Hydrogeochemical characteristics and genesis of geothermal water in Xiaoyangkou, Nantong, China

To identify the mechanism of geothermal water in Xiaoyangkou Geothermal Field, the earliest medium-temperature, high-yield geothermal resource found in Subei Basin, based on the water quality indicators of four geothermal water and one groundwater samples, hydrogeochemical and isotopic methods were used to analysis the hydrochemical characteristics, recharge sources and circulation depth of geothermal water, and to determine the reservoir temperature. Results show that the temperature of the thermal reservoir of the Neogene Yancheng Formation is about 39.5–41.64 °C. The geothermal waters are HCO3 type with TDS of 616–638 mg/L. The temperature of the Paleogene Funing Formation and Devonian–Silurian sandstone reservoir is about 95.45–110.78 °C. The geothermal waters are SO4 type with TDS of 1446–1812 mg/L. The concentrations of trace elements F, Sr and Li increased rapidly with the increase of temperature. The isotopic compositions of δ18O and δD indicates that the geothermal water is mainly recharged by atmospheric precipitation from the elevation of 145.5–524.5 m. The atmospheric precipitation infiltration is heated by the heat flow in the deep part and transported upward along the water-conducting structure through deep circulation (3.37 km) to the karst fissure at the top of the underlying strata of Yancheng Formation, forming a convective geothermal system. The upper Yancheng formation is an associated-conduction geothermal system. The research results provide reference and basis for the rational development and utilization of geothermal resources in the region.