Effects of thermal maturation and thermochemical sulfate reduction on compound-specific sulfur isotopic compositions of organosulfur compounds in Phosphoria oils from the Bighorn Basin, USA. (January 2017) - Ellis, Geoffrey S.
Highlights: Compound-specific sulfur isotopes distinguish TSR and non-TSR altered oils. Thermal maturation of source rocks may lead to 34 S-enriched sulfur compounds. δ 34 S results suggest charge of TSR-derived H2 S in a reservoir in the Bighorn Basin. δ 34 S of sulfur compounds can constrain sulfate sources in TSR reservoirs. Abstract: Compound-specific sulfur isotope analysis was applied to a suite of 18 crude oils generated from the Permian Phosphoria Formation in the Bighorn Basin, western USA. These oils were generated at various levels of thermal maturity and some experienced thermochemical sulfate reduction (TSR). This is the first study to examine the effects of thermal maturation on stable sulfur isotopic compositions of individual organosulfur compounds (OSCs) in crude oil. A general trend of 34 S enrichment in all of the studied compounds with increasing thermal maturity was observed, with the δ 34 S values of alkyl-benzothiophenes (BTs) tending to be enriched in 34 S relative to those of the alkyl-dibenzothiophenes (DBTs) in lower-maturity oils. As thermal maturity increases, δ 34 S values of both BTs and DBTs become progressively heavier, but the difference in the average δ 34 S value of the BTs and DBTs (Δ 34 S BT-DBT) decreases. Differences in the isotopic response to thermal stress exhibited by these two compound classes are considered to be the result of relative differences in their thermal stabilities. TSR-altered Bighorn Basin oils have OSCs that are generally enriched in 34 S relative to non-TSR-altered oils, with the BTs being enriched in 34 S relative to the DBTs, similar to the findings of previous studies. However, several oils that were previously interpreted to have been exposed to minor TSR have Δ 34 S BT-DBT values that do not support this interpretation. The δ 34 S values of the BTs and DBTs in some of these oils suggest that they did not experience TSR, but were derived from a more thermally mature source. The heaviest δ 34 S values observed in the OSCs are enriched in 34 S by up to 10‰ relative to that of Permian anhydrite in the Bighorn Basin, suggesting that there may be an alternate or additional source of sulfate in some parts of the basin. These results indicate that the sulfur isotopic composition of OSCs in oil provides a sensitive indicator for the extent of TSR, which cannot be determined from other bulk geochemical parameters. Moreover, when combined with additional geochemical and geologic evidence, the sulfur isotopic composition of OSCs in oils can help to identify the source of sulfate for TSR alteration in petroleum reservoirs.