Superhot Hydrothermal Fluid Reservoirs: Targeting Through Boron and Chlorine Volatility Geothermometry

Abstract

Superhot geothermal reservoirs, with fluid temperatures ranging from ~360°C to greater than 500°C, have been encountered in active geothermal systems worldwide. These fluids hold the potential to significantly enhance power production in geothermal fields. However, identifying such resources without drilling has remained a major challenge. In this study, we present a new and novel geochemical method for targeting superhot geothermal reservoirs by analyzing boron (B) and chlorine (Cl) abundances in surface hydrothermal fluids. The volatility of B and Cl is temperature-dependent, allowing their concentration to be used to infer superhot formation conditions. Our findings reveal that near-constant B concentrations coupled with increasing Cl concentrations and vapor fractions in surface well fluids indicate the input of superhot fluids into overlying subcritical geothermal reservoirs. In contrast, decreasing B and Cl concentrations with increasing vapor fractions suggest depressurization boiling at subcritical temperatures, followed by liquid-vapor separation. Geochemical modeling reveal that the superhot water dominated fluids exhibit low concentrations of non-volatile elements (Si, Na, K, Ca, Mg, Al), controlled by salt, oxide, and aluminosilicate solubility in high-temperature ( greater than 400°C), low-density ( less than 300 kg m⁻³) fluids. In contrast, volatile elements (C, S, B) show similar abundances to those found in subcritical geothermal fluids. This method was successfully applied to target deep, superhot geothermal reservoirs in three geothermal fields in Iceland (Krafla, Hengill and Theistareykir), where such fluids have been observed or suggested based on subsurface borehole temperature measurements.