Comprehensive assessment of pore structure and multiphase water distribution is critical to the flow and transport process in coalbed methane (CBM) reservoirs. In this study, nuclear magnetic resonance (NMR) and multifractal analysis were integrated to quantify the multiscale heterogeneity of nine medium- and high-rank coals under water-saturated and dry conditions. By applying the box-counting method to transverse relaxation time (T₂) spectra, multifractal parameters were derived to characterize pore heterogeneity and residual water distribution. The influencing factors of pore heterogeneity were also discussed. The results show that pore structures in high-rank coals (HCs) exhibit a broader multifractal spectrum and stronger rightward spectrum than those of medium-rank coals, reflecting micropore-dominated heterogeneity and the complexity induced by aromatization in HCs. The vitrinite content enhances micropore development, increasing the heterogeneity and complexity of pore structure and residual water distribution. Inertinite content shows opposite trends compared to vitrinite content for the effect on pore structure and water distribution. Volatile yield reflects coal metamorphism and thermal maturity, which inversely correlates with pore heterogeneity and complexity. Residual water mainly distributes to adsorption pores and pore throats, shortening T₂ relaxation (bound water effect) and reducing spectral asymmetry. The equivalence of the multifractal dimension and singularity spectrum validates their joint utility in characterizing pore structure. Minerals enhance pore connectivity but suppress complexity, while moisture and ash contents show negligible impacts. These findings provide a theoretical reference for CBM exploration, especially in optimizing fluid transportation and CBM production strategies and identifying CBM sweet spots.

How shale reservoirs and gas contents are affected by the pore structure of shale is very important. Low-temperature nitrogen isothermal adsorption experiments were conducted by us to investigate the pore structure of the Dalong Formation shale. We measured the specific surface area and fractal dimension of the pores and also considered the mineral fraction and organic matter content of the rock. The results show that the Dalong Formation shale contains a lot of organic carbon, with a total organic carbon (TOC) value between 1.20 and 10.82% (mean: 5.02%). Quartz and clay minerals are the main components of the shale, with quartz making up 40.30 to 85.60% (mean: 67.21%) and clay minerals making up 9.20 to 34.10% (mean: 20.26%) of the shale. Most of the pore space in the shale of the Dalong Formation is formed by intragranular and intergranular pores, organic matter pores, and some microfissures. The pore structure is complex, with parallel-plate and ink-bottle pores being the most common types. Most of the pores are 0-2 or 2-5 nm in size. D₁ and D₂ are the fractal dimensions, with averages of 2.66 and 2.81, respectively. D₁ can range from 2.55 to 2.78, while D₂ can range from 2.66 to 2.94. The TOC content, mineral composition, and pore structure characteristics determine the fractal dimension. Higher levels of the TOC content, quartz mineral content, and specific pore surface area result in a higher fractal dimension, while higher levels of feldspar content result in a lower one. There is no apparent correlation to clay minerals or other mineral compositions.

Understanding shale deposition processes and the mechanisms controlling organic matter enrichment is crucial for analyzing lithofacies genesis, reservoir heterogeneity, and identifying shale gas exploration targets, yet these aspects remain insufficiently understood. In this study, the Wufeng–Longmaxi black shales from the Yangtze Basin were systematically analyzed to reconstruct sedimentary conditions and evaluate the impact of paleoenvironmental factors on organic matter accumulation. Major and trace element geochemistry (e.g., Th/Sc, Co/Th, La/Sc, Fe₂O₃/TiO₂, and Al₂O₃/(Al₂O₃ + Fe₂O₃)) indicates that the Wufeng–Longmaxi shales were derived from a felsic igneous provenance within a continental margin setting. Paleoclimate proxies (CIA, αAlE), primary productivity indicators (P/Al ratios), and Co(ppm) × Mn(%) values suggest a predominantly semi-warm to warm climate with moderate productivity during intervals A and B. In contrast, interval C experienced climatic cooling and elevated primary productivity associated with enhanced upwelling, whereas interval D was marked by rapid warming, reduced productivity, and weakened upwelling. The results demonstrate that paleoclimate influenced primary productivity by modulating oceanic upwelling intensity. Cross-plots of Co(ppm) × Mn(%) and Cd/Mo ratios reveal that organic matter enrichment in interval A was mainly driven by high productivity, while preservation conditions played a dominant role during intervals B and D. Although cooling during interval C enhanced upwelling and productivity, a concurrent sea-level fall increased bottom-water oxygenation, indicating that both high productivity and redox conditions jointly controlled organic matter accumulation at this stage. Overall, the findings suggest that climate-driven changes in ocean circulation and sea level significantly influenced fine-grained sedimentation and organic matter enrichment, providing insights into shale depositional processes under varying paleoenvironmental conditions.

Biogas production was conducted using samples from different coal beds and laboratory-domesticated microbes to investigate the effect of the addition of benzoic acid on biogas production. Furthermore, the response properties of produced organic substances at different gas production stages were analyzed with ultraviolet-visible (UV-Vis) spectroscopy and three-dimensional fluorescence spectroscopy. The results showed that adding benzoic acid significantly enhanced the microbial gas production with different rank coals. Obvious spectroscopic differences were observed in the gas production effects and liquid-phase composition across varying rank coals. The UV-vis spectroscopy findings indicated that soluble organic matter gradually increased in molecular weight during gas production, leading to increased aromatization and an increase in aromatic ring substituents with hydrogen and oxygen functional groups. Fluorescence spectroscopy revealed changes in protein-like substances during gas production, indicating the involvement of humic acid-like substances from coal in microbial gas production. The results of the fluorescence index supported the biological origin of humic acid during the gas production process. Benzoic acid augmentation promoted biogas production in different coal grades, and distinct differences were observed in the organic spectral properties during gas production, suggesting that the metabolic pathways of the same microbes acting on different coal grades vary.