Constraints of molecular and stable isotopic compositions on the origin of natural gas from Middle Triassic reservoirs in the Chuanxi large gas field, Sichuan Basin, SW China

•The Middle Triassic natural gas in the Chuanxi gas field is of oil-cracking gas.•The gas experienced weak TSR alteration without the involvement of ethane.•The gas pools were mainly formed by direct filling of secondary cracking gas.•The gas was originated from source rocks of Late Permian mudstones and marlstones.