Geology and Evolution of The Geysers Field, Coast Ranges, California

Abstract

After 20 years of mapping, drilling, producing, modeling and researching, the Geysers field offers always new problems concerning its geology, limits, evaluation, and life span. However, several points are now clear: (1) the source of heat is a magmatic intrusion; (2) the cap rock has been formed by self sealing; (3) the fractured Franciscan complex is the reservoir rock; (4) during the early Tertiary, the Franciscan complex has been pushed under the Great Valley basal oceanic crust by subduction; (5) the Great Valley Sequence is a poor geothermal reservoir; (6) a well drilled in the Great Valley, east of its oceanic crust outcrops, will penetrate an unpredictable thickness of the Great Valley Sequence, its oceanic crust, and, under it, the Franciscan; (7) more or less vertical impermeability barriers exist; (8) at the beginning, the reservoir contained liquid water; surface hot springs decreased reservoir pressure till the boiling point.