Hydrogeochemical and Isotopic Analyses of Deep Geothermal Fluids in the Wumishan Formation in Xiong’an New Area, China

Abstract

Abstract Medium-low temperature geothermal resources in the Wumishan Formation, which is the geothermal reservoir, are local enrichment resources in Xiong’an New Area, North China. In this study, 35 water samples were collected from the bedrock of Taihang Mountains and Wumishan Formation in Xiong’an New Area and display the chemical compositions of water samples as well as the stable isotope compositions for hydrogen, oxygen, carbon, sulfur, and strontium. Hydrogeochemical characteristics and isotope compositions of water samples are analyzed to understand the origin and circulation processes of these geothermal fluids. Our results of cold groundwaters in the bedrock of Taihang Mountain indicate a more open oxidation environment, and the HCO3-Ca·Mg-type groundwater also indicates a prevailing carbonate dissolution condition. The deep geothermal fluids in the Wumishan Formation beneath Xiong’an New Area indicate a closed reduction condition, and their hydrochemical types are mainly Cl·HCO3-Na type. The diagram of hydrogen vs. oxygen isotope indicates that the recharge for the deep geothermal fluids in the Wumishan Formation of Xiong’an New Area is mainly from atmospheric precipitation. The high δ13C values (-3.4‰−-4.9‰) are notably controlled by the eluviation of the carbonate rock layers. The δ34S values vary from 18.02‰ to 27.01‰; the relatively high values indicate the eluviation of sedimentary rock layers. The high 87Sr/86Sr ratios (0.70806−0.71270) and the high Sr2+ concentrations (0.69−2.92 mg/L) suggest that the Sr in the deep geothermal fluids originates from the eluviation of both silicates and carbonates. According to the multimineral equilibrium diagram, chalcedony is saturated at the measured temperature of geothermal wells; therefore, we chose chalcedony as a geothermal thermometer for the calculation of the reservoir temperature of the Wumishan Formation, and the results vary from 68.63 to 89.10°C. Our study identifies the geothermal type of the deep medium-low temperature hydrothermal systems and also recognizes their water-rock interaction processes. We get a comprehensive understanding that the geothermal resources in the Wumishan Formation beneath Xiong’an New Area is convection-conduction type, for which potential of geothermal development and utilization is enormous.