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phyllite metamorphosed impure quartzite with irregular chlorite crystals chlorite schist and hornfels; this unit appears to be contact metamorphosed phyllite with some interbedded mica schist and impure quartzite or argiflite chlorite schist and hornfels; this unit has been intruded by 4 granite dikes. The first granite is present at 5730 feet. Chips of biotite schist are abundant granite, very low in mafic minerals; composed of feldspar, quartz, and biotite altered to chlorite chlorite schist and hornfels with granite dikes and some biotite schist granite chlorite schist, hornfels, and granite granite granite, ch orite schist, and hornfels chlorite schist and hornfels granite chlorite schist and hornfels granite chlorite schist and hornfels granite granite, chloritic schist and hornfels, and fault gouge granite; on the logs this unit appears to be fresh and relatively homogenous; however, the cuttings are a mixture of fresh granite, iron-stained granite, and fault gouge. The bottom of the interval appears to be relatively fresh. Fault gouge is especially abundant between 8470 and 8630 feet. Below 9120 feet fault gouge appears to be less common location: SW/4 NW/4 S23, T22N, R27E

Churchill County, Nevada started: March 19, 1979 completed: May 30, 1979

FIGURE 25. Lithologie log of Desert Peak well B23-1.

the subunits can be further divided. The entire unit is generally dacitic in composition with lesser amounts of rhyodacite, rhyolite, and andesite. Two thin basalt units are present. The Tertiary rhyolitic unit in well B23-1 is considerably different from that found in wells 29-1, B21-1, and B21-2. In well B23-1 there is very little rhyolite. Dacite is the most common lithology. Also, distinctive marker units found at or near the base of the rhyolitic section in wells 29-1, B21-1, and B21-2 are not present in well B23-1. The rhyolitic unit is 2900 feet thick in well B23-1, making it the thickest section yet encountered in the area.

Regionally metamorphosed pre-Tertiary sedimentary rocks are present from 4225 to 5640 feet. Phyllite is the dominant lithology. Interbedded with the phyllite are layers or lenses of impure quartzite, argillite, chlorite schist, and possibly minor amounts of metavolcanic rocks. The pelitic and generally homogeneous nature of this unit suggests that it could be part of the upper Triassic Auld Lang Syne group (Johnson, 1977; Burke and Silberling, 1973).

It is important to note that no thick sections of greenstone similar to those found in wells B21-1 and B21-2 are present in well B23-1. The Mesozoic section in well B23-1 is also substantially different from that in well 29-1, as the sequence is mostly pelitic. Well 29-1 has a much greater variety of rock types than does well B23-1.

From 5640 to 7300 feet the section consists of roughly equal amounts of granite and chlorite schist or hornfels. Near a depth of 6000 feet chips of biotite schist are common in the cuttings. The nature of the contact relations between the chloritic schist and hornfels and the granite over a range of about 1700 feet suggests that the hole may be located close to a steeply dipping margin of the granitic intrusive or that the contact is gradational with many dikes extending out from the pluton and many large xenoliths or roof pendants imbedded within the pluton. The chlorite schist and hornfels unit is interpreted as part of a contact metamorphic aureole associated with the margins of the granite and is part of the Mesozoic pelitic sequence. Similar rocks are present in the nearby Truckee and Trinity Ranges (Willden and Speed, 1974).

Below a depth of 7300 feet, granite is the dominant lithology. Between 7270 and 8170 feet the granite is moderately altered. The biotite has been partially altered to chlorite in all of the samples, and much of the feldspar has been altered to clay. The granite is rich in quartz and poor in mafic minerals. No hornblende is present.

Between a depth of 8170 and 9641 feet the rocks consist of a mixture of moderately altered granite, fault gouge, minor chloritic schist, and some iron-stained granite. In this interval lost-circulation problems were chronic. Up to 2500 barrels of drilling fluid were lost every day. However, circulation was never completely lost as in wells B21-1 and B21-2. Many samples have a significant amount of brown to orange iron staining; however, most of these samples also contain abundant metal shavings from the casing, bit, or drill string, and some of the iron stain may be a result of the metal shavings. The fault gouge is a highly sheared, soft, waxy-looking clay ranging in color from blue-green to brown.

