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6 hours after circulation ceased 10 hours after circulation ceased 20 hours after circulation ceased 35/4 hours after circulation ceased 15 days after circulation ceased (5-27-74) estimated equilibrium profile (pi. 6)

FIGURE 8. Temperature profiles of Desert Peak well 29-1.

thermal conductivities (from cuttings) are present in this interval. This is about 30% above the average for the Basin and Range physiographic province. There is no evidence for convective transfer of heat below 1700 feet. The conductive temperature profile is additional evidence that well 29-1 did not intersect a geothermal reservoir.

The temperature reversal at 700 feet was initially believed to result from a thin subhorizontal thermal aquifer less than 10 feet thick. It was initially assumed that the area is at thermal equilibrium. If so, the only way to maintain the negative temperature gradient between 700 and 1700 feet would be by movement of cooler water through the area below the thermal aquifer. Because the temperature profile is smooth in the interval between 700 and 1700 feet it is unlikely that there is movement of cooler water below the aquifer. A dynamic system with both cool and warm waters moving would not produce such a smooth, regular temperature profile. Also, in this desert basin there is no identifiable source for rapidly moving, laterally flowing cool water.

Blackwell (1975) and Ziagos and Blackwell (1981) have suggested a nonequilibrium thermal model which closely fits the equilibrium temperature profile. In this model a thermal aquifer is assumed to be present at 700 feet and the temperature decrease below 700 feet is the result of the area not having attained thermal equilibrium. The thermal aquifer became active on the order of several thousand years ago and is still warming the underlying rocks. This interpretation is compatible with the smoothness of the equilibrium temperature profile. With time the temperature reversal should be replaced by a linear temperature profile with a low thermal gradient, assuming the thermal aquifer remains active.

The temperature reversal at 700 feet is located at the contact between overlying fine-grained sediments and the basalts of the Chloropagus Formation. Reservoir fluids apparently rise from depth through fractures or faults in the basalts, hit the impervious sediments which act as a cap, and flow either laterally along the contact or in the top few feet of the basalt. At the site of well 29-1 the aquifer must have limited permeability as no lost-circulation problems were encountered when the aquifer was penetrated. No evidence of this aquifer was detected in the cuttings; therefore, the significance of this contact was realized only after the hole had approached thermal equilibrium.

Although well 29-1 is correctly regarded as a commercial failure, it provided extremely useful information on the thermal structure, hydrology, and stratigraphy of the area. Analysis of data from this well made it possible to develop and use exploration tools and techniques suited to previously unknown conditions and problems in the area.

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