GAS GEOTHERMOMETRY IN SELECTED ICELANDIC GEOTHERMAL FIELDS WITH COMPARATIVE EXAMPLES FROM KENYA

Using data from selected geothermal fields in Iceland, several gas geo thermometers are applied to calculate reservoir temperatures. Results for well fluids are compared to estimated inflow temperatures. The eff ects of gases from other sources, e.g. magmatic fluid entering reservo irs, condensation/boiling, loss of components and mixing of different fluids during upflow, are discussed. Those geothermometers that give r esults reasonably close to the inflow temperatures, and are not constr ained by the need to know thermodynamic parameters, are used to estima te subsurface temperatures from fumarole steam composition. It is sugg ested that geothermometers based on H-2 and H2S concentrations are rel atively effective. The CO2, H2S and H-2 geothermometers are also evalu ated by thermodynamic simulation using plausible mineral buffers. The CO2 geothermometer generally gives slightly low temperatures for value s in the range 100-200 degrees C. The calibration of the existing H2S and H-2 geothermometers needs improvement when reservoir temperatures are higher than 220 degrees C. The mineral buffer controlling H2S in r eservoirs seems to be pyrite + magnetite f epidote + prehnite changing into pyrite + pyrrhotite + epidote + prehnite with increasing tempera ture. Copyright (C) 1996 CNR.