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Organic Matter Variations within the Bakken Shales of Saskatchewan: With Implications Upon Origin and Timing of Hydrocarbon Generation

Abstract

This study presents a detailed temporal and spatial geochemical analysis of the Upper and Lower Bakken shales within the northern part of the Williston Basin. High-resolution sampling of drill core from 28 wells was achieved by obtaining 20 to 25g core from up to 30 sampling intervals per well throughout the Upper and Lower Bakken. Analyses show a significant variation in TOC, S1, S2 and HI exists both depth-wise within the Upper and Lower Bakken in individual wells, and across the study area. For example, within the Upper Bakken at 01/5-4-16-31W1M (86H080) TOC ranges from 2wt% to 23wt%TOC and at 01/15-25-10-8W2M (65G163) within the Lower Bakken the TOC ranges from 4wt% to 16wt% TOC. This depth-wise pattern is mirrored in the yield of extractable organic matter (EOM), which are in agreement with Rock-Eval S1 values, showing a temporal variation in the proportion of saturate, aromatic and NSO compounds. High proportion of NSO compounds in the EOM are consistent with the thermal maturity 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. 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 a high total sulphur content of 13.0wt%. Using conventional maturity parameters, the Type II kerogen would be considered relatively immature, however Tmax shows a strong inverse relationship with total sulphur. The implication being that the presence of sulphur may influence the early generation of hydrocarbon at relatively low levels of thermal maturity. Analysis reveals that the Type II kerogen within the Bakken are maceral assemblages and not a single organic matter type as defined by bulk pyrolysis. This study has identified the molecular fingerprints of recently defined major and minor Organofacies within the Upper and Lower Bakken, using saturate and aromatic biomarkers; for example Organofacies 2A, dominated by Bituminite A, has a Gammacerane/C31-hopane ratio between 0.36 to 0.46, suggesting the presence of bacterivorous ciliates that feed upon green sulphur bacteria. This study relates the regional variability in hydrocarbon potential within the Bakken to compositional differences in OM and depositional environment.