Unraveling the reservoir heterogeneity of a complex hybrid play by integrating results from core, outcrops and well logs: A case study from the bakken formation in Williams and McKenzie Co., ND Williston basin

The Devonian-Mississippian Bakken Formation in the Williston Basin is a complex hybrid play exhibiting both unconventional and conventional traits. While typical wireline log stacking patterns show very little change in character for the Middle Bakken Member across the study area, production results show significant variability across the central basin. These differences are attributable in part to changes in lithofacies, which in turn affect reservoir quality. This study focuses on the integration of results from detailed core, outcrop, and wireline log characterization that provides a more comprehensive understanding of the in-situ reservoir architecture and allows for the predictability of both vertical and lateral reservoir heterogeneity. This level of comprehension is crucial for determining optimal development and lateral placement and enhancing well performance in unconventional plays. The Bakken Formation in the study area is composed of 24 distinct lithofacies as determined from the detailed facies description of eight sediment cores, and supplemented by x-ray diffraction (XRD), petrography, LECO total organic carbon (TOC), and mercury injection capillary pressure (MICP) analysis. The organic-rich source rocks associated with the Upper and Lower Bakken Shales can be subdivided in six distinct core lithofacies, each reflecting unique depositional processes and/or composition. The Middle Bakken reservoir interval is composed of 14 distinct core lithofacies and the organic-lean Pronghorn Member is composed of four distinct core lithofacies. To achieve a meaningful correlation of facies between wells it is essential to develop an up-scaling technique that honors the lithofacies distribution observed from cores, as well as the architectural information observed from outcrop. This was achieved by employing Linear Discriminant Analysis (LDA) to extrapolate core facies across the area of interest using a standard quad combo log suite. The LDA technique represents Principle Component Analysis in the presence of pre-defined groups or in this case, the described core facies. By choosing the relevant wireline logs that are maximally correlated with the first linear discriminant from the eight cored wells the core facies were propagated over 1,100 vertical wells. The predicted core facies are mapped across the study area using standard gridding algorithms. These maps are further enhanced through the integration of the detailed outcrop studies. The orientation and geometries of sedimentary bodies observed in the Middle Member of the Sappington Formation (Bakken-equivalent strata) in the Bridger Range, Montana, allowed for enhanced mapping in key target areas across the broader region of interest. The results from this integrated study significantly impact decision making processes with regard to well placement within the study area and can serve as a model for improving reservoir characterization techniques in other hybrid plays.