The sealing capacity of the cap rock above the Torre Alfina geothermal reservoir (Central Italy) revealed by soil CO sub(2) flux investigations

Torre Alfina is a medium enthalpy (T = 140 degree C) geothermal field in Central Italy, hosted in buried fractured Mesozoic limestones, extensively explored in the 1970s and 1980s, but which so far has not been exploited. A detailed diffuse soil CO sub(2) flux investigation (1336 measurements over a surface of 12.6 km super(2)) and the periodic monitoring of soil CO sub(2) flux from target areas indicate that in most of the area, even above a pressurized gas cap existing at the reservoir top, the soil CO sub(2) flux is low and mostly within the background threshold (48 g m super(- 2) day super(- 1)) and is likely generated by biological soil respiration. Anomalous values (up to 30.250 g m super(- 2) day super(- 1)) are found only in the proximity of a small zone with gas vents whose composition is identical to that of the gas produced by the geothermal wells tapping the reservoir. 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 degree C), where anomalous soil CO sub(2) flux is recorded above the productive reservoir. The difference of soil CO sub(2) release in the two fields is attributed to the differences in the impervious cover (allochthonous 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 soil CO sub(2) flux investigations are useful in geothermal exploration, but only high flux values likely indicate the presence of a geothermal reservoir at depth, whereas low flux values can indicate either the lack of an active geothermal reservoir at depth or the presence of a very effective impervious cover above the reservoir.