Hydrochemical Characteristics and Water-Rock Interaction of Geothermal Fluids In Deep Karst Reservoir of Tianjin

Tianjin is rich in geothermal resources, the development level has been in a leading position for a long time in China. As of 2021, the city's hydrothermal geothermal heating area will reach 38 million square meters, making it the largest city in the world with hydrothermal geothermal heating. Tianjin geothermal utilization is dominated by karst reservoir in the deep Jixian Wumishan Formation, accounting for about 2/3 of the total geothermal development in the city, the karst reservoir has the characteristics of shallow burial, well-developed fissures, large water output, easy development and easy recharge. Hydrogeochemical characteristics of geothermal water, water-rock interaction and the formation process of geothermal water in the study area were analyzed and studied, to better support the scientific development of geothermal resources. The results show that the geothermal water has obvious zoning. TDS increases along the runoff direction from north to south, and the chemical type gradually evolves from HCO3·SO4·Cl-Na type to Cl·SO4·HCO3-Na, the anion is gradually changed from HCO3- to Cl-·SO42-and finally to Cl-; The chemical composition of karst water is mainly formed by the dissolution of carbonate rock and dolomite, and there is an excess of SO42-, indicating that SO42- has other sources, which may be formed by the oxidation of deep H2S gas and the oxidation of pyrite. Years of monitoring data show that the main ion concentration and total salinity of geothermal fluid are basically stable, and there is no obvious change in ion composition due to man-made mining disturbance. This study can provide a scientific basis for the exploration, development and protection of deep karst geothermal resources in Tianjin.