geothermometers

Geothermometers are tools used in hydrogeology and geothermal exploration to estimate the temperature of a reservoir or aquifer from the chemical and isotopic composition of its fluids. They rely on the principle that certain minerals dissolve or exchange with water at temperatures characteristic of the system, so measured concentrations of dissolved species or gas ratios can be interpreted as reservoir temperatures after calibration against laboratory or field data.

Chemical geothermometers rely on aqueous equilibrium with minerals. The silica geothermometer uses dissolved silica as a function of temperature, since silica solubility in water increases with temperature. Alkali–feldspar geothermometers, such as Na–K and Na–K–Ca formulations, relate concentrations of sodium, potassium, and calcium to temperature, but their accuracy depends on water–rock equilibrium, mixing, and ion exchange, and they tend to be most reliable in relatively simple, high-temperature systems.

Gas geothermometers use ratios of dissolved gases, for example CO2/H2 or CO2/H2S, to estimate formation temperatures because gas speciation changes with temperature and depth. Isotope geothermometers use stable isotope ratios in water, such as 18O/16O and D/H, to inform recharge conditions and the extent of water–rock interaction, often in conjunction with chemical geothermometers.

Limitations include assumptions of equilibrium and single-component end-members; mixing, dilution, boiling, degassing, and rapid cooling can bias results. Therefore, geothermometers are typically applied in combination with hydrogeochemical modeling and other reservoir data to constrain temperatures and fluid histories. They remain useful for preliminary temperature estimates, exploration, and reservoir characterization.