Research in the Geysers-Clear Lake geothermal area, northern California

Abstract

DE82 905361 COVER View southeastward at geothermal development in the Geysers steam field. Large steam plumes rise from condensing towers of numerous electrical powerplants: smaller plumes rise from geothermal wellheads. Northwest-flowing Big Sulphur Creek has excavated the canyon that transects the photograph from upper right to lower left. Hydrothermal leakage from the steam reservoir occurs dong faults that cut rocks of the Franciscan assemblage along the northeast side of Big Sulphur Creek canyon. The altered rocks are prone to extensive landslides, many of which are visible here. Prominent flat-topped peak on the central horizon is Cobb Mountain, a 1.1-million year-old silicic volcanic center of the Clear Lake Volcanics. . RESEARCH IN THE GEOTHERMAL=, NORTHERN CALIFORNIA GEYSERS-C-LAKE Landsat image of a part of northwestern California, showing Clear Lake in the center, Lake Berryessa to the southeast, Suisun Bay still farther south (lower right corner), and the Pacific Ocean to the west. Agricultural development in the western Sacramento Valley is visible northeast of Clear Lake. Two smaller closed valleys, visible west of Clear Lake, that are dined along a northwestward trend are Little Lake Valley to the northwest and Ukiah Valley with Lake Mendocino on the southeast.. The present area of geothermal development in the Geysers steam field is encircled. Northwest-trending hogback ridges, composed of late Mesozoic sedimentary rocks of the Sacramento Valley that mark the west side of the Great Valley physiographic province,contrast with the more heterogeneous intricately dissected terrane of the Coast Range province to the west. Sutter Buttes, a center of late Cenozoic volcanism in the center of Sacramento Valley, is visible in the upper right corner. Along the southwest coast can be seen the rift of the San Andreas fault, trending southeast from Point Arena and passing through Bodega Head and Tomales Bay in the lower part of the photograph.

Reservoir depletion at The Geysers geothermal area, California, shown by four‐dimensional seismic tomography

“…The low‐ V p / V s anomaly observed at The Geysers correlates with the volume where the pore fluid is thought to be vapor‐dominated, where pore pressure is relatively low and argillaceous minerals relatively dry [e.g., McLaughlin , 1981]. Julian et al [1996] concluded that The Geysers might have had an associated low‐ V p / V s anomaly prior to development, since such conditions probably existed naturally then.…”

Plate motions recorded in tectonostratigraphic terranes of the Franciscan complex and evolution of the Mendocino triple junction, northwestern California

“…However, this argument does not seem to apply to the Coast Ranges because initiation of volcanism at 2.6-2.0 Ma in the Clear Lake area lagged behind passage of the Mendocino triple junction by only 0.7 to 1.3 m.y. (Donnelly-Nolan and others, 1981;McLaughlin, 1981;McLaughlin and others, 1985b ). This shorter lag may be attributable to the likelihood that thermal anomalies in the upper 10 km of crust above the slabless window (slab gap) are more directly tied to extensional strain adjacent to newly lengthened segments of the transform than to conductive heat transfer.…”

The Fluid Inclusion and Mineralogic Record of the Transitionfrom Liquid- to Vapor-Dominated Conditions in The Geysers GeothermalSystem, California

“…1). During the Late Mesozoic (Franciscan) to Early Tertiary, this region was the site of deep oceanic sedimentation, northeast-directed subduction, and regional metamorphism, which locally grades up to the blueschist facies (McLaughlin, 1981). Modern hydrothermal activity at The Geysers and in the surrounding area is associated with shallow intrusions related to the Pliocene to Recent Clear Lake volcanic field .…”

“…Most of the steam is produced from a thick succession of metagraywacke that underlies a chaotic suite of serpentinite, argillite, cherts, and greenstone. Excellent reviews of the geologic and geophysical features of the area can be found in McLaughlin and Donnelly-Nolan (1981) and Donnelly-Nolan et al (1993).…”