Geochemical Exploration: Application of Fluid Geochemistry to the Utilization of Geothermal Energy

Geothermal fluid heated by a magma heat source is available as a geothermal resource when the fluid or vapor is obtained continuously from the subsurface and is introduced into power gen-eration facilities. Exploration of geothermal resources using a combination of various techniques is an essential process to successful developing geothermal energy, which has become attractive as a renewable and sustainable resource. Geochemical exploration provides direct information on the properties and behaviors of geothermal fluid, based on chemical analysis of hot spring waters, fumaroles collected at geothermal areas, and high-temperature fluids obtained from the heads of drilled wells. Chemical properties of those fluids reflect histories of various chemical processes during fluid circulation within a subsurface geothermal system where groundwater evolves into a high temperature fluid. Compositions of major cations (Na, K, Ca, Mg) and SiO2 reflect the course of fluid-mineral interactions, and function as a geothermometer when compo-sitions are controlled by chemical equilibria within the reservoir. Meanwhile, compositions of major anions (Cl, SO4, HCO3) and B reflect contribution from various sources, such as seawater, magmatic volatiles, and crustal fluid. Gas species (CO2, H2S, H2, N2, CH4, Ar, He) are also de-rived from various sources such as the atmosphere, organic matter decomposition, and a magma heat source. These species provide information on the history of fluid circulation, because they are unaffected by fluid-mineral interactions in spite of some exceptions. Hydrogen and oxygen isotopes of H2O mainly provide an index of the origin of a fluid, such as meteoric water, seawater, and magmatic volatile, as well as evidence for fluid-mineral interactions and boiling processes. Isotopic compositions of light elements such as carbon and sulfur of gas species are useful for discriminating contributions from magmatic volatiles and from microbiological activities. Recent technical advances in mass spectrometry have enabled the isotopic composition of several ele-ments to be obtained. These newly proposed geochemical indices would provide information that is difficult to extract using conventional geochemical tools. Developments in new geochemical extrapolation techniques are expected in the light of the present situation where expansion in the use of geothermal resources is demanded.