--> Distribution of Depositional Environment, Diagenetic Features, and Reservoir Quality of the Middle Bakken Member in the Williston Basin, North Dakota

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Distribution of Depositional Environment, Diagenetic Features, and Reservoir Quality of the Middle Bakken Member in the Williston Basin, North Dakota

Abstract

The Upper Devonian–Lower Mississippian Bakken formation in the Williston Basin is an important source rock for oil production in North America. The Bakken formation comprises three units: upper and lower black shales, and a middle member. Upper and Lower Bakken shales are high quality source rocks for reservoirs in the Middle Bakken, upper Three Forks, and lower Lodgepole formations. The Middle Bakken Member—which consists of gray, interbedded siltstone and sandstone with shale, dolostone, and limestone—is under investigation in this study. The goals here are to determine the regional distribution of lithofacies and the depositional environment of the Middle Bakken Member and to explain diagenetic sequence and reservoir-quality parameters in the Williston Basin. The reservoir quality of the Middle Bakken Member is primarily influenced by mineralogy and cementation, which caused low porosity and permeability and are intrinsically linked to facies distribution in the basin. Pore types include primary intergranular, intragranular, and fracture. Secondary moldic and vugular porosity resulted from dissolution of biogenic fragments. Dolomitization is pervasive throughout the Middle Bakken, although we also see facies-exclusive concentrations of dolomite cement that were mapped regionally. Secondary cementation includes quartz overgrowths, K-feldspar, clay cement, and pyrite as both cement and nodules. This study will present the vertical and lateral distribution of dolomite and pyrite cementation zones correlated to lithofacies and depositional environment of the Middle Bakken Member via core and petrographic thin-section analysis in the Williston Basin.