Hydrogeochemical Characteristics and Geothermometry Applications of Thermal Waters in Coastal Xinzhou and Shenzao Geothermal Fields, Guangdong, China

Two separate groups of geothermal waters have been identified in the coastal region of Guangdong, China. One is Xinzhou thermal water of regional groundwater flow system in a granite batholith and the other is thermal water derived from shallow coastal aquifers in Shenzao geothermal field, characterized by high salinity. The hydrochemical characteristics of the thermal waters were examined and characterized as Na-Cl and Ca-Na-Cl types, which are very similar to that of seawater. The hydrochemical evolution is revealed by analyzing the correlations of components versus Cl and their relative changes for different water samples, reflecting different extents of water-rock interactions and clear mixing trends with seawaters. Nevertheless, isotopic data indicate that thermal waters are all of the meteoric origins. Isotopic data also allowed determination of different recharge elevations and presentation of different mixing proportions of seawater with thermal waters. The reservoir temperatures were estimated by chemical geothermometries and validated by fluid-mineral equilibrium calculations. The most reliable estimates of reservoir temperature lie in the range of 148-162 degrees C for Xinzhou and the range of 135-144 degrees C for Shenzao thermal waters, based on the retrograde and prograde solubilities of anhydrite and chalcedony. Finally, a schematic cross-sectional fault-hydrology conceptual model was proposed.