The Geysers is the world's largest geothermal field spanning an area of around 30 square miles (78 km2) in Sonoma, Lake and Mendocino counties in California, centered in the area of Geyser Canyon and Cobb Mountain. Power from The Geysers provides electricity to Sonoma, Lake, Mendocino, Marin, and Napa counties. It is estimated that the development meets 60% of the power demand for the coastal region between the Golden Gate Bridge and the Oregon state line. Unlike most geothermal resources, the Geysers is a dry steam field which mainly produces superheated steam.
Steam used at The Geysers is produced from a greywacke sandstone reservoir, capped by a heterogeneous mix of low permeability rocks and underlaid by a silicic intrusion. Gravity and seismic studies suggest that the source of heat for the steam reservoir is a large magma chamber over 4 miles (6.4 km) beneath the surface, and greater than 8 miles (13 km) in diameter.
Power plants at the Geysers are of the dry steam power plant type, where the steam directly powers the generator. In general, the Geysers has 1517 MW of active installed capacity with an average production factor of 63% (955 MW).
According to wikipedia