CONTROLS ON FRACTURE NETWORK CHARACTERISTICS IN THE BAKKEN FORMATION AT ELM COULEE SUPERGAINT

The Bakken formation in the Elm Coulee supergiant of the Williston Basin consists of three members: 1) upper shale member, 2) middle silty dolostone member, and 3) lower shale member. The Upper and Lower members act as excellent source rocks and the main reservoir is the Middle member with an average porosity of 3 to 9% and average permeability of 0.04md. The Bakken Oil play in this area is a stratigraphic trap with a pinch-out to the southwest and a diagenetic facies change in the northeast. The main production is interpreted to come from matrix permeability in the field area. However, the first year production trends from the Elm Coulee supergiant show areas of anomalously high production.

This study used 3D seismic data from Crane Field, which is located in the southeastern part of the Elm Coulee supergiant. We used post stack filtering and seismic attributes to amplify the structure of the basement. The basement structure shows clear evidence of reactivated strike slip faults with a dip slip component. These faults have 3 orientations: NE-SW, NW-SE, and N-S. This is interpreted to be a wrench faulting system driven by regional scale NE-SW faults such as Brockton-Froid strike slip system. The fractures in the Bakken Middle member are controlled by local stresses generated by these reactivated oblique slip faults. Our conceptual fracture model incorporates the fracture set generated due to curvature and fracture set generated due to the extensional stresses caused by strike slip motion along these faults. The analogue outcrop of the Cottonwood formation from the Shell Canyon in the Bighorns Mountains shows a fracture density of 22.3 fractures per meter. We built our fracture model using orientations from the conceptual fracture model and fracture density from the analogue outcrop.

The Crane Field is located on a major strike-slip system in the 3D area, with potential for other strike-slip faults elsewhere in the Elm Coulee supergiant. The first year production trends from the Elm Coulee supergiant show that there are compartments with better reservoir performance. Our results show that this can be explained by the presence of reactivated basement faults. Prediction of compartments with better versus poorer reservoir performance (EUR’s) may be possible by detailed fault mapping in 3D seismic data set from Elm coulee supergiant.