The characterization of genetic crude oil types within a given petroleum province can be accomplished readily by 4 commonly used techniques--carbon isotope ratio measurement, gas chromatography, optical rotation measurement, and infrared spectrophotometry. Obtaining carbon isotope ratios on both the whole oil and its aliphatic fraction is desirable. Gas chromatography is carried out on the C₄-C₇ fraction and the C₁₅+ fraction of each oil. A semilog plot of carbon isotope ratio versus optical rotation is helpful in characterizing oil types.

Application of these techniques to approximately 200 oils from the Williston basin has revealed the presence of 3 major oil types within the basin. One type occurs primarily in Ordovician reservoirs, but is found in some Silurian, Devonian, and Mississippian reservoirs as well; a second type is found almost entirely in Mississippian Madison reservoirs, and the third type is restricted to Pennsylvanian Tyler reservoirs. A few oils either were derived from minor sources or were modified by contributions from minor sources. Other oils appeared to be mixtures of 2 major types. A classic example of commingled Ordovician and Madison types was found in Weldon field. The effects of thermal maturation are evident on the carbon isotope optical rotation plot.

The source of the Madison-type oils appears from geochemical data to be the Upper Devonian-Lower Mississippian Bakken Shale. The Tyler-type oil seems to have originated in the Tyler shales. Geochemical data have not established clearly the source for the Ordovician-type oil, but its prevalence in Ordovician reservoirs suggests the Ordovician Winnipeg Shale as the probable source.

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