Hydrogeochemical characteristics and genesis of the geothermal fields in Batang of Sichuan

Geochemical characteristics of geothermal water can reveal deep geothermal processes. The Batang area in western Sichuan is taken as an example, where geothermal resources are relatively abundant, but few studies were carried out previously. Based on the existing researches, this paper aims to further identify the occurrence and source of geothermal resources in western Sichuan, reveal the mechanism of thermal cycle, and quantitatively evaluate the temperature of the geothermal reservoirs, the mixing ratio of cold water and depth of thermal cycle in the study area. Hydrochemical analyses and hydrogen and oxygen isotopes of 20 hot spring water samples from two geothermal fields in Batang are used to calculate quantitatively and analyze the evolution process of thermal groundwater in the Batang area. The results show that the thermal groundwater is mainly of HCO3-Na type in the Batang area. F-, Li2+, As+ and Cl- in the hot water have poor correlation, indicating that F-, Li2+, As+ only come from the dissolution of minerals in the rocks. The hot water is mainly recharged by atmospheric rainfall. The temperature and depths of geothermal reservoirs before mixture with cold water are obtained by the Si-enthalpy model as 218-229 ℃ and 4 546.32-4 777.89 m and the mixing ratio of cold water ranges from 64% to 68%. The changes in concentrations of Na+, HCO-3 and SiO2 in the Batang area are relatively large, indicating that the geothermal water only comes from the dissolution of minerals in rocks, including dissolution of calcium feldspar and silicate precipitation. Water-rock interactions and ion exchange occur between the geothermal water and surrounding rocks in western Sichuan. After completing a series of hydrochemical action and water-rock interaction, the hot water rises to the land surface and eventually emerges in the form of hot springs. The above analyses of the characteristics of the geothermal reservoirs can provide data support and theoretical support for the future geothermal research in the Batang area in western Sichuan, and can also provide methodological references for the overall regional geothermal research in western Sichuan, which is of important reference significance for the study of groundwater development and utilization in the study area.