To analyze the pore structure and fractal characteristics of marine shale in the lower Cambrian Niutitang Formation in northwestern Hunan Province, China, the pore characteristics of shale were characterized using total organic carbon (TOC) content, field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), low temperature nitrogen adsorption (LT-N₂GA) and methane adsorption experiments. The pore surface and pore space fractal dimensions of samples were calculated, respectively. The influencing factors of fractal dimensions and their impact on the adsorption of shale reservoirs were discussed. The results indicate the Niutitang Formation shale mainly develops four types of pores: organic pores, intragranular pores, intergranular pores and microcracks. The pores have a large specific surface area (SSA), primarily consisting of mesopores. The fractal dimensions are calculated using the FHH model and the XS model. The fractal dimensions (D₂ and D_f) are greater than D₁, indicating that the pore surface with larger pore size is rougher, and the pore structure of shale is complex. The pore volume (PV), SSA, and TOC show positive correlations with the fractal dimensions but negative correlations with APS. There is no obvious correlation between fractal dimensions and quartz content, while clay minerals show a negative correlation with D₂ and D_f. This is mainly because clay mineral particles are small in size and have weak resistance to compaction. The pyrite content is positively correlated with the fractal dimensions because pyrite promotes the development of organic, intergranular, and mold pores. According to Pearson correlation analysis, the main influencing factors of the pore surface fractal dimension are PV, SSA, and APS. The main influencing factors of the pore space fractal dimension are APS and the content of clay minerals. Further analysis of the influence of the fractal dimension on the adsorption capacity of shale reveals that the fractal dimensions are positively correlated with Langmuir volume, indicating that fractal dimensions can be used as a quantitative target for evaluating shale gas reservoirs.

We report on the effects of magmatic hydrothermal fluids on the mineralogical and geochemical compositions of 12U and 12L Coals from the Meitian Mine in the Meitian Coalfield, southern China. The minerals in 12U Coal are predominantly chlorite, quartz, and calcite, while the minerals in 12L Coal consist mainly of illite, quartz, chlorite, kaolinite, and mixed-layer illite/smectite (I/S). The vesicle- and fracture-filling illite, chlorite, I/S, pyrite, and fluorite, cleat- and fracture-filling carbonate minerals (i.e., calcite, and dolomite), and cleat-filling tremolite, diopside, and talc have epigenetic hydrothermal origins. Tremolite, diopside, and talc were probably formed from the reaction between dolomite and Si-rich magmatic hydrothermal fluids. Elevated Pb–Zn–Sn–Cd assemblages are characteristic for the 12U Coal, while 12L Coal is enriched in W, Rb, Cs, Th, V, Zn, and Zr, most notably W, Rb, and Cs. REY (Rare Earth Elements and Yttrium) plots for almost all coals, partings and host rocks are similar, showing an M-type REY distribution, Gd-maximum, positive Y anomalies, and negative Ce anomalies, suggesting acid hydrothermal circulation in the coal-bearing strata. Rubidium and cesium in the coal is clearly associated with K-rich clay minerals (illite + I/S), and to a lesser extent with silicate minerals that were precipitated from hydrothermal solutions. W in the coals mainly occurs in the inorganic constituents of illite and pyrite, especially illite. Enrichment of W, Rb, and Cs in the coal and host rocks is genetically associated with magmatic hydrothermal fluids. Specifically, magmatic hydrothermal fluids of relatively high temperatures that are rich in volatile matter can extract abundant W, Rb, and Cs from granitic melts. The enrichment of these rare metals in the coal is mainly related to illitization. Our study results suggest that, for coal intruded by magmatic rocks, the type of hydrothermal alteration may greatly influence the enrichment of elements.

To further understand the pore structure characteristics and their effect on CH₄ adsorption capacity for high-rank coals. Based on 11 fresh coal samples from the Zhina coalfield of South China. We analyzed the pore structure characteristics of coal samples by low-temperature liquid-nitrogen adsorption (LP-N₂A) measurements. On the basis of nuclear magnetic resonance (NMR), we obtained the fractal dimensions of different types of pores by the new model, studied the relationship between the fractal dimensions, and the characteristic parameters of coals (composition and pore characteristics) and discussed the influence of the fractal dimensions on CH₄ adsorption. The results show that according to LP-N₂A isotherms, all coals can be classified into three types. The micropores provide the largest proportion of the specific surface area (SSA) of coals. Two fractal dimensions, D_a (adsorption pore) and D_s (seepage pore), ranged from 2.471 to 2.805 and from 2.812 to 2.976, which were acquired in the saturated water condition by NMR. Furthermore, D_a and D_s have different correlations with ash yield, carbon contents, moisture, SSA and irreducible fluid porosity. The coal composition and pore parameters have much greater control over fractal dimensions. Moreover, the different fractal dimensions have different influences on methane adsorption. With the increase of D_a, the methane adsorption capacity is enhanced, but it is weakened with the increase of D_s. The high-rank coals have more SSA with higher D_a and provide more adsorption sites for CH₄. Langmuir pressure P_L has different correlations with fractal dimensions. D_a decreases with the increase of P_L. The adsorption velocity is faster with higher D_a. Thus, the fractal dimensions are the comprehensive reflection of differences among the physical properties of coal and are able to show the effect of coal properties on methane adsorption fully.

