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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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The Geysers area is about 90 mi north of San Francisco. The rocks in the area are Jurassic-Cretaceous Franciscan graywacke, sandstone, shale, greenstone, serpentine, and extrusive rocks belonging to the Clear Lake volcanic series of late Pliocene to late Pleistocene age. Volcanic activity is believed to be the cause of the abnormal high heat flow.
Drilling was started by Magma and Thermal Power Companies in 1955 and Pacific Gas and Electric's first plant of 12,500 kilowatt-hours (KWH) electrical generating capacity went into operation in 1960. The power output was more than doubled in 1963 and again in 1967 for a total of 55,000 KWH. A fourth plant of 27,500 KWH is under construction and new generating units of 55,000 KWH each are being planned.
To date Magma-Thermal have drilled 42 wells in three producing areas comprising a total of 300 acres. The wells range in depth from 1,500 to 5,000 ft. Steam capable of generating about 150,000 KWH is actually developed. The reservoir is a fractured sandstone and the occurrence of thermal alterations in a linear trend have led many to conclude that the fracturing is associated with faulting. The fluid exists in the reservoir as superheated steam with a pressure of about 500 lb. Successful stepouts drilled by Union, Signal Oil and Gas, and by Geothermal Resources International have expanded significantly the potentially productive area and the early estimates of 600,000 KWH power potential for the area may be conservative.
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