Geochemistry of geothermal well gases from superhot zones of Larderello, Italy

Abstract

At Larderello, granitic intrusions into a thinned continental crust act as the heat source for the regional thermal anomaly, and a major seismic reflector (K-horizon) extends all over the field at 3–6 km depth. The geological meaning of the K-horizon is still under debate, and some authors believe that the presence of high-P, supercritical fluids may explain this feature. In late 2017, a deep well was deepened at Larderello to search for supercritical fluids. Temperatures more than 500 °C were estimated at the bottom hole, associated with a leak-off pressure of about 30 MPa, though an exploitable reservoir was not proven. The lack of relevant fluid entries in concomitance with the crossing of the local major seismic reflectors, during the deepening of the well, raised fundamental questions about the geothermal significance of these seismic reflectors, as well as their relevance as possible targets for drilling and industrial exploitation of supercritical fluids.

To gain some insight into the super-hot horizons of the Larderello geothermal field, we present new geochemical data on gases from productive wells approaching the K-horizon, and/or closely related to granitic bodies in the southern and eastern sectors of the geothermal field. The wells considered here discharge a single gaseous phase, draining a vapor-dominated reservoir at temperatures up to 375 °C. The stable isotope signature of steam is indicative of enhanced water-rock interactions at depth (oxygen-shift), and the C–He isotope systematics allowed the identification of a predominant crustal source for CO₂. The He isotope distribution at the surface mirrored a dynamic balance between crustal- and mantle-derived fluids, as also indicated by the relative abundances of N₂, Ar, He, and Ne.