The Geysers geothermal field, located in Lake, Sonoma, and Mendocino Counties, California is the largest developed geothermal system in the world. Electric power generation started at The Geysers in 1960 with a 12 MW (gross) plant. The total installed capacity in the field peaked in 1989 at 2,043 MW. As more and more power plants were built during the 1980s and net mass withdrawals increased, reservoir pressures at The Geysers declined, eventually resulting in steam shortfalls and declining generation levels. This net withdraw is due to the fact that geothermal power plants at The Geysers typically lose about 70% to 80% of produced mass to evaporation in cooling towers, with the balance of mass being returned to the reservoir through injection of steam condensate.

This report presents the results of an investigation, by the California Division of Mines and Geology, of the geothermal resources of the northern Sonoma Valley area, California. A report on the geothermal resources of the southern Sonoma Valley area, (Youngs and others, 1983) indicated several patterns, trends, and anomalies that appeared to extend north and northwest in the valley. Study of the northern valley area was conducted to attempt to follow these trends and to determine whether significant geothermal resources exist in the area.