Water-gas-rock interactions and their variations in coal measures during evolution of the North China Craton Basin

Abstract Under China's “dual carbon” goal, exploring and developing coal measure gas is crucial for advancing the national energy transition. However, coal measure gas exploitation is often accompanied by the production of substantial volumes of formation water with varied compositions. The Carboniferous Permian coal measures in North China formed during the late Paleozoic era and subsequently experienced complex geological processes throughout the Mesozoic and Cenozoic. These geological transformations have resulted in significant regional variations in the structural and hydrogeological characteristics of the North China Craton Basin. Consequently, the hydrogeological conditions governing the coal seams and their surrounding strata are highly complex, necessitating a comprehensive investigation into the water-gas-rock interactions and their spatial variability during basin evolution. Water-gas-rock interactions in coal-bearing formations exert a fundamental control on the hydrogeochemical characteristics of coalbed methane (CBM) reservoirs, thereby influencing CBM production. This study investigates the geochemical properties of formation water in five representative CBM reservoirs within the North China Craton Basin. Through systematic analyses of major ions, trace elements, and hydrogen oxygen isotopes in coal measure water samples, this study elucidates the spatial variability of hydrogeochemical characteristics, the underlying water-gas-rock interactions, and their controlling mechanisms. The results show that the coal measure water in these reservoirs is rich in Na+, Cl−, and HCO3−, while the contents of Ca2+, Mg2+, and SO42− are low. Na+ and Cl− mainly come from primary water, NaCl dissolution, cation exchange reaction, and evaporation. HCO3− mainly comes from the dissolution of carbonate minerals. Ca2+ and Mg2+ come from the dissolution of carbonate minerals, silicate weathering, and ion exchange processes. SO42− comes from the oxidation of sulfide minerals. The Linxing and Liulin blocks exhibit groundwater retention and increased mineralization, whereas the Panhe, Shizhuangnan, and Luling blocks display strong groundwater mobility with relatively low mineralization. Isotopic analyses of δD and δ18O indicate that the recharge sources vary across the study sites. The Linxing block is primarily recharged by groundwater, while atmospheric precipitation is the dominant recharge source for the other blocks. Notable isotopic shifts, such as δD drift in the Panhe block and δ18O drift in the Linxing, Liulin, and Shizhuangnan blocks, suggest influences from evaporation, deep-fluid mixing, or reservoir geochemical processes. Additionally, the Luling block exhibits a pronounced evaporation effect along with partial recharge from a sandstone aquifer. The migration and distribution of trace elements, including Li, Rb, Sr, and Ba, further highlight the role of water-rock interactions. Carbonate weathering and dissolution predominantly control water chemistry in the Linxing, Panhe, and Shizhuangnan blocks, while evaporite dissolution and carbonate weathering exert greater influence in the Liulin and Luling blocks. These findings provide critical insights into the hydrogeochemical evolution of CBM reservoirs and offer valuable guidance for optimizing resource exploration and development in the North China Craton Basin. Moreover, they contribute to the advancement of sustainable energy strategies, playing a pivotal role in accelerating China's progress toward carbon neutrality.