Knowledge of in situ stress distribution is fundamental for coalbed methane production; however, it is poorly understood in the eastern Yunnan region, South China. In this study, the horizontal maximum (SHmax) and minimum (Shmin) principal stress and vertical stress (Sv) were systematically analyzed for the first time. The results indicated that the magnitudes of SHmax, Shmin, and Sv showed positive correlations with burial depth. In general, three types of in situ stress fields were determined: (1) SHmax > Sv > Shmin in shallow layers with burial depths less than approximately 600 m (∼1970 ft) below ground level (bgl), indicating a dominant strike-slip faulting stress regime; (2) in medium layers approximately 600–800 m (∼1970–2625 ft) bgl, the in situ stress state followed multiple relationships, suggesting that the in situ stress regime was transformed; and (3) Sv > SHmax >Shmin in deep layers with burial depths greater than approximately 800 m (∼2625 ft) bgl, indicating a dominant normal faulting stress regime. Coal permeabilities obtained from injection–falloff well tests showed that they were widely distributed, and no obvious relationships were found between coal permeability and effective in situ stress magnitude. In the study area, the development and orientation of previously generated natural fractures combined with the present-day in situ stress distribution controlled the permeability in coal reservoirs. Differential stress and presence of natural fractures significantly affected the geometry and pattern of hydraulic fractures. In addition, in the eastern Yunnan region, locations with relatively deep depths in vertical wells and approximately west–northwest/east–southeast-trending horizontal wells suffered high potential of borehole instability because of the high differential stress.

Effects of in-situ stress and joint on permeability of the coal bed depend on orientation relationships between in-situ stress and joint. In-situ stress orientations of Linfen block of southeastern Ordos Basin were determined by systematical measurements of the loess joints. In-situ stress magnitudes were calculated based on well logging and hydraulic fracturing data. Joint orientations of the No. 5 coal bed and density distributions of the overlying sandstone bed were investigated. The results show that the NE-oriented maximum horizontal principal stress, approximately parallel to the predominant joint orientation of the coal bed, is favorable to the openness of the predominant joints. The minimum horizontal principal stress, with its orientation perpendicular to the predominant joint orientation, tends to induce the closure of the predominant joints. The stress state of No. 5 coal bed is o_v > 0_H > O_h, which is favorable for the openness of subvertical joints and an increase in the permeability. The permeability of No. 5 coal bed increases obviously with increasing effective vertical stress and effective horizontal maximum stress and decreases with increasing effective horizontal minimum stress. Besides, the permeability of No. 5 coal bed exhibits a good exponential relationship with the density of predominant joint set. Therefore, predominant joints with the orientation of 45° mainly contribute to increases in the permeability of the No. 5 coal bed in Linfen block. Multidimensional analysis show good exponential relationships among the permeability, predominant joint density, and effective stress of the No. 5 coal bed.

The phenomenon of coalbed-methane synclinal accumulation in the Qinshui Basin has been widely reported, but it has mainly been observed in the core block of the Qinshui Syncline. The questions arise: does this phenomenon exist in the wing of the Qinshui Syncline and, if so, what is the mechanism behind it? Further study is required to answer these questions. This paper focuses on the South Anze No. 3 coal seam in the Qinshui Basin as an example. It conducts a systematic sorting of coalbed-methane geological characteristics and an analysis of the effects of structural assemblage characteristics, genetic mechanisms, and structural control on coalbed-methane accumulation. Additionally, it examines the basin structure and evolution during the critical period of the Qinshui Basin, as well as the gas geological characteristics of adjacent areas, in order to discuss the gas-rich mechanism of the syncline in the Qinshui Basin. Key insights obtained from the study include the following: (i) The whole South Anze is a nosing structure that plunges from west to east and superposes secondary folds and faults in different directions. Four deformation zones can be identified based on the characteristics of structural assemblage, including NEN-oriented compressive structures, ENE-trend shear fractures, EW-trend compressive fractures, and EW-trend compressive folds. The formation of structural assemblage in the study area is attributed to the compression in the Indosinian and Yanshanian, and the fault inversion in the Himalayan period. (ii) The ENE-trend shear fracture deformation area located in the nosing uplift is a low CBM (coalbed methane) content area due to gas diffusion during the Himalayan extension. The syncline in the combination of NEN-trend and EW-trend “ejective folds” in the west and south of the study area is a high-value area of coalbed-methane content. It is further verified that the law of syncline gas accumulation in the Qinshui Basin is also applicable to the wing of the Qinshui Syncline. (iii) Since the formation of the Qinshui Syncline, the main coal seam has been in an extensional environment below the neutral plane, resulting in the main dissipation of coalbed methane. During its geological history, surface water penetrated the aquifer above the main coal seam through two channels: the extensional area above the neutral plane of the adjacent anticline and the shear fracture. A hydrostatic pressure seal is formed in the Qinshui Syncline and the secondary syncline is superimposed upon it, which is the cause of gas enrichment in the syncline of the Qinshui Basin. (iv) Weak deformation in the syncline basin is the focus of global coalbed-methane exploration and development. The mechanism proposed in this paper can provide ideas and references for further understanding of coalbed-methane enrichment in this type of basin.

