Geologic Structure And The Geothermal Reservoir

Post-discovery geologic mapping in the northern part of the Hot Springs Mountains shows that the area underlain by the Desert Peak geothermal field is structurally higher than other parts of the northern Hot Springs Mountains. Most of the surface in this area is covered by alluvium, but where bedrock is exposed it consists mostly of rocks of the lower Chloropagus Formation or of units of the unnamed rhyolite sequence. The subsurface temperature distribution is areally nonlinear; based on present hole spacing, it does not indicate an obvious association with faults observed at the surface. In fact, the lack of this association strongly suggests that some mechanism other than high-angle faulting serves to localize the heat.

Defining and locating the geothermal reservoir boundaries poses critical problems in the study of the geothermal reservoir. Solutions to the problem at Desert Peak are complicated since the rocks in which the reservoir occurs are not exposed nearby. Where similar pre-Tertiary rocks crop out in neighboring ranges they are highly deformed. Lithology appears to exert some influence on the reservoir configuration, but structural controls obviously exist but remain almost totally unidentified. It is possible that geologic structure observed at the surface at Desert Peak has no relationship to reservoir boundaries. However, it will eventually be necessary to choose possible reservoir boundaries based on geology, if only as a first approximation, in order to facilitate planning and future work. Possible geologic boundaries have been selected (fig. 38), but these are subject to change as additional subsurface information becomes available. Possible reservoir boundaries based on temperature and gravity data were discussed previously in the section of this report on the Desert Peak reservoir. The proposed geologic boundaries were chosen to conform with the temperature data insofar as possible.

The most credible boundary at present is the southwest boundary. This boundary is defined by both gravity (pi. 14) and temperature data (fig. 20; pi. 12). The exact location and limits of this boundary are uncertain. It may be an unexposed fault or fault zone.

A possible candidate for the northwestern boundary is the Desert Peak Fault (fig. 34; pi. 13). The significance of this fault to the geothermal reservoir is questionable; however, the fault is part of a major topographic lineament in the Hot Springs Mountains. The southeastern boundary of the field could be the Desert Queen Fault. The gravity contours confirm the presence of this fault, and it is the first substantial known structure east of well B23-1. A northern reservoir boundary is even more speculative at present. Possibly the northern margin of the N60°E-trending fault system also acts as a field boundary (fig. 35; pi. 13).

Silo Field Fault Map

Field mapping, temperature data, and detailed gravity work suggest that the Desert Peak geothermal field is contained in the en echelon rhombohedral horst system described earlier. Two apparent stages of uplift along two distinct fault sets have resulted in an area of structural elevation at the intersection of the two structural trends. Structural elevation in this area is significant to the geothermal anomaly in at least two ways: 1) the thermal aureole surrounding the geothermal reservoir was brought up to depths shallow enough to be detected by shallow drill-hole exploration methods, and 2) Mesozoic rocks which contain the reservoir are nearer the surface and within range of economic exploitation. If the reservoir were too deep, the Desert Peak discovery would have been an exploration success but an almost certain economic failure.

Although the proposed geologic boundaries agree with the approximate areal limits of the deep thermal anomaly, they do not constitute the geothermal reservoir, as is the case with Brady's Fault. Tertiary highangle faults probably act only as reservoir boundaries, although at least one Tertiary fault taps the geothermal reservoir. This fault functions as the conduit for thermal water which leaks from the reservoir into the Southwest Aquifer system (previously described). Other than this leakage, the Tertiary faults in the Desert Peak area apparently do not transmit large amounts of thermal fluids.

The geothermal reservoir waters are contained in fractures in pre-Tertiary rocks. Tertiary volcanic rocks form a somewhat leaky but generally effective cap over the reservoir. The Tertiary rocks are not appreciably hydrothermally altered, indicating that thermal water circulation in the younger rocks has been minor, in spite of pervasive Tertiary high-angle faulting. Productive fractures are apparently confined to pre-Tertiary rocks whose stratigraphy and structure are not well known. Correlation of pre-Tertiary units between the four deep tests is still speculative, which also hampers the interpretation of the reservoir fracture system. However, there are several implications from the available evidence.

One implication is that Tertiary fault zones might intersect pre-existent Mesozoic fracture systems. Tertiary structural intersections (such as the north-northeast and east-northeast trends at Desert Peak) superimposed on Mesozoic structures could well provide sufficient porosity and permeability for deep, high-volume circulation of thermal waters, as well as a basis for localization. It is not known why some Tertiary faults act as borders whereas others remain permeable.

The Humboldt Lopolith (Speed, 1976) contains bedding repetitions, comagmatic greenstones, and intrusion-related thrusting in the nearby West Humboldt Range and Mopung Hills. This relationship, along with the presence of hornblendite in well 29-1, has led to consideration of the possible involvement (lithologically and structurally) of the lopolith in the localization of the geothermal reservoir, but this is highly conjectural.

It has also been suggested that the Desert Peak geothermal field occurs in the faulted nose of a south-plunging anticline. The surface distribution of rocks is suggestive of this, as are the numerous cross sections that have been drawn. Numerous folds of possible deep-

seated nature occur both east and west of the geothermal field (pi. 13). Thermal waters could be localized by fracturing where flexure has been most severe, perhaps at the anticlinal crest. The fault block in which well B21-2 was drilled could be slightly down-dropped, in a keystone effect (fig. 36).

The source of heat is presently unknown. The available information does not support the inference of a shallow magma chamber below or tangential to the Hot Springs Mountains. Heat flow outside the Brady's and Desert Peak thermal anomalies is near average for the Basin and Range physiographic province (Olmsted and others, 1975). The thermal waters must circulate to depths greater than 10,000 feet to attain the observed temperatures (Hose and Taylor, 1974). Geologically, the relationship between Brady's Hot Springs and the Desert Peak thermal area is uncertain. Their proximity suggests a common heat source, but water chemistry and temperature distribution indicate that the two systems are not connected at shallow levels. Because the high temperatures observed imply circulation as deep as 10,000 feet or more, the systems are probably not connected at depth as well. It is suspected, therefore, that both thermal anomalies reflect deep unconnected thermal water convection systems and that their proximity is geologically coincidental. Thus, the two areas may have the heat source in common, but the thermal convection systems are separate entities.

The Desert Peak geothermal field occurs at the intersection of two structural trends. Structural elevation has brought the reservoir, contained in pre-Tertiary basement rocks, to depths shallow enough to permit detection and potential economic exploitation. Highangle Tertiary faults may serve in part as boundaries for the field. The thermal fluids are capped by a thick Tertiary volcanic sequence which functions effectively despite pervasive basin-and-range faulting; the cap allows a relatively small amount of thermal water leakage. Fractures which allow the movement of fluid within the geothermal reservoir may be a consequence of the intersection of Tertiary faults and pre-Tertiary basement tectonic elements.

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