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.
The 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 o C 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 o C to 325 o C) that range from several tens of
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.

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