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, ^dgr C¹³ and ^dgr S³⁴ all increase whereas the percentage of sulfur, nitrogen, and asphaltenes decreases. Except for increases in ^dgr S³⁴ and S/N ratio, these changes are generally recognized as typical of the thermal-maturation process.

^dgr C¹³ increases are reasonably explained by C¹² enrichment in evolved gas. Profiles of ^dgr C¹³ versus B.P. show systematic changes with maturation. In particular, a ^dgr 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 ^dgr S³⁴ of organic sulfur. ^dgr 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 ^dgr S³⁴ toward that of the reservoir sulfate (about 15^pmil 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 n ^dgr S³⁴ and S/N ratio would not be expected in reservoirs devoid of sulfate.

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