ASSESSMENT OF GEOTHERMAL ENERGY RELEASED FROM UMBRIA REGION (ITALY): A GEOCHEMICAL INVESTIGATION

The recent development of new exploitation technologies has allowed to use the geothermal energy, which derives from the internal heat of the Earth, as a source of renewable energy with a limited environmental impact respect to non-renewable energies. The increasing number of direct applications of geothermal energy has reawakened the interest on its potentialities and has extended the researches in areas considered not exploitable before. In this regard, the geothermal potentialities of Umbria region, always considered not suitable for electricity production, should be substantially reviewed in the context of direct uses of this energy. Based on its geological-structural characteristics, it is possible to divide Umbria region into two distinct sectors: the eastern sector, where the compressive structures are better preserved and the western sector characterised by a strong crustal thinning due to an intense extensive deformation (Barchi et al., 1998; Collettini et al., 2000; Pauselli et al., 2006). The western sector is very interesting from a geothermal point of view because the normal faults that have dislocated the compressional structures allow the rise of deep NaCl-rich fluids with high PCO2, and their mixing with more shallow circulating waters. The occurrence of these saline fluids is related to the presence of a deep regional aquifer at the base of Mesozoic carbonates, in which are also located the main deep geothermal reservoirs of the region. The high CO2 content is due to the influx of deeply derived CO2 related to a regional process of mantle degassing. This sector is also characterized by the presence of thermal springs, gas emissions, areas of diffuse soil degassing and middle-low enthalpy geothermal systems (Chiodini et al., 2000, 2004; Frondini, 2008). It seems highly improbable that the heat content of thermal waters, circulating at relatively shallow depths (some tents of meters), could be caused by conductive heating only and a process of advective mixing with deep hot fluids can be hypothesized. Recent works (Frondini, 2008; Frondini et al., 2012; Chiodini et al., 2013) clearly showed that, in this area, deep fluids transport a significant amount of heat towards the surface causing the observed thermal anomaly of spring waters. The extent of the thermal anomaly of each spring is essentially a function of the relative proportions of deep fluids and shallow groundwater. The geothermal prospecting was accomplished through geochemical investigations focused on fluids, both liquid phase and gases, which characterize the study area. Groundwater and gas samples were collected and analysed for the determination of the chemical and isotopic compositions. Geochemical data interpretation coupled with information from hydrogeological and geological-geophysical data allowed to estimate the reservoir temperatures, to estimate the composition of the thermal fluid, to evaluate the input of deep gas dissolved in the waters, and to reconstruct the model of local and deep circulation. This study includes the detailed investigation of three low and middle-low enthalpy systems located in the Umbria region (Fonti di Tiberio, Parrano, and Triponzo) and the investigation of the advective heat flow in Umbria at regional scale.