The Occurrence of Coalbed Methane in the Deep No. 8 Coal Reservoirs of the Ordos Basin: Insights from Deep Exploratory Wells

Coal reservoirs in the central-eastern Ordos Basin are important targets for deep coalbed methane exploration. Based on core samples and pressurized-coring gas-content data from nine exploratory wells at depths of 1800–2800 m, multiscale pore structures, methane isothermal adsorption, gas geochemistry and production response were analyzed to investigate methane occurrence and its controlling factors. The results show that the deep coal reservoir differs from shallow adsorption-dominated reservoirs by its dominance of thermogenic dry gas, adsorption oversaturation (avg. 118%), and coexistence of adsorbed gas and free gas (0.02–10.89 m3/t). Free gas is preferentially mobilized during the early stage of pressure depletion, contributing to rapid gas breakthrough and a high early production rate. Regionally, coal-rank differentiation controls the variation in gas content (2.98–30.97 m3/t) by regulating micropore development and methane adsorption capacity. The high-rank coal reservoirs with higher total gas content exhibit greater gas production potential because of a stronger adsorbed-gas supply, as indicated by peak daily gas rates exceeding 15 × 104 m3/d in horizontal wells. Vertically, intraseam ash-yield heterogeneity controls sweet-interval distribution. The upper low-ash intervals exhibit better-developed micropores, stronger adsorption capacity, and higher gas content and should therefore be preferentially targeted during horizontal-well geosteering. This study clarifies the controlling effect of physical parameters on the gas occurrence state in deep coal reservoirs, providing crucial theoretical support for the precise targeting of sweet spots to achieve efficient exploitation of resources.