川西藏东地区隧道围岩有害气体孕育机制及逸出风险评价研究

Formation Mechanism and Releasing Risk Assessment of Hazardous Gas in Tunnel Surrounding Rocks in Western Sichuan and Eastern Xizang,China

青藏高原复杂的地质构造使区域内断裂分布密集、岩性变化巨大,现有的研究成果无法有效预测岩层内有害气体赋存种类与状态。基于川西藏东地质演化相关研究成果,对区域内岩石、断裂构造以及水热循环影响下有害气体的成因及组分特征进行研究。通过对区域内地层构造及岩石分布进行分析,明确区域内主要气藏类型及分布范围;建立复杂地层有害气体逸出风险评价体系,并通过现场监测对评价效果进行验证。结果表明:1)川西藏东地区岩层主要有害气体孕育类型包括岩浆岩脱气孕育气体、变质岩化学反应释放气体、沉积岩油气矿藏富集气体、金属矿伴生气体等;其中,岩浆岩孕育有害气体主要赋存于入侵通道形成的气囊内,变质岩孕育有害气体主要赋存于酸性岩浆岩与碳酸岩接触面以及金属矿附近,沉积岩孕育有害气体主要赋存于油气盖层内。2)岩石圈断裂对深部有害气体的赋存最有利,水热循环通过物质扩散和水力运移2种方式加速深部有害气体的运移,其中深大主断裂控制型水热循环模式影响最显著。3)基于川西藏东地区地层岩性及断裂构造特征,对CO_(2)、CH_(4)、SO_(2)及H_(2)S等主要有害气体的赋存区域进行划分。4)根据研究区域地层岩性、断裂构造、水热分布特点,建立复杂地层有害气体逸出风险评价体系;根据隧道开挖高浓度有害气体逸出可能性划分为5个风险等级,并针对不同逸出风险提出相应防治建议。

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_(2)S,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.