Abstract: New Geologic Framework for Mining of Oil Sands

R. P. Sinha

Syncrude Canada Ltd. is committed to produce eventually 125,000 bbl/day of crude oil by mining oil sands. The heavy oil, known as bitumen, is confined in the northern Alberta area to the McMurray Formation of Cretaceous age. Lateral and vertical variations of the bitumen distribution are significant within the formation and, therefore, it is essential to know and to maintain the quality of the oil sands being mined.

The bitumen concentration is related to the various lithofacies recognized within the McMurray Formation. These lithofacies have been grouped in three major zones. The bitumen concentration is fairly consistent within each zone, but varies considerably from one zone to another. These zones are, from the base up, (1) basal clay zone, (2) main oil-sand zone, and (3) lean oil-sand zone.

The basal clay zone is as much as 50 ft (15 m) thick and fills depressions in the Devonian erosional surface. The clays were deposited in a nearshore, marshy, lagoonal environment. The overall bitumen saturation is very low and the whole unit has been outlined as a "reject" for mining purposes.

The main oil-sand zone overlies the basal zone and consists of two main units: (1) the fluvial-sand unit and (2) the deltaic unit. The fluvial sands are well-sorted point-bar deposits and average about 50 ft (15 m) in thickness. The deltaic unit is essentially estuarine, nearshore-distributary or stream-mouth bar deposits. These are approximately 60 ft (18 m) thick and overlie the fluvial sands. The bitumen saturation of both the fluvial and deltaic units is high and together they constitute the main orebody which is more than 100 ft (30 m) thick at the Syncrude mine.

The uppermost unit is the lean oil-sand zone which consists essentially of offshore deposits and represents transition from deltaic to marine environment. It consists of interbedded sand, silt, and clay. Owing to the argillaceous nature of this zone, the bitumen content is low and the unit is outlined as a "reject."

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma