Estimation of the geothermal potential of the Caldara di Manziana site in the Mts Sabatini Volcanic District (Central Italy) by integrating geochemical data and 3D-GIS modelling

Abstract

The Tyrrhenian margin of central Italy is an area characterized by crustal thinning (<25 km) and a high heat flow (>300 mW/m2), which makes it attractive for medium to high-enthalpy geothermal projects. The main difficulties encountered in the geothermal exploitation of the area are not related to the thermal conditions, but rather to the lack of an adequate rock permeability at depth. The medium-high enthalpy geothermal reservoirs of central Italy are hosted in Mesozoic carbonate-evaporitic rocks. These rocks exhibit secondary fracture permeability, locally reduced by self-sealing processes, especially in areas of low seismicity. Also, a low partial pressure of CO2 (PCO2) may facilitate the complete sealing of the reservoir fractures, preventing the ascent of hot fluids and resulting in a low CO2 flux at the surface. Conversely, a high CO2 flux reflects a high pressure of CO2 at depth, that is suggestive of the presence of an active geothermal reservoir. Despite the possibility that also part of CO2 be dissolved into groundwater, a large amount of this non-condensable gas will reach the surface being emitted into the atmosphere by discrete manifestations (gas vents) or through diffuse soil emissions. This is particularly true in sites -such as Caldara di Manziana, hereafter (CM) characterized by cold-gas emissions, which have CO2 concentrations up to 97 vol. %. In this paper we present the results of a study carried out in the western zone of Sabatini Volcanic District (SVD; north of Rome, Italy) that hosts CM (and also Solfatara di Manziana-SM), one of the most spectacular CO2 gas manifestations of central Italy. This study estimated the temperature and pressure conditions of the reservoir and the depth of its top using geological, geochemical and geophysical data and the TIN (triangulated irregular networks) interpolator provided in the ArcGIS for Desktop software. A new structural setting of the Mesozoic carbonates in the CM site is proposed, and an estimation of its geothermal potential is presented on the base of the total (diffuse and viscous) CO2 release.