Gas geothermometry in selected icelandic geothermal fields with comparative examples from Kenya

Using data from selected geothermal fields in Iceland, several gas geothermometers are applied to calculate reservoir temperatures. Results for well fluids are compared to estimated inflow temperatures. The effects of gases from other sources, e.g. magmatic fluid entering reservoirs, condensation/boiling, loss of components and mixing of different fluids during upflow, are discussed. Those geothermometers that give results reasonably close to the inflow temperatures, and are not constrained by the need to know thermodynamic parameters, are used to estimate subsurface temperatures from fumarole steam composition. It is suggested that geothermometers based on H 2 and H 2S concentrations are relatively effective. The CO 2, H 2S and H 2 geothermometers are also evaluated by thermodynamic simulation using plausible mineral buffers. The CO 2 geothermometer generally gives slightly low temperatures for values in the range 100–200°C. The calibration of the existing H 2S and H 2 geothermometers needs improvement when reservoir temperatures are higher than 220°C. The mineral buffer controlling H 2S in reservoirs seems to be pyrite + magnetite + epidote + prehnite changing into pyrite + pyrrhotite + epidote + prehnite with increasing temperature.