The Origin of Natural Gas and Influence on Hydrogen Isotope of Methane by TSR in the Upper Permian Changxing and the Lower Triassic Feixianguan Formations in Northern Sichuan Basin, SW China

Abstract

After analyzing the molecular and stable isotope compositions of 45 natural gas samples in the Changxing and Feixianguan Formations in northern Sichuan basin, significant differences have been observed between the samples in the northwest Kaijiang-Liangping trough and those in the northeast. The δ13C2 values of the gas in the northeast of Kaijiang-Liangping trough (δ13C2=−25.0‰∼−32.3‰ with an average of −28.2‰)are lighter than those in the northwest of Kaijiang-Liangping trough (δ13C2=−21.3‰∼−28.9‰ with an average of −24.8‰). The δD1 values of the gas in the northeast (δD1=−114‰∼−129‰ with an average of −121‰) are also lighter than those in the northwest (δD1=−107‰∼−115‰ with an average of −113‰). By analyzing the kinetic fractionation of carbon isotope in methane and ethane, we find that gas in the northwest of Kaijiang-Liangping trough originated from the P2l coal measures while those in the northeast were sourced from the thermal cracking of oil, which are most likely generated from the marine source rocks in the Upper Permian Longtan formation(P2l) and the Silurian. The CO2 content varies from 0 to 30.20% with the δ13CCO2 values ranging from −2.9‰ to 3.3‰, which suggests that the CO2 may possibly originate from the carbonate reservoirs dissolved by acidic fluids. Thermochemical Sulfate Reduction (TSR) has resulted in the richness of deuterium in the methane.