A Review of the Mt. Amiata Geothermal System (Italy)

Exploration of the Mt. Amiata area started at the beginning of the 50’s, and the first overview of the geothermal system was carried out on 1970. After more than 30 years, all the data acquired during the exploration and exploitation activities allowed a significant update of this system. Data from more than 100 drillings (500-4000 m depth), as well as from many geophysical, geological, structural, and hydrogeological studies have been utilized for defining the geothermal model. Two distinct geothermal water dominated reservoirs are present. The first is hosted in Mesozoic, carbonatic formations at depths ranging between 500-1000 m, the second, and deeper one in the Paleozoic, metamorphic basement at depths of 2500-4000 m. These two reservoirs are characterized by different temperatures (150-220°C the shallower one, 300-350°C the deeper one) and, although they are in hydrostatic equilibrium, they are separated by a low permeable layer as evidenced by the temperature profiles of deep wells. These characteristics are common for both the Bagnore and the Piancastagnaio fields presently under exploitation. An important phreatic aquifer is hosted in the Mt. Amiata volcanic complex that overlies the cap rock of the system, and is separated from the shallow carbonatic geothermal reservoir by a few hundred meters of impermeable shaly formations in flysch facies. Hydrological, structural, and production data demonstrate that no interferences occur between the phreatic aquifer and the geothermal reservoir. Furthermore, the numerical modeling of the whole geothermal system demonstrates that a hypothetic water recharge from the phreatic aquifer to the shallow geothermal reservoir can be at most 0.01 m 3 /s, in order to preserve the currently observed thermal anomaly.