Geological and geophysical modelling for geothermal assessment: the VIGOR case study of Guardia Lombardi Area, Campania.

Within the framework of the VIGOR project, the geothermal assessment of medium enthalpy resources in the "Guardia dei Lombardi" area, located in the Campania region (Southern Italy, Province of Avellino), has been undertaken. Due to the intense hydrocarbon exploration, carried out particularly during the 1956-1996 period, a wide dataset exists in the study area. This dataset, made up by seismic reflection profiles, well data, logs and cores, has been accesible to our research group thanks to the cooperation agreement with the Italian ENI oil company. The overall structural-stratigraphic setting of the reservoir/caprock system has been defined on the basis of the integrated interpretation of the available composite dataset of surface and subsurface geological, geophysical and geochemical information. The main geothermal reservoir is made up by a fractured carbonate reservoir, that is constituted by the Cretaceous-Eocene sequence of the Apulian shallow water carbonate platform (e.g., Scrocca, 2010 and reference therein). Following the Apennine orogenesis, the Apulian platform was deformed to shape a buried antiformal stack (Nicolai & Gambini, 2007). In the study area (Fig. 1) the culmination of the uppermost thrust unit reaches a depth of about -250 m below the sea level (i.e. about -1125 m below the ground level). According to well test results, the reservoir fluids are mainly composed of CO2, which rests above an accumulation of brackish water in the central and upper part of the culmination of the deep carbonatic aquifer (i.e., Monte Forcuso 1 and 2 wells). The reservoir fluid do not show a free gas phase but only saline water along the flank of the buried anticline (e.g., Bonito 1 Dir, Ciccone 1 wells). As already suggested by previous assessments (e.g., ENEL, 1987; ENEL/ENI/CNR/ENEA, 1994), available well data temperature highlight the presence of reservoir fluid that reaches temperature of 120°C at depth of 3000 m below ground level (e.g., Bonito 1 Dir well). This conclusion is also in agreement with estimations of reservoir fluid temperature up to about 124°C calculated using different geothermometers by Duchi et alii (1995) on water samples collected from thermal springs (e.g., Terme di San Teodoro). On the basis of well tests, cores analysis and mud losses/absorptions, it has been possible to re-estimate the reservoir properties, confirming quite good permeabilities for the fractured carbonate reservoir. One of the main results of this research is the development of an integrated 3D geological model which provides the base for a detailed assessment of the possible geothermal exploitation of the carbonate reservoir. The preliminary results of our analysis suggest that the "Guardia dei Lombardi" site could be indicated as an interesting area for the geothermal medium enthalpy exploitation, although the presence of the CO2 gas cap should be carefully evaluated.