Abstract

The Bakken Formation (Upper Devonian-Lower Mississippian) of western Saskatchewan has lower and upper members dominated by black shales and a heterolithic middle member, similar to southeastern Saskatchewan and North Dakota. However, the middle member in western Saskatchewan contains fine-grained sandstone where porosity and permeability values commonly exceed 25% and 1 Darcy respectively that are absent in southeastern Saskatchewan and North Dakota. Although focus has been in southeastern Saskatchewan and North Dakota, the Bakken Formation of west-central Saskatchewan has conventionally produced heavy gravity oil (API approximately 12 degrees) since the 1950s using vertical wells. The sequence-stratigraphic framework of the Bakken Formation of west-central Saskatchewan includes deposition under open-marine and marginal-marine conditions. Open-marine black shales were deposited during late-stage transgression (TST1) and progradation of offshore siltstones during the subsequent highstand systems tract (HST). Fine-grained sandstone, siltstone, and mudstone showing evidence of tidal influence was deposited during a falling-stage systems tract (FSST), initiated under forced-regressive conditions. Increased fluvial influence and dilution of normal marine salinity established marginal-marine conditions, evidenced by the low bioturbation index (BI: 0−2). Subsequent transgression (TST2) led to increased marine influence, reflected in a sudden increase in biogenic activity (BI: 4−5). Offshore siltstone and black shale were deposited through continued transgression (TST2) resulting in the re-establishment of open-marine conditions. Preserved thickness of the Bakken Formation in west-central Saskatchewan has been affected by post-Mississippian erosion during the creation of the sub-Cantuar drainage network, which was later filled by Lower Cretaceous clastics. The Bakken middle marginal-marine interval in west-central Saskatchewan differs from that in southeastern Saskatchewan. Fine-grained sandstones associated with tidal channels in west-central Saskatchewan comprise a reservoir characterized by porosity and permeability values that commonly exceed 25% and 1 Darcy, respectively. In contrast, in southeastern Saskatchewan, the Bakken Formation produces from a dolomitized silty sandstone where low porosity and permeability values have led to the widespread use of unconventional exploitation techniques to produce economically.