An integrated sedimentological, ichnological and sequence stratigraphic study in the Devonian-Carboniferous Bakken Formation of subsurface southeastern Saskatchewan

Abstract

In spite of the economic importance of the Bakken Formation as one of the most prominent oil-producing units in North America (Canada and USA), an integrated sedimentological, ichnological and sequence stratigraphic analysis of the Upper Devonian - Lower Carboniferous Bakken Formation is presented for the first time in this thesis.

The Bakken Formation has been subdivided into three members: lower, middle and upper. The lower and upper members are homogeneous and consist of only one sedimentary facies (facies 1); in contrast, the middle member is much more heterogeneous, both vertically and laterally, and comprises several sedimentary facies (facies 2 to 10). For this project, sixty-two well-cores from southeastern Saskatchewan were examined in detail. Eleven sedimentary facies were defined based on lithology, sedimentary structures and trace-fossil content (F1 to F11). From these, two facies were subdivided into subfacies (F3 and F8). These facies were grouped into two facies associations: open marine and brackish-water marginal marine. According to the sequence-stratigraphic framework, the orientation and geometry of the sedimentary bodies and the distribution of the sedimentary facies, the brackish-water interval is interpreted as a marginal-marine embayment.

The Upper Devonian–Lower Carboniferous Bakken Formation records a complex depositional history involving several relative sea-level changes and open marine and brackish-water marginal-marine conditions. The depositional history of the Bakken can be summarized within three systems tracts: (1) a lower transgressive systems tract, which comprises black shelf shales (facies 1) of the lower part of the lower member; (2) a highstand systems tract, which embraces from bottom to top black shelf shale (facies 1) of the upper part of the lower member, and lower-offshore muddy siltstone (facies 2), upper-offshore sandy siltstone (subfacies 3A), offshore-transition silty very fine-grained sandstone (facies 4) and locally lower-shoreface, silty, very fine-grained sandstone (facies 5) of the lower part of the middle member; and (3) an upper transgressive systems tract, which encompasses barrier-bar fine-grained sandstone (facies 6), margin-bay, very fine-grained sandstone (subfacies 8A), very thinly laminated mudstone, siltstone and very fine-grained sandstone (facies 9), distal-bay, thinly interlaminated mudstone and very fine-grained sandstone (facies 10), very fine-grained sandstone (subfacies 8B), tidal-flat very fine-grained sandstone (subfacies 8C), wave-dominated tidal-flat very fine-grained sandstone (facies 7), transgressive lag (facies 11), and upper-offshore siltstone interbedded with very fine-grained sandstone (subfacies 3B) of the upper part of the middle member, and black shelfal shale (facies 1) of the upper member.

Deposition of the Bakken Formation was controlled mostly by salinity, oxygen content and storm action. While open-marine deposits are generally characterized by high degrees of bioturbation, moderate ichnodiversity and the “distal” Cruziana ichnofacies, brackish-water marginal-marine deposits are distinguished by low levels of bioturbation, lower ichnodiversity, and the “impoverished” Cruziana ichnofacies. The lack of bioturbation, black color, high organic matter content, thin lamination, and scarce benthic fauna indicate anoxic conditions in shelf deposits, whereas the rest of the open-marine sediments accumulated under well-oxygenated conditions. Depending on the frequency and intensity of storms, tempestites were preserved or not in the upper-offshore deposits.

Based on the petrophysical characterization of the sedimentary facies of the Bakken Formation, facies 6, 7, 4 and subfacies 8C have the best reservoir rock quality with the highest porosities (8.6% to 12%) and permeabilities (0.09 md to 0.27 md). Lithology, diagenesis and bioturbation played a key control on the reservoir quality of the rock. However, the importance of the spatial distribution of the sedimentary facies in reservoir potential should not be overlooked. While facies 4, deposited in an open-marine realm, has the best reservoir potential in southeastern Saskatchewan due to its wide distribution; facies 6, 7 and subfacies 8C, deposited in the brackish-water dominium, can constitute good local targets.