Gas-phase reaction of CH₄ and H₂S – Evidence from pyrolysis experiments and case study from the Sichuan Basin

The application of routine geochemical analyses for identification of dry natural gas sources and post generation processes poses a challenge as its hydrocarbon composition is typically dominated by CH₄ (∼≥99 %) while the presence of non-hydrocarbons may be limited. In particular, the possibility of dry gas interaction with H₂S at reservoir conditions (low temperatures, high pressures, millions of years residence time) is not established as previous studies of this reaction focused on conditions typical for industrial reactors (high temperature, ambient pressure, residence time of seconds). To address these issues, we studied the molecular and isotopic (δ³⁴S) composition of volatile organic sulfur compounds (VOSC) formed during pyrolysis experiments between CH₄ and H₂S at 360 °C (4–96 h), reaching %Ro equivalent of 0.80–1.25 which represents thermally mature oil and gas reservoirs. The results demonstrated that methanethiol (MeSH) is the main product of the reaction, while its δ³⁴S value suggest equilibrium isotopic effect with its parent H₂S. Subsequent analysis of the molecular and isotopic (δ¹³C, δ²H, δ³⁴S) composition of thermogenic, H₂S containing, dry natural gases from the Jiannan gas field in China revealed the presence of short thiols and sulfides dominated by MeSH. The δ³⁴S of the VOSC identified suggests they all formed by gas-phase reaction of alkanes (mainly CH₄) with the associated H₂S. This study demonstrates the applicability of VOSC as a proxy for identification of interaction between H₂S and dry gas and identification of H₂S sources within a gas reservoir or a basin.