More mileage for the isotope gas geothermometers: the case of the Larderello geothermal field

New isotopic analyses of fluids were performed in the Larderello field about 10 years after reinjection tests began, and the results compared with data obtained in the pre-reinjection period. Whereas the isotopic composition of the steam is clearly affected by mixing with injected waters in the vicinity of the reinjection sites only, the isotopic composition of the gases appears to be modified over much wider zones. The isotopic temperatures derived from H and C isotopic exchanges between gas molecules such as CO2, CH4, H2 and H2O were all lower than the values obtained from the same geothermometers in the pre-reinjection surveys. The presence of a liquid water phase in the reservoir and the local formation of new CO2 and H2 with a quite different isotopic composition from the relatively deep components, are both considered in the interpretation of the field data. The kinetics of the isotope exchange reactions involved in the considered geothermometers are also discussed.