Below a depth of 9120 feet the fault gouge in the cuttings decreases.

The gamma log from well B23-1 shows that the gamma ray activity of the granite is not homogenous. (Benoit and others, 1980). In the interval between 5730 and 6740 feet, where most of the intercalation of granite and chlorite schist and hornfels occurs, the granite has a radioactivity greater than 200 API units. Below 6740 feet, where the granite is more massive, the radioactivity drops sharply to an average of 130 API units. To determine the reasons for this difference would require additional chemical and (or) petrologic studies.

Although no granite or granodiorite is exposed in the Hot Springs Mountains, Cretaceous granites are common in the nearby mountain ranges (fig. 2). The granite in well B23-1 is also expected to be Cretaceous but a Tertiary age cannot be excluded with the available data.

An equilibrium temperature profile of well B23-1 is shown on plate 6. This single profile, however, does not present a clear picture of the thermal structure in the well. The additional nonequilibrium temperature profiles shown on figure 26 aid greatly in understanding this well.

To a depth of 3200 feet the equilibrium temperature profile is nearly linear and has an average gradient of 9.3°F/100 feet (pi. 6). Between 3200 and 5200 feet the gradient smoothly decreases to zero at a temperature of 409°F. Between 5200 feet and 8900 feet the temperature is nearly isothermal, varying only about 6°F. Below 8900 feet the temperature gradient is consistently positive at 0.8°F/100 feet. The bottom-hole temperature of 413°F makes this the hottest hole drilled to date at Desert Peak. Previously unpublished temperature surveys were also run in well B23-1 by the USGS (T. C. Urban, personal commun., 1979) (fig. 26). These surveys showed temperatures from 9 to 15°F colder below 3200 feet than the wireline surveys obtained by Phillips. The wireline data are used in this report to maintain data consistency between all the deeper holes and wells. No USGS data were obtained below 7000 feet.

There is some correlation between changes in the temperature gradient and major changes in lithology. The temperature profile becomes isothermal at a depth of 5200 feet. At a depth of 5180 feet the Mesozoic rocks change from predominantly phyllite to schist. The schist may be slightly more competent than the phyllite and able to maintain fractures allowing reservoir fluids to circulate freely. Fractures in the softer phyllite probably heal too quickly to allow significant convection.

At 8900 feet the temperature gradient changes from isothermal to 0.75°F/100 feet. At about 8400 feet the granite has abundant fault gouge, and chronic lost-circulation problems developed with losses as great as 2500 barrels per day throughout the drilling of the rest of the hole. There may or may not be a relationship between the two features but it is interesting to speculate whether or not this fault zone at 8400 feet represents the bottom of the reservoir. The positive gradient segment at the bottom of the hole could indicate a deeper and hotter source of heat in the area.

Two nonequilibrium temperatue profiles were run 8 !/2 and 37 Vi hours after drilling circulation ceased (fig.

temperature in °F

temperature in °F

26). The most significant feature on these profiles is the sharp temperature reversal which occurs at a depth of 3400 feet. This reversal must have formed quickly after circulation ceased, because it is well developed after only 8I/2 hours. Mass transfer of heat via moving thermal water is required for this reversal to form so quickly. Evidence from the profile taken 37 Vi hours after circulation and the equilibrium profile indicate that once the reversal has formed, this interval appears to heat by conduction at about the same rate as the rest of the hole. The equilibrium temperature of this aquifer is 372°F, yet 37 Vi hours after circulation ceased the maximum temperature of the reversal was 340°F.

This aquifer, at 3400 feet, plays an important role in determining the shape of the equilibrium temperature profile in well B23-1. As previously noted, the temperature profile begins to decrease from 9.3°F/100 feet to 0°F/100 feet at a depth of 3200 feet. Therefore, nearly all of the interval between the aquifer and the surface is at a steady-state thermal equilibrium. This indicates that the aquifer has been active for a considerable period of time.

Between 3150 and 5200 feet the temperature gradient smoothly decreases, giving a convex upward profile. This part of the temperature profile represents a late stage in the approach to thermal equilibrium in the rocks between the aquifer at 3400 feet and the top of the isothermal section of the reservoir at 5200 feet. Urban and others (1978) have described a similar case in the East Mesa geothermal anomaly.

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