Genetic-specific biomarker ratios and carbon isotopes of Williston Basin produced oils were used to identify three oil families derived from Mississippian carbonate source rocks. Multivariate statistical analyses of these geochemical parameters provided information on source rock type, depositional environment and oil thermal maturity. These carbonate families differ due to algal sterane distributions and stable carbon isotopic compositions. Family 3 oils are sourced from the most carbonate-rich facies and have the highest sulfur content, while Family 2 oils were generated from more of a ‘marl’ facies. Family 1 is intermediate with respect to source rock lithology. The most mature oils of Family 1 & 2 tend to be associated with Williston Basin depocenters. Oil Family 3 members are the least thermally mature. Family 1 oils are generally located along an east to west trend in northern ND and MT while Family 2 oils occur in a north to south trend along the ND and MT border. Family 3 oils predominantly occur in Saskatchewan. Three Madison oil families suggest three discrete Madison source horizons and more exploration opportunities, especially for unconventional resources.
The molecular and isotopic composition of produced oil may be used to predict various geological and geochemical aspects of the oil's corresponding source rock, including organo-facies, lithology, depositional environment, source rock age, thermal maturity, and, at times, migration distance and relative direction (Peters et al., 2005; Zumberge et al., 2013). Biomarkers, such as terpanes and steranes, function as molecular fossils, and even though the oil may have migrated from its source, fossil evidence as to the nature of the source is carried with the oil. In our continuing evaluation of oils and source rocks from the Williston Basin, we have identified three distinct groups or families of Mississippian-reservoired oils that were derived from carbonate-rich source rocks. This identification resulted from multivariate statistical analyses of genetic-specific terpane and sterane biomarker ratios (molecular fossils) as well as stable carbon isotope values of the C15+ hydrocarbon fractions (cluster analysis dendrogram in Figure 1 and principal component analysis in Figure 2). For details of the analytical methodology and statistical approach, refer to Zumberge, et al., 2005. Specifics concerning the various biomarker ratios are discussed in Peters, et al., 2005.