GEOLOGY AND MINERAL PARAGENESIS STUDY WITHIN THE COSO-EGS PROJECT

摘要

The Coso Geothermal Field is a large, high temperature system located in California on the western edge of the Basin and Range province. The East Flank of this field is currently under study as a DOE-funded Enhanced Geothermal Systems project. he reservoir rocks generally consist of a complex, interfingering sequence of diorite, granodiorite, and granite. The diorites show weak to strong alteration representing multiple hydrothermal events. The work described here was undertaken to evaluate the geology and thermal history of the East Flank, in order to better understand how the rocks will behave during hydrofracturing. Eight East Flank wells have been studied. The alteration assemblages present indicate that the granites are younger than the diorites. The oldest alteration sequence consists of epidote, pyrite, sphene, chlorite, and calcite; this sequence developed before the granite was emplaced. This alteration appears to be due to fluids of a deep, crustal origin. The granites are less common and more weakly altered than the diorites; the younger alteration assemblage as found in the granites consists of clays, calcite, chlorite, and sericite. Fluid inclusion studies were implemented to separate different thermal events. Fluid inclusions were studied from three wells: 83-16, 38B-9, and core hole 64-16. Inclusions from calcite, quartz, feldspar, and epidote were Studied. Quartz- and calcite-hosted inclusions from the upper 256m in 64-16 are characterized by salinities usually less than ～2 weight percent NaCl equivalent and homogenization temperatures that differ by less than ～15℃ from the down hole measurements. Minerals in these veins, which typically contain significant open-space, display euhedral forms. These relationships suggest that the inclusions could be related to the modern geothermal system. Inclusions from 83-16 and 38B-9 display a broad range of salinities from ～0 to 7 wt % NaCl equivalent and homogenization temperatures (202.5℃ to 325℃) that range from several tens of degrees below to tens of degrees above the present day values. The temperature-salinity relationships suggest that many of these inclusions represent older thermal events. Taken together, the mineral assemblages and fluid-inclusion data suggest a long and complex thermal history for the East Flank.