Constraints of Tectono-Hydrothermal Alteration on Permian Volcanic Reservoir Formation as Recorded by Fluid Inclusions

The origin of effective porosity in the Permian volcanic reservoirs of the Sichuan Basin remains poorly understood, particularly regarding the contribution of deep-sourced hydrothermal fluids. This study presents an integrated analysis of the Emeishan basalts, combining petrography, fluid-inclusion microthermometry, and laser Raman spectroscopy. Three broad hydrothermal fluid characteristics were identified, ranging from low-temperature CH4-H2S-bearing to medium–high-temperature hydrocarbon-rich alkaline fluids. Their influence on porosity was tectonically modulated: fluids associated with the Indosinian and Yanshanian orogenies primarily degraded the pore system through clay infill and siliceous cementation. In contrast, the Himalayan orogeny may have driven a constructive shift. Sodic (Na-rich) fluids channeled along deep faults drove extensive local dissolution, creating secondary, micron-scale pore occurrence. Simultaneously, the temperatures recorded by this medium–high-temperature hydrothermal system fall within a range permissive for in situ hydrocarbon thermal alteration. This is tentatively consistent with models of vertical hydrocarbon migration and accumulation, but does not constitute direct evidence for in situ cracking or charging. Therefore, the Himalayan tectono-hydrothermal event is regarded as a possible contributor to the development of potential volcanic gas reservoirs. This work proposes a unified interpretive genetic model that couples late-stage alkaline-fluid dissolution with possible hydrocarbon alteration effects, offering a predictive framework for exploring volcanic reservoirs in similar geodynamic settings.