The pronghorn member of the Bakken Formation, Williston Basin, USA: Lithology, stratigraphy, reservoir properties

The Bakken Formation is present in the Williston Basin of North Dakota, Montana, Saskatchewan and Manitoba. The organic-rich upper and lower shale charge the unconventional, Late Devonian-Early Mississippian Bakken Petroleum System. While the middle member of this formation, sandwiched between the two shales, has proven to be a prolific producer, recent drilling has expanded to focus on the upper Three Forks Formation and the irregularly occurring interval of variable lithology above the Three Forks and below the lower Bakken Shale. Nomenclature has evolved from 1954 to present. This interval has in previous studies been referred to as the Sanish sand, the Lower Bakken Silt, the Grassy Butte Member, and the Lower Bakken bench. It has recently been renamed the Pronghorn Member and placed within the Bakken Formation. This interval corresponds with the Big Valley Formation of Canada, which lies between the Torquay and Bakken Formations in south Saskatchewan. While previous work has described locally occurring lithologies in various plays in the Williston Basin, the purpose of this study is to develop a basin-wide sequence stratigraphic framework for the facies by identifying related lithofacies and attempting to recognize the most significant sequence boundaries. The focus area of this study is the Pronghorn Member in the Williston Basin of northeastern Montana in comparison to the facies of the Pronghorn Member in the North Dakota portion of the basin. Reservoir properties of the variable lithologies will be compared throughout the basin in order to determine the most economic facies. Preliminary work, including core description, well-log analysis, X-ray diffraction, and petrographic thin-section analysis have indicated a transgressive surface of erosion and a significant sequence boundary between the underlying Three Forks Formation and the Pronghorn Member. In Montana, this lag is made up of fragmented brachiopods, echinoderms, stromatoporoid, coral, dolomite clasts, and v. fine to fine-grained sandstone, within mud, clay, microcrystalline quartz, or peloidal dolomite matrix. An additional erosional surface is also present in some cores between the Pronghorn Member and the overlying lower shale member of the Bakken Formation. A silty shale to siltstone facies present in Montana may be an up-dip equivalent of the lower shale member, but the relationship between the silty facies and the shale facies in North Dakota has not yet been determined. Porosity and permeability appear to be highly variable, with the best reservoir quality expected in the sandiest facies. In comparison to the prolific Sanish sand facies of North Dakota, the facies in Montana have much higher illite/mica content, finer grain sizes, and therefore greatly reduced porosity. Difficulties in correlation among the various lithofacies that may belong in Pronghorn Member of the Bakken Formation exist due to limited thickness and distribution throughout the basin. Further work will investigate the reason for isolated deposition and variable thickness. Proposed causes include accommodation space created by Prairie Salt dissolution or deposition within paleo-graben areas. ABSTRACT