PREFERRED ORIENTATION AND SEISMIC ANISOTROPY OF SHALES WITHIN THE BAKKEN FORMATION, MONTANA AND NORTH DAKOTA

As unconventional oil and gas reservoirs become viable through the advancement in drilling technology and geologic understanding, petroleum systems such as the Bakken Formation have become more important for oil production. Clay-rich shales, such as the members 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, whereas conventional X-ray analysis can only extract limited information on fine-grained aggregates. Crystallographic preferred orientation can be determined through synchrotron X-ray diffraction and the interpretation of two-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. Synchrotron X-ray diffraction measurements were conducted at BESSRC 11-ID-C beamline of the Advanced Photon Source at Argonne National Laboratory. Orientation distribution functions were determined with the program Material Analysis Using Diffraction by matching a calculated model with the experimental data. Samples of the Bakken shales from wells in North Dakota and Montana, and outcrop analogs from southwestern Montana were investigated. Individual phyllosilicate minerals such as illite, smectite, muscovite, biotite 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. Calculated models of intrinsic anisotropy were produced by the Berkley texture package BEARTEX. A better understanding of shale anisotropy could help improve exploration and production of unconventional shale oil reservoirs.