--> A petrographic analysis of the Lower Bakken Shale and Pronghorn Member of the Bakken Formation, Williston Basin

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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A petrographic analysis of the Lower Bakken Shale and Pronghorn Member of the Bakken Formation, Williston Basin

Abstract

Recent studies have revealed that mudstones are significantly variable in lithology, texture, structure, and depositional setting. The black pyritic organic-rich Bakken shales, however, have been documented to be homogenous in appearance and contain little physical sedimentary structures. Therefore, the anoxic, stagnant, deep-water condition has been a long-held depositional model for the Bakken shales. Although the Lower Bakken Shale and the Pronghorn Member have been intensively investigated via geochemical methods, few studies have systematically characterized their lamina-scale petrographic properties. This project presents an integrated stratigraphic, petrographic and geochemical investigation to evaluate depositional environments, diagenetic history, and stratal architecture of the Lower Bakken Shale and the Pronghorn Member in the Williston Basin. Detailed petrographic studies in different scales (core slab, hand-specimen and thin-section) have been undertaken to characterize the studied succession. The results show that the previously proclaimed homogeneous Lower Bakken Shale show significant heterogeneity in decimeter to centimeter scale. Abundant pyritic radiolarian-rich bandings/laminae are normally calcite rich and develop ptygmatic folding of calcite-healed fractures. The mature/over-mature Bakken Shale samples also show abundant lenticular-shaped solid bitumen associated with these bandings/laminae. Multiple distinct lithofacies have been identified within the Lower Bakken Shale, thereby suggesting a complex depositional conditions. General euxinic condition did exist during the deposition of Lower Bakken Shale. However, the evidence of reworked fossil debris, benthic fecal pellets, agglutinated foraminifera, and possible burrows at certain levels may indicate dysoxic conditions under intermittent bottom current activities. Additionally, detailed facies analyses of the Pronghorn Member have been undertaken to understand its stratigraphic relationships with the overlying Lower Bakken Shale. In this study, a suite of wireline logs, X-ray fluorescence (XRF), X-ray diffraction (XRD), thermal maturity, and organic enrichment (TOC) data have also been integrated to support the interpretation of sedimentary processes, depositional environments and their stratigraphic variations within the Williston Basin.