| Abstrakt: |
East Sichuan is rich in shallow natural gas; however, such gas substantially influences tunnel construction in the region. Accordingly, we analyzed the geological characteristics of shallow natural gas in eastern Sichuan and evaluated the risk the gas poses to tunnels along the Chongqing–Wanzhou (YW) high-speed railway employing a fuzzy synthetic evaluation approach. We conducted isothermal adsorption, gas chromatography, total organic carbon (TOC), carbon isotope, porosity and permeability, and field gas pressure test experiments and thin section examination of rock. Our results showed that the potentially harmful components in shallow natural gas in East Sichuan are mainly CH4, with small amounts of H2, CO, CO2, and H2S. The shallow natural gas mainly derives from shale and limestone in the Xujiahe and Ziliujing formations, but migrates to adjacent reservoirs through intraformational shear fractures in the strata. Sandstones of the Xujiahe and Shaximiao formations are porous reservoirs, with the best reservoir physical properties, belonging to the main enrichment strata of shallow natural gas in the region. Compared with the west of the study area, shallow natural gas is more developed in the east. According to synthetic evaluation of shallow natural gas influence presented in the paper, tunnels along the YW high-speed railway number eight in high-gas zones, four in medium gas zones, and two in no or low-gas zones. [ABSTRACT FROM AUTHOR] |
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