Indicative significance of hydrochemical characteristics in geothermal resource investigations in the Enshi Basin

Objective Geothermal resources are valuable clean resources, and the Enshi Basin contains abundant geothermal resources. It is very important to expore the origin of geothermal resources. Methods Starting from the chemical and isotopic characteristics of geothermal water, combined with the hydrological and geothermal geological conditions of the Enshi Basin, the following works have been performed. The geochemical characteristics and main ion recharge sources of geothermal water in the Enshi Basin are discussed and analyzed by the Piper diagram method and main ion correlation analysis method. In this study, the appropriate silica temperature scale method is used to solve the problem that it is difficult to accurately measure the heat storage temperature. Using hydrogen and oxygen isotope testing technology, the recharge source, circulation depth and recharge elevation of geothermal water are determined. Results Results show that the hydrochemical type of geothermal water in the Enshi Basin is mainly SO4·Cl-Na water, and the main ions in the groundwater are SO42-, Cl- and Na+. There is a good positive correlation between TDS in geothermal water and Na+, Ca2+, Mg2+, Cl- and SO42-, while TDS in geothermal water is much higher than that in cold springs.It is due to the large burial depth, long runoff path and strong dissolution leaching action, which makes it easier to extract relevant ions from the surrounding rock and results in ion concentrations much higher than those of surface water. The hot water in the study area is mainly supplied by atmospheric precipitation, and the 14C and 34S isotopic characteristics of geothermal water show that the storage environment of geothermal water from the edge of the basin to the center of the basin is gradually closed, the retention time of geothermal water is gradually longer, and the degree of water rock reaction is gradually stronger. The results of the water-rock balance show that the concentration of SiO2 in hot water is controlled by the dissolution balance of quartz.Using the SiO2 geothermal temperature scale, the estimated thermal storage temperature is 55.74-58.24 ℃, the burial depth of thermal storage is 1 793-1 906 m, and the circulating depth of hot water is 1 823-1 936 m. The recharge elevation of geothermal water is estimated to be 1 022.64-1 109.00 m, according to the elevation effect of δO of atmospheric precipitation. Conclusion According to the elevation range of the study area, the geothermal water recharge area is mainly the low and middle mountain area of Cambrian Ordovician carbonate rocks in the western part of the basin.