The Bakken Formation within the Northern Part of the Williston Basin: a Comprehensive and Integrated Reassessment of Organic Matter Content, Origin, Distribution and Hydrocarbon Potential

Summary This paper presents the findings of a integrated and comprehensive assessment of the oil generative potential for the Lower and Upper Bakken within the northern portion of the Williston Basin, using high resolution sampling of core from over 40 boreholes, analyses show the total organic carbon (wt% TOC) content for the Bakken Formation is not constant throughout any cored depth interval, but exhibits an extreme degree of variability both with depth and across northern portion of the Williston Basin. This is replicated by a variation in S1, S2, T max and HI across the study area and mirrored by the variation in yield of extractable organic matter (EOM), saturate, aromatic and NSO compounds as well as total sulphur, g.c.-fingerprint analysis, Pr/Ph ratios, short-chain/long-chain ratios, and the abundance and distribution in biomarkers. The dominant type of organic matter is a Type II fluorescing Bituminite, but relatively high amounts of total sulphur and organic sulphur within the kerogen indicate the localized presence of a Type IIs kerogen, with implications for the early generation of hydrocarbon at low levels of thermal maturity. The presence of sulphur is supported by the relatively high abundance of aryl isoprenoids within the total extract. A depth-wise and basin-wide variation in transition metal concentration, noteably Molybdium, Chromium, Nickel and Vanadium, key molecular ‘fingerprints’ and diagnostic biomarkers within the Upper and Lower Bakken shale, indicate the existence of a stratified water column characterized by photic-zone anoxia during deposition. Key biomarkers also indicate the periodic occurrence of cyanobacteria, green sulfur bacteria (e.g., Chlorobiaceae) within the water column as the primary OM producers with anaerobic Bacterivorous Ciliates acting as ‘decomposers’; suggesting that amorphous kerogen within the Upper and Lower Bakken (identified as Bituminite) was bacterially reworked by the Bacterivorous Ciliates. Some of the broader implication from this study includes support for a paleo-water depth during the Upper and Lower Bakken that is at least 100m a mechanism and process that explains the origin and composition of the amorphous kerogen, an explanation for the presence of high amounts of sulphur and pyrite within the shale, the variation in organic matter content, variation in biomarker distribution and presents a challenge to the established notion that precursor organic matter is extensively reworked, and hence transformed into amorphous kerogen, exclusively within the sediment.