Big Horn basin (Wyoming) Paleozoic oils are believed to have been similar in composition initially, but they now differ greatly as the result of maturation caused by variations in thermal history. With increasing maturity, API°, GOR, S/N, δ C13 and δ S34 all increase whereas the percentage of sulfur, nitrogen, and asphaltenes decreases. Except for increases in δ S34 and S/N ratio, these changes are generally recognized as typical of the thermal-maturation process.
δ C13 increases are reasonably explained by C12 enrichment in evolved gas. Profiles of δ C13 versus B.P. show systematic changes with maturation. In particular, a δ C13 maximum in the 50-125°C B.P. range increases with maturity, suggesting that molecules in this size range have undergone more cleavages, on the average, than higher MW-components.
Isotopic evidence indicates that H2S produced by microbial reduction of sulfate in shallow reservoirs (low temperature) generally does not react sufficiently with associated oil to alter δ S34 of organic sulfur. δ S34 of oils and H2S are essentially unrelated in these cases.
Thermal desulfurization of organic sulfur compounds occurs with negligible isotopic fractionation. However, isotopic evidence indicates that, in the temperature range of 170-300°F, sulfate reduction (probably nonmicrobial) occurs slowly without isotopic fractionation, and the produced sulfide is incorporated into both oil and H2S. Organic sulfur thus becomes a dynamic system with competing sulfurization and desulfurization leading to changes in δ S34 toward that of the reservoir sulfate (about 15‰ heavier isotopically than S in initial oils). The percentage of sulfur in oil, thus may attain a steady-state concentration although the percentage of nitrogen continues to decrease, resulting in increasing S/N ratios with increasing maturity. These changes in δ S34 and S/N ratio would not be expected in reservoirs devoid of sulfate.