Sedimentology, ichnology 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 present in the subsurface of the Williston Basin in northeastern Montana, North Dakota, southwestern Manitoba and southern Saskatchewan. In the southeastern corner of Saskatchewan, the Bakken Formation either conformably overlies the Upper Devonian Big Valley Formation or unconformably overlies the Torquay Formation, and is conformably overlain by the Lower Carboniferous Souris Valley (Lodgepole) Formation. The Bakken Formation typically includes three members: the lower and upper organic-rich black shale, and the middle calcareous/dolomitic sandstone and siltstone, which makes a “perfect” petroleum system including source rock, reservoir, and seal all within the same formation. According to detailed core analysis in the southeastern corner of Saskatchewan, the Bakken Formation is divided into eight facies, and one of which (Facies 2) is subdivided into two subfacies: Facies 1 (planar cross-stratified fine-grained sandstone); Facies 2A (wavy- to flaser-bedded very fine-grained sandstone); Facies 2B (thinly parallel-laminated very fine-grained sandstone and siltstone); Facies 3 (parallel-laminated very fine-grained sandstone and muddy siltstone); Facies 4 (sandy siltstone); Facies 5 (highly bioturbated interbedded very fine-grained sandstone and siltstone); Facies 6 (interbedded highly bioturbated sandy siltstone and micro-hummocky cross-stratified very fine-grained sandstone); Facies 7 (highly bioturbated siltstone); and Facies 8 (black shale). Our integrated sedimentologic and ichnologic study suggests that deposition of the Bakken occurred in two different paleoenvironmental settings: open marine (Facies 4 to 8) and brackish-water marginal marine (Facies 1 to 3). The open-marine facies association is characterized by the distal Cruziana Ichnofacies, whereas the brackish-water marginal-marine facies association is characterized by the depauperate Cruziana Ichnofacies. Isochore maps shows that both open-marine and marginal-marine deposits are widely distributed in this study area, also suggesting the existence of a N-S trending paleo-shoreline. The Bakken strata in this study area represent either one transgressive systems tract deposits or two transgressive systems tracts separated by a 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.