Geochemical characteristics and sources of natural gas reservoired in Lower Triassic Jialingjiang Formation in Naxi-Hejiang area,Southern Sichuan Basin

Based on the combination of chemical and C and H isotopic compositions of the natural gases reservoired in the Jialingjiang Formation(Fm) and characteristics of the source rocks developed in the Naxi-Hejiang area,the natural gases of the Jialingjiang Fm were thought to be a kind of mixed gas.It mainly consisted of the oil-type dry gas generated from the carbonate source rocks of Qixia Fm-Maokou Fm of Lower Permian,and the coal-derived gas from the Liangshan Fm of Lower Permian,and the Longtan Fm of Upper Permian.Additionally,the Longmaxi Fm of Lower Silurian possibly had a certain contribution.Hydrogen isotopes of the Jialingjiang Fm gases in the studied area were relatively enriched in D,while the alkane gases in the Naxi gas field present a normal carbon isotope distribution and partial reversal of hydrogen isotope.It was widely found that both carbon and hydrogen isotopes of the alkane gases occurred reversal for the Jialingjiang Fm natural gases in the Miaogaosi,Hejiang and Erlichang gas fields,showing the characteristics of δ13C1 δ13C2 δ13C3 and δD1 δD2 δD3,respectively.The reversal of carbon and hydrogen isotopes for alkane gases was probably caused by two reasons: the first one was the mixing of the oil-type gas and the coal-derived gas,the other one was mixing of the oil-associated gas and the oil-cracking gas.