Sedimentology and stratigraphic architecture of the Bakken Formation (Devonian-Mississippian), west-central Saskatchewan

The Late Devonian to Early Mississippian (Fammenian-Tournaisian) Bakken Formation is a predominantly siliciclastic marine deposit consisting of black shale upper and lower members, with a mudstone, siltstone, sandstone and carbonate middle member. The Bakken Formation occurs in the subsurface of the Williston Basin. In west-central Saskatchewan, the Bakken Formation contains hydrocarbon-producing, linear sandstone accumulations that are not completely and accurately explained in previously published interpretations. The sandstone accumulations are up to 25 m thick, between 1.4 to 5.5 km wide, up to 40 km long, have crest-to-crest spacings between 3 and 10 km, and are separated by intervening thins. These sandstone bodies have a common basal surface and form a ridge and swale morphology, with the ridge width and thickness increasing toward the northwest. Several ridges are asymmetric with a steeper southeast-facing slope and a gentle northwest-facing slope. Mudstone and siltstone, laminae and beds within the ridges parallel the steeper, southeast-facing slope and dip toward the southeast. The Bakken Formation sandstone ridges have many similarities to modern active and moribund, tidally-influenced, shelf sand ridges in the North Sea and East China Sea. The modern shelf sand ridge depositional environment is an analogue for the Bakken Formation sandstone ridges, the distribution of reservoir-quality sandstone is more accurately explained and predicted by this model. Four, transgressive-regressive, relative sea-level cycles are interpreted to have influenced deposition of the Bakken Formation in west-central Saskatchewan, with at least two cycles related to alternating, glacial-interglacial events that occurred in the Southern Hemisphere during the Late Devonian.