Stratigraphic Test No

Strat. test no. 5 was drilled between July 7 and 26, 1976. The purpose of strat. test no. 5 was to prove that high temperatures in the vicinity of strat. test no. 2 could be areally extensive and to provide a possible alternative site to strat. test no. 2 for a second deep geothermal test well.

The top 50 feet of this hole are composed of Quaternary alluvium (fig. 13). Lacustrine rocks of the Truckee(?) Formation are present between 50 and 200 feet. The samples show intense hydrothermal alteration between 200 and 440 feet, and the original material is unrecognizable. The top of the Chloropagus Formation may be at a depth of 200 feet. The bottom is at 667 feet, resulting in a thickness of 467 feet. The top of the rhyolitic unit was encountered at a depth of 667 feet. About five subunits in the rhyolitic unit appear to be present between 667 and 2000 feet.

A temperature profile of strat. test no. 5 is shown on plate 6. The accuracy of this profile is questionable, but it is shown because it was obtained by the same contractor and equipment as most of the other profiles on plate 6. The bottom-hole temperature is 339°F and the thermal gradient near the bottom of the hole is 9.0°F/100 feet. A temperature survey by Phillips personnel to a depth of 550 feet (fig. 14) shows a substantial difference, as an isothermal section exists between 310 and 460 feet with a temperature of 207°F. The Phillips temperature survey is believed to be more accurate.

This temperature profile is more complex than the temperature profile of any other stratigraphic test at

total depth: 1675 feet

andesitic flows, somewhat iron stained and kaolinitized; many different flows are present; Chloropagus Formation lacustrine sediments, mostly clay, may contain some water-laid tuffs basalt, gray-black, generally fairly fresh; below 950 feet some iron staining is present.¡Vfery little pyrite is present sand, oft'3n tuffaceous with minor layers of ash(?), clay, and basaltic gravel location: NW/4 SE/4 S17, T22N, R27E

Churchill County, Nevada started: April 22, 1976 completed: May 6, 1976

FIGURE 12. Lithologie log of Desert Peak stratigraphie test no. 4.

Desert Peak. The isothermal section in the profile between 310 and 460 feet corresponds with the intensely altered interval, and some lost-circulation zones were encountered while drilling this interval. The temperature Phillips obtained in this interval is at the boiling point, and water-level data from nearby well B21-2 suggest the water level is at a depth of 480 feet. Therefore, this isothermal and altered section probably is the result of a boiling water table. A thermal aquifer is present at a depth of 880 feet and has a temperature of 298°F. The temperature profile is nearly linear between the bottom of the isothermal zone and the aquifer, suggesting this section of the hole has essentially reached a steady-state equilibrium. Below the aquifer there is a 25°F drop before the thermal gradient becomes positive again. The section of hole below the aquifer has not yet reached thermal equilibrium. This aquifer apparently is not the same as the one encountered in well 29-1, yet both appear to have become active recently. The aquifer is located within the rhyolitic unit but no lithologic feature has been identified as a possible caprock.

TEMPERATURE CROSS SECTION A-A'

A temperature cross section between well 29-1 and strat. test no. 5 clearly reveals the gross thermal structure in the area (fig. 15). The thermal aquifer present in well 29-1 and strat. test no. 1 is one of the major features on the cross section. The closely spaced isotherms near the surface are largely the result of the thermal aquifers. This cross section shows that the higher isotherms plunge steeply to the southwest between strat. test nos. 1 and 2. The isotherms probably reflect a steeply dipping reservoir boundary, although a fault similar to Brady's Fault in this location cannot be disproved. The isotherms indicate that there is a high horizontal thermal gradient caused by lateral conduction of heat away from the reservoir. The near-surface aquifer has totally hidden this boundary from detection by shallow temperature-gradient holes. This cross section (fig. 15) clearly shows the area near strat. test no. 2 is an obvious location for a second deep geothermal test.

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