Structure and coalbed methane occurrence in tectonically deformed coals

Research on structure of tectonically deformed coals (TDC) is a key issue in coal and gas outburst prevention and coalbed methane (CBM) exploitation. This paper presents a summary on the research progress in TDC's structural-genetic classification, tectonic strain influence on coal microstructure, coal porosity system, coal chemical structure and constituents, and their relationship with the excess coalbed methane. Previous studies suggested that tectonic deformation had significant influence on coal microstructure, coal super microstructure, and even chemical macromolecular structure. The main mechanisms of coal deformation are the tectonic stress degradation and polycondensation metamorphism (dynamical metamorphism). Besides, under different deformation mechanisms, the ultra- and micro-structure and chemical constituents of TDC presented distinct characteristics. Based on these achievements, we propose one possible evolutionary trend of TDC with different deformation mechanisms, and suggest that the coal and gas outburst in the TDC, especially in the mylonitic coals, may be not only controlled by geological structure, but also influenced by the tectonic stress degradation of ductile deformation. Therefore, further study on TDC should be focused on the controlling mechanism of deformation on structure and composition of coal, generation conditions and occurrence state of excess coalbed methane from deformation mechanism of coal.