Organic Matter Variations within the Upper and Lower Bakken Shales of Saskatchewan, with Implications for Origin and Hydrocarbon Generation

Summary This study presents a detailed temporal and spatial geochemical analysis of the Upper and Lower Bakken shale within the northern part of the Williston Basin. The high-resolution sampling of drill core from 28 wells shows a significant variation in TOC, S1, S2 and HI exists both across the study area and with depth within the Upper and Lower Bakken. For example, within the Upper Bakken the TOC ranges from 2.0wt% to 23.0wt%TOC and within the Lower Bakken the TOC ranges from 4.0wt% to 16.0wt% TOC. This depth-wise pattern is mirrored in the yield of extractable organic matter (EOM), which is in agreement with Rock-Eval S1 values, showing a temporal variation in the proportion of saturate, aromatic and NSO compounds. There is also a depth-wise variation in total sulphur, ranging from 1.3wt% to 8.1wt% for the Upper Bakken, and from ~0.25wt% to typically 6.5wt% for the Lower Bakken, although two samples show high total sulphur content of 13.0wt%. High proportion of NSO compounds in the EOM is consistent with the thermal maturation state, but may also reflect the incorporation of high proportion of sulphur into the kerogen during diagenesis. This later interpretation is supported by the relatively high abundance of aryl isoprenoids, likely derived from green sulphur bacteria Chlorobiaceae. This study has identified key molecular ‘fingerprints’ and the existence of significant variation in aliphatic and aromatic biomarkers within the Upper and Lower Bakken shale, indicating existence of green sulfur bacteria Chlorobiaceae and the presence of anaerobic Bacterivorous Ciliates during the deposition of the Upper and Lower Bakken. This indicates that the amorphous organic matter within the Bakken (identified as Bituminite) represents the bacterial reworking of primary organic matter by 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 (thereby accommodating the photic zone and euxinic bottom water), a mechanism and process that explains the origin and composition of the amorphous organic matter within the Upper and Lower Bakken, an explanation for the presence of high amounts of sulphur and pyrite within the shale 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.