The Geysers Geothermal Field
Like many of the volcanic areas in the U.S. the Geysers-Clear Lake region has been drilled to determine its potential for geothermal energy. Drilling projects revealed a large silicic intrusive body known as “the felsite” which reaches to within about 1 km (0.62 mi) of the surface, south of Cobb Mountain. Dating of this rock body indicates a minimum crystallization (solidification) age of 1.3 to 1.4 Ma, which is in accord with the age distribution of the Clear Lake Volcanics. Volume estimates of the felsite indicate that the intrusive body is probably larger than the erupted volume of the volcanic field, which is estimated at 100 km3 (24 mi3). Steam production at The Geysers seems to be related to the distribution of the felsite - the shallowest steam is near the shallowest reports of the felsite. Today, geothermal power production is entirely from steam in The Geysers vapor-dominated field, which is located on the southwest margin of the volcanic field. Studies of hydrothermal mineralogy in drill cores and cuttings indicate that a hot water system exists in The Geysers field prior to the current vapor-dominated system.
Despite the name of the steam field, no natural geysers exist anywhere in the Geysers-Clear Lake area. The name was suggested to early explorers by the fumarolic activity and steam rising from hot springs in what later became the earliest developed part of the steam field. The steam field is adjacent to the southwest edge of the Quaternary Clear Lake volcanic field. The geothermal field includes the 1.66 Ma basalt of Caldwell Pines and wraps around the northwest and south sides of the 1.0 to 1.1 Ma rhyolite and dacite domes of Cobb Mountain. Otherwise, the geothermal field is entirely within an area of the Franciscan complex and rocks of the Coast Range ophiolite that are juxtaposed along fault zones. The reservoir rock is primarily a relatively brittle greywacke (type of sandstone) unit that is broken by fractures, which can transmit the steam. To the northeast and southwest, respectively, are the major northwest- trending Collayomi and Mercuryville faults that appear to act as boundaries to the vapor dominated system.
Numerous geothermal wells are located in an area about 30 km northwest-southeast by approximately 5 km northeast- southwest in rugged Coast Range terrain. Power production began in the 1960’s and increased to nearly 2,000 MW before declining in the late 1980’s to about 1,500 MW.