Comparative study of stable carbon and hydrogen isotopes of alkane gases sourced from the Longtan and Xujiahe coal-bearing measures in the Sichuan Basin, China

• The δ13C2 value is a useful parameter to distinguish coal-derived and oil-associated gases.
• Secondary migration has little influence on the carbon and hydrogen isotopic composition of gas.
• The origin of CO2 in the P2l gases may be erosion of the carbonate reservoir.

Two sets of coal-bearing measures, the Upper Triassic Xujiahe Formation (T3x) and the Upper Permian Longtan Formation (P2l), have been extensively developed in the Sichuan Basin, SW China and some large gas fields related to these coal-bearing measures have been discovered. The stable carbon and hydrogen isotopic composition of alkane gases sourced from the two sets of coal-bearing measures (T3x and P2l gases) have been investigated in this study. The main controls on the gas composition and isotopic abundances of gases in the Sichuan Basin have been identified and distinguished on the basis of their molecular and isotopic composition. The carbon and hydrogen isotopic compositions of methane (δ13CC1: − 42.5‰ to − 26.8‰; δDC1: − 173‰ to − 107‰) vary widely and show a positive correlation with the thermal maturity of the source rocks. The δ13CC2 value of the T3x and P2l gases in the Sichuan Basin ranged from − 28.2‰ to − 21.3‰, and showed less variations with increasing maturity from Ro values of 1.2% to 2.8%. However, these δ13CC2 values differ greatly from that of oil-associated gas in the Sichuan Basin. Thus we provide further evidence that the value of δ13CC2 is a useful parameter to distinguish coal-derived and oil associated gases. The hydrocarbon gases in the Xujiahe and Jurassic reservoirs in contrast showed little isotopic difference indicating that secondary migration has little influence on their carbon and hydrogen isotope compositions. In addition, the δ13C values of P2l CO2 (− 2.6− to 3.3‰) may reflect an origin from dissolution of the carbonates in the reservoir by acidic fluids containing H2S.