Super-high-organic-sulfur (SHOS) coals are limited in their utilization because they are rich in sulfur and other toxic trace elements. SHOS coals are generally rich in V, Se, Mo, U, Re, rare earth elements and yttrium (REY). This paper investigates the causes of elemental enrichment in the Naliang SHOS coals (Xian’an Coalfield, Guangxi Province, China) and analyzes geological controls on elemental/mineralogical anomalies in coal and associated rock samples. Compared with world hard coal averages, the Naliang coals are rich in V, and Se, highly rich in U, and unusually rich in Mo. Minerals in the Naliang coals include quartz, kaolinite, illite, mixed-layer illite, pyrite, calcite, dolomite, gypsum, anhydrite, albite, heulandite, and diaspore. Minor iron sulfate minerals, Fe(Al, Si)-oxysulfate minerals, and ammonia illite were observed under the scanning electron microscope with an energy dispersive spectrometer. Based on geochemical indicators such as Al ₂ O ₃ /TiO ₂ ratios and REY distribution patterns, the source material compositions of Naliang coals are felsic igneous rocks from the Yunkai Upland. The ratio of Sr/Ba is > 1, the high concentration of B (67.9 µg/g-429 µg/g), and the presence of fossil foraminifera provide strong evidence that the Naliang coals were affected by seawater. The vein-filled occurrences of quartz, pyrite, calcite, and kaolinite, along with the similar distribution of elemental assemblage of V- Mo-U and Se-Hg, indicate that the Naliang coals may have been subjected to multi-stage hydrothermal fluid injections. The discovery of hydrothermal-origin pyrite offers new evidence for sulfur origins in SHOS coals.

Understanding the shear mechanical properties of shale reservoirs is of great significance in the study of the formation stability around horizontal shale wells and the propagation and evolution of fractures for shale fracturing. However, the existing direct shear test results are limited due to small sample sizes and low shear rates. Based on previous experimental research results, the mechanical properties of anisotropic reservoir shale in direct shear tests with different experimental conditions were explored in this study. It was found that the shear mode, shear strain rate, and normal stress have a significant impact on the deformation and failure characteristics of shale. The peak shear displacement, peak shear strength, and shear stiffness of shale present an increasing trend of fluctuation, with an increase in the bedding angle. The peak shear strength of shale decreases with an increase in the shear strain rate, and this decrease trend descends with an increase in the shear strain rate. The shape of the shear fracture zone and the shear fracture mode of shale exhibit bedding effect characteristics. The fractal dimension of the shale shear fracture surface morphology shows a trend of fluctuation with the variation in the bedding angle. Further, the shear strain rate was found to play a dominant role in the fractal dimension of the shear fracture surface. The larger shear strain rate strengthens the bedding effect of the roughness for the shear fracture surface morphology. The results of this study provide a theoretical reference for determining the engineering geomechanics characteristics of shale reservoirs.

The Chaijiagou Mo deposit (0.11 Mt Mo @ 0.07%) is located along the northern margin of the North China Craton. This study integrates ore geology, S–Pb–He–Ar–H–O isotopes, and fluid inclusion (FI) analyses to constrain the sources of ore-forming fluids and metals, as well as mineralization mechanisms. Three principal inclusion types were identified: liquid-rich, vapor-rich, and saline FIs. Microthermometry documents a progressive decline in homogenization temperatures and salinities from early to late mineralization stages: Stage 1 (360–450 °C; 5.3–11.3 and 35.4–51.5 wt.% NaCl equation), Stages 2.1–2.2 (320–380 °C and 260–340 °C; 5.4–11.8 and 33.8–44.5 wt.% NaCl equation), and Stage 4 (140–200 °C; 0.4–3.9 wt.% NaCl equation). Noble gas and stable isotope data reveal that the ore-forming fluids were initially dominated by crustally derived magmatic–hydrothermal components with a minor mantle contribution, subsequently experiencing significant meteoric water input. S–Pb isotopic compositions demonstrate a genetic relationship between mineralization and the ore-bearing granite porphyry, indicating a magmatic origin for both sulfur and lead. Fluid–rock interactions and fluid boiling were the dominant controls on molybdenite and chalcopyrite deposition during Stage 2, whereas mixing with meteoric waters triggered galena and sphalerite precipitation in Stage 3.

To understand the macroscopic fracture behavior of shale under different confining pressures, it is necessary to study the process of microcrack propagation from a microscopic perspective. In this study, a cohesive zone model for heterogeneous shale based on mineral distribution was constructed. Numerical simulation experiments were conducted under different confining pressures to investigate the effects of confining pressure on the extension of microcracks under tensile and shear loading from three perspectives: microcrack morphology, acoustic emission characteristics, and mechanical responses of different minerals. This study reveals the influence of different confining pressures on the extension of microcracks under tensile and shear loading conditions in shale and their microscopic mechanisms, which holds theoretical and practical significance for the exploitation of deeply buried shale gas.

Experimental studies have confirmed the permeability reduction of coal samples upon the adsorption of CO₂. However, these studies were carried out under limited experimental conditions. In this study, CO₂ flow behaviors in a macro-scale coal sample were numerically simulated using a coupled gas flow, mechanical deformation, and sorption-induced deformation finite element model. The simulation results show that the effect of the reduction of effective stress on the enhancement of permeability is greater than the negative effect of permeability reduction due to CO₂ adsorption for low injection pressures. CO₂ pressure development in the sample increases with increasing injection pressure due to the enhanced advection flux for sub-critical CO₂ injections, while for super-critical CO₂ injections, CO₂ pressure development, as well as concentrations in the sample, decreases compared to sub-critical CO₂ injections because of greater density and viscosity of super-critical CO₂ as well as coal matrix swelling induced by the adsorption of super-critical CO₂. Increasing axial stress (buried depth) obstructs CO₂ migration in the sample due to the increased effective stress, and this effect is more influential for low injection pressures, which indicates that high CO₂ injection pressures are preferred for CO₂ sequestration in deep coal seams.

Deep coal seams are generally preferred for CO₂ sequestration, during which the saturation fluids and high-stress condition involved can significantly alter the mechanical attributes of coal. To understand the effect of stress conditions on the mechanical properties of coal during CO₂ sequestration, a finite element model was developed and subsequently validated using experimental data. The results indicate that coal strength increases from 10.35% for a 5 MPa CO₂-saturated sample to 114.54% for an 8 MPa CO₂ + water-saturated sample as the confining pressure rises from 0 to 30 MPa, due to reduced porosity. However, this effect diminishes with higher confining pressures as dilation decreases. The critical confining pressure determined in this study is approximately 20 MPa, at which all samples exhibit similar failure strength (around 48.50 MPa). Moreover, the strengthening effect caused by applied stress is especially pronounced in CO₂-saturated samples, particularly in those saturated with super-critical CO₂ and CO₂ + water. This suggests that the reduction in coal strength resulting from the adsorption of saturation fluids can be counterbalanced by the strength gain resulting from applied stress. The aforementioned results highlight the effectiveness of injecting high-pressure super-critical CO₂ into deep coal seams for carbon sequestration purposes.