Application of Mineralogical Methods to Assess the Thermal Stabilities of Geothermal Reservoirs

Estimates of temperatures, past and present, in geothermal reservoirs can be made by using now standard mineralogical techniques, including fluid inclusion geothermometry, vitrinite reflectance, calc-silicate and clay occurrence, the extent of clay interlayering, and measuring clay crystallinity. Recent studies of clays in 60 drillcores from 6 wells at Wairakei, for example, show an inverse relationship between reservoir temperatures and crystallinities from 90' to 225'C (195 to 435°F) (Kiibler Indices: 1.40 to 0.44 A 2 8 ) . Fluid inclusion geothermometry results require careful interpretation but the method need not be calibrated with respect to the reservoir, as do other geothermometric methods. More useful are mineral geothennometers and methods that are of wider application, and some of these are summarized in this paper. For convenience, the thermal signatures of a geothermal reservoir may be considered, in mineralogical terms, as belonging to one of three categories: (1) minerals whose presence in a reservoir has a thermal significance; (2) minerals whose compositions or structures vary with temperature; (3) determinative methods that depend upon some property of a mineral or material present within the reservoir rocks. TEMPERATURE SENSITIVE MINERALS