The coupling relationship between the depth effect of coalbed gas content and the formation process

Depth is a comprehensive factor influencing coalbed methane (CBM) enrichment, and the depth effect of gas content is an important basis for understanding the storage state and accumulation mechanism in both deep and shallow zones. Based on the current status of CBM exploration and analyzing the data from exploration wells in the eastern margin of the Ordos Basin, the coupling relationship between depth effects of gas content, adsorption capacity, gas saturation, and reservoir formation process were discussed using both conventional and unconventional petroleum geology theories. It is pointed out that the CBM formation is a coupled result of hydrocarbon generation during the structural subsidence phase and phase transformation and dissipation during the uplift phase, which is manifested as a deep coupling of self-sealing storage and buoyancy storage. The variation in gas content involves two critical depth thresholds:the turning point of saturated adsorption capacity and the depth of retained free gas. Importantly, these two thresholds do not exhibit absolute synchronicity:The saturated adsorption capacity is an intrinsic property of coal under specific temperature and pressure conditions, not strictly constrained by preservation conditions. Its dynamic evolution process controls the phase transition and is influenced by pressure gradients and rank compensation effects, leading to a noticeable lag in the turning depth (zone) of current regional saturation adsorption capacity. The accumulation of free gas is controlled by the covering conditions during the stratum uplift phase, involving the comprehensive impact of burial depth-structure-hydrology tri-coupling effects, as well as the effects of buoyancy, reservoir/caprock capillary force. Super-saturated gas reservoirs can form only with small uplift amplitude and the weak transformation intensity, while the weaker sealing capacity of shallow strata leading to widespread loss of free gas. In the area from Liulin to Yanchuannan in the eastern margin of Ordos Basin, the total gas content continues to increase with depth, with a gradual convergence trend in the deep zones being less pronounced. The theoretic turning depth of in-situ saturated adsorption capacity is in the range of 1600-2200 m, but the regional differentiation of coal rank results in a continuous increase in saturated adsorption capacity with depth. In Daning block, the critical depth of free gas retention is approximately 2000 m, the average gas saturation is 120% at 2500 m, and it is estimated to reach 136% at 3000 m. Different regions exhibit variations in the geological background and conditions, necessitating a specific analysis of the depth effects of gas content. The analysis should focus on the comprehensive impact of the spatial-temporal evolution of methane phase transitions and formation sealing conditions on the current distribution of gas and water. This is crucial for achieving zonal evaluation and efficient development design of deep CBM.