Analyses of fifteen oils produced from fractured shales of the Upper Devonian and Lower Mississippian Bakken Formation in the Williston Basin were compared to the same analyses for eighteen Williston Basin oils produced from conventional reservoirs in the middle of the Lower and Upper Mississippian Madison Group. All oils came from the U.S.A. portion of the Williston Basin. These comparisons demonstrate that, contrary to previous studies and the models developed from those studies, the Bakken and mid-Madison oils are different oil families from different source rocks. Our findings thus agree with recent work on the Canadian side of the Williston Basin (Osadetz et al., 1992). Within the saturated hydrocarbons (HCs) the Bakken oils are more naphthenic, less paraffinic (especially less waxy), have different n-paraffin and isoprenoid-HC profiles, and have different compound ratios for ＇＇generic＇＇ saturated HCs, as compared to the Madison oils. Within the aromatic HCs the mid-Madison oils have an unusual trimethylnaphthalene distribution, high concentrations of dibenzothiophenes and alkylated variants thereof, and different aromatic-HC peak distributions in general (and thus different aromatic-HC compound ratio plots), as compared to the Bakken oils. Because of wide maturity ranges in both oil sets, biomarkers were of limited utility as source-facies indices and thus as correlation tools. We have not yet found any oil generated by the Bakken shales or any mixing of Bakken and Madison oils in a Madison reservoir. We believe the range of variation in our correlation indices to be too large among the Madison oils for these oils to have a single source rock. For this and other reasons, we hypothesize that the Madison oils are from separate source rocks interbedded with the different mid-Madison reservoirs in the basin, rocks which appear to have the same, or very similar, depositional environments. Thus, the original Bakken/mid-Madison petroleum system appears dysfunctional. The results of this study have significant implications for: 1) models of both oil expulsion and deep-basin fluid flow; and 2) the possible existence of very large oil-resource bases in mature, organic-rich, fractured, fine-grained rocks in petroleum basins worldwide.