The effectiveness of the impervious cover above the Torre Alfina geothermal reservoir (Central Italy) revealed by CO2 soil flux investigations

Torre Alfina is a medium enthalphy (T=140 °C) geothermal field in Central Italy discovered and extensively explored in the years ’70 and ’80. A detailed CO2 soil flux investigation (1336 measurements over a surface of 12.6 km2) and periodic monitoring of CO2 soil release from target areas, indicate that in most of the area, even above a pressurized gas cap existing at the reservoir top, the CO2 soil flux is low and mostly within the background (48 g/m2*day) and is likely generated by biological soil respiration. Anomalous values (up to 30.250 g/m2*day) are found only in the proximity of a gas manifestation whose composition is identical to that of the gas produced by the geothermal wells. This is the only zone where a fault connecting the deep geothermal reservoir with the surface does exist. These data are compared with those of the near Latera high enthalpy geothermal field (T = 210 °C), where anomalous CO2 soil flux is recorded above the productive reservoir. The difference of CO2 soil release in the two fields is attributed to differences in the impervious cover (flysch deposits) above the carbonate reservoir, which is thin and locally lacking at Latera, and continuous and thick (>400 m) at Torre Alfina. Results demonstrate that CO2 soil flux investigations are useful in geothermal exploration, but only high flux values likely correspond to buried active geothermal reservoirs, whereas low flux values can indicate either the lack of inactive geothermal reservoir at depht or the presence of a very effective impervious cover above the reservoir.