The coupling relationship between the depth effect of coalbed gas content and the formation process
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摘要: 埋深是影响煤层气富集程度的综合要素，理解含气性深度效应是认识深浅部煤层气赋存状态与聚集机制的重要基础。基于煤层气勘探现状，在剖析鄂尔多斯盆地东缘煤层气探井资料的基础上，综合常规-非常规油气成藏地质学理论，探讨了煤层含气量、饱和吸附量、含气饱和度深度效应及其与成藏过程的耦合关系。煤层气成藏是构造沉降阶段生烃供气和回返抬升阶段相态转化、逸散的耦合结果，体现为自封闭成藏和浮力成藏的深度耦合，含气性变化存在饱和吸附量转折和游离气滞留两个关键深度界限，且二者不具备绝对同步性：(1)饱和吸附气量是煤在特定温压条件下的固有属性，不受保存条件的严格限制，其随深度的演化过程是控制相态转换的基础，压力梯度和变质程度补偿效应会引起现今区域饱和吸附量转折深度（带）的明显滞后；（2）游离气的运聚成藏与改造定型受控于地层回返抬升阶段的遮盖条件，涉及埋深-构造-水动力场三元耦合效应及浮力、储盖层毛管力的综合影响，抬升幅度小且改造强度弱时方可具备游离气滞留保存条件，滞留深度以浅地层封闭性降低，游离气普遍散失。鄂尔多斯盆地东缘柳林—延川南一带煤层总含气量随埋深增大近乎线性增高，深部收敛趋势不明显，不同变质程度煤理论饱和吸附量转折深度为1 600~2 200 m，但煤阶的区域分异致使原位饱和吸附量随埋深持续增大；大宁-吉县区块游离气滞留临界深度约2 000 m，2 500 m处含气饱和度平均120%，3000 m处含气饱和度预计可达136%。不同地区煤层气成藏背景和地质条件存在差异，含气性深度效应需具体分析，分析重点应聚焦于甲烷相态转换、地层封闭条件的时空演化对现今气、水分布的综合影响，以实现深部煤层气的分区分带评价和高效开发设计。Abstract: 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.
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Keywords:
- deep CBM /
- gas content /
- conversion depth /
- compensating effect
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