Abstract

The Lower Cretaceous McMurray/Wabiskaw stratigraphic interval contains approximately 142 × 10⁹ m³ (892 × 10⁹ bbls) of bitumen in the Athabasca Oil Sands Area, with the McMurray Formation containing most of the reserves. However, in the western portion of the area (Townships 70-90, Ranges 19W4-5W5), the Wabiskaw Member of the Clearwater Formation contains the bulk of the resources with an estimated 6 × 10⁹ m³ (38 × 10⁹ bbls) of bitumen reserves and significant gas accumulations.

Prominent topographic features in Athabasca West, including the north trending Grosmont High, east trending Wainwright Ridge, and the intervening Southern and Central sub basins, played a major role in controlling the thickness and sand distribution in the McMurray Formation and Wabiskaw Member. The Grosmont and Wainwright highs acted as effective barriers to the major, northward flowing drainage system in the eastern part of the Athabasca area. As a result, the McMurray Formation in Athabasca West is generally thin and argillaceous, with poor reservoir potential. In contrast, the Wabiskaw Member consists of relatively continuous, sand dominated units associated with paleoshorelines that developed adjacent to the paleohighs.

A new sequence stratigraphic framework, which is applicable to the entire Athabasca area, was implemented in this study. This stratigraphic scheme consists of two types of regionally correlatable surfaces: E surfaces, which are erosional surfaces with pronounced relief and T surfaces, which are transgressive erosional surfaces with low relief. The new stratigraphic scheme provides the basis for a consistent oil sands database, fundamental to regional resource assessment, and facilitates detailed reservoir continuity studies.

The Wabiskaw Member is subdivided into the Wabiskaw D, C, B and A intervals, from the base up, respectively. The Wabiskaw D interval (E10 to T10.5), a newly defined unit situated in the northern portion of the study area, consists of a blocky to fining upward estuarine valley fill which attains a cumulative sand thickness of 10 m. The Wabiskaw C (T10.5 to T11), B (T11 to T15) and A (T15 to T21 or the Clearwater/Wabiskaw) intervals are characterized by relatively well defined coarsening upward successions that were deposited by progradational shorelines. The cumulative sand thickness of these intervals reaches 22 m within the Central and Southern sub basins.

In the Southern sub basin, the McMurray Formation and Wabiskaw Member are relatively thick but the sands are water saturated. Oil sands in the Wabiskaw C, B and A intervals are situated in the Central sub basin whereas oil sands in the D interval occur in the northeast part of the study area. Most of the gas sands are associated with the Wainwright Ridge that separates the Central and Southern sub basins.