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

Abstract

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.