Pronghorn Member, Bakken Formation, Williston Basin, USA: lithology, stratigraphy, and reservoir quality

Rebecca Johnson

The Pronghorn Member of the Bakken Formation has previously been referred to as the "Sanish" sand, lower Bakken silt, Grassy Butte, or lower Bakken bench, but has recently been renamed the Pronghorn and placed within the Bakken Formation. The Bakken Formation is present in the Williston Basin of North Dakota, Montana, Saskatchewan and Manitoba. The organic-rich upper and lower shales 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. The lower Bakken silt or lower Bakken bench 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 will be to develop a basin-wide sequence stratigraphic framework for the facies by objectively identifying related lithofacies and attempting to recognize the most significant sequence boundaries. 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, fusulinids, sponges, microcrystalline dolomite and dolomite clasts, v. fine to fine-grained sandstone, and mud to siltstone. However, 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.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012