Geochemistry in geothermal exploration

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Abstract
Techniques based on the variations in composition of water, gas and stable isotopes in the liquid and gas phases of the geothermal fluids have been applied for some time now in the major geothermal fields and are now also used regularly in geothermal exploration. There are numerous processes capable of modifying isotopic composition after infiltration of water from the surface, such as water‐rock exchanges, formation of secondary minerals and exchange with the gaseous phase (CO2 and H2S). During ascent to the surface, the two main processes are steam separation and dilution and mixing with shallower waters. This paper also deals with the chemical characteristics of the waters, their classification and the water‐rock interaction producing hydrothermal alteration. During exploration the chemical and isotopic geothermometers represent a unique method for investigating the deep system. The choice of geothermometer and interpretation of geothermometric data are two crucial steps in geothermal exploration. Finally, the paper discusses the geochemistry of gas mixtures, especially the origin of the gas species and the main chemical reactions that produce semi‐empirical geothermometers and some recent non‐empirical geothermometers based on models of a two‐phase system in the reservoir. Gas‐geothermometers can be developed to calculate the reservoir temperature for natural manifestations.