Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 1 Anatomy of Granite Intrusions in the Travale Geothermal Field (Italy): a First Geochemical- Petrographic-spectral Gamma Ray Log Approach Rebecca Spinelli 1 , Michele Casini 1 , Nicola Costantino 1 , Geoffrey Giudetti 1 , Simonetta Ciuffi 1 and Andrea Dini 2 1. Enel Green Power, via Andrea Pisano, 120, 56122 Pisa (Italy) 2. Istituto di Geoscienze e Georisorse – CNR, Via Moruzzi 1, 56124 Pisa, Italy michele.casini@enel.com Keywords: Granite, Travale, geothermal field, geophysical logs, Spectral Gamma ray log, Th-U minerals 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 18 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 K 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. 1. INTRODUCTION Exploration activity and scientific investigation of the Larderello-Travale geothermal field (Tuscany, Italy), indicate that the deeper part of the system (below ca. 2500-4000 m) is constituted by a plutonic complex built up incrementally (between 3.8 and 1.3 Ma) by progressive stacking of several different granite magma batches (Dini et al., 2005; Bertini et al., 2006; Casini et al., 2008). Cooling models for these intrusions imply thermal equilibration with the ambient crust within ca. 10-100 ka, indicating that they cannot be responsible for the active geothermal system. The present day geothermal system and potential supercritical reservoirs detected by geophysics at depth (the so-called K-horizon at 5-6 km depth; e.g., Batini et al., 2003; Bertini et al., 2006) must be related to the emplacement of very recent granite magmas at shallow depth. Currently exploited superheated steam reservoirs are hosted by both granites and their contact metamorphic aureoles. Reservoir productivity is linked to fractured and permeable levels that are rather confined and mostly scattered. Although most of the geothermal wells have reached the intrusive bodies in total loss of circulation (TLC), a detailed 3D reconstruction of the top of the granite intrusion was reconstructed by means of 3D and 2D reflection seismic prospection, well logs and through the observation of cutting from the wells drilled with return of circulation. Moreover, in spite of the high cost, some granite cores were collected throughout the entire geothermal field. On the basis of petrographic, geochemical and isotopic investigation of few core samples, Dini et al. (2005) provided a preliminary classification of Larderello-Travale buried granites. Major and trace element patterns coupled with isotopic data suggest that in the root zone of the Larderello geothermal field (ca. 20 km depth), P–T conditions for crustal melting were intermittently reached, and low fractions of heterogeneous crustal melts were locally produced and sequentially transferred/emplaced (between 3.8 and 1.3 Ma) at shallow levels (4-6 km depth). Multiple emplacement of geochemically distinct magma batches characterize also the older granite complexes of the Tuscan Magmatic Province, like those cropping out in Elba Island (8.4-5.9 Ma; Dini et al 2002) and in Campiglia Marittima. In the case of multi-pulse magmatic complexes, several contact metamorphic and hydrothermal effects can overlap through time on a relatively small crustal portion (e.g. Dini et al., 2005). The net result of the described complex evolution is a continuously renewed magmatic system, where exsolution of magmatic fluid, heat flow and triggering of meteoric fluid circuits follow cyclically transient patterns with strong implication on geothermal field lifespan. Understanding the “anatomy” of such a complex intrusive systems has strong impact on both scientific and industrial sides. In fact, deciphering the story of granite complexes provide crucial knowledge of generation, segregation, transfer and emplacement of granite magmas within the continental crust: i.e. all the distinct stages of crustal recycling/growth, a fundamental process that involves mass and heat transfer from lower-middle crust (and mantle) to the upper crust/surface. On the other hand, defining