Geochemical characteristics and source of natural gases from Southwest Depression of the Tarim Basin, NW China

•The alkanes were derived from humic source rocks with varying thermal maturities.•The gases from Kekeya and Kedong fields are mixed with oil-associated gas.•CO2 from Akemomu field was derived from the thermal decomposition of limestones. The molecular compositions and stable carbon and hydrogen isotopes of natural gases from the Kekeya, Kedong and Akemomu gas fields in the Southwestern Depression of the Tarim Basin were investigated to distinguish between coal-derived and oil-associated gases. The carbon and hydrogen isotope ratios indicate that the gaseous alkanes from these three fields were mainly coal-derived and sourced from the Middle–Lower Jurassic humic source rocks. The gaseous alkanes in natural gas from the Kekeya and Kedong fields were generated from low maturity humic source rocks. However, the light hydrocarbons display the characteristics of marine sapropelic gas, and the high δD1 values and partial carbon isotopic reversal of alkanes indicated that the gases had been mixed with oil-associated gas derived from the Carboniferous–Permian sapropelic source rocks. The alkanes in natural gas from the Akemomu Field display high carbon isotopic ratios and were probably derived from the Middle–Lower Jurassic humic source rocks in high to overmature stage. The high contents of CO2 with high δ13CCO2 values were mainly derived from the thermal decomposition of Carboniferous limestones. The high content of N2 was likely derived from highly overmature coal measure source rocks.