The three major types of crude oil in the Williston basin--the type I oils of the Winnipeg-Red River system, the type II oils of the Bakken-Madison system, and the type III oils of the Tyler-Pennsylvanian system--can be distinguished by their sulfur isotope compositions. They have characteristic ^dgr³⁴S values of 5.8 ± 1.2 parts per thousand (ppt), 2.8 ± 0.8 ppt, and -4.0 ± 0.7 ppt respectively. Highly mature oils have less typical values.

Type II oils which have migrated over a distance of some 150 km beyond the region of generation have maintained their characteristic ^dgr³⁴S values even though sulfur may have been lost. This indicates little or no interaction with reservoir sulfates under normal circumstances.

On the periphery of the basin, type II oils altered by water washing and biodegradation have altered ^dgr³⁴S values which increase from +2.9 to +9.4 ppt with the increasing degree of crude oil degradation.

The Bakken shales, source of the type II oils, have ^dgr³⁴S distribution patterns in the reduced sulfur typical of marine sediments. The ^dgr³⁴S values for the type II oils match most closely the ^dgr³⁴S value of organic sulfur in the black bituminous shales of the lower Bakken.