Fluid geochemistry and geothermometry in the western sector of the Sabatini Volcanic District and the Tolfa Mountains (Central Italy)

Evaluation of the Theoretical Geothermal Potential of Inferred Geothermal Reservoirs within the Vicano–Cimino and the Sabatini Volcanic Districts (Central Italy) by the Application of the Volume Method

“…This hypothesis is confirmed by the δ 13 C-CO 2 values of the sampled waters, ranging from −27.6 to +2.3‰ vs. VPDB [19,39,40], which suggest that CO 2 has a two-fold origin: relatively negative δ 13 C-CO 2 values of the low pCO 2 waters imply dominant CO 2 contribution from soil respiration and aerobic decay of organic matter [44]; conversely, less negative δ 13 C-CO 2 values of the medium-to-high pCO 2 waters point to CO 2 production from thermo-metamorphic reactions involving carbonate formations (δ 13 C-CO 2 values from −2.0 to +2.3‰ vs. VPDB [45]) and minor contribution from mantle degassing (δ 13 C-CO 2 values from −7.0 to −3.0‰ vs. VPDB [46]). Consistent with this hypothesis, the different populations highlighted in the QQ plot (Figure 3) can be interpreted as representative of pCO 2 values fed by both biological and endogenous sources.…”

“…They are both associated with (1) thermal waters upwelling from the deep reservoir, where the CO 2 -rich gas phase is originally dissolved, and (2) cold waters from the shallow aquifers, which receive the input of endogenous CO 2 that separates from the parent deep fluid and upwells as a single gas phase from the deep reservoir. These areas are generally located in correspondence with positive gravity zones, which have been interpreted as buried structural highs of the carbonate basement [19] and represent the sectors where the top of the geothermal reservoir is located at shallower depths. On the other hand, pCO 2 values < 0.045 bar, i.e., those reflecting the shallow production of biological CO 2 , are associated with gravity lows and are typical of cold waters circulating within the shallow aquifers, which have no relation with the tectonic framework and do not receive contributions from the geothermal reservoir.…”

“…A = area of the geothermal reservoir calculated by kriging; h = average thickness of the geothermal reservoir; V = volume of the geothermal reservoir; Q = discharge rate of the geothermal fluid; T min-max = minimum and maximum temperatures of the geothermal reservoir; W = theoretical thermal power. 1 [12]; 2 [19]; 3 [39]; 4 [40]; 5 [41].…”

“…The average specific productivity of 40 t/h km 3 , calculated for the liquid-dominated geothermal systems of Latium by [14] has been used for calculations. Reservoir temperatures have been derived taking into account, hierarchically, data from: (1) deep drilling (i.e., bottom-hole temperatures); (2) geothermometric evaluations based on chemical equilibria of gas species [19,[39][40][41]; and (3) the map of temperature of the potential reservoir top [12]. Minimum and maximum temperatures (Table 2) have been considered to obtain the minimum and the maximum theoretical thermal power, respectively.…”

“…In more recent years, the old approach has been reviewed with the introduction of the CO 2 surveyed in the regional aquifers as a tracer for reservoir productivity [14], as typical geothermal systems exhibit an anomalous CO 2 degassing of deep provenance in areas of high heat flux [15][16][17][18][19]. In this paper, a revised version of the volume method has been applied to estimate the theoretical geothermal potential [20] of inferred geothermal reservoirs within the Vicano-Cimino Volcanic District (VCVD) and the Sabatini Volcanic District (SVD) in the Latium region.…”