Labile Organosulfur Compounds as Potential Geochemical Organocatalysts in Thermochemical Sulfate Reduction: A Perspective

Abstract:

Thermochemical Sulfate Reduction (TSR) converts sulfate and petroleum-derived organic matter into H2S, CO2, altered hydrocarbons, carbonate and sulfide minerals, solid bitumen, and organosulfur products in deeply buried, high-temperature petroleum systems. Although geologically important, TSR is chemically demanding because water-solvated sulfate is kinetically resistant to reaction with hydrocarbons in neutral to weakly acidic aqueous media. Experimental evidence shows that labile organosulfur compounds, particularly thiols, sulfides, and disulfides, can substantially accelerate TSR. H2S can react with hydrocarbons to generate such labile organosulfur species, whereas density-functional calculations and recent isotopic-computational studies support activation of protonated or contact-ion-paired sulfate during the first S-O bond-cleavage step. These observations support a proposed dynamic labile organosulfur pool-based organocatalytic model, in which labile organosulfur compounds function as reactive sulfur components within a regenerating reservoir- scale network. However, complete molecular turnover has not yet been demonstrated for every organosulfur species. The proposed model provides an operational framework for interpreting the disproportionate rate effects of trace thiols and disulfides, the initiating role of H2S, and the organosulfur molecular fingerprints retained in TSR-altered oils and gases.