total depth (in feet)

granite intrusive-