Using Hydrochemical Geothermometers Calculate Exchange Temperature of Deep Geothermal System in West Coastal Area of Guangdong Province

The estimated temperatures by traditional geochemical geothermometers and borehole temperatures are always different in practice. It is important to decide the equilibrium between minerals and fluids, determine the application of geothermometers for calculating heat exchanging temperatures, and choose the most suitable results. In this study, both Na-K-Mg triangular diagram and PHREEQCI code that simulate minerals saturation indexes were used to judge the equilibrium state between hot waters and minerals. The results show that: (1) the 23 water samples collected from the deep geothermal system in west coastal area of Guangdong Province are characterized by middle-low temperatures with weak alkalinity, high content of fluoride derived from the water-rock interaction between underground water and granite, and the transition of hydrochemical type from HCO₃·Cl-Ca·Na to Cl-Na in groundwater from inland to coastal area; (2) Hydrochemical geothermometer temperature estimations show that the mixture of shallow water with deep water leads to the lower silica geothermometer temperatures than the actual ones. Only sample 21 with a subsurface equilibrium temperature at 150-170 ℃ can be calculated by cation geothermometers. K-Mg geothermometers yield minimum temperatures of heat exchange for samples 1 and 19 at 136.2 and 151.6 ℃, respectively. log(Q/K)-T plot proves better for other samples. The mixing (with cold water) process occurs when thermal waters ascend to the surface after a deep circle.