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

摘要： 煤储层含气性是供气能力、储集能力与封闭能力三者耦合作用下的非线性响应系统,深部和浅部煤储层的本质差异在于甲烷的赋存状态。“游离气稳定滞留、连续聚集的门限深度带或地质边界”(游离气滞留深度)可作为深部和浅部煤储层含气系统划分的关键节点。游离气滞留深度标志着甲烷的赋存由范德华力主导的吸附平衡阶段,转向毛细管压力与压力封闭协同作用的复合控制阶段。同时,煤层气开发的主导机理亦由排水降压转为排气降压。煤储层探井取心的实测含气量统计结果分析表明:1游离气滞留深度在鄂尔多斯盆地与四川盆地为1 500~1 800 m,在准噶尔盆地下移至2 000~2 300 m,沁水盆地当前勘探深度范围内的游离气显示极弱且投产初期均存在单相排水阶段。2煤阶-埋深耦合作用对生-储潜力、含气性分布和游离气占比起决定性约束作用;自生自储型富游离气煤储层可能存在临界镜质体反射率阈值;岩浆热叠加引起的温度不对称性演化可促使煤储层中的游离气滞留界限加深;异常超压体系和外源气输入可通过强化储集或供气能力拓展煤储层富气的深度下限。3综合压力体系、煤阶及气源特征,可将深部煤储层含气系统划分为常压自生型中—高煤阶系统(鄂尔多斯盆地)、常压自生型中—低煤阶系统(准噶尔盆地南部)、超压自生型中—高煤阶系统(四川盆地)、常压外源型中—低煤阶系统(准噶尔盆地白家海地区西山窑组)和岩浆热叠加型中—高煤阶系统(沁水盆地)。研究成果以期为深部资源的分带评价与“甜点”优选提供理论指导和数据支撑。

关键词: 深部煤储层, 煤层气, 煤岩气, 含气系统, 临界界限