Organic geochemical assessment of the Upper and Lower Members of the Bakken Formation, southern Saskatchewan

Abstract

The Upper and Lower Members of the Bakken Formation (Late Devonian- Early Mississippian) are some of the most prolific petroleum source rocks of the Williston Basin. However, despite their excellent petroleum potential, the available geochemical data appears insufficient to provide a complete understanding of variations in organic matter content, kerogen Type and petroleum generative potential. Using a combination of bulk geochemical analysis, augmented by gas chromatography (GC) and gas chromatography-mass spectrometry analyses (GC-MS), as well as high-resolution sampling of drill core from 32 boreholes, results clearly show that a significant degree of variation exists in every geochemical parameter, both depth-wise and across the Bakken Formation sub-crop. For example, the total organic carbon (TOC) can vary from 8.0 wt.% to 35.0 wt.% within a single borehole and from less than 1.0 wt.% up to 35.0 wt.% in several boreholes across southern Saskatchewan. This depth-wise variability in TOC is echoed across the sub-crop with variations in sulphur, extractable organic matter (EOM), saturate, aromatic, nitrogen-, sulphur- and oxygen- (NSO) bearing compounds, biomarker compounds and biomarker ratios. Analysis of total sulphur, organic sulphur and alkylthiophene compounds suggests the localized presence of organic sulphur (i.e. Type II-S kerogen), which appears to influence Rock-Eval Tmax, with possible implications for the localized early generation of petroleum. Therefore depth-wise and spatial variations in Rock-Eval Tmax and biomarker thermal maturity appears to be governed by variations in kerogen composition with the kerogen Type being an assemblage of amorphous organic matter, as resolved using a multi-proxy biomarker approach. The GC-MS analyses of the saturate and aromatic fractions also clearly reflect variations within amorphous organic matter related to the influence of the depositional setting at the time of sedimentation. Variations in biomarker concentrations, biomarker ratios and the presence or absence of gammacerane and isorenieretane, augmented by trace element analyses, are resolved through the presentation of a descriptive model of the palaeodepositional environment that relates the periodic existence of photic zone anoxia, and the existence stratified water to variations in biologic precursor and preservation potential. Emerging patterns within the Upper and Lower Bakken palaeoecology, as interpreted through biomarker analysis, provides an understanding of the distribution and occurrence of organic matter within the Bakken Formation, with implication for petroleum potential within southern Saskatchewan. Keywords: Bakken Formation shale, hydrocarbon, organic matter, biomarkers, Tmax, TOC, photic zone anoxia