Evaluation of source rocks in the 5th member of the Upper Triassic Xujiahe Formation in the Xinchang Gas Field, the Western Sichuan Depression, China

The 5th member of the Upper Triassic Xujiahe Formation (T3x5) is one of the important targets for terrigenous natural gas exploration in the Xinchang Gas Field in the Western Sichuan Depression, however, the quality of the T3x5 source rocks is poorly studied and the understanding of it is controversial. Studies have been conducted on the development and organic geochemical characteristics of the T3x5 mudstone in the Xinchang Gas Field, and the results indicate that, the T3x5 mudstone in the Xinchang Gas Field display an average thickness of 285.3 m, and the thickness of each submember increases from north to south. The average TOC content, hydrocarbon potential (S1+S2) and chloroform bitumen “A” content of T3x5 mudstone are 2.17%, 2.26 mg/g and 0.264‰, respectively, with the organic abundance mainly reaching the standard of medium source rocks. The kerogen type index (TI) is lower than 0 with the mean δ13C value of −25.0‰, indicating mainly humic and sapropelic-humic types of organic matters. The average vitrinite reflectance (RO) value is 1.17%, indicating the stage after the peak of oil generation. The gas generation intensity of T3x5 mudstone is generally in the range of (3-16) × 108 m3/km2, which suggests the resource basis for the generation of medium-small gas fields. The total amount of natural gas generated by T3x5 mudstone in the Xinchang Gas Field is relatively low, and the generated gas cannot completely displace the water in the formation, therefore, the fullness of gas in the reservoirs is generally low, which causes the simultaneous production of gas and water and the low gas production in the process of production test.