Effects of depth on gas-bearing properties of coal reservoirs and their coupling relationships with coalbed methane accumulation

Burial depth serves as a comprehensive factor influencing coalbed methane (CBM) accumulation. Understanding the effects of depth on gas-bearing properties is critical for ascertaining the occurrence states and accumulation mechanisms of deep/shallow CBM. Based on the present situation of CBM exploration and the dissection of data from CBM exploration wells on the eastern margin of the Ordos Basin, this study explored the effects of burial depth on the gas content, saturated adsorbed gas capacity, and gas saturation of coal seams, as well as their coupling relationship with accumulation processes, using conventional and unconventional petroleum accumulation. The results indicate that CBM accumulation results from the coupling of hydrocarbon generation and gas supply in the tectonic subsidence stage with the phase transformation and dissipation of gas during the rollback and uplift of strata. This is manifested as a deep coupling of self-sealing- and buoyancy-driving gas accumulation. The variations in gas-bearing properties involve two critical depth thresholds: one for the turning of saturated adsorbed gas capacity and one for free gas retention. Notably, the two thresholds do not exhibit absolute synchronicity: (1) The saturated adsorbed gas capacity is an intrinsic property of coals under specific temperature and pressure conditions, not strictly constrained by preservation conditions. Its depth-varying evolutionary process governs the phase transformation of methane. The compensating effects of pressure gradients and metamorphic grades lead to a significantly deeper turning point (zone) of current regional saturated adsorbed gas capacity. (2) The migration and accumulation, along with transformation and shaping, of free gas are dictated by the cap rock conditions during the rollback and uplift of strata, involving the comprehensive influence from burial depth - structure - hydrodynamic field coupling effects, as well as buoyancy and reservoir/cap rock capillary pressures. Minor uplift amplitude and low transformation intensity are necessary for free gas retention and preservation. Free gas will dissipate extensively in strata shallower than the retention depths due to reduced sealing properties. In the Liulin-southern Yanchuan area on the eastern margin of the Ordos Basin, the total gas content of coal seams increases almost linearly with the burial depth, exhibiting a subtle convergence trend in deep layers. Coals with different metamorphic grades in this area show depth thresholds ranging from 1600 to 2200 m for the turning of theoretical saturated adsorbed gas capacity. However, regional differentiation of coal ranks causes a continuous increase in the in-situ saturated adsorbed gas capacity with the burial depth. The Daning-Jixian block has a critical depth of approximately 2000 m for free gas retention, with an average gas saturation of 120% at a depth of 2500 m and an estimated gas saturation of 136% at a depth of 3000 m. Different areas exhibit differential CBM accumulation settings and geological conditions, necessitating a specific analysis of the effects of depth on gas-bearing properties. The analysis should focus on the comprehensive influence of the spatio-temporal evolution of both the phase transformation of methane and the sealing conditions of strata on current gas and water distributions. This is crucial for achieving the zone-orientated evaluation and efficient production design of deep CBM.