Mixed Biogenic-Thermogenic Gas Accumulation: New Insights into the Source-Reservoir-Caprock System of Permafrost Gas Hydrate in the Quemocuo Area, Qiangtang Basin

The Quemocuo area in the Qiangtang Basin is a key prospect for permafrost gas hydrate exploration in China. This study investigates source-reservoir-caprock characteristics and their control on gas hydrate accumulation based on drilling results from wells QK-8 and QK-9, integrated with multiple analytical methods. Two high-quality marine source rocks with cumulative thickness ~1000 m exhibit TOC values of 0.74–2.5%, Type II2 kerogen, and vitrinite reflectance (Ro) of 1.37–2.94%, indicating high to over-mature thermal evolution primarily generating dry thermogenic methane. Gas logging shows hydrocarbon anomalies with a maximum desorbed gas content of 90 mL, confirming strong gas generation capacity. Although reservoir matrix properties are poor (porosity mostly <5%, permeability < 0.2 × 10−3 μm2), multi-phase tectonics and dissolution formed a secondary fracture-vug system. Permafrost conditions are favorable (thickness 100–120 m; geothermal gradient 4.5–4.7 °C/100 m), with extremely low permeability at high ice saturations, forming an effective multi-level seal together with thick mudstones. A key novel finding is the significant mixing of biogenic and thermogenic gases, with the biogenic component interpreted to originate from overlying Jurassic-Quaternary low-maturity strata, facilitated by late tectonic uplift and fault conduits. NW-trending faults connect deep thermogenic reservoirs and provide pathways for shallow biogenic gas migration. For the first time, this study establishes a region-specific composite accumulation model for the Qiangtang Basin, characterized by “lower generation and upper storage, fault-fracture conduit and permafrost sealing”, which reveals fault-controlled migration, fracture-vug-controlled storage, permafrost-controlled sealing, and mixed gas enrichment under a high geothermal gradient.