Organosulfur Compounds: Molecular and Isotopic Evolution from Biota to Oil and Gas

Abstract

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.

Sulfur Cycling in an Iron Oxide-Dominated, Dynamic Marine Depositional System: The Argentine Continental Margin

“…In the uppermost surface sediments of organic-rich, sulfide-dominated marine sedimentary systems, sulfurization of labile organic compounds, such as humic acids (François, 1987;Ferdelman et al, 1991), can precede the formation of pyrite (Mossmann et al, 1991;Vairavamurthy et al, 1992Vairavamurthy et al, , 1995Filley et al, 2002;Werne et al, 2008). The isotopic composition of the precursor inorganic sulfur species, such as hydrogen sulfide, polysulfides, or elemental sulfur, should thus be recorded in the resulting organic sulfur compounds (Werne et al, 2008;Amrani, 2014). Unfortunately, the bulk organic sulfur fraction, with an isotopic signature representing a mixture of the distinct organic sulfur compounds, does not allow us to distinguish between different sulfurization pathways (Werne et al, 2003); only compound-specific approaches may provide some further insights (Amrani and Aizenshtat, 2004;Raven et al, 2016).…”

“…The TOS content of sediments located in the sulfidic zone between the two TOS enrichment peaks fall to almost background levels, likely indicating that sulfurization of particulate organic matter occurs at the sulfidic fringes of the sulfide zone but not in the center of the SMT where hydrogen sulfide displays the highest concentrations. Polysulfides have been shown to be stronger nucleophiles for the sulfurization of organic matter, and nucleophilic substitution has also been recognized to be the dominant sulfurization mechanism (Amrani, 2014, and references therein). If sulfurization of organic matter is indeed tied to polysulfides, our findings imply that the center of the SMT has lower polysulfide concentrations than the border region of the SMT.…”

Effect of thermal maturity on remobilization of molybdenum in black shales

“…Experiments have identified significant kinetic isotopic fractionation during microbial sulfate reduction due to initial breaking of the S-O bond in sulfate (Harrison and Thode, 1958;Cross et al, 2004). Hence, MSR is generally associated with large sulfur isotope fractionation between sulfate and H 2 S ( MSR = −15 to −65h depending on the type of bacteria; Canfield, 2001;Amrani, 2014). Compared to MSR, TSR results in a much smaller degree of sulfur isotopic fractionation between sulfate and H 2 S. TSR is temperature dependent and ranges from −15h to −10h at 150-200 • C (Krouse et al, 1988;Machel et al, 1995).…”

Variations of Stable Isotope Ratios in Nature

“…These results indicate that the origin and fate of organic compounds are far more complicated than was previously assumed. Individual organosulfur compounds may reveal 34 S-variations of up to 50 ‰ in oils from the same source rock (Amrani et al 2012;Amrani 2014).…”