Deep-carbonate gas reservoirs are a significant field for conventional gas exploration in the Sichuan Basin, and industrial-grade gas reservoirs have recently been discovered in the second member of the Dengying Formation in the Deyang–Anyue rift. Drilling results have indicated that the large amount of solid bitumen that is commonly found in pores and fractures in the reservoir of the second member of the Dengying Formation. However, systematic studies have not yet been conducted on the solid bitumen of the second member of the Dengying Formation and its genesis and parent source remain unclear. This study is based on the systematic analysis of solid bitumen samples collected from new exploratory wells, with the aid of optical microscopy, scanning electron microscopy, and gas chromatography–mass spectrometry to clarify the genesis, thermal maturity, and parent source deposition environment of the solid bitumen. Electron microscopy and biomarker results indicate that bitumen is in a high-maturity stage and is a product of the thermal cracking of crude oil. Energy dispersive spectrometer results and the continuous distribution of n-alkanes indicate that the bitumen is not subjected to strong thermochemical sulfate reduction and biodegradation. Isoprenoids and terpenes suggest that the palaeoreserves associated with the bitumen of the second member of the Dengying Formation originate in reductive marine carbonate sediments. The combination of regional stratigraphic contact relationships, lithology, and biomarker characteristics implies that the Maidiping Formation muddy dolomite and the Dengying Formation algal dolomite are the potential parent sources of solid bitumen in the second member of the Dengying Formation.

Microbial-Enhanced Coalbed Methane (MECBM) is a technology that generates new methane gas in coal seams through the action of microorganisms, thereby improving the efficiency of coalbed methane development. In this study, low-temperature CO₂ adsorption, low-temperature N₂ adsorption, and isothermal adsorption experiments were conducted to systematically characterize the changes in the pore characteristics of low-rank coals in Xinjiang before and after degradation. The results show that microbial action increases the average pore diameter and enhances pore connectivity. Meanwhile, it reduces the fractal dimension of the pore surface and simplifies the complexity of the pore structure. The modification of the pore structure effectively promotes the efficiency of methane desorption and migration, thus improving the exploitation potential of coalbed methane. Microbial degradation avoids the risk of deterioration of reservoir physical properties through biological modification, and reduces carbon emissions and environmental pollution. This study provides an environmentally friendly solution for the low-carbon development of coal resources, and has important scientific significance for promoting the transformation of energy structures and achieving the goal of carbon neutrality.

The majority of known coalbed methane (CBM) production worldwide comes primarily from high-abundance CBM-enrichment areas or ‘fairways.’ The high-abundance CBM-enrichment areas are primarily characterised by large CBM resources with high single-well productions. CBM accumulation areas from the medium- to high-rank coals in the southern Qinshui Basin and the Hancheng CBM fields in the Ordos Basin were investigated based on regional geological analyses and physical analogue experiments. The results show that gas contents in the study areas increase with depths over the range from approximately 300 to 800 m, while permeabilities generally decrease with depths. Intervals with optimal gas content and permeability exist at a moderate depth along an inclined coal seam under the coupled control of temperature and stress. Brittle–ductile transition deformation increases the permeability and the pore-specific surface areas of coals. The gas content and permeability of the CBM reservoirs are shown to be two key factors determining the formation of high-abundance CBM areas. The coupling of gas enrichment and high permeability provides a favourable combination for CBM accumulation and high production. Combining CBM exploration and development practices in the study areas with physical analogue experiments, two CBM-enrichment models for medium- to high-rank coal have been recognised for different geological conditions, including (1) the model controlled by the depth in the slope zone and (2) the model controlled by the coal brittle and ductile in the deformation zones.

The temperature or maturity limit of methane (CH₄) cracking is very useful for the determination of the most depth or the highest maturity in natural gas exploration owing to the composition of over mature gas. In this work, three series of CH₄ cracking experiments were conducted under different conditions of N₂ + CH₄, N₂ + CH₄ + montmorillonite and N₂ + CH₄ + shale, respectively, in a gold tube system. The experimental results show that some heavy gas with negative carbon isotope composition could be generated in the three series experiments and that shale has more intense catalysis for CH₄ cracking than montmorillonite. The catalysis of metal elements distributed in the minerals of shale is attributed to CH₄ cracking acceleration. The shale catalysis makes the maturity threshold of CH₄ substantial cracking decrease from 6.0%R_o under no catalysis, to 4.5%R_o under a shale system in a geological setting. Nevertheless, we suggest not to lightly practice natural gas exploration in shale with the maturity range of 3.5–4.5%R_o, as the maturity threshold of gas generation from oil prone organic matter distributed extensively in shale is 3.5%R_o.