Application of Geochemical Methods in Geothermal Exploration

Subsurface waters have been classified into meteoric water, ocean water, evolved connate water, magmatic water and juvenile water. Geothermal water is mostly meteoric and oceanic water although andesitic waters near subduction areas often contain significant proportions of evolved connate and magmatic waters. Geothermal waters have been classified with respect to their anion and cation contents into alkali-chloride water, acid sulphate water, acid sulphate-chloride water and bicarbonate water. Acid waters are generally unsuitable for elucidation of subsurface properties. Conservative constituents are used for tracing origin and flow of geothermal fluids, stable isotopes (especially 2 H and 18 O) along with B and Cl being most important. Radioactive isotopes and chlorofluorocarbons (CFCs) have been used for dating. Rock forming constituents (e.g. SiO2, Na, K, Ca, Mg, CO2, H2) are used to predict subsurface temperatures and potential production problems such as deposition and corrosion. Triangular diagrams and log(Q/K) diagrams are introduced as powerful tools in the interpretation of geothermal geochemistry.