Reinterpretation of the Upper Devonian-Lower Mississippian Bakken Formation of Subsurface Saskatchewan: Integrating Sedimentologic and Ichnologic Data within a Sequence-Stratigraphic Framework
The Upper Devonian-Lower Mississippian Bakken Formation is one of the most prominent oil-producing units in Saskatchewan. Recent Bakken discoveries in northeastern Montana and North Dakota have led to much-increased attention from the petroleum industry, substantially augmenting the drilling of oil wells in southeastern Saskatchewan in the past five years (by 2004 fewer than 80 wells had been drilled, a count that rose to 895 wells by July 2008).
Several sedimentologic interpretations have been made for the Bakken Formation suggesting open-marine conditions for the entire unit which comprises Lower, Middle and Upper members. However, integration of ichnologic and sedimentologic data indicates deposition under open-marine and brackish-water settings for the Middle Member. Open-marine deposits record shelf, offshore and offshore-transition subenvironments in a low-gradient, shallow epicontinental sea. These deposits are commonly intensely bioturbated, and are characterized by a “distal” Cruziana ichnofacies in which dominant elements are Phycosiphon incertum and Nereites missouriensis. Brackish-water deposits are characterized by the presence of syneresis cracks and sedimentary structures indicative of tidal influence, the small size of trace fossils, low to moderate degree of bioturbation, and low ichnodiversity from an “impoverished” Cruziana ichnofacies in which dominant elements are Planolites montanus and Teichichnus rectus.
Further work needs to be done to fully understand the sedimentary environments and facies distribution of the brackish-water interval. Better comprehension of the sedimentologic conditions that prevailed during deposition of the Bakken Formation is expected to improve predictions regarding the distribution of the most oil-prospective facies, thereby impacting petroleum-industry activity in both exploration and production.
AAPG Search and Discovery Article #90094 © 2009 AAPG Foundation Grants in Aid