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DEEP EXPLORATION FOR LOCATING A SECOND GEOTHERMAL RESERVOIR IN THE VITERBO AREA-LAZIO.

Objective

THE WELL WILL GIVE UNDER ALL CIRCUMSTANCES AN IMPROVED VIEW ON THE DEEP GEOLOGY OF THE REGION.IN CASE A PRODUCTIVE RESERVOIR IS FOUND A CONSIDERABLE GEOTHERMAL RESERVE WILL BE OPEN TO EXPLOITATION.
The well target was the exploration of the metamorphic basement of the Tuscan Series. The well crossed 1072 m of limestone mixed with clay of the Rhaetian age belonging to Rhaetavicula limestone. From this layer to the total depth of the Vetralla 1 well, it passed through a remarkable sequence of dolomitic limestone, dolomite and anhydrite belonging to the Burano formation (Norian). The metamorphic basement was not reached.

A permit was granted to the joint venture (Agip and Italian Electricity Board (ENEL)) in 1979. Activities consisted in detailed geological surveys, geophysical surveys, geochemical surveys and gradient wells. In the central southern part of the permit area (on the eastern edge of the Vico caldera) a deep well was drilled through volcanics, shales and limestone formations, to a depth of 3000 m.

The presence of active thermal springs (temperature 50 to 60 C) and travertine deposaits in the western point indicated thermal activity and, previously, this has stimulated the undertaking of geothermal exploration. This search led to the drilling of futher wells.

The maximum depth reached was 1100 m. The maximum measured temperature was 78 C. The geothermal reservoir explored by these wells is the same one which feeds the thermal springs and is located in limestone formations at a depth on the order of hundreds of metres.

From the geological and structural points of view, the western area is characterized by a buried ridge, located west of the city of Viterbo and recognized as being a high, confirmed by the study carried out on the outcroppings. This is very clearly evidenced by the gravimetric survey.
The outlaying areas, having analogous geological and structural characteristics, are also undergoing geothermal exploration. This exploration has evidenced structural repititions in several wells and has led to the hypothesis that tectonic repetitions of the stratigraphic series also exist in the Mount Razzano Ridge area.

It was decided to test the hypothesis that on the ridge a lower section of the Mesozoic carbonates, present owning to repitition of the beds by faulting and intercalcated with thick layers of impermeable sediments, constitutes a separate deep geothermal system unconnected with the upper one, containing fluids at temperatures exceeding 200 to 250 C. An alternative possibility is that under the Mesozoic carbonatic section lies a metamorphic unit (b asically impermeable) which, especially in Mount Amiata, contains a productive fractured reservior. The deep Vetralla 1 exploration well will be drilled at the expected final depth of 3600 m.
THE OBJECTIVE OF THE STUDY IS THE DRILLING OF A WELL TO VERIFY THE HYPOTHESIS THAT THE REPETITION OF THE MESOZOIC CARBONATE SERIES,AT A DEPTH OF ABOUT 3000 M,CONSTITUTE A DEEP GEOTHERMAL SYSTEM CONTAINING FLUIDS AT TEMPERATURES EXCEEDING 250 C.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Ente Nazionale per l'Energia Elettrica SpA (ENEL)
Address
Via Andrea Pisano 120
56100 Pisa
Italy

Participants (1)

Agip SpA
Italy
Address
Via Emilia 1
20097 San Donato Milanese