IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1A1_1E-O3Room A3

Date/Time: July 21 9:15-9:30

The bolsena-torre alfina geothermal field (Italy): a case of caldera-related blindgeothermal resource
GUIDO GIORDANO1 , GABRIELE BERARDI1 , CLAUDIO CHIARABBA3 , ARNALDO DE BENEDETTI1 ,MARINA PASTORI3 , ANNAMARIA PINTON1 , FEDERICO ROSSETTI1 , ANDREA SCHITO1 , MICHELESOLIGO1 , INES TESCIONE1 , MICOL TODESCO2 , GIANLUCA VIGNAROLI1
1UNIVERSITA’ ROMA TRE, ITALY, 2 INGV - BOLOGNA, ITALY, 3 INGV - ROMA, ITALY
E-mail: guido.giordano@uniroma3.it

The Torre Alfina geothermal field is located about 10 km north of the Quaternary Bolsena caldera. Thereservoir is a buried structural high made of Mesozoic-Cenozoic carbonatic sequences characterised bydiscontinuous secondary permeability and sealed by clay-rich alloctonous syn-orogenic flysch successions
and Pliocene neo-authoctonous marine clays. The reservoir is locally highly productive, water-dominatedand CO2-rich, with T at its top of 120 degrees C. The origin of the heat source was not investigated in detail but
generally attributed to some unidentified deep magmatic body. Our field structural data document fracturing
and faulting that affect both the reservoir and the seal units with dominant NE-SW and a subordinate E-W as
maximum extension directions. The surface pattern and kinematics indicate that fault systems are part of the
Pliocene-Quaternary tectonic episode in Central Italy, where strike-slip kinematics transfers deformation
between main extensional shear zones. The fault network controls intense hydrothermal manifestations andtravertine deposition north of the Torre Alfina area. 230Th/234U dating allows distinguishing three different
stages of hydrothermal pulses bracketed between 200 and 90 ka. To improve the current understanding of
the geometry and density of subsurface fracture system we applied the Shear Wave Splitting (SWS)
technique studying the waveforms from local microearthquakes recorded in the area from 2008 to 2011. The
analysis of SWS provides parameters directly related to the strike of the subsurface fluid-filled fractures andtheir density. According to the extensive-dilatancy anisotropy hypothesis (Crampin, 1984) SWS is generated
by propagation through distributions of fluid-filled cracks, microcracks, and preferentially oriented porespace aligned according to the active stress field in the area.
The analysis was made using the software ANISOMAT and gives a major direction NW-SE. The results
agree both with structural data and with the focal mechanism of the earthquakes collected in the area
between years 1977 and 1992. From the analysis on the crack density our data gives
as result a good index of fracture for the reservoir’s rocks. Based on the updated conceptual model we
performed numerical simulations in TOUGH2 code. Results indicate that deep circulation is forced by thegeometry of the reservoir and by T and P gradients. We interpret the Torre Alfina field as a "blind" system,
formed dominantly by lateral advection of heat and mass from the Bolsena caldera deep system driven bothby high T and by the greater depth at which reservoir rocks are downthrown by the volcano-tectonic
collapse. Preferential fracture fabric in the deep carbonates allows the lateral heat transfer. The cap-rocks
have excellent sealing characteristics allowing the preservation of heat in correspondence with positive
structural traps.

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