Northeastern Sichuan Basin is one of the most significant areas for natural gas exploration in China, and Puguang and Maoba are the two most important and typical gas fields there. The common characteristics of the natural gases in the Puguang and Maoba gas fields include extremely high dryness, high H2S contents, high δ13C1 values, with a wide range of δ13C2 values. Based on gas composition and stable carbon isotope values, natural gases in the study area are classified into three genetic groups, derived from type III kerogen, type II kerogen and those with a complex origin. The latter group is most likely the result of mixing between low maturity ethane inherited from earlier oil accumulation and high maturity methane generated from subsequent in-reservoir oil cracking and thermochemical sulfate reduction. In conjunction with regional source rock data, the Upper Permian Longtan Formation is considered to be the main source rock for the gases in the Feixianguan-Changxing reservoirs of the Puguang and Maoba gas fields.
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