Abstract:

The complex geological structure of the QinghaiXizang plateau yields a dense distribution of faults and considerable lithology variations. Existing research fails to effectively predict the types and states of hazardous gases within rock layers. Therefore, based on the research results of the geological evolution of the western Sichuan and eastern Xizang regions, the origin and composition characteristics of hazardous gases under the influence of rock, fault structure, and hydrothermal cycle are examined. Based on the analysis of the geological structure and rock distribution in this region, the types and distribution ranges of hazardous gas reservoirs are established and risk assessment system for hazardous gas leakage in complex formations is established and verified. The findings are as follows: (1) The main hazardous gases in this region include gases released by magmatic rocks, those generated by the chemical reactions of metamorphic rocks, those enriched by oil and gas sedimentary rock deposits, and those associated with metal mines. Hazardous gases generated in igneous rocks are primarily generated within the gas pockets formed along the intrusion channels, those generated in metamorphic rocks are mainly present within the contact zones between acidic volcanic rocks and carbonates, as well as near metal ore deposits, and those generated in sedimentary rocks are predominantly accumulated within oil and gas cap layers. (2) The hydrothermal cycle model of the fault zone in this region could be categorized into five types: deep major fault control, secondary fault control, secondary fault and soluble rock control, deep major fault and high permeability rock control, and shallow fault control. The migration of hazardous gases in the fault zones is mainly categorized into two types: material diffusion of soluble gases and hydraulic transport of insoluble gases. (3) Based on the stratigraphic lithology and fault structural characteristics of this region, the storage areas for major hazardous gases, such as CO2, CH4, SO2, and H2S, are divided. (4) Based on the characteristics of stratum lithology, fracture structure, and hydrothermal distribution in this region, a risk assessment system for hazardous gas leakage in complex formations is established. The system is divided into five risk levels based on the possibility of highconcentration hazardous gas release during tunnel excavation, and the corresponding prevention and control suggestions are proposed for different risk levels.

Key words: western Sichuan and eastern Xizang regions, hazardous gas, tunnel surrounding rock, fault zone, assessment system, formation mechanism