Chemical, carbon and sulfur isotopic compositions constrain the origin of Upper Carboniferous-Lower Triassic gases in eastern Sichuan Basin, SW China

Abstract

Methane dominated gas is one of the cleanest energy resources; however, there is no direct method to determine its source rock. Natural gases produced from the eastern Sichuan Basin together with seismic data were studied for their sources and secondary alteration by thermochemical sulfate reduction (TSR). Our results demonstrate that Upper Permian to Lower Triassic (P₃ch-T₁f) gases in the surrounding of the Kaijiang-Liangping area show volatile organic sulfur compounds (VOSCs) δ³⁴S values close to those of the associated H₂S, and may have been altered by methane-dominated TSR, resulting in positive shift in methane δ¹³C₁ values with increasing TSR extents. Other (or group 2) gases produced from the P₃ch-T₁f reservoirs from the southern area and the Upper Carboniferous to Middle Permian (C₂h-P₂q) from the eastern Sichuan Basin are not significantly changed by TSR, show similar δ³⁴S values between the kerogens and some VOSCs, and may have been derived from the Lower Silurian and Middle Permian source rocks. This study demonstrates a case for the first time showing the δ³⁴S values of VOSCs can be used as a tool for direct correlation between non-TSR altered gas and source rocks. Methane-dominated gas pools can be found using gas and source rock geochemistry combined with seismic data.