Chemical characteristics, formation mechanisms, and geological evolution processes of high-salinity coal reservoir water in the Binchang area of the southern Ordos Basin, China

The chemical characteristics of coal reservoir water are important for studying the formation and enrichment of biogenic coalbed methane (BCM). Based on geological and sampling test data, this paper studied the geochemical characteristics and formation mechanisms of high-salinity coal reservoir water (CRW) in the Jurassic Yan'an Formation of the Binchang area in the southern Ordos Basin. The results show that the TDS contents of the CRW in the Binchang area are between 7577.38 and 15,138.61 mg/L (av. 13,268.95 mg/L), which is high-salinity brackish water. The ion types of CRW are mainly Na⁺, Cl⁻ and HCO₃⁻, and the correlations between TDS and Na⁺ and Cl⁻ are close to 1. The Piper trilinear diagram indicates that the evolution direction of the CRW is deep concentrated brine, and the hydrochemical type is the NaCl type. The ¹²⁷I concentrations of CRW are between 285 and 484 μg/L, which are much higher than the values of 55.88 μg/L for seawater. The results of ¹²⁹I dating show that the minimum age of the CRW in the study area is between 6.7 Ma and 39.97 Ma, which is much younger than the actual geological age of the Yan'an Formation. The hydrogen and oxygen isotope results show that the CRW in the study area experiences an apparent oxygen drift, indicating that the coal reservoir of the Yan'an Formation has good sealing and a long retention time for the CRW. The hydrodynamic factors show that the hydrodynamic conditions of the coal reservoir are weak, and the primary ions in the CRW originated from the dissolution of salt rocks. The main ion differentiation indices show that high-salinity coal seam water is mainly formed by evaporation, and the ion exchange between CRW and the surrounding rock and the alternating adsorption of cations in water are very weak. Evaporation and diagenesis lead to an increase in the contents of Na⁺, Cl⁻ and I⁺ in coalbed water, which in turn leads to an increase in the total dissolved solids contents of CRW and its evolution toward concentrated brine. The genesis and evolution of the CRW in the study area are affected by the combination of the relationships among the paleoclimate, aquifers and aquifuges, and tectonic evolution processes. The CRW in the study area has experienced five evolution stages, i.e., sedimentary water and diagenetic water, high-salinity infiltration water, primary mixed water, paleoatmospheric precipitation recharge water, and secondary mixed water. The above understanding can provide a basis for studying the formation period and accumulation mechanism of BCM and provide a hydrogeological basis for water resource utilisation and pollution prevention and the control of high-salinity water.