Reinterpretation of the Upper Devonian-Lower Mississippian Bakken Formation of Subsurface Saskatchewan: Integrating Sedimentologic and Ichnologic Data Within a Sequence Stratigraphic Framework

Solange Angulo¹, Luis A. Buatois¹, and Steve Halabura^2
1Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada
²North Rim Exploration Ltd, Saskatoon, SK, Canada

The Bakken Formation (Upper Devonian - Lower Mississippian) of the Williston Basin, Canada, is one of the most significant hydrocarbon reservoirs in Southeastern Saskatchewan and represents a perfect hydrocarbon system - from source rock to trap within the same formation. This unit is subdivided into three members: the lower and upper members (shelf black shale) and the sandy-silty middle member (interpreted previously as open marine). Based on core analysis, a new interpretation for the middle member is proposed. An open marine progradational parasequence is interpreted from the lower member to the lower interval of the middle member. The parasequence consists of shelf black shale, lower to upper offshore siltstone interbedded with thin layers of silty very fine-grained sandstone, offshore transition regularly interbedded siltstone and very fine-grained sandstone and lower shoreface very fine-grained sandstone. With the exception of the anoxic to dysaerobic shale, these open marine deposits are intensely bioturbated, containing Phycosiphon, Nereites missouriensis, Planolites, Asterosoma, Chondrites and Teichichnus. Above this succession, an erosive-based high-angle planar cross-stratified sandstone is deposited, followed by thinly interlaminated sandstone and siltstone. A decrease in bioturbation index, the small size of trace fossils, low ichnodiversity, syneresis cracks and mud drapes suggest tidal influence and brackish-water. Above, a transgressive lag occurs and open marine conditions are re-established with deposition of upper offshore interbedded highly bioturbated siltstone and microhummocky cross-stratified sandstone, followed by the shelf black shale of the upper member. Integration of sedimentologic and ichnologic data within a sequence stratigraphic framework is essential to provide a more robust depositional model for the Bakken Formation.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas