--> Compound-Specific Sulfur-Isotopic Composition of Organosulfur Compounds in Oil: A Case Study From the Bighorn Basin, WY USA

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Compound-Specific Sulfur-Isotopic Composition of Organosulfur Compounds in Oil: A Case Study From the Bighorn Basin, WY USA

Abstract

Determining the extent of thermochemical sulfate reduction (TSR) in petroleum reservoirs is critical for assessing the risk of hydrocarbon loss through oxidation and reservoir souring via hydrogen sulfide formation. However, distinguishing between the effects of TSR and thermal maturation or other alteration processes (e.g., biodegradation or water washing) can be difficult, particularly during the early stages of TSR. Compound-specific sulfur-isotopic analysis (CSSIA) is a new tool that has the potential to provide additional information related to petroleum formation and alteration. This study applied CSSIA to a suite of 16 Phosphoria Formation-sourced oils from the Bighorn Basin of Wyoming, USA that had undergone TSR to a variable extent. The target analytes included benzothiophene (BT) and dibenzothiophene (DBT), and their methyl-, dimethyl-, and trimethyl-alkylated forms. A general trend of 34S enrichment in all of the studied compounds with increasing source thermal maturity was observed. The δ34S composition of BTs and DBTs in low-maturity oils tends to be lighter than that of the bulk oil, but at higher maturities, they are nearly equivalent. This may suggest that sulfur isotopic homogenization among different sulfur-containing petroleum fractions (e.g., asphaltenes, NSOs, pyrobitumen) occurs during thermal maturation. However, oils that had experienced slight to moderate TSR exhibited a pronounced 34S enrichment in the BT compounds relative to the DBTs, with the BTs being 34S enriched and the DBTs 34S depleted relative to the bulk oil composition. For most of the oils that experienced little or no TSR, the sulfur-isotopic composition of the majority of compounds examined correlated with the bulk δ34S of the oil. In contrast, oils that had experienced the greatest amount of TSR generally do not conform to the compound-specific versus bulk δ34S trends defined by the other oils. All of the oils exposed to the highest degree of TSR (four samples) showed a consistent pattern of δ34S compositions in the alkylated BTs and DBTs that was not observed in the other oils. The possibility that this is an inherited signal from a distinct facies in the Phosphoria source rocks (e.g., Meade Peak versus Retort Member) can largely be discounted based on biomarker and other geologic data. It is most likely to be a reflection of the TSR mechanism, whereby specific BT and DBT isomers are preferentially involved in the reaction.