A fracture and texture analysis of the Bakken Formation, Montana

Abstract

The Bakken Formation underlies much of eastern Montana, North Dakota and Saskatchewan, with correlative units extending in the subsurface beyond these regions. It is composed of three informal members: an upper shale member, a middle silty limestone/dolostone member, and a lower shale member. The Bakken petroleum system acts as a conventional and unconventional reservoir within the Williston Basin and fractures that occur naturally within the Bakken petroleum system can either help or hinder reservoir characteristics. Unconventional reservoirs, such as the Bakken Formation, rely heavily on fracture enhancement (hydraulic fracturing) to become producible oil plays. Pre-existing fractures and weaknesses open more readily with fracture stimulation than the creation of new fractures, and have been correlated to increased early production in shale plays. To determine the influence of these fractures on the reservoir in the Bakken Formation and its correlative units, fractures in core and outcrop were examined. Clay-rich shales, such as those within the Bakken Formation, display high intrinsic anisotropy, which can be helpful in interpreting seismic profiles. Despite the importance of shale oil reservoirs, the contribution of preferred orientation of minerals to shales is not well constrained. These constituent clay minerals are phyllosilicates that acquire preferred orientation during sedimentation and early diagenesis. Hard X-rays produced from a synchrotron source are effective at extracting orientation distributions of individual mineral components within a shale. Crystallographic preferred orientation can be determined through synchrotron X-ray diffraction and the interpretation of three-dimensional images by using a Rietveld refinement method. This method incorporates a least squares approach to produce a calculated model of the degree of preferred orientation. Samples of the Bakken shales from wells in North Dakota and Montana, and outcrops from southwestern Montana were investigated. Individual phyllosilicate minerals such as illite, smectite, muscovite, and chlorite yield individual orientation patterns. The elastic properties of each shale sample were determined by averaging the calculated properties of each mineral phase over their orientation distributions. The presence of specific clay minerals and degree of anisotropy is highly variable from well to well. A better understanding of shale anisotropy could help improve exploration and production of unconventional shale oil reservoirs.