Abstract: Deep coalbed methane (CBM) will become an important field for China to increase the large-scale natural gas reserves and production in the future. It is of great significance to review the history and progress of the geological research on deep CBM propose and evaluate the existing problems and exploration directions, which can provide a reference for developing applicable exploration and development technologies. Analyses reveal that China has made three major advances in the geological research of deep CBM in the past 20 years. First, the basic concept and its scientific connotation of deep CBM have been defined. It is found that there is a critical depth for the absorbed gas content of deep coalbeds, which mainly depends on the coupling relationship between geothermal gradient and geo-stress gradient, and other geological factors can adjust the critical depth. A decrease in the adsorbed gas content may lead to an increase in free gas content, resulting in the orderly accumulation of CBM in the depth sequence and the formation of highly to super saturated reservoirs with abundant free gas in the deep coal. Second, remarkable progress has been made in research of the geological properties of deep coal reservoirs, and it has been recognized that the weakening adsorption of deep coal reservoirs and the increase of free gas content are resulted from the dynamic equilibrium between the positive effect of pressure and the negative effect of temperature. Moreover, it has been found that there is a "highly permeability window" of coal reservoirs near the transition zone of geo-stress state on the depth profile, and the formation temperature and pressure indices related to the reconstruction of deep coal reservoirs may have a threshold property, and the temperature compensation and variable pore compressibility effects may significantly lower the decay rate of permeability for deep coal reservoirs. Third, an in-depth research is gradually implemented on the accumulation and geological evaluation of deep CBM reservoirs, and the exploration on accumulation mechanism focuses on CBM gas-bearing property formed by buried depth changes, vertical permeability distribution and its geological control, thus initially revealing the "depth effect" for CBM reservoir formation. Through on-site case analysis, the relevant understandings have been deepened and expanded from basin to favorable zone, then to sweet spot and from reservoir control to production control. The analyses suggest that the organic connection and deep coupling of basic geology (reservoir-forming process), exploration geology (evaluation optimization) and development geology (dynamic process) are key directions for the geological-engineering integration in deep CBM exploration and development. Therefore, it is suggested future research should focus on "depth effect", including the systematic description of deep CBM reservoir and the characterization of gas reservoir engineering responses to geological conditions.

Key words: deep coalbed methane, critical depth, reservoir properties, accumulation, geological evaluation, exploration direction

摘要： 深部煤层气是中国未来天然气规模性增储上产的重要领域，回顾深部煤层气地质研究历史与进展，评述面临问题和探索方向，可为发展适应性勘探开发技术提供借鉴。分析表明，中国在20余年的深部煤层气地质研究中主要取得3方面进展。第一，界定了深部煤层气基本概念及其科学内涵，发现深部煤层吸附气含量存在临界深度，这一深度主要取决于地温梯度和地应力梯度两者的耦合关系，其他地质因素对临界深度具有调整作用；吸附气含量降低可能导致游离气含量随之增高，结果是形成煤层气在深度序列上的有序聚集，在深部形成富含游离气的高饱和—超饱和煤层气藏。第二，对深部煤层地质属性的研究进展显著，认识到深部煤层吸附性减弱及游离气含量增高是地层压力正效应与地层温度负效应之间动态平衡的结果；发现在深度剖面上地应力状态转折带附近存在煤层"高渗窗"，与深部煤层可改造性相关的地层温度、压力指标可能具有"门限"性质，温度补偿效应和变孔隙压缩系数效应可能使得深部煤层渗透率衰减速率显著降低。第三，对深部煤层气成藏作用与地质评价的研究逐渐深入，对成藏作用原理的探讨聚焦在由埋深变化引起的煤层含气性、渗透率垂向分布及其地质控制等5个方面，初步揭示了煤层气成藏"深度效应"。对现场案例进行的剖析经历了由盆地到有利区、由有利区到"甜点"、由控藏到控产的认识深化过程。分析认为，基础地质（成藏作用）、勘查地质（评价优选）、开发地质（动态过程）3方面有机衔接和深度耦合，是面向深部煤层气勘探开发地质—工程一体化所需探索的关键方向，建议未来研究应聚焦"深度效应"，包括深部煤层气藏特征的系统描述和气藏工程对地质条件响应的刻画。

关键词: 深部煤层气, 临界深度, 储层属性, 成藏作用, 地质评价, 探索方向