The methods proposed in the past to evaluate the geothermal potential of a 
given area can be grouped in the following four classes: (a) the heat-flow 
method; (b) the volume method; (c) the planar-fracture method and (d) the 
magmatic-heat-balance method (Muffler, 1973; Bodvarsson, 1974; Nathenson 
and Muffler, 1975; Renner et al., 1975; Smith and Shaw, 1975; Muffler and 
Cataldi, 1978; Cataldi et al., 1978; Cataldi and Squarci, 1978; Cataldi and 
Celati, 1983; Wohletz and Heiken, 1992). 
They were critically re-examined by Marini et al. (1993) who: (1) modified the 
volume method by taking into account the geochemical data available for 
shallow groudwaters and gas emissions from soils; (2) calibrated this method 
based on the production data available for the geothermal fields either under 
exploitation or explored through deep drilling (Larderello, Travale, Bagnore, 
Piancastagnaio, Torre Alfina, Latera, Cesano and Mofete); (3) applied the 
modified volume method to several areas of Italy for which geochemical data 
are available.
In the original volume method, the heat stored underneath the considered 
area, until a given depth (which depends on economical and technological 
parameters), is computed for each of the recognisable geological-
hydrogeological units, by means of simple heat balances (e.g., Cataldi et al., 
1978). This exercise requires knowledge of the following parameters: volume, 
porosity and temperature of each unit, density and heat capacity of the rocks, 
density and heat capacity of the fluids and reference temperature (usually 25°
C). A recovery factor is then used to compute the extractable geothermal 
energy.
In the modified volume method, the areas of high CO2 flux (recognisable at the 
surface based on the presence of high-PCO2 waters and/or CO2-rich gas 
emissions from soils) are used to bound the extension of geothermal 
reservoirs. This approach is based on the fact that a continuous flux of deep 
CO2 occurs through all known geothermal systems of medium-high enthalpy in 
the same manner as the heat flux (e.g., Mahon et al., 1980; Marini and 
Chiodini, 1994; Gambardella et al., 2004).
Here we present an example of application of the modified volume method in 
Central Italy, through the superposition of the maps depicting the geographical 
distribution of the following parameters:
(a) depth of the potential geothermal reservoir (from Buonasorte et al., 1995);
(b) temperature at the top of the potential geothermal reservoir (from 
Buonasorte et al., 1995);
(c) PCO2 in shallow groundwaters (from Gambardella et al., 2004).
Suitable assumptions (Marini et al., 1993) are adopted to quantify the total 
volume of the geothermal reservoir and its productivity. This exercise leads to 
the identification of:
(1) a total extractable thermal power of 440-550 MW from conventional 
geothermal reservoirs with temperature higher than 200°C;
(2) a total extractable thermal power of 1070-1540 MW from conventional 
geothermal reservoirs with temperatures of 45-90°C.
In order to apply this approach to areas where low-enthalpy geothermal 
resources are present (e.g., the sedimentary basin of the Po Valley in Northern 
Italy), research must be focussed towards the identification and use of 
permeability-indicator(s) other than carbon dioxide.

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