Chemofacies Characteristics of Six Bakken Cores From the Northeastern Williston Basin: Insights to Paleodepositional Conditions From Trace Element Variability in Mudrocks

Abstract

X-ray fluorescence (XRF) analyses provide a quantitative approach to defining stratigraphic changes in successions lacking distinct lithological variations, such as indiscriminate mudrocks in unconventional plays like the Bakken Formation in the Williston Basin. The Devonian-Mississippian age Bakken Formation is composed of a silty dolomite-limestone middle member bounded by upper and lower organic-rich, fine grained mudrocks in which subfacies are difficult to identify visually. High resolution (2-inch interval) XRF analyses of the lower and upper Bakken shales reveal significant enrichment and variability of redox-sensitive trace elements such as Mo, Ni, and Zn associated with anoxic, reducing basin bottom waters and the preservation of organic matter. Hierarchical cluster analysis (HCA) of the XRF data defines subfacies within the Bakken shales by the enrichment or depletion of major and trace elements that correspond to changes in available micronutrients (Ni, Zn, Cu) and deep water oxygenation conditions (V, Mo) during Bakken deposition in the Williston Basin. Six North Dakotan Bakken cores, including one from the central Williston Basin and five concentrated in the northeast portion of the Basin, were scanned for major and trace element concentrations at a 2-inch interval and supplemented with x-ray diffraction (XRD) and total organic carbon (TOC) analyses at a 2-foot interval. HCA identified two geochemically distinct lower Bakken subfacies and three distinct upper Bakken subfacies defined by variable enrichments in V, Ni, Zn, and Mo. The lower Bakken is characterized by a high Mo (900 ppm) subfacies with correspondingly high TOC content (5-18%) and a significantly less enriched (150 ppm Mo) and lower TOC (1-5% TOC) that becomes dominant as the lower Bakken thins to the northeast. The upper Bakken is characterized by high TOC content (10-20%) and three subfacies with similar geochemical signatures differentiated by their enrichment in Zn, V, and Mo. These clusters allow for quantitative discrimination between subfacies within the Bakken shales and provides an effective means of identifying changes in paleodepositional conditions, as well as correlating similar environments and their geochemical signatures between wells.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016