Sequence-Stratigraphic and Depositional Framework of the Middle Bakken Formation, Williston Basin, North Dakota
The Devonian-Mississippian Bakken Formation located in the Williston Basin, North Dakota is a significant oil reservoir with a complex depositional history. The Bakken Formation is composed of three members: (1) mudrock of the lower member; (2) dolomitic silty sandstone of the middle member; and (3) mudrock of the upper member. This study provides a high-resolution sequence-stratigraphic and depositional framework to explain the distribution of facies within the middle Bakken Formation. A total of 37 cores were evaluated in detail over an area of approximately 400 mi2 (644 km2). Core descriptions were paired with petrophysical logs to provide lateral correlations between core locations.
The middle Bakken Formation is composed of 11 facies indicative of a tidally-influenced-shallow-marine system. It is divided into three packages (lower, middle, and upper) based on facies and their stacking patterns. The lower package is composed of shallowing-upward offshore to transitional marine facies. The middle package is composed of subtidal ooid facies, and the upper package is composed of fining-upward tidally-influenced facies. Sequence-stratigraphic analysis is used to divide the entire Bakken Formation (lower, middle, and upper members) into five systems tracts: (1) a highstand systems tract of the lower member; (2) two lowstand systems tracts within the lower and middle packages; (3) a transgressive systems tract of the upper middle package; and (4) a highstand systems tract of the upper member. Two sequence boundaries are present: (1) between the lower and middle members; and (2) between the lower and middle packages of the middle Bakken Formation.
Facies-distribution maps within a sequence-stratigraphic context show a stratigraphic pinch-out to the east in the study area, and provides a link to reservoir productivity. The sequence-stratigraphic framework proposed provides a predictive component helpful in determining play elements within the hydrocarbon system, including reservoir size, location, and quality.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California