School of Energy Resources, China University of Geosciences (Beijing)
Coal Reservoir Laboratory of National Engineering Research Center of Coalbed Methane Development and Utilization
National Engineering Research Center of China United Coalbed Methane Co., Ltd.
PetroChina Coalbed Methane Company Limited

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处含气饱和度平均约120%，3 000 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。

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.

国家自然科学基金项目(42130802)；中央高校基本科研业务费专项资金项目(2652022036)；中石油煤层气有限责任公司科技项目(2023-KJ-18)

陈世达，侯伟，汤达祯，等. 煤储层含气性深度效应与成藏过程耦合关系[J]. 煤田地质与勘探，2024，52(2)：52−59.

CHEN Shida，HOU Wei，TANG Dazhen，et al. Effects of depth on gas-bearing properties of coal reservoirs and their coupling relationships with coalbed methane accumulation[J]. Coal Geology & Exploration，2024，52(2)：52−59.