Application of sulfur isotopes of volatile organic sulfur compounds to determine the natural gas secondary alterations and possible sources in the Tarim Basin, NW China

Abstract

Currently, there are very limited methods for direct gas-source rock correlation studies. Herein, we applied sulfur isotope analyses of individual volatile organic sulfur compounds (VOSC) in natural gases and dibenzothiophenes (DBTs) in the associated oils from Ordovician reservoirs in Shunbei and Tahe areas of the Tarim Basin, and coupled them with characteristics of gaseous hydrocarbons to elucidate gas-source correlation and secondary alterations in the reservoirs. The results show that the VOSCs in some of the gas samples have reached isotopic equilibrium with the associated H2S but with distinct sulfur isotope compositions, suggesting that they may be related to the thermal cracking of kerogen and/or oil, or altered by thermochemical sulfate reduction (TSR). The δ34S values of VOSCs in the gases unaffected by TSR range from +14.9 to +21.3‰, which are correlated well with the Cambrian source rock kerogen and DBTs from its derived oils, indicating the gases may have a Cambrian source. Few gas samples from the Tahe area exhibit higher δ34S values of VOSCs (from +22.9 to +31.6‰) but with no evidence for TSR alteration in the associated oils. This suggests that exogenous TSR-H2S was charged into the reservoirs, and altered the δ34S values of the VOSCs. The sulfur isotopes of VOSCs in a gas sample range from +27.3 to +34.8‰ which are close to the Cambrian anhydrite but are not in isotopic equilibrium with the associated H2S. This suggests that the VOSCs have interacted with recently migrated H2S produced by TSR and the increasing concentrations of H2S might have prevented the system from reaching isotopic equilibrium with the VOSC. Thus, VOSC can provide an additional layer of information when interpreting gas generation and migration processes within a complex basin such as the Tarim Basin.