Controls on fracture network characteristics of the middle member of the Bakken Formation, Elm Coulee field, Williston Basin, United States

Although most of the production from the Bakken Formation in the Elm Coulee field of the Williston Basin is interpreted to come from matrix permeability, plots of reservoir performance indicators, such as estimated ultimate recovery (EUR), show areas of anomalously high production believed to be caused by secondary (fracture) permeability. Recent studies have alluded to basement tectonics controlling the structure of the overlying sedimentary cover and thus the reservoir performance in the Elm Coulee field, but the exact controlling mechanism has not been demonstrated. This paper describes an antiformal structure overlying a zone of deformation in a seismic data set. The zone of deformation consists of small vertical offset faults displaying evidence of wrench faulting and involves the Bakken petroleum system and deeper units. An analog model is used to guide interpolation and extrapolation of the faults when they are traced through the data cube. With the help of analog models, it is established that the antiformal structure and the associated deformation zone evolved over a stepover in a northwest-southeast-oriented left-lateral-strike-slip system in the rigid basement. This strike-slip system may be secondary to the regional left-lateral Brockton-Froid fault zone, which lies to the north of the study area. The fracture network in the Elm Coulee field is likely to be enhanced around such antiformal structures, providing a plausible explanation for the locations of the EUR sweet spots. Thus, it is proposed that locating such deformation zones forms a potential new exploration trend in this field.