Light hydrocarbon (gasoline range) parameter refinement of biomarker-based oil–oil correlation studies: an example from Williston Basin

We evaluated geochemical compositions of 189 crude oils produced from Paleozoic reservoirs across the Williston Basin. Emphasis is placed on compositional variations in the gasoline range (i-C5H12-n-C8H18) to verify the biomarker-based classification of oil families. The oils belong to four distinct compositional oil families — A, B, C and D—broadly confined to specific stratigraphic intervals. The unique character of each oil family, evident from their n-alkane and biomarker signatures, is supported by distinctive gasoline range characteristics in general, and C7 (“Mango”) parameters in particular. An invariance in the K1 parameter among oils from a single compositional group is observed for most of the oils. The K1 ratio, although relatively constant within each suite of oils, is different for each oil family, clearly indicating their compositional distinction. Other Mango parameters (N2, P2, P3) show a similar reflection of the oil families. However, while C7 parameters provide excellent evidence for distinct familial association of oils from families A, B and D, family C often overlaps with the latter two families, perhaps indicating greater genetic and source heterogeneity in the family C oils. Nevertheless, differences in the gasoline range composition suggest that the existing biomarker-based classification of oil families can be more universally applied throughout the entire Williston Basin. Moreover, because the light hydrocarbon parameters prove very useful in refining oil–oil correlations, routine gasoline range analysis shows good potential as a supplementary component in geochemical correlation of crude oils, especially when high levels of thermal maturity decrease the usefulness of biomarker compounds.