ST

TD: 7662

2000 feet

Lines are solid where known and dashed where inferred.

temperature interval 50 F

2000 feet

0 600 meters no vertical exagération DP29-1 Desert Peak well 29-1 ST-1 stratigraphie test no. 1 TD total depth (in feet)

Lines are solid where known and dashed where inferred.

FIGURE 15. Temperature cross section A-A' based on stratigraphie tests (see plates 12 and 13 for location of A-A').

Strat. test nos. 2 and 5 proved that geothermal potential at Desert Peak was high enough to warrant a second deep test well. In July 1976 it was decided that well B21-1 would be drilled near strat. test no. 2 to a maximum depth of 6000 feet. Once again the well was located on private land. Geology, geochemistry, and geophysics (other than the temperature-gradient work) played no part in locating well B21-1. Prior to spudding well B21-1 very little was known about the reservoir except that it would be liquid-dominated. Thermal fluids in the shallow aquifers could not originate as steam condensate or from a hot-dry-rock reservoir. The few water samples available were sodium-chloride waters (see the section of this report on geochemistry) which also indicate a liquid-dominated reservoir (White, 1970). Geothermometry suggested temperatures near 400°F, but these geothermometers could have been readjusted to lower temperatures in the aquifers; hopefully the reservoir would be significantly hotter than 400°F. The reservoir host rock was unknown, as was the possible size, depth, and thickness of the reservoir. It was crucial that this well be a commercial producer, as another unsuccessful well would make it virtually impossible to convince management that the area warranted or deserved additional exploration.

Well B21-1 was spudded on October 25, 1976, about 400 feet away from strat. test no. 2. The rig was moved off location on November 18, 1976. Well B21-1 is 4150 feet deep and produces 450,000 lbs per hour of fluid (fig. 16) with a wellhead pressure of 103 psig (pounds per square inch gage). The flow rate was calculated using the lip pressure method described by James (1966). The reservoir temperature in well B21-1 is 406°F.

A lithologic log of well B21-1 is shown on figure 17. Upper Tertiary and Quaternary sediments and sedimentary rocks are present to a depth of 520 feet. Between 250 and 430 feet they are intensely hydrothermally altered. The Chloropagus Formation is present between 520 and 1270 feet, resulting in a thickness of 750 feet. Fresh black basalts are found only between 900 and 1050 feet. The rest of the Chloropagus Formation is composed of generally soft, green basaltic and andesitic flows and (or) tuffs.

In well B21-1 the rhyolitic unit is 1675 feet thick and is found between depths of 1270 and 2945 feet. The rhyolitic unit in this well has a 500-foot-thick sequence of andesitic to dacitic rocks present near the middle of the unit. These intermediate composition rocks are not present in well 29-1.

Mesozoic metamorphic rocks are found below a depth of 2945 feet. In well B21-1 the Mesozoic rocks are predominantly metamorphosed volcanic rocks of intermediate composition which are referred to as greenstone. Between depths of 3740 and 3900 feet a section of limestone and impure quartzite is present. At the base of this unit numerous slickensides and considerable fault gouge were observed in the cuttings, suggesting a fault contact. The Desert Peak geothermal reservoir has productive zones consisting of fractures in the greenstone. In well 29-1 the greenstone accounted for roughly 30% of the Mesozoic section present; in well B21-1 the greenstone comprises more than 80% of the known Mesozoic section. The stratigraphic and structural relationships between these two greatly different sections have not been resolved. No cores were taken in well B21-1, and it was terminated at 4150 feet because production had been proven and additional drilling would have been difficult, risky, and expensive. Circulation was lost at depths of 3638, 3891, 3970, and 4000 feet. Below 3881 feet the well was drilled with air and water. The well was difficult to control and produced 1800 barrels of water while drilling between 3990 and 4140 feet.

The equilibrium temperature profile of well B21-1 is shown on plate 6. The temperature gradient is 42°F/100 feet to a depth of 480 feet. This interval corresponds to the relatively impervious upper Tertiary and Quaternary sedimentary rocks. Between 560 and 1300 feet the temperature gradient gradually decreases to 0.22°F/100 feet with a slight temperature reversal between 870 and 1060 feet. This type of convex upward temperature profile is usually caused by nearby ascending thermal waters (White, 1973). The long, nearly isothermal section of the profile from 1360 to about 3638 feet is believed to result from lateral conduction of heat from reservoir water ascending a nearby steeply dipping fault. The location of this nearby fault is not precisely known, but evidence for it is discussed later in this report. The isothermal section between 3638 and 4192 feet is interpreted to result from convective movement of water within a network of fractures in the greenstone.

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