Stratigraphic Test No

Strat. test no. 1 is located near shallow temperature-gradient hole no. 37 principally because hole no. 37 had such a high bottom-hole temperature and thermal gradient. Hopefully, the 64°F/100 foot gradient would continue to a depth of 500 or 600 feet and possibly would give the base temperature of the geothermal system. There was also some meager geological and geophysical evidence (ground magnetics, gravity, and heat flow) that a northwest-southeast trending structure was located near strat. test no. 1. This structure is discussed in more detail later in this report.

Strat. test no. 1 was drilled between March 6 and 18, 1976, to a depth of 2000 feet. Unconsolidated gravel and clay were the dominant lithologies present to a depth of 120 feet (fig. 9). The rocks are intensely hydrothermally altered between 120 and 250 to 290 feet. There were several lost-circulation zones encountered in this interval. It is surprising that this hot, highly permeable, altered interval does not show up on the temperature profile (pi. 6) as an isothermal zone.The Chloropagus Formation is present between 300 and 2000 feet and contains numerous thin lacustrine intercalations of sandstone and claystone which were not present in well 29-1. The first basaltic material is present at a depth of 45 feet and could be either part of the Chloropagus Formation or one of several younger basalts (pi. 13). The bottom 200 feet of strat. test no. 1 consist of lacustrine sedimentary rocks. It is possible that these are correlative with the sedimentary rocks found near the bottom of the Chloropagus section in well 29-1 (fig. 7).

The equilibrium temperature profile for strat. test no. 1 is shown on plate 6. It has a bottom-hole temperature of 259°F, but the temperature gradient at the bottom of the hole cannot be reliably extrapolated. The profiles for well 29-1 and strat. test no. 1 have a similar shape. In strat. test no. 1 the aquifer was intersected at a depth of 360 feet and has a temperature of 299°F. Below the aquifer a minimum temperature of 257°F is present at a depth of 1750 feet. The temperature decrease below the aquifer is 42°F. Below a depth of 1000 feet strat. test no. 1 and well 29-1 have remarkably parallel temperature profiles, with strat. test no. 1 averaging 63°F hotter than well 29-1 at the same depth. The two wells are about 2600 feet apart; the horizontal temperature gradient between these two holes below 1000 feet is 2.4°F/100 feet. Although 2.4°F/100 feet is not a high vertical temperature gradient, it is an extremely high horizontal gradient. This gradient suggests that strat. test no. 1 is located a short distance south of a near-surface and steeply dipping thermal boundary.

The thermal aquifer in strat. test no. 1 occurs in the Chloropagus Formation at a depth of 360 feet. The aquifer appears to be capped by a thin clay unit in-terbedded within the basalt flows. Apparently there are a number of relatively local caps for this aquifer instead of one widespread lithologic cap. Strat. test no. 1 and well 29-1 showed that the water in the thermal aquifer is indeed flowing in a southerly direction.

Strat. test no. 2 was drilled between March 19 and 28, 1976, to a depth of 1293 feet. This hole was terminated short of 2000 feet due to severe lost-circulation problems and high subsurface temperatures. The drilling mud reached a temperature of 180°F during drilling. This hole was located northeast of strat. test no. 1 in the SE/4 S21,T22N,R37E near hole no. 39, which has a temperature gradient of 44°F/100 feet. Data were also needed from this area to evaluate the surrounding Federal land and to provide a basis for future bids for geothermal leases.

The top 30 feet of strat. test no. 2 are composed of Quaternary sand and gravel. From 30 to 705 feet the rocks are fine-grained, upper Tertiary lacustrine units of the Truckee and (or) Desert Peak Formations (fig. 10). The underlying basalts and andesites of the Chloropagus Formation are only 520 feet thick and contain only a few minor lacustrine intervals. In well 29-1 the Chloropagus Formation is 2610 feet thick. This 2090-foot difference in thickness over a horizontal distance of 1.4 miles was unexpected. At 1225 feet the top of the rhyolitic unit is present, and circulation was completely lost during drilling in this unit. This is the only hole to date at Desert Peak which has encountered severe lost-circulation problems in the rhyolitic unit. Strat. test no. 2 bottomed in this unit.

The equilibrium temperature profile for strat. test no. 2 is shown on plate 6. The bottom-hole temperature is 369°F and the temperature gradient at the bottom of the hole is approximately 16°F/100 feet. The temperature profile is characterized by a low temperature-gradient interval sandwiched between much higher temperature gradients. This low-gradient interval corresponds well with the Chloropagus Formation.

Thermal conductivity variations can have a great influence on the shape of temperature profiles and their subsequent interpretation. Heat flow is the product of the temperature gradient and the thermal conductivity of the surrounding rocks. In many cases temperature profiles similar to those from strat. test no. 2 can originate from thermal conductivity variance with lithology simply because different strata have different thermal conductivities. If a stratum with a relatively high thermal conductivity is sandwiched between strata with much lower thermal conductivities, the middle stratum should show a lower thermal gradient, assuming the heat flow is constant.

However, in the case of strat. test no. 2, the large gradient decrease within the Chloropagus Formation is believed to reflect lateral conductive and (or) convective movement of heat. No thermal conductivity measurements have been obtained for strat. test no. 2; therefore, accurate heat flow calculations are not available. In the absence of the conductivity measurements, only a small part of the very large decrease in thermal gradient is attributable to variations in thermal conductivity. Strat. test no. 7 penetrated a very similar stratigraphic sequence, yet the profile is linear throughout the hole (pi. 6). This indicates that the collar elevation: 4275 feet

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