Changes in sulfur content and sulfur-isotope ratios with "thermal maturation" have been studied using Big Horn basin (Wyoming) Paleozoic oils as examples of "single source" oils which have attained different stages of maturity as a result of variations in thermal history. With increasing maturity, API°, GOR, S/N, ^dgrC¹³, and ^dgrS³⁴ all increase, whereas percentage S and percentage N decrease. Except for the increase in ^dgrS³⁴ and S/N, these changes generally are recognized as typical of the thermal-maturation process.

Hydrogen sulfide produced in low concentrations by microbial sulfate reduction in shallow reservoirs varies in ^dgrS³⁴ and generally does not appear to change ^dgrS₃₄ of associated oil. Isotopically unrelated H₂S and organic sulfur may remain for long times because of negligible reaction between H₂S and oil at low temperatures and low H₂S pressures.

The major new conclusion from this study is that thermal maturation in high-temperature reservoirs (more than 80-120°C) with sulfate present may involve nonmicrobial sulfate reduction with a negligible isotopic fractionation, producing reduced sulfur species with nearly the same isotopic composition as the reservoir sulfate. In this case, sulfurization and desulfurization of oil compete in kinetically controlled processes resulting in isotopic exchange. The ^dgrS³⁴ of H₂S and organic sulfur in oils change toward that of reservoir sulfate. Exchange is faster for H₂S than for oil. Initial oils with a homogeneous ^dgrS³⁴ distribution with boiling point and compound type become heterogeneous; the lower boiling fractions approach reservoir s lfate values faster than high-boiling fractions. The large increase in S/N ratio with maturation is attributed to percent S being maintained at a significant level by competing sulfurization and desulfurization processes, whereas percent N continues to decrease.

The mechanism for high-temperature sulfate reduction is proposed to be the reaction of H₂S and SO₄⁼ to produce elemental sulfur and polysulfides, which react rapidly to oxidize and dehydrogenate organic compounds and distribute the sulfur between oil and H₂S. High concentrations of H₂S may accumulate in this case, and oils or condensates may develop abnormally high concentrations of thiols. H₂S is a catalyst as well as a product of the reaction; the process, therefore, may be autocatalytic.