Genetic types and sources of natural gases reservoired in the second member of the Xujiahe Formation in the Xinchang area of the Western Sichuan Depression

The second member of the Upper Triassic Xujiahe Formation is an important exploration target for terrestrial natural gases in the Sichuan Basin.The geochemical characteristics of natural gases indicated that the gases reservoired in the second member of the Xujiahe Formation in the study area have high dryness coefficients generally over 0.95.The δ¹³C₁, δ¹³C₂ and δD₁ values are in the ranges of -34.5‰~-30.3‰, -29.1‰~-23.4‰ and -168‰~-157‰, respectively.The alkane gases mainly display positive stable carbon and hydrogen isotope values with several samples being partially reversed between ethane and propane carbon isotopes.These gases are of mixed origin, and the heavy hydrocarbon gases were mainly derived from the secondary cracking of oil augmented with coal-type gas, whereas methane was mainly a coal-type gas.Gas-source correlations indicated that the natural gases reservoired in the second member of the Xujiahe Formation were derived mainly from the source rocks in the Ma'antang and Xiaotangzi formations and augmented by those in the second member of the Xujiahe Formation itself.