Structural Interpretation of Coso Geothermal Field, Inyo County, California

Carl F. Austin, Barbara P. Bishop, James L. Moore

The Coso Geothermal field, located east of the Sierra Nevada at the northern edge of the high Mojave Desert in southern California, is an excellent example of a structurally controlled geothermal resource. The geothermal system appears to be associated with at least one dominant north-south-trending feature which extends several miles through the east-central portion of the Coso volcanic field. Wells drilled along this feature have encountered production from distinct fractures in crystalline basement rocks. The identified producing fractures occur in zones which range from tens to hundreds of feet in extent, separated by regions of essentially unfractured rock of similar composition.

Proposed origins of the zones of vertical fracturing are diverse and provide additional fuel for the continuing debate concerning structural control. Theories, which invoke both extensional and compressional tectonic settings, include a system of individual breccia pipes, networks of intersecting through-going faults and fracturing associated with dome emplacement, and localized zones of extensive hydraulic fracturing.

Wells in the Devil's Kitchen area have encountered fluids in excess of 450°F and flow rates of 1 million lb/hr at depths less than 4,000 ft. A step-out exploration/production well drilled in 1986 to a depth of 6,553 ft located several miles south of the Devil's Kitchen region along the identified north-south feature produced fluids with a temperature greater than 640°F.

While the extent of the resource is not yet known, reservoir tests estimate 240 MWe of power will be available from the Coso field. Construction of the first 25 MWe power plant, begun in the first part of 1986, is scheduled to be completed with power on line by mid-1987.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.