David R. Taylor
Oil sand deposits of the Clearwater Formation in northeastern Alberta, Canada, are estimated to have reserves of 11 billion m3 of bitumen in
place. From examination of core data, ten sedimentary facies were defined that are interpreted to have been deposited in two specific depositional settings: deltaic and foreshore/shoreface. Deltaic sediments are predominantly sandstones deposited in distributary-channel, inner distributary-mouth bar, outer distributary-mouth bar, and delta-front environments. Sandstone-body geometries and lateral facies associations suggest that the deltas were fluvially dominated and prograded northward into an embayment of the Clearwater Sea. Marginal to the main delta lobes are foreshore/shoreface deposits that change facies northward into interbedded sandstones and shales of the offshore transition zone.
Deltaic and foreshore/shoreface deposits often display thickening-and/or coarsening-upward trends. These trends indicate shoreline progradation, and each represents an individual parasequence within the Clearwater Formation. Parasequences can be correlated throughout the deposit at Cold Lake and these commonly stack aggradationally in parasequence sets. This aggradational stacking pattern represents the lowstand systems tract within a depositional sequence. The lowstand systems tracts of two high-frequency sequences comprise the majority of the Cold Lake oil-sand deposit. Aggradational parasequence sets develop at the mouths of incised paleovalleys, the lower confining surface of which is a well-defined sequence boundary. Sequence boundaries were recognized within the Clearwater Forma ion by erosional truncation of underlying parasequences, onlap of flooding surfaces, and a basinward shift in depositional environments.
Paleogeographic mapping of depositional environments within individual sequences and parasequences is critical to understanding the distribution of thick, bitumen-saturated, reservoir-quality sandstones and provides a firm geological framework for reservoir engineering simulation modeling. Outer distributary-mouth bar deposits have the best reservoir quality due to a high sandstone:shale ratio. The most prospective areas for production and future development occur where outer distributary-mouth bar sandstones are stacked vertically within parasequence sets.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990