Typical coalbed methane (CBM) enrichment and production modes under the control of regional structure and evolution

Coalbed methane (CBM) exploration and development is of great significance to optimize China’s energy structure and cope with coal mine disasters and ecological environment problems. In this paper, combined with regional structure and evolution and CBM geological conditions, the enrichment and recovery modes of CBM are innovatively divided into four categories: simple structural fracture system, fold system (shallower synclinal shaft, fold limb, secondary structural high position), thrust system (fault-thrust-fold belt, high-steep thrust structure) and structural superposition system. Among them, simple structural fracture system mode is often encountered in the areas with relatively stable structure, where the CBM is mainly derived from deep thermogenesis, and can also be reformed by magma thermal contact, forming high-yield enrichment zones with moderately developed fracture. In the fold system mode, the extrusion stress at the shallower synclinal shaft is conducive to the preservation of CBM. The fold limbs with uniform pressure distribution and superior sealing performance are compatible with both gas content and permeability, thus forming high-yield enrichment zones. The secondary structural high position mode refers to the structural traps formed in the high part of the associated structures (anticlines, nose-shaped uplift, fault blocks, etc.) under compression, which generate “gas cup”. In the fault-thrust-fold belt mode, the thrust fault prevents the escape of CBM, and captures gas at the relative structural high position close to the thrust fault. The high-steep thrust structure mode is developed in the complex fault area. Subjected to the specific temperature and pressure fields, the desorbed deep CBM migrates to the upper strata, and is re-adsorbed or partially still in a free state and accumulated. The structural superposition system mode, in contrast to single main control mode, is formed in the coal reservoirs affected by multi-stage tectonic movements. The structural superposition caused by different stress directions and mechanisms superposition achieves optimal matching of gas content and permeability, which is also favorable for CBM enrichment. Therefore, regional structural characteristics control the mode of CBM enrichment and recovery, which is fundamentally determined by tectonic evolution. Conclusions drawn in this study are of great reference value to systematically understanding the law of CBM enrichment in China and guiding the exploration and exploitation of CBM during the 14th Five-Year Plan.