LAUSR

Abstract The 5th member of the Upper Triassic Xujiahe Formation (T 3 x 5 ) 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 T 3 x 5 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 T 3 x 5 mudstone in the Xinchang Gas Field, and the results indicate that, the T 3 x 5 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 (S 1 +S 2 ) and chloroform bitumen “A” content of T 3 x 5 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 δ 13 C value of −25.0‰, indicating mainly humic and sapropelic-humic types of organic matters. The average vitrinite reflectance ( R O ) value is 1.17%, indicating the stage after the peak of oil generation. The gas generation intensity of T 3 x 5 mudstone is generally in the range of (3–16) × 10 8  m 3 /km 2 , which suggests the resource basis for the generation of medium-small gas fields. The total amount of natural gas generated by T 3 x 5 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.