Gabbro zone, located north of the o l d geothermal field of Larderello, was explored after 1960 by several wells, as shown in Fig. 1. The geology of the area can be summarized by grouping the different terrains into three main complexes as follows.
1. An upper complex, consisting of Neogenic deposits, and allochthonous flysch facies formations with ophiolites (Jurassic-Eocene), Because of its predominantly argillaceous nature, this complex is practically impermeable.

2. A discontinuous layer represented by brecciated carbonate rocks associated with evaporitic deposits (Trias), characterized by high secondary permeability.

3. A lower complex, consisting mainly of Triassic and Paleozoic metamorphic clastic formations (quartzites and phyllites). In this case too the permeability is tied to the existence of fractures.

Gabbro zone represents a structural high, separated from the culmination of the main Larderello-Castelnuovo structure.

On the E-NE side, this structure borders on an important direct fault which, on a regional scale, represents the western boundary of a large tectonic depression trending NW-SE (Era Graben).

On the north side of the Gabbro structure, the layer of Triassic breccias is lacking. This is probably one cause of the decrease in permeability observed in the northern marginal zone.

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