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Sedimentary Facies, Depositional Environment and Sequence Stratigraphy of the Upper Devonian-Lower Carboniferous Bakken Formation in the Southeastern Corner of Saskatchewan

Abstract

The Upper Devonian-Lower Carboniferous Bakken Formation is deposited in the subsurface of the Williston Basin in northeastern Montana, North Dakota, southwestern Manitoba and southeastern Saskatchewan. Now it is one of the most important oil-producing units in Saskatchewan. In the southeastern corner of Saskatchewan, the Bakken Formation unconformably overlies either the Upper Devonian Big Valley Formation or the Torquay Formation, and is conformably overlain by the Lower Carboniferous Souris Valley (Lodgeopole) Formation. The research area includes Townships 1 through 17, Ranges 30 of the First Meridian to Ranges 1 of the Second Meridian, which is just west of the border between Saskatchewan and Manitoba. The lower black shale member is essentially absent in this area, except for the Rocanville oil-field area. The Bakken succession typically consists of the middle sandy to silty member and the upper black shale member. According to detailed core analysis, the Bakken Formation is divided into eight facies and two subfacies: facies 1 (flaser-bedded very fine-grained sandstone); facies 2 (wavy-bedded very fine-grained sandstone); facies 3 (parallel-laminated very fine-grained sandstone and siltstone); facies 4 (sandy siltstone with rip-up clasts); facies 5 (Interbedded highly bioturbated very fine-grained sandstone and siltstone); facies 6 (interbedded highly bioturbated sandy siltstone to silty very fine-grained sandstone and sparsely bioturbated micro-hummocky cross-stratified very fine-grained sandstone); facies 7 (highly bioturbated siltstone); and facies 8 (black shale). Our integrated sedimentologic, ichnologic, and sequence-stratigraphic study suggests that the deposition of Bakken Formation occurred in two different paleoenvironmental settings: open marine (facies 4 to 8) and brackish-water marginal-marine (facies 1 to 3). The base of the marginal-marine interval is represented by a sequence boundary (coplanar surface or amalgamated sequence boundary and transgressive surface). This surface has been identified in previous studies west-southwest of our study area, therefore assisting in high-resolution correlation of Bakken strata.