ABSTRACT: Localization of the Geysers Steam Field

GABELMAN, JOHN W., John W. Gabelman and Associates, Inc., Danville, CA, and EDWARD B. TOWNE, JR., Towne Exploration Company, San Francisco, CA

The reservoir is a fracture system in competent metagraywacke, capped by incompetent shaly graywacke, in two northeast-tilted normal-fault blocks between major wrenches. The field was sensed by the airborne Daedalus Thematic Mapper Simulator and 157 spectrally different images were produced using a G.E. IMAGE-100 digital analyzer. Eleven spectral bands variously combined differentiated background signatures for the lithologic and vegetative suites. Anomalous signatures portray hydrothermally altered equivalents. Emanations from the deeply trapped steam reservoir are suggested by coincident unexplained anomalies. These anomalies mimic the reservoir in shape. Volcanic vents are negative heat anomalies. Positive heat anomalies lie over or alongside the reservoir, suggesting an underlying source. Thrust faults are obscure, but wrenches are apparent. Most outstanding are unmapped fractures, lineaments, and circles. Swarms of aligned or en echelon short fractures are designated "integrated"fractures, and considered larger deep structures dying upward. Their pattern supports wrench-induced second-order deformation between the major Maacama and Collayomi wrenches. The pattern is compartmented into mostly shear, compression, or tensional domains. The single tensional domain roughly outlines the steam reservoir. Circles, which are abundant over the reservoir, suggest shale or serpentine diapirs, which helped seal the cap. Magma type appears influential. The Clear Lake volcanic field is mostly mafic and water dominated northeast of the Collayomi wrench. Fluid mafic/intermediate avas promote hydrofracturing and groundwater migration. Viscous rhyolite retards fracturing and nonexplosive volatile release promotes sealing of reservoirs.

AAPG Search and Discovery Article #91016©1992 AAPG-SEPM-SEG-EMD Pacific Section Meeting, Sacramento, California, April 27-May 1, 1992 (2009)