Sedimentological and stratigraphic characterization of Cretaceous upper McMurray deposits in the southern Athabasca oil sands, Alberta, Canada

Abstract

The Lower Cretaceous McMurray Formation located in northeastern Alberta, Canada, is host to one of the largest bitumen resources in the world. In this paper, we provide an integrated sedimentological and stratigraphic analysis of the upper McMurray member located in the Pike and Jackfish project areas (Townships 73–76 and Ranges 4–7 W4M) of the southern Athabasca oil sands. Using core data, geophysical well logs, and seismic data, two main facies associations (embayment and fluvial) are evident. The embayment deposits were likely sourced by up-dip distributary channels and are commonly expressed in the subsurface as a series of coarsening-upward parasequences. The facies expressions in core range from bioturbated mudstones to wave-rippled sandstones. Noticeable, however, is the fact that there are areas where strata within the parasequences are instead substituted with sand, inclined heterolithic stratification, and mud. These deposits, in the form of fluvial channels with brackish-water overprint, occur at three different parasequence horizons within the study area. The removal of these embayment deposits is believed to be initiated, in part, by halite dissolution of the Middle Devonian Prairie Evaporite Formation deposits during upper McMurray deposition. Notably, the halite dissolution edge on the eastern half of the study area is believed to have created a subregional shelf–slope break during upper McMurray deposition. As such, the distributary channels likely re-equilibrated themselves to the slope created by the salt dissolution edge and thus proceeded to incise into previously deposited upper McMurray parasequence and underlying middle McMurray deposits.