Chemical and Isotope Geothermometers to Estimate Geothermal Reservoir Temperature and Vapor Fraction

Abstract

According to thermodynamics, the equilibrium constant of a chemical or isotopic fractionation reaction is a state function and in general is a function of two state variables (e.g. temperature and pressure). Depending of the type of species involved in the reaction, it can be used as geothermometer and/or geobarometer. In this work various chemical and isotopic geothermometers: silica solubility, cation-exchange, methane breakdown, and oxygen-18 in dissolved sulfate and water are revised from the thermodynamic point of view. Their applications are illustrated for the calculation of geothermal reservoir temperature and vapor fraction of Los Azufres and Los Humeros geothermal fields, Mexico.