Differences in gas sources of the Changxing–Feixianguan formations around the Kaijiang-Liangping Trough and favorable exploration directions for coal-formed gas generated by the Upper Permian Longtan Formation, Sichuan Basin, China

Abstract

Through the analysis of natural gas composition, carbon and hydrogen isotopes of alkane gases, reservoir bitumen, source rock conditions, and source–reservoir combinations, this study clarifies the differences in gas sources and the origins of natural gas in the Permian Changxing Formation–Triassic Feixianguan Formation on both sides of the Kaijiang–Liangping Trough. Additionally, it identifies favorable exploration directions for coal-formed gas generated by the Longtan Formation in the Sichuan Basin. The following understanding was obtained: (1) The natural gas in the Changxing–Feixianguan formations mainly composed of alkane gas, typical of dry gas. (2) The carbon isotope values of methane and ethane in the Changxing–Feixianguan formations on the east side of the trough are lower than those on the west side. Specifically, the ethane carbon isotope value in the Longgang Gas Field on the west side is higher than that in the Yuanba Gas Field, while the methane hydrogen isotope value is lower in the Longgang Gas Field compared to the Yuanba Gas Field. (3) The natural gas in the Dongyuezhai, Puguang, and Yuanba gas fields predominantly originates from sapropelic organic matter of the Wujiaping Formation, with kerogen types II₁–I; in contrast, the Longgang Gas Field contains a mixture of coal-formed gas and oil-type gas, with a slightly higher content of coal-formed gas, originating from mixed organic matter of the Wujiaping Formation, with kerogen type II₁–II₂. (4) Multiple types of gas, such as coal rock gas, tight sandstone gas, and shale gas, can be formed within the Longtan Formation. The Suining–Luzhou and Langzhong–Guang'an–Fuling areas are identified as favorable zones for the exploration of coal rock gas and marine–continental transitional shale gas, respectively. Additionally, the reef and shoal development areas of the Changxing Formation in the Suining–Hechuan and Guang'an–Nanchong regions are also favorable exploration areas for coal-formed gas.