Geological controls and migration mechanisms of gas in fractured non- coal-measure strata: Insights from mountainous tunnel investigations in the western Sichuan Plateau
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Gas migration through fractured geological media is controlled by the coupled effects of gas sources, geological structures, and excavation-induced disturbances. However, the occurrence patterns and migration mechanisms of gas in non-coal-measure fractured strata remain poorly understood, particularly in mountainous regions with complex tectonic conditions. This study investigates gas occurrence and migration characteristics in representative non-coal-measure tunnels located in the western Sichuan Plateau, China, by integrating field investigation, geological interpretation, gas monitoring, and three-dimensional numerical modelling. Based on engineering geological observations, three typical gas occurrence patterns are identified, including direct exposure of gas-bearing strata, structural connection between deep gas sources and underground openings, and indirect exposure through secondary migration pathways. A geological source-pathway-opening framework is proposed to describe the progressive process of gas accumulation and migration under excavation disturbance. A three-dimensional gas transport model is subsequently established to quantify the effects of exposed gas-bearing areas and gas-conducting structures on the spatial distribution and migration behavior of gas. The results indicate that gas migration is strongly controlled by the connectivity of geological pathways and the interaction between gas pressure gradients and excavation-induced openings. Larger exposed gas-bearing areas and more favorable structural pathways significantly enhance local gas accumulation and accelerate migration toward underground openings. Field monitoring results from a representative tunnel further verify the reliability of the proposed migration mechanism and numerical analysis. This study provides new insights into gas migration in fractured non-coal-measure geological media and offers a geological framework for evaluating gas-related risks in deep underground engineering.
