Ionic composition data of coalbed methane (CBM) produced water from five CBM blocks in China were used to investigate the variability in and the geological controls on water quality across geologic basins and the resulting environmental impacts. The assessment tools included the Schoeller diagram, the Na-K-Mg ternary diagram and the Cl-Na-HCO₃ ternary diagram. Techniques such as principal component analysis (PCA) and R-mode hierarchical cluster analysis were also applied to the ionic database to reveal the relationships between the ions. The results from this study indicate that three major ions (Na, Cl, and HCO₃) and two primary processes (sulfate reduction and mixing of the coal seam water with the fracturing fluid) affect the ionic compositions of CBM produced water. The burial depth of the coal seam, the fresh water influence and the fracturing fluid contamination jointly control the water quality variability of produced water. The Na-K-Mg ternary diagram provides a potential method to evaluate the origin of CBM produced water, such as coal seam water in the partially equilibrated region and exotic water (including the fracturing fluid or shallow cold groundwater) in the immature water region. Most of the CBM produced water in China has high concentrations of Na, HCO₃, Cl and TDS compared to the standards for drinking water and irrigation water. The produced waters from deep formations (>1000 m), or those produced during the early drainage periods, are always saline, “Na-Cl” type waters. Appropriate treatment methods and regulations for CBM produced water in China are urgently needed.

Geothermal fields in coal-bearing strata significantly influence coal mining and coalbed methane accumulation and development. Based on temperature data from 135 coalfield exploration boreholes and thermophysical tests of 43 rock and coal samples from the Upper Permian coal-bearing strata of the Bide-Santang basin in western Guizhou, South China, the distribution of terrestrial heat flow and the geothermal gradient in the study area are revealed, and the geological controls are analysed. The results show that the thermal conductivity of the coal-bearing strata ranges from 0.357 to 3.878 W (m K)⁻¹ and averages 1.962 W (m K)⁻¹. Thermal conductivity is controlled by lithology and burial depth. Thermal conductivity progressively increases for the following lithologies: coal, mudstone, siltstone, fine sandstone, and limestone. For the same lithology, the thermal conductivity increases with the burial depth. The present geothermal gradient ranges from 15.5 to 30.3°C km⁻¹ and averages 23.5°C km⁻¹; the terrestrial heat flow ranges from 46.94 to 69.44 mW m⁻² and averages 57.55 mW m⁻². These values are lower than the averages for South China, indicating the relative tectonic stability of the study area. The spatial distribution of the terrestrial heat flow and geothermal gradient is consistent with the main structural orientation, indicating that the geothermal field distribution is tectonically controlled at the macro-scale. This distribution is also controlled by active groundwater, which reduces the terrestrial heat flow and geotemperature. The high geothermal gradient in the shallow strata (<200 m) is mainly caused by the low thermal conductivity of the unconsolidated sedimentary cover. The gas content of the coal seam is positively correlated with terrestrial heat flow, indicating that inherited palaeogeothermal heat flow from when coalbed methane was generated in large quantities during the Yanshanian period due to intense magmatic activity.

As the co-exploration and co-production of unconventional gas have become a new trend of the unconventional gas exploration and development in the eastern margin of the Ordos Basin, this study focuses on the distribution, pore characteristics, geochemical properties, and mineral composition of the mud shale in the coal-bearing strata. The results show that the mud shale in the coal-bearing strata is distributed relatively stably with an average cumulative thickness of 77.60 m and a buried depth of 200–2400 m, increasing from east to west. The total organic carbon content of mud shale is relatively high, ranging from 0.14% to 39.9%, and the thermal maturity ranges from 0.23% to 2.49%, affected by both buried depth and magmatic intrusion. The organic matter type of the mud shale is dominated by type III, favorable for gas generation. The mineral composition of mud shale is mainly clay minerals and quartz, with low brittle minerals content (averaging 30.34%) and high kaolinite content (averaging 59.23%). Pores and micro-fissures are relatively developed in the mud shale, dominated by intergranular pores, organic pores, and the micro-fissures developed between brittle mineral crystals, within crystals and between clay mineral particles, with a porosity of 1.05%–1.59% and a permeability of 0.001–0.142 (×10⁻³µm²). The specific surface area of pores in mud shale is 12.31–28.99 m²/g, and the total pore volume is 0.0146–0.0483 cm³/g. Mesopores and micropores (mainly slit shaped pores) provide the main pore volume of mud shale, and micropores contribute most of the specific surface area. The controlling factors on pore development include the total organic carbon, thermal maturity, and inorganic minerals composition. The total amount of shale gas resources is 2.49 × 10¹² m³, and the Linxian-Xingxian area and Shilou-Daning area are the potential favorable zones for shale gas exploration and development.