关键词: 深部煤储层, 煤层气, 煤岩气, 含气系统, 临界界限

Abstract: Coal-reservoir gas-bearing capacity is a nonlinear response system based on the coupling of gas supply, storage, and sealing capacities. The fundamental distinction between deep and shallow coal reservoirs lies in the methane occurrence state. The threshold depth zone or geological boundary where free gas can stably retain and continuously accumulate, namely the free-gas retention depth, can serve as a critical node for differentiating deep and shallow coal-reservoir gas-bearing systems. The free-gas retention depth marks the transition of methane occurrence from an adsorption-equilibrium stage dominated by van der Waals forces to a composite control stage governed by the synergistic effects of capillary pressure and pressure sealing. Meanwhile, the dominant mechanism of coalbed methane development shifts from dewatering-induced depressurization to gas-production-driven depressurization. Statistical results of gas content in coal reservoir measured by exploration well coring show as follows. (1) The free-gas retention depth ranges from 1 500 to 1 800 m in Ordos Basin and Sichuan Basin, shifts downward to 2 000-2 300 m in Junggar Basin. The free gas in the Qinshui Basin shows extremely weak indications, and all production wells exhibit a distinct single-phase drainage stage. (2) The coupling effect of coal rank and burial depth exerts a decisive constraint on the hydrocarbon generation-storage potential, gas-bearing distribution, and proportion of free gas. A critical vitrinite reflectance threshold may exist for self-sourced free-gas-rich coal reservoirs. The asymmetric thermal evolution induced by magmatic thermal superposition can deepen the free-gas retention boundary, while abnormal overpressure systems and exogenous gas input can extend the lower depth limit of gas-rich zones in coal reservoir by enhancing storage or gas-supply capacities. (3) By integrating the pressure systems, coal ranks, and gas-source characteristics, the gas-bearing systems in deep coal reservoirs can be classified into the following types:normal-pressure autogenic medium-high rank coal system (in Ordos Basin), normal-pressure autogenic medium-low rank coal system (in southern Junggar Basin), overpressured autogenic medium-high rank coal system (in Sichuan Basin), normal-pressure exogenous medium-low rank coal system (in Xishanyao Formation of Baijiahai area), and magmatic-thermal superposition medium-high rank coal system (in Qinshui Basin). The findings are expected to provide theoretical guidance and data support for the zonal evaluation and "sweet spot" optimization of deep resources.

Key words: deep coal reservoir, coalbed methane, coal-rock gas, gas-bearing system, critical boundary