wave-reworked and bioturbated beach-shoreface sands, and eventually to offshore-transition siltstones and shales. The depositional sand axes in each stratigraphic unit are offset from those in older units because of the influence of differential compaction on sedimentation patterns.

The reservoir quality and continuity of the oil-bearing sands are intimately related to the environmental facies. Statistical analyses show the distributary-type sands are coarser-grained, thicker-bedded, and have a lower percentage of muddy matrix and higher oil saturations than any of the other identified facies types. Maximum oil-sand development occurs in the Middle Unit, which has the highest proportion of distributary facies.

Petrographic and X-ray analyses reveal that the McMurray oil sands in the Athabasca area have a grain-and-matrix composition that is significantly different from those in the Cretaceous sands of the Cold Lake and Peace River heavy-oil areas. The clay/silt ratio in the matrix fines of the quartzose McMurray sands is about one-fifth that found in the lithic-feldspathic and chert-rich sands of the other two areas, and the percentage of swelling clays is insignificant.

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