GNGTS 2019 - Atti del 38° Convegno Nazionale

180 GNGTS 2019 S essione 1.3 Thermal evidences. The Ischia Island is an example of an active volcano hosting a large hydrothermal system. Geothermal exploration provided information about rocks and temperatures down to a maximum depth of about 1 km. The temperature profiles display a typical conductive trend in the upper sections due to the occurrence of impermeable, mainly argillic, layers acting as caprock. The deeper tuff and lava formations constitute the deep-seated geothermal reservoir along which a roughly isothermal profile is observed down to a maximum depth of 900 m. Below this convective layer, the thermal gradient increases and a maximum temperature of 225 °C is recorded. In Long Valley Caldera, the measured thermal profiles show characteristic thermal picks at very shallow depths systematically followed by temperature reversals. The numerous thermal surveys suggest that the upwelling fluid locates in the western sector of the caldera where a maximum temperature of 216°C at a depth of 1 km has been measured. From the western sector, the geothermal fluid flows laterally eastward throughout the intra-caldera volcanic sequence. The magnitude of the thermal anomaly decreases from >200°C to less than 50°C in the thermal springs located in the easternmost sector. The Acoculco caldera complex is characterized by a widespread hydrothermal alteration but nowadays only two exploratory wells have been drilled. They are very close each-other and recorded a similar thermal profile with a maximum temperature slightly above 300°C at 2 km depth. The drilled formations resulted impermeable, likely due to self-sealing processes, and a mainly conductive temperature distribution has been observed in both wells. The Larderello-Travale geothermal field hosts two main reservoirs, the shallow reservoir develops within Mesozoic evaporite-carbonate units with temperatures from 150°C to 260°C, the deep reservoir is hosted in the uplifted Paleozoic metamorphic basement and Neogene granitoids with temperatures from 300°C to 350°C. Fluids dominantly of meteoric origin at vapor phase circulate in both reservoirs. A recent exploration program investigated the possible occurrence of high-pressure and high-temperature fluids in the proximity of a deep seismic reflector at about 3 km depth. Drilling revealed an unexpectedly high temperature >500°C near the bottom at 2.65 km but no fluid entered into the well. Fig. 1 - (a) Comparison between the measured (red line) and the simulated thermal profiles (black lines) for different reservoir permeability of Ischia Island. The thickness of the geothermal reservoir (light grey) and of the magmatic intrusion (dark grey) are also reported. (b) Matrix of the NRMSE resulting from the multi-parametric study. Each pixel corresponds to a numerical model solved using specific combination of permeability and magma temperature.