ABSTRACT: Structural and Alteration Controls on Thermal Fluid Flow at the Coso Geothermal Field, California

D. L. Nielson, J. B. Hulen, J. Copp

Analysis of downhole pressure and temperature data from the Coso geothermal field demonstrates a deep zone of upwelling that is displaced several miles from both the near-surface heat flow anomaly and the area of extensive surficial acid-sulfate alteration (the Devil's Kitchen). A series of temperature/pressure slice maps at progressively deeper levels shows that the plume extends from south to north along faults and fractures defined at the surface by detailed geologic mapping. The plume generally follows a north-northeast trend that is parallel to one of the Coso field's major mapped fault sets (this set is seismically active in the area of the geothermal field). Additional controls on fluid flow are thought to be provided by relatively flat faults whose origin is equiv cal at present. These features have been mapped at the surface, but they are difficult to trace due to poor exposure and extensive younger faulting. Our data also reveal the importance of hydrothermal illite and mixed-layer illite/smectite in development of a high-level, impermeable zone that channels fluid flow northward and inhibits the development of surface thermal manifestations over the principal zone of upwelling.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990