Sedimentology, ichnology, and stratigraphic architecture of the upper Devonian-lower Mississippian Bakken Formation, west-central Saskatchewan

The Upper Devonian-Lower Mississippian Bakken Formation has recently become a prolific producer of light gravity oil in southeastern Saskatchewan since the advent of horizontal drilling and multi-stage hydraulic fracture technologies, which has resulted in an increase in geological studies within the area. However, the Bakken Formation of west-central Saskatchewan has been producing heavy oil since the 1950s, and has comparatively received much less attention than its southeastern counterpart. The Bakken Formation is the youngest member of the Three Forks Group and unconformably overlies the Big Valley Formation. In west-central Saskatchewan, the Bakken Formation can be conformably overlain by the Mississippian carbonates of the Madison Group or unconformably overlain by the Lower Cretaceous Mannville Group. A tripartite subdivision is applied to the Bakken Formation, with a mixed clastic/carbonate Middle Member deposited between Lower and Upper Black Shale Members. Based on detailed core description, eight facies have been defined for the Bakken Formation of west-central Saskatchewan: Facies 1 (Lower and Upper Black Shale members), Facies 2 (bioturbated siltstone/sandstone), Facies 3 (wave-rippled sandstone), Facies 4 (bioclastic grainstone), Facies 5 (interbedded mudstone, siltstone, and very fine-grained sandstone), Facies 6 (very fine- to fine-grained sandstone), Facies 7 (bioturbated siltstone/sandstone), and Facies 8 (massive and brecciated siltstone). Deposition of the Bakken Formation in west-central Saskatchewan occurred under either open-marine or marginal-marine conditions. Facies association 1 (open-marine interval), which is made up of F1 through F4, is characterized by the distal Cruziana Ichnofacies. It was deposited within a wave-dominated shallow-marine depositional environment. Facies association 2 (marginal-marine interval), which is comprised of F5 through F8, shows scarce biogenic structures, most likely as a result of brackish-water conditions. Geological mapping (structure surface and isopach) of the facies and facies associations has aided in illustrating their lateral distribution. However, mapping of the overlying Mississippian carbonates and sub-Mesozoic unconformity shows that post-Mississippian erosion was a controlling factor in the distribution and preservation of Bakken Formation deposits, which creates uncertainty when interpreting geological maps and stratigraphic cross-sections. Although post-Mississippian erosion causes problems when reconstructing the depositional history and stratigraphic architecture of the Bakken Formation, it illustrates the importance of not performing stratigraphic studies within a vacuum, only focusing on the formation of interest. Rather, underlying and overlying units must be studied to see whether or not the unit of interest’s deposition and distribution has been affected by pre- and post-depositional events