Geothermometry

Geothermometry is the set of methods used to estimate the temperature of geothermal reservoirs from chemical and isotopic data of thermal fluids, minerals, and rocks. The goal is to reconstruct the conditions at depth where the fluids last equilibrated with the surrounding rock or minerals, providing estimates of reservoir temperature for exploration, production, and research.

Liquid-based geothermometers are the most commonly applied. The silica (SiO2) geothermometer uses dissolved silica concentration, which generally increases with temperature in water-rich systems; careful sampling and post-collection corrections are required to account for cooling, dilution, and degassing. The Na-K-Ca geothermometer relies on the relative abundances of sodium, potassium, and calcium in high-temperature waters that have equilibrated with feldspar-bearing rocks. These empirical calibrations provide temperature estimates that are widely used for assessing reservoir conditions. Other mineral-based approaches use crystallization or dissolution relations in feldspars and other hydrothermal minerals.

Gas geothermometers interpret the temperature from the composition of volatiles in steam or fluids, such as CO2, H2S, and other gas species, which reflect high-temperature equilibria in the vapor phase or in the aqueous phase in contact with the reservoir. In some systems, mineral assemblage geothermometers compare observed hydrothermal minerals to temperature conditions implied by their stable equilibria, offering complementary temperature estimates.

Practical use requires corrections for mixing with cooler waters, boiling and steam loss, and non-equilibrium effects. Geothermometry provides approximate reservoir temperatures and is often integrated with petrological, isotopic, and hydrological data for a robust interpretation.