Bulletin 018

Bulletin 018

BUL 018

Lithology of the Athabasca Oil Sands

Author(s) Carrigy, M.A. Date 1966-01-01

Coarse grained quartzose oil sand with a conspicuous clay matrix cemented in places by quartz and goethite lie at the base of the McMurray Formation and are probably pre-McMurray in age.

Most of the lithologic features observable in the Athabasca Oil Sands can be described to a succession of depositional environments that developed as a consequence of an extensive marine transgression in early Cretaceous time. The lower part of the McMurray Formation consists of fluviatile deposits and the middle and upper parts of foreset and topset beds of an ancient delta. The parts of the McMurray Formation observable in outcrop on the lower Athabasca River and its tributaries are the deltaic plain deposits; the submarine portion of this ancient delta should be found northwest of Fort McMurray beneath the Birch Mountains. The lower Clearwater Formation consists of nearshore continental shelf sediments deposited as the shoreline moved across the area formerly occupied by the delta.

The heavy minerals of the McMurray Formation were derived from a single provenance underlain by metamorphic and igneous rocks. The heavy minerals are distributed among the lithofacies of the McMurray Formation according to their grain size and shape at the source. Thus, coarse-grained sands contain an assemblage dominated by garnet, kyanite, tourmaline and staurolite; medium grained sands have mostly tourmaline and zircon; in very fine grained sands and silk, chloritoid tourmaline and zircon are most abundant.

The distribution of oil within the Athabasca Oil Sands is controlled by the petrographic properties of the reservoir rocks. The porosity is intergranular and was established by the sorting action of the traction currents and the winnowing action of oscillatory current at the time of deposition; it has been modified only locally by compaction and cementation.

Most of the oil in the Athabasca Oil Sands is contained in the fluviatile sands and in the fine-grained micaceous sands of the foreset beds of the ancient delta of the McMurray Formation. So pockets of clean sand in argillaceous beds produced by the action of a soft-bodied bottom fauna are commonly impregnated with heavy oil. Post-Cretaceous uplift and subsequent dissection of the reservoir by streams and the collapse of large areas due to salt removal from underlying strata have failed to induce secondary migration in the reservoir except on a local scale at outcrop faces.

Carrigy, M.A. (1966): Lithology of the Athabasca Oil Sands; Research Council of Alberta, RCA/AGS Bulletin 18, 55 p.