The aim of the present multidisciplinary study is to explain the changes in 
resistivity observed in the deep reservoir of the Travale area taking into 
account the lithology and alteration affecting the reservoir rocks, with particular 
regard to conductive and clay minerals, the physico-chemical characteristics of 
the fluids, and their distribution and evolution with time. The study is also 
directed at calibrating petrophysical experiments in order to reproduce realistic 
physical conditions on a small scale. The deep reservoir consists of 
metamorphic Paleozoic units and younger granite. The metamorphic units 
include: i) the Phyllitic–Quartzitic Complex (metagreywacke with minor 
metabasite levels and locally carbonate–siliciclastic metasediments); ii) the 
Micaschist Complex (almandine-bearing albite micaschist with minor 
amphibolite); and iii) the Gneiss Complex (gneisses with minor amphibolite 
layers and rare calc-silicate rocks). Deep drillings have encountered Pliocene-
Quaternary granites at depths between ~2 and 4 km below ground level 
(b.g.l.). All the crystalline units are affected by contact and hydrothermal 
metamorphism originated by the granite intrusions. 
X-ray diffraction was carried out on one well in the Travale area (63 cutting 
samples in 2400 m of drilled depth) to identify the types of minerals present, 
their relative abundance and to compare the mineral characterisation with 
resistivity values. The Phyllitic–Quartzitic Complex is characterised by clay 
minerals (chlorite and mica types), quartz, plagioclase, calcite, anhydrite and 
rare dolomite: the relative abundance of these minerals is not homogeneous 
throughout the complex. A clear correlation between the abundance of clay 
minerals and a change in resistivity was not observed.
The study of fluid inclusions provided information on the fluids that interacted 
with the reservoir rocks, their composition, physico-chemical nature, origin and 
evolution. A multi-stage fluid circulation was observed, consisting of an early 
magmatic stage characterised by high-salinity fluids (around 50 wt. % eq. NaCl) 
of magmatic origin, and vapours and liquids resulting from heating of the 
Paleozoic rocks during contact metamorphism. The hydrothermal stage that 
follows is characterised by low- to high-salinity aqueous fluids with vapours 
produced by boiling processes. The high salinities can be explained by the 
interaction of these fluids with evaporites and/or connate waters.
The present-day geothermal fluid is superheated steam with similar gas/steam 
ratios. The components of the geothermal fluids are H2O, CO2, CH4, H2S, N2 
and H2, with part per million (ppm) amounts of He, Ar, O2 and CO.
Since the state of the geothermal fluid produced cannot explain the observed 
reduction in resistivity, the latter could be related to the abundance and type of 
i) heterogeneities in the reservoir rocks, ii) the abundance and type of 
alteration minerals, and iii) the presence of brines similar to those evidenced by 
the fluid inclusion study, whose interconnection would be sufficient to produce 
electrolytic conduction.

Campus SIAF, SP del Monte Volterrano
Localita' Il Cipresso
Volterra, Italy