f 3D Surface seismic and well seismic applied to the delineation of geothermal reservoirs in metamorphic formations in the Larderello area

The delineation of deep (>3000m) reservoirs is a primary objective in the exploration of the LardereIlo and Mt Amiata (Italy) geothermal fields. Drilling wells at such depths in relatively hard rocks is expensive, therefore risk must be minimized through optimum application of surface exploration techniques. Since 1974 ENEL has performed extensive reflection seismic surveys. They made it possible to identify well defined groups of reflectors within a metamorphic basement composed of phyllites, micoschists and gneisses (Batini et at, 1978). An exploratory well, the San Pompeo 2 well, revealed the presence of a permeable zone filled with geothermal fluids at very high temperature (>400°C) and pressure (>240 bars) and corresponding to the main reflector or K marker. Subsequent deep drilling targeted at reflections with lateral continuity over 1-2 km above the K marker demonstrated the existence of productive zones associated with these reflections. Because these reflections are less continuous, their delineation is a major objective. A 3D seismic reflection survey combined with multi-offset well seismic profiles, well logs and measurements on core samples (ERL/MIT, 1989) was therefore planned as a pilot study. It was carried out at the Monteverdi test site where an extensive drilling programme was to take place.