Anatomy of Granite Intrusions in the Travale Geothermal Field (Italy): a First Geochemical-Petrographic-Spectral Gamma Ray Log Approach

Abstract

Since the 80’s, the exploration in the eastern portion of Larderello - Travale geothermal field (southern Tuscany, Italy) has been targeted to depths of 3000-4000 m b.s.l., where the first deep drillings discovered a superheated steam reservoir (temperatures of 300-350°C). The deep reservoir is hosted within, such as metamorphic and/or intrusive formations. The best productivity seems to be linked to deep fractured levels related directly or indirectly, to the emplacement of granitic bodies. Most of the deep wells drilled in the eastern portion of the geothermal field reached deep Pliocenic granites finding different characteristics and ages. Granitic bodies permeability is still under investigation. A long term activity, aimed at the geological/petrophysical/geophysical characterization of these granite bodies has been planned. Core samples analyses (both petrographic and chemical), geophysical logs, seismic and well testing data will be integrated in order to identify a possible correlation with steam production and to provide an update of the geothermal conceptual model. In this work the first results of this activity are presented. New data regarding chemistry and petrological description are tentatively correlated to spectralog response on 16 wells from Travale area. Mineralogical and chemical data point out to the presence of four different granitic facies which are also autonomously identified by spectra log analyses on the basis of U, Th and K2O measurement. Since coring is an high costly and risky operation and most of the drilling is done under total loss of circulation geophysical log shows an optimum capability as a continuous stratigraphic tool to identify different granitic bodies in Travale area and possibly, using geospatial modeling software tool, reconstruct their geometry.