wWMmt wWMmt no sample basalt or andesite, altered

*diatomite and other fine-grained lacustrine sediments, altered and iron stained; possibly Truckee Formation andesitic flows; Chloropagus Formation rhyodacite ash-flow tuff with quartz, biotite, and feldspar rhyolite ash-flow tuff with quartz, biotite, and feldspar, white rhyolite ash-flow tuff, white to light green andesite flows; cuttings from this interval were quite complex rhyolite ash-flow tuff; there could be 3 separate units in this interval, white to pink in color, with abundant biotite, feldspar, and quartz crystals dacite ash-flow tuff, biotite and feldspar, white, gray, and pink dacite lava flovi/(?), gray with abundant plagioclase and biotite; this unit is thick and homogeneous dacite ash-flow tuffs, white to light green, feldspar and biotite rhyolite ash-flow tuff, very fine grained, light green to light gray in color dacite ash-flow tuff, purple with biotite and plagioclase greenstone, metamorphosed andesitic lavas and breccias total depth: 3192feet

FIGURE 19. Lithologie log of Desert Peak well B21-2.

location: NE/4 NE/4 S21, T22N, R27E

Churchill County, Nevada started: November 23, 1976 completed: December 20, 1976

not run below a depth of 2900 feet. A core consisting of greenstone was taken between depths of 2937 and 2949 feet; it is a highly fractured metamorphosed andesitic breccia consisting of angular fragments up to 3 inches in length. The rock has been fractured but not sheared, as the fractures have no slickensides or fault gouge. Quartz or calcite vein fillings are also absent. The entire pre-Tertiary section penetrated in well B21-2 is suspected to consist of greenstone.

The temperature profile of well B21-2 is the most complicated to date in the Desert Peak area (pi. 6). The top 2000 feet are very similar to the profile from strat. test no. 5, previously discussed. The temperature profile shown on plate 6 has the same problem as the strat. test no. 5 profile; the temperature of the isothermal section near the top of the hole is probably 12°F too low. A partial temperature log of well B21-2 by a second operator showed this isothermal section to have a temperature of 207°F. At a depth of 2540 feet the temperature gradient abruptly decreases to nearly 1°F/100 feet. On plate 6 a small temperature reversal is shown at 2900 feet. This reversal is the result of severe lost- circulation problems (see section of this report on the Desert Peak reservoir), and the lowest part of the temperature profile does not represent equilibrium conditions.

This nearly isothermal section represents the geother-mal reservoir. It is interesting that this nearly isothermal section extends about 245 feet up into the rhyolitic unit and about 330 feet above the known top of the production zone. Reservoir fluids may be convecting a couple of hundred feet up into the Tertiary rhyolitic rocks, and it may be possible to intersect the productive zones nearby at slightly shallower depths.

After well B21-2 was completed another cross section similar to figure 15 was constructed (fig. 20). This cross section shows some deeper temperature data not available on figure 15. The 400°F isotherm on figure 20 is probably the best representation of the reservoir boundary presently available.


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