In order to predict in advance the coal and gas outburst risks in the No. 3 coal seam in the non-mining area of the Xinjing mine, the strata were divided into seven rock assemblage types based on the lithologic characteristics of strata in the research area, and eight geological profile models were constructed. The finite element methods were used to simulate the ground stress field of the No. 3 seam floor. Based on the log curve, the coal structural type of the No. 3 coal seam was identified, with the thicknesses of the coal body under different types of damage being marked off in each borehole. A damage index of the coal structure (DV) was also proposed, and the DV indexes for all boreholes were calculated. The coal and gas outburst risks in the research area were comprehensively evaluated and predicted through a superposition analysis of the spatial distribution states of three indexes, namely: ground stress, coal structure damage degree, and coal seam gas content. The results show that the equivalent stress of the No. 3 coal seam floor is usually within the range of 9–26 MPa. The high-stress zone presents a strip distribution along the northeast–southwest (NE–SW) direction. The distribution of ground stress is mainly subject to the folds and buried depth of the coal seam. The distribution range where the damage degree of the coal structure falls under Types II and III is DV ≥ 22. The gas outburst risk in the mid-southern and northeastern parts of the research area is high, whereas that in the mid-western part is low. The zones with a high and low degrees of gas outburst risks are all mainly present as strips in the NE–SW direction. The gas outburst risk in the northwest and southeast is moderate. The research results can provide guidance for gas control in non-mining areas.

Tight sandstone gas constitutes a strategically significant resource in the exploration of unconventional hydrocarbon systems. Current understanding of the geochemical composition and genesis of tight sandstone gas in the Daning–Jixian Block, southeastern Ordos Basin, is insufficient, which hampers a comprehensive assessment of its resource potential. This study is the first to systematically investigate the geochemical characteristics and genetic origin of high-maturity tight sandstone gas in the southeastern Ordos Basin’s Daning–Jixian Block. Gas specimens were systematically acquired from multiple stratigraphic units within the reservoir interval and subjected to compositional and carbon–hydrogen isotope analysis. Compared with other gas fields in the Ordos Basin, the Daning–Jixian Block has higher average methane concentration, and notably lower ethane and propane concentrations; its average δ¹³C₁ and δ²H-CH₄ is heavier, while δ¹³C₂ and δ¹³C₃ are lighter. Based on the δ¹³C₁-δ²H-CH₄ diagram, all gas samples from the block and other basin gas fields fall into the geothermal, hydrothermal and crystalline gas domain, indicating gas genesis associated with over-mature organic matter interacting with external hydrogen. Milkov genetic diagram analysis reveals that the natural gas consists of primarily early-stage kerogen-cracking gas, with a minor contribution from crude oil-derived gas originating from Carboniferous–Permian source rocks. Notably, samples from Daning–Jixian exhibit a unique δ¹³C₁ > δ¹³C₂ reversal, attributed to mixing effects between gas from highly mature kerogen and gas from secondary cracking of crude oil. Consequently, ethane carbon isotopes alone are insufficient for definitive genetic classification. These findings provide a new geochemical interpretation framework for analogous high-maturity tight gas reservoirs.