The pore structure of coal reservoirs is the main factor influencing the adsorption–diffusion rates of coalbed methane. Mercury intrusion porosimetry (MIP), low-pressure nitrogen adsorption (LP-NA), low-pressure carbon dioxide adsorption (LP-CA), and isothermal adsorption experiments with different macerals were performed to characterize the comprehensive pore distribution and methane adsorption–diffusion of coal. On the basis of the fractal theory, the pore structures determined through MIP and LP-NA can be combined at a pore diameter of 100 nm to achieve a comprehensive pore structural splicing of MIP, LP-NA, and LP-CA. Macro–mesopores and micro-transitional pores had average fractal dimensions of 2.48 and 2.18, respectively. The Langmuir volume (V_L) and effective diffusion coefficients (D_e) varied from 31.55 to 38.63 cm³/g and from 1.42 to 2.88 × 10⁻⁵ s⁻¹, respectively. The study results showed that for super-micropores, a higher vitrinite content led to a larger specific surface area (SSA) and stronger adsorption capacity but also to a weaker diffusion capacity. The larger the average pore diameter (APD) of micro-transitional pores, the stronger the diffusion capacity. The diffusion capacity may be controlled by the APD of micro-transitional pores.

Due to the importance of the wide occurrence of thick coal seams for Chinese coal resources, the origins of these seams have received considerable attention. Using the Early Cretaceous No. 5 coal seam with a thickness of 16.8 m in Inner Mongolia as a case study, this paper presents a systematic investigation of the coal petrology, geochemistry, and palynology of 19 coal samples to explain the origin and evolution of peat accumulation. The results indicate that the No. 5 coal seam is generally characterized by low rank (lignite), dominant huminite (average = 82.3%), intermediate ash yield (average = 16.03%), and sulfur content (average = 1.12%). The proportion of spores generally increases from the bottom to the top of the coal seam, whereas the proportion of pollen decreases. The vegetation in the coal seam is dominated by gymnosperms at the bottom and by ferns at the top. The paleographic precursor peat was most likely accumulated in the lakeshore where herbaceous and bushy helophytes were dominant. The total sulfur content was positively related to the huminite content. The sulfur content was possibly derived from bacterial action with sulfur brought in via marine incursions. Three overall declining-increasing values of carbon isotopes within the No. 5 coal seam possibly indicated three general cooling trends during peat accumulation. The environment of peat accumulation included three cycles, including one drying-wetting-drying in the bottom part and two drying-upwards cycles in the upper part. These cycles of the peat-accumulation environment could likely be ascribed to climate change because of their good agreement with humidity signals from plant types at that stage.

Graphite microcrystals are the product of coal graphitization and widely exist in the graphitized coal of Yongan Coalfield, Fujian Province, China, which is direct mineralogical evidence for the transformation of coal to graphite. Optical microscopy, scanning electron microscopy (SEM) and micro-Raman spectroscopy were used to detect the morphology and microstructure of the in situ graphite microcrystals. The results show that the volume proportion of graphite microcrystals in graphitized coal samples is between 2.39% and 7.32%, and the optical anisotropy of graphite microcrystals is stronger than that of coal macerals. Graphite microcrystals show the occurrence of attached microcrack inner walls or infilling the cell cavity, with several forms of flakes, needles or aggregates. Under optical microscopy of polarized light and with a retarder plate of 1λ, graphite microcrystals show the color of primary yellow and secondary blue, and the two kinds of colors appear alternately when the microscope is rotating. Additionally, flake-like graphite microcrystals with an isochromatic zone diameter of 10−50 μm are the most widely distributed in graphitized coal. Under SEM, graphite microcrystals show a rough and irregular edge and are characterized by flow or bubble film structures along with several pores, which is the product of cooling crystallization after the softening and melting of carbon-containing substances. Moreover, flake-like graphite microcrystals developed interlayer pores with a clear outline of loose stacking and were almost entirely composed of pure carbon; a small amount of oxygen is related to oxygen-containing functional groups or structural defects. The micro-Raman spectra of graphite microcrystals in the first-order region are characterized by low-intensity D1 and D2 bands and a high-intensity G band, and the parameters R1 and R2 vary from 0.21–0.39 and 0.60–0.74, respectively. The second-order micro-Raman spectra of graphite microcrystals are characterized by a higher intensity of the 2D1 band and a lower intensity of the other three bands. The parameter R3, derived from the area ratio of the 2D1 band to all the bands in the second-order region, was proposed. The value of R3 ranges between 0.78 and 0.86, and both of them indicate a higher percentage of graphene plane with a highly internal crystallographic structure. Similar to the parameters R1 and R2 in the first-order micro-Raman spectrum, the parameter R3 is an effective parameter to characterize the ordering degree of the microstructure, which may be used to evaluate the graphitization degree of graphitization coal.