Gas Tunnels in Non-Coal Strata in China: A Systematic Review of Conceptual Framework, Genetic Typology and Regional Distribution

The expansion of transportation infrastructure into geologically complex mountainous regions of China has increased encounters with gas-bearing non-coal strata during tunnel construction. Unlike coal-measure gas systems, gas occurrences in carbonate, clastic, and metamorphic rocks present distinct challenges due to multi-source origins, structurally controlled accumulation, and abrupt outburst mechanisms. This review examines existing research to establish a conceptual framework for this hazard category. Analysis of gas sources, reservoir characteristics, and geological controls indicates that non-coal gas tunnels require theoretical approaches distinct from coal-gas methodologies. A genetic classification system distinguishes three fundamental types: tectonically migrated, stratum-self-generated, and composite origins. Regional distribution patterns reveal clustering around petroliferous basin margins, particularly the Sichuan Basin and surrounding provinces, governed by the interaction of source rocks, migration pathways, and structural traps. Critical gaps remain in multi-field coupling theory, concealed gas detection technology, and dedicated technical standards. Development of quantitative predictive models and intelligent monitoring systems represents the primary direction for future research.