Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 1 Application of the Revised Volume Method for the Evaluation of the Geothermal Potential in the Vicano-Cimino Hydrothermal Reservoir (Central Italy) Daniele Cinti 1 , Monia Procesi 1 , Pierpaolo Poncia 2 , Franco Tassi 3,4 , Orlando Vaselli 3,4 and Fedora Quattrocchi 1 1 Istituto Nazionale di Geofisica e Vulcanologia (INGV), via di Vigna Murata 605, 00143 Roma, Italy 2 Po Valley Operations pty ltd, via Ludovisi 16, 00187 Roma, Italy 3 Dipartimento di Scienze della Terra, Università di Firenze, via G. La Pira 4, 50121 Firenze, Italy 4 CNR - Istituto di Geoscienze e Scienze della Terra, via G. La Pira 4, 50121 Firenze, Italy daniele.cinti@ingv.it Keywords: volume method, geothermal potential, Vicano-Cimino volcanic district, central Italy ABSTRACT The revised Volume Method is used here to evaluate the geothermal potential of the practically exploited Vicano-Cimino hydrothermal reservoir. This method is based on the distribution of pCO 2 (partial pressure of CO 2 ) in shallow and deep-originated waters to define areas of geothermal interest, according to the hypothesis that anomalous degassing zones of endogenous CO 2 , from either soil degassing and spring and well waters, are spatially related to deep fluids raising up from underlying hydrothermal reservoirs. On the whole, 333 fluid discharges (cold waters, thermal waters and bubbling pools) were collected within the Vicano-Cimino Volcanic District (VCVD) for chemical and isotopic compositions, in an area of approximately 1,400 km 2 . From this large hydro- geochemical dataset the pCO 2 values were computed and then processed to obtain a map of its distribution by using geostatistical techniques (kriging). The resulting thematic map was then used to draw up the boundaries of high pCO 2 areas within the VCVD, potentially corresponding to the exploitable sectors of the deep hydrothermal reservoir. The estimated potential productivity and the total extractable thermal power of the VCVD are approximately of 10×10 3 t/h and 517- 979 MW, respectively. This makes the VCVD suitable both for direct and indirect exploitation of the geothermal resources, in view of the target to reduce electricity generation from conventional and poorly sustainable energy sources. 1. INTRODUCTION Starting from the pioneering experiment of power generation from a geothermal source in 1904 at Larderello, extensive exploration surveys for geothermal energy exploitation were carried out in Italy in the last century. Geothermal exploration mainly focused along the peri-Tyrrhenian sector of central Italy, where active or recent magmatism and high heat flow occur (Barberi et al., 1994; Cataldi et al., 1995). In the VCVD, exploration geophysical and geochemical surveys were carried out from 1951 to the 1990’s, resulting in the drilling of numerous test-holes and some wells whose depth was up to 3,000 m. However, despite the significant potential (a maximum temperature of 218°C at the depth of 2,153 m in the Cimini 1 well; Figure 1), operations always stopped at the preliminary phases. In the last years, Italy is experiencing a renewed interest for geothermal energy, favored by the recent technological advances and encouraged by the growth of energy demand and the need to reduce CO 2 emissions to the atmosphere (Procesi et al., 2013). At present, 108 new research permits have been requested by private companies in Italy, and 34 of these are in the Latium region, including the VCVD (http://unmig.sviluppoeconomico.gov.it/unmig/istanze). A revised version of the classical Volume Method (Muffler and Cataldi, 1978) was recently applied on a regional scale for estimating the geothermal potential of the volcanic areas of Latium region (Doveri et al., 2010). Based on a larger and more detailed geochemical dataset, the aim of this contribution is to refine the estimates of potential productivity and total extractable thermal power on a bigger scale, in order to identify the best areas within the VCVD for high-to-low enthalpy resources exploitation. 2. AREA DESCRIPTION The VCVD is characterised by Pleistocenic volcanic products overlying a sedimentary sequence constituted, from top to bottom, by Plio-Pleistocene clays, Cretaceous-Oligocene flyschoid sediments (Ligurian s.l.) and a thick Mesozoic carbonate-Triassic evaporite formation (Barberi et al., 1994). The Cimino and Vicano volcanic complexes belong to two distinct magmatic cycles: the acid cycle (1.35-0.94 Ma) of the Tuscan Magmatic Province and the silica-undersaturated K-alkaline cycle (0.42-0.09 Ma) of the Roman Magmatic Province (Sollevanti, 1983; Peccerillo, 1985; Cimarelli and De Rita, 2006; Aulinas et al., 2011), respectively. Extensional tectonics, which interested the peri-Tyrrhenian sector of central Italy during the Neogene (Barberi et al., 1994), formed preferentially formed by NW-SE oriented horst and graben structures with clastic marine sediments filling the structural lows. The main hydrogeological pattern is related to a pressurized hydrothermal reservoir, of regional extension, hosted in the carbonate- evaporite unit, which is separated from a shallow, mainly unconfined, regional cold aquifer hosted in the volcanic products by low- permeability Plio–Pleistocene deposits and/or the Ligurian Units s.l. acting as aquicludes (Capelli et al., 2005). Perched aquifers of limited and discontinuous extent are also present in the more permeable strata of the sedimentary sequences.