Coal structure refers to the internal structural characteristics of coals including the degree of macroscopic and microscopic deformation, pore structure, and mechanical properties after various geothermal and geological stresses, which have substantial impacts on coal mining safety and coalbed methane (CBM) exploitation. The coal structure is an indicator of coal mechanical strength, which determines the petrophysical properties of CBM reservoirs and gas extraction or CBM production efficiency from coals. Coals with different structures are divided into primary coals and tectonically deformed coals (TDCs). To explore the macro- and microscopic petrophysics of TDCs, a wide range of techniques including geophysical logging, seismic inversion, amplitude variation with offset (AVO), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), and Fourier transform infrared spectroscopy (FTIR) have been employed. From a macroscopic perspective, the geological and geophysical methods are generally used to distinguish the degree of deformation of whole coal seams. Microscopic methods including SEM, NMR, and FTIR, are normally used to investigate the internal petrophysics (e.g., pore and fracture, macromolecular structure, and fluid performance) of TDCs. Herein, the macro- and micro-petrophysical properties of TDCs and their impacts on CBM exploration and exploitation are systematically reviewed and potential opportunities of future directions are recommended. Extensive studies have shown that the development zone of TDCs in the brittle series, notably cataclastic coals, has high potential for CBM exploration and exploitation, while that of TDCs in the ductile series is a dangerous region for coal mining and subject to gas outbursts. This review aims to provide background on the coal structure, summarize evaluation methods, highlight its geological drivers, and explain its significance in CBM exploitation. We will end with proposals for future research directions.

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The pore-fractures network plays a key role in coalbed methane (CBM) accumulation and production, while the impacts of coal facies on the pore-fractures network performance are still poorly understood. In this work, the research on the pore-fracture occurrence of 38 collected coals from Sangjiang-Muling coal-bearing basins with multiple techniques, including mercury intrusion porosimetry (MIP), micro-organic quantitative analysis, and optic microscopy, and its variation controlling of coal face were studied. The MIP curves of 38 selected coals, indicating pore structures, were subdivided into three typical types, including type I of predominant micropores, type Ⅱ of predominant micropores and macropores with good connectivity, and type Ⅲ of predominant micropores and macropores with poor connectivity. For coal facies, three various coal facies were distinguished, including lake shore coastal wet forest swamp, the upper delta plain wet forest swamp, tidal flat wet forest swamp using Q-cluster analysis and tissue preservation index–gelification index (TPI-GI), and wood index–groundwater influence index (WI-GWI). The results show a positive relationship between tissue preservation index (TPI), wood index (WI), and mesopores (102 nm–103 nm), and a negative relationship between TPI, WI, and macropores/fractures. In addition, groundwater level fluctuations can control the development of type C and D fractures, and the frequency of type C and D fractures show an ascending trend with increasing groundwater index (GWI), which may be caused by the mineral hydration of the coal. Finally, from the perspective of the pore-fractures occurrence in CBM reservoirs, the wet forest swamp of upper delta plain is considered to be the optimization areas for Sangjiang-Muling coal-bearing basins by a comparative study of various coal facies.