Geochemical methods of exploration for geothermal energy

Abstract

The conditions presiding to the production of geothermal energy are first reviewed in order to determine how and how far the different geochemical data are relevant to the objectives of geothermal exploration. Two-phases convection appears to be the natural process that is expected to be most closely correlated with prospective production of geothermal energy. It is also apt to bring about the most extreme chemical changes in the involved geologic setting. The geochemical data that appear more relevant to date include • (a) fundamental water chemistry, for the purpose of waters classification • (b) chemical parameters related to reservoir temperatures • (c) gases which point to leakages from highs of permeability • (d) “geochemical indicators” of two-phases processes, more specifically related to the flashing off of steam at high reservoir temperatures. Furthermore, the geochemical methods are classified according with the scale of the required operations: regional reconnaissance, includes mercurimetry, hydrogeochemical survey of surface waters, and isotopic study of water. These data convey information having regard to large thermal anomalies, to drainage, to water storage and supply, respectively; strategic exploration puts more emphasis on springs systems. It possibly includes broad gas surveys, mostly CO 2 surveys. Strategic work next to regional reconnaissance aims to closer understanding of the hot spring systems related to selected geological units. It aims specifically to the detection of geochemical anomalies related to leakage manifestations, that is to two-phases convection: tactical exploration would include close investigation of specific hot springs and fumaroles, as well as of their environment. Detailed investigation of free and dissolved gases could be referred to this scale. In principle geochemical thermometry belongs to this group so far as it requires that water samples be available from a specified reservoir. Thus exploration shades towards reservoir studies, through steps like spring water study, geochemical logging on drilling, geochemical monitoring of well head fluids. Specific geochemical methods are discussed. Based on the geochemical distribution of gases, boric acid and ammonia, the occurrence of leakage manifestations (two-phases convection) in some geothermal as well as volcanic areas is reported. Some techniques are also destribed, especially on field techniques.