Coal texture is important for predicting coal seam permeability and selecting favorable blocks for coalbed methane (CBM) exploration. Drilled cores and mining seam observations are the most direct and effective methods of identifying coal texture; however, they are expensive and cannot be used in unexplored coal seams. Geophysical logging has become a common method of coal texture identification, particularly during the CBM mining stage. However, quantitative methods for identifying coal texture based on geophysical logging data require further study. The support vector machine (SVM), a machine-learning method, has received great interest due to its remarkable generalization performance, and it has been used to quantitatively identify hard and soft coal using geophysical logging data. In this study, four well-logging curves, the acoustic time difference (AC), caliper log (CAL), density (DEN), and natural gamma (GR), were used for coal texture analysis. Hard coal (undeformed and cataclastic coal) exhibited higher DEN, GR, lower CAL, and lower AC than soft coal. The accuracy rate of coal texture identification was highest (97%) when the linear kernel function was applied, and the maximum training accuracy rate was achieved when the penalty parameter value of the linear kernel increased to 1. The results of verification with a newly cored CBM exploration well indicated that the SVM-based identification method was effective for coal texture analysis. With the increasing availability of data, this method can be used to distinguish hard and soft coal in a coal-bearing basin under numerous sample learning conditions.

The water produced from coalbed methane (CBM) wells contains abundant geochemical information; therefore, analyzing the geochemical information available therein is of great significance for the efficient exploitation of CBM wells. Based on the geochemical characteristics of water from four CBM wells in eastern Yunnan, this paper analyzes the relationship between the geochemical characteristics of the produced water and gas production. The results indicate that the underground environment in which water is produced in the four CBM wells in the study area exists in a closed state. The water sourced from wells L-1 and L-2 is of the Na-Cl-HCO₃ type, whereas the water sourced from wells L-3 and L-4 is of the Na-HCO₃ type. Gas production is approximately positively correlated with high concentrations of HCO₃⁻, up to concentrations of 2800 mg/L, beyond which gas production decreases. Both D drift and ¹⁸O isotope drift in the produced water are beneficial for the production of CBM. The trace element content of the well water is influenced by the trace elements found in the coal seams and surrounding rocks, especially at peak trace element contents. The dissolved inorganic carbon isotope (¹³C_DIC) in the well water is affected by microbial methanogenesis and carbonate dissolution, and gas production is high when ¹³C_DIC is approximately −4‰.

The continental shale of the Jurassic Xiahuayuan Formation in the Xuanhua Basin of the Yanshan area displays widespread occurrence and notable thickness, indicating good potential for industrial-scale shale gas production. However, there are still many uncertainties regarding the characteristics of shale reservoirs, their gas-bearing properties, and the main controlling factors. To characterize the Xiahuayuan Formation shale, this study employed total organic carbon (TOC) analysis, X-ray diffraction (XRD), low-temperature N ₂ adsorption (LTN ₂ A), and in-situ gas content measurement to systematically investigate organic matter (OM) composition, reservoir properties, and gas occurrence characteristics. Moreover, the principal influencing factors of the gas-bearing characteristics were discussed. The results indicate that the Xiahuayuan Formation shale exhibits relatively high TOC content and is in the mature to high-mature stage. The OM type is mainly Type Ⅲ kerogen, providing a favorable foundation for the formation and storage of shale gas. The in-situ desorption gas content is predominantly composed of free gas. Both the adsorbed and free gas contents gradually increase as burial depth increases. Shale gas content is influenced by multiple geological factors. Among them, the TOC is conducive to hydrocarbon generation in shale. It promotes the development of OM pores, increases pore volume (PV) and specific surface area (SSA), and thus shows a positive correlation with various gas contents. As R _o increases, both the hydrocarbon adsorption capacity and the ability to store free gas are enhanced. In addition, quartz has a remarkable resistance to compaction and can effectively protect the pore structure. When its content is relatively high, it tends to have a slight positive relationship with various gas contents. Conversely, carbonate minerals, by filling the original pores and microfractures, show a negative association with various gas contents. In contrast, clay minerals contribute to the formation of numerous secondary pores and show a positive correlation with various gas contents. The pore structure of shale primarily consists of mesopores and micropores. An increase in micropore volume leads to a reduction in the average pore size (APS), promoting an increase in SSA and PV, and further enhancing the adsorption capacity and storage space of shale. This research are intended to offer a robust geological foundation for future exploration and development efforts related to continental shale gas resources.