Kinetics and fate of organosulphur compounds during the metagenesis stage of thermal maturation: hydrous pyrolysis investigations on dibenzothiophene

Abstract Organosulphur compounds (OSCs) play important roles in the formation, preservation, and thermal degradation of sedimentary organic matter associated with petroleum generation. However, with the ongoing develop of hydrocarbon prospecting in ultra-deep reservoirs, the thermal maturation and geochemical behaviours of OSCs in deep sedimentary basins during the metagenesis stage of thermal maturation has not been well understood. In this study, we modelled the thermal maturation of OSCs by a combination of gold-tube hydrous pyrolysis studies and quantum mechanics calculations based on density functional theory (DFT) to investigate the reaction pathways and kinetic properties. Dibenzothiophene (DBT) was selected as an over-mature model molecule due to its high degree of aromaticity and lack of side chains. The energy barrier of aryl-aryl bonding is estimated to be approximately 60 kcal/mol. Dehydrogenation of OSCs through aryl-aryl bonding mainly occurs in the Easy%Ro range of 2.3%–3.8%. We modeled the contribution of OSCs to organic-derived H2S at advanced maturity based on numerical simulations in MATLAB. The contribution of OSCs to H2S can be quantitatively predicted based on specific thermal histories and the kinetics of H release from biaryl formation. Taking the Sinian reservoirs in the central uplift of the Sichuan Basin as an example, the yield of organic-derived H2S contributed from S-rich solid bitumen (S/C atomic ratio ~0.06) was estimated to be approximately 1.58 mg/g. These results suggested that OSCs participated as carriers of organic S from the surface to deep strata. Desulphurisation of OSCs together with thermochemical sulphate reduction, could contribute to the subsurface accumulation of H2S, to the detriment of. the quality of hydrocarbon reservoirs and the durability of the drill.