Organosulfur compounds: Molecular and isotopic evolution from biota to oil and gas

Organosulfur compounds (OSCs) play important roles in the formation, preservation, and thermal degradation of sedimentary organic matter and the associated petroleum generation. Improved analytical techniques for S isotope analysis have recently enhanced our understanding of the mechanisms for OSC formation and maturation and their associated S isotope distributions. The close interaction of OSCs with inorganic S species throughout their formation and maturation affects their ³⁴S/³²S isotopic ratio (δ³⁴S), forming specific signatures for distinct sources and processes. Ultimately, thermal maturation homogenizes the δ³⁴S values of different fractions and individual compounds. Reservoir processes such as thermochemical sulfate reduction (TSR) introduce exogenous and isotopically distinct S into hydrocarbons and can significantly change the δ³⁴S of petroleum or kerogen. Specific OSCs react at different rates and thus can be used to evaluate the extent of processes such as TSR. This article reviews factors that affect the ³⁴S/³²S isotopic distribution of OSCs along pathways of formation, diagenesis, and thermal alteration.