The characterization of genetic crude-oil types within a given petroleum province can be accomplished readily by use of common techniques such as carbon-isotope ratio measurement, gas chromatography, optical-rotation measurement, and infrared spectrophotometry. 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 quite helpful in characterizing oil types. It is very important that a variety of techniques be applied when characterizing a group of oils.

Application of these techniques to 184 oil samples from the Williston basin has revealed the presence of three major oil types. One type is present primarily in Ordovician reservoirs, but occasionally is found in Silurian, Devonian, and Mississippian reservoirs as well; a second type is 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 two major types. A classic example of commingled types was found in Weldon field. The effects of thermal maturation are evident on the carbon isotope-optical rotation plot.

Geochemical data suggest the Winnipeg shale to be the source for the first oil type. The source of the second type appears to be the Upper Devonian-Lower Mississippian Bakken shale. The third oil type seems to have originated in the Tyler shales.