ORIGINAL RESEARCH article
Front. Earth Sci.
Sec. Cryospheric Sciences
Volume 13 - 2025 | doi: 10.3389/feart.2025.1573369
Sources of shallow natural gases in the Longgang marine gas field, NE Sichuan Basin: An example from the Water Conveyance Tunnel
Provisionally accepted
Wang Shilin1*
Xiangqian Guo1Peidong Su1Long Li2Mohamed S. Ahmed3Xiaojin Chu4Xiangbing Zhang4
Thomas Gentzis5
Ahmed Mansour1*- 1Southwest Petroleum University, Chengdu, China
- 2Sichuan Water Development Investigation, Design & Research Co.,Ltd., 610000, ., Chengdu, China
- 3King Saud University, Riyadh, Riyadh, Saudi Arabia
- 4China Railway 18th Bureau Group Corporation Limted , 300222., Tianjin, China
- 5Core Laboratories (United States), Houston, Texas, United States
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With the advancement of the Tingzikou Irrigation Area Water Diversion Project, the issue of harmful shallow underground gases in the tunnel has become increasingly prominent. Identifying the causes, sources, and migration processes of these harmful gases is crucial to prevent possible catastrophic impacts and mitigation during tunnel construction. Here, we apply an integrated approach of carbon isotope composition to analyze the geochemical characteristics of shallow natural gas components from a high-gas water diversion tunnel in the Tingzikou Irrigati Area.Results are integrated and compared with data from the Sichuan Basin fault system and isotopic values from different strata to provide further evidence of the source of shallow natural gas in the area. The geochemical characteristics of the gas samples reveal that the methane content is the most abundant, and constitutes as high as 45.16%, while carbon oxides are in the range of 0.02-0.1%.Heavier hydrocarbons (C2+) account for 2.27%, and the dryness coefficient is 0.96. These major components indicate the presence of wet gas with relatively low thermal maturity. The alkane gases exhibit a normal carbon sequence, with δ 13 C1 values less than -25‰, indicating that the gas originates from oil-type organic. The carbon dioxide is classified as Type II inorganic gas, and the hydrogen sulfide is attributed to thermochemical sulfate reduction (TSR) origin. In the tunnel area, δ 13 C2 ranges from -29.53‰ to -28.11‰, with an average of -28.80‰, which are similar to the ethane isotope values of the J1l and J1m strata in the central Sichuan Basin, but heavier than those of the J1dn, J1d, and T2l strata, and significantly different from T3x. This reveals that the shallow natural gas originates from the shallow oil and gas source rock layers of the Ziliujing Formation at the Ziliujing anticline, making it a "self-generated and self-stored" gas.
Keywords: Carbon Isotopes, Harmful gases, Sichuan Basin, Shallow gas characteristics, Water diversion tunnel
Received: 08 Feb 2025; Accepted: 03 Jun 2025.
Copyright: © 2025 Shilin, Guo, Su, Li, Ahmed, Chu, Zhang, Gentzis and Mansour. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence:
Wang Shilin, Southwest Petroleum University, Chengdu, China
Ahmed Mansour, Southwest Petroleum University, Chengdu, China
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