Geology of the Middle Member of the Bakken Formation in Divide County, North Dakot

The Upper Devonian-Lower Mississippian Bakken Formation of Williston Basin is a large unconventional oil and gas play consisting of a lower shale member, a middle member, and an upper shale member. The middle member is a production target because it contains porosity and petroleum expelled from the shale members around it. Variable production and sweet spots of the Bakken in the study area of Divide County, ND and 15 miles around Divide County in the USA prompted a closer look at middle member Bakken sediments. For this study, 606 wells were correlated and thin sections from two wells located 10.17 miles apart were point counted for mineralogy, grain size, and grain angularity, as well as analyzed for types of cements and diagenetic features using a scanning electron microscope. Minor variations in mineralogy, grain size, and grain angularity indicate similar depositional environments between the two wells. Similar dolomite cement compositions between the two wells indicate that the same fluids and diagenesis occurred in both well localities. The Bakken Formation locally thickens where underlying Prairie Formation evaporite locally thins, indicating that the evaporite underwent dissolution during lower shale and middle member Bakken deposition. The Bakken middle member clean sand (clean gamma ray response) facies and overlying facies correlate well across the study area, while underlying facies are heterogeneous and too variable to correlate across the study area.