Geological structure of a long-living geothermal system, Larderello, Italy.

We have examined the data of 600 geothermal wells and re-interpreted approximately 500 km seismic profiles through the field of Larderello, Italy. We conclude that the two main seismic reflectors present below the geothermal area host two different fluids: (1) superheated steam in the upper H-horizon (reached by drillholes) and (2) supercritical fluid in the deeper K-horizon (reached by few unproductive or damaged wells). The superheated steam has the physical and chemical connotation of the geothermal fluid exploited so far at Larderello, whereas the supercritical fluid represents a potential unconventional deep-seated resource still to be assessed. The high temperatures existing in correspondence of the K-horizon suggest that the silica-rich rocks are close to a plastic state and the fluids should remain confined in a medium sealed to the confining rocks, unless occasional fluid overpressure and abrupt high strain rates occur.