ABSTRACT: Trapping Mechanism and Time of Oil Migration in the Athabasca Oil Sands Deposit

RANGER, MICHAEL J., and GEORGE PEMBERTON, University of Alberta, Edmonton, Alberta, Canada

A discrete bitumen-water contact exists under much of the eastern and southwestern portion of the Athabasca oil sands deposit. This contact appears to be a single continuous horizon. The bitumen (6-10 degrees API) is highly viscous and has a density similar to the formation water. As a consequence, the bitumen is essentially "frozen" in position, preserving the structural attitude of the trap at the time of degradation of light oil to bitumen. Mapping the bitumen-water interface has generated new evidence regarding the time of migration and degradation of the Athabasca bitumen and clarifies the regional trapping mechanism. It also puts constraints on the migration mechanism itself.

The present-day bitumen-water interface has a regional dip to the southwest as a result of tectonic development of the Rocky Mountains (Laramide orogeny). Assuming that the interface was originally horizontal, and applying what is known about the timing of basin tilting, it can be concluded that migration, trapping, and degradation must have occurred very early in Laramide time; i.e., late Upper Cretaceous. Little or no oil accumulation could have occurred in the Tertiary, otherwise the dip of the bitumen-water interface would be closer to horizontal today.

Restoring the structure of the Athabasca basin to remove the southwest dip reveals a broad regional anticlinal trap, which fully accounts for the trapping mechanism in the southern part of the deposit. North of Fort McMurray (Township 89), bitumen extends eastward beyond the structural closure of the anticline, but a simple stratigraphic mechanism can easily explain the trap: During early Clearwater sea level highstand, overlying shales overstepped the reservoir sands, sealing them by onlap onto Devonian carbonates and the Precambrian shield.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)