Abstract

The examination of over 400 well logs and 34 cores has provided sufficient data to establish the geological setting, lithofacies patterns and depositional history of the McMurray Formation and Wabiskaw Member of the Clearwater Formation in the central region (Townships 80 to 90, Ranges 1 to 18W4) of the Athabasca Oil Sands Area.

The principal control on sand distribution in central Athabasca during the Lower Cretaceous was the topography of the underlying sub-Cretaceous unconformity. This surface is generally low in the east and higher in the west toward the Paleozoic Grosmont High. The McMurray Formation displays a corresponding thinning, from greater than 100 m in the east to less than 5 m in the western part of the study area. A number of Paleozoic ridges controlled McMurray Formation sedimentation styles and can be used to divide the study area into an eastern region with two unequal valley systems and a western region. Two cored wells, one from the more prominent or main valley system and one from the western region provide examples of some of the sedimentological features within each region.

The eastern region is dominated by channelized McMurray Formation sands occurring within two valley systems. Deposition within the valley systems reflects transgressive valley-fill sedimentation. Amalgamated fluvial channel and off channel deposits grade upward into muddy, brackish water deposits. Marine Wabiskaw Member siltstones and mudstones blanket the eastern valleys.

The western region shows no influence of the eastern valley systems. Its strata reflect abrupt changes from marine or brackish water to continental deposits that probably accumulated within a bay fill setting. Here, thin tabular sands were developed in the upper part of the McMurray Formation, and thicker, laterally extensive sands in the Wabiskaw Member.