Structural and stratigraphic factors influencing hydrocarbon accumulations in the Bakken petroleum system in the Elm Coulee field, Williston Basin, Montana

Abstract

The Bakken Petroleum System in the Elm Coulee field is an upper Devonian to lower Mississippian system consisting of the upper Three Forks, Bakken, and lower Lodgepole formations. The Elm Coulee Field is located at the southwest margin of the Williston Basin in Richland County, Montana, and covers over 500 sq. miles. Today, the Elm Coulee Field is one of the largest oil fields in Williston Basin and has an estimated ultimate recovery of 200-250 MMBO. This study focused on the structural and stratigraphic components of the Bakken Formation and Prairie Salt Formation in the Elm Coulee Field and addressed the basement structural trends and their influence on the Bakken pay interval and possible salt dissolution in the Prairie Salt. The latter is significant in explaining whether the anomalous thickness of the Middle Bakken Member is due to salt dissolution that may have occurred during Bakken deposition. The study utilized subsurface cores, well log data, and 3-D seismic. The Core data identified the main reservoirs in the Bakken Petroleum System to be within the Bakken Formation's Upper Bakken Member and Middle Bakken Member: Facies B (upper interval), C, D, E, and F. The reservoir quality in the Middle Bakken Member is enhanced by dolomitization. Well log data identified a NW-SE trending thickness anomaly of the Bakken Formation's Middle Bakken Member in the Elm Coulee Field. Whether or not this anomaly was caused by basement tectonics and/or Prairie Salt dissolution was unable to be determined, but indirect evidence for Prairie Salt dissolution was identified. The 3-D seismic data identified sub-seismic discontinuities/faults with two dominant trends, NE-SW and NW-SE, which correlates with the regional fractures present throughout the Williston Basin. The fracture network present plays a critical role in the enhancement of the reservoir quality.