Changes in Sulfur Content and Isotopic Ratios of Sulfur during Petroleum Maturation—Study of Big Horn Basin Paleozoic Oils

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, δC13, and δS34 all increase, whereas percentage S and percentage N decrease. Except for the increase in δS34 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 δS34 and generally does not appear to change δS34 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 δS34 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 δS34 distribution with boiling point and compound type become heterogeneous; the lower boiling fractions approach reservoir sulfate 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 sulfurlzation 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.