Factors Controlling Organic-Richness in Upper and Lower Bakken Shale, Williston Basin: An Application of Inorganic Geochemistry

Abstract

Organic-richness in shale is controlled by the interplay between the three factors viz., organic productivity, preservation and dilution. This study is focused on identifying the effect of these factors in the context of upper and lower (U&L) Bakken shales in the Williston basin. Lithologically, the U&L shales are siliceous mudstones with the Total Organic Carbon (TOC) varying between 3 to 20 weight percent. X-ray fluorescence results, TOC information, X-ray diffraction mineralogy and stable isotope data (C, O and S) collected from 10 well cores are used to determine the influence of detrital sediment influx, paleoproductivity and paleoceanographic conditions in controlling organic content in the U&L shales. The mineralogical composition of the U&L shales may vary as follows: 30–45% of biogenic, detrital and authigenic quartz silt, 20- 30% clay, 5–10% feldspar, 2–11% detrital and authigenic dolomite and 3–9% pyrite. So, to distinguish biogenic silica-rich intervals from detrital sediment-rich intervals, inorganic geochemical proxies (Si/Ti and Al/Ti ratios) are used. The vertical profiles show positive covariance between TOC-rich and the detrital sediment-rich intervals, which demonstrate the beneficial effect of detrital sediment input in the organic matter (OM) accumulation. Therefore, an optimum sedimentation rate instead of creating a dilution effect, appears to have helped in quick preservation of OM. Paleoproductivity and paleonutrient utilization was determined by using stable Nitrogen isotope data as a proxy. In U&L shales, the covariance between lighter values of deltaN15 and higher TOC reveals that OM accumulation relies on increased nutrient supply for a higher organic productivity. The nutrients required to sustain the organic productivity was supplied by the detrital sediment influx and the surface water currents within the basin. Anoxic-euxinic basin condition results in higher preservation of OM. Enrichment of redox-sensitive trace elements (like Mo, V, U, Ni and Cu) and their relation with TOC, values of C/S ratio and degree of pyritization suggests that the Bakken shales were deposited in an anoxic-euxinic basin with higher OM preservation potential. Therefore this study concludes that higher paleoproductivity driven by critical nutrient supply, optimum detrital sedimentation rate and anoxic-euxinic basin condition resulted in higher accumulation and better preservation of the OM within the Bakken shales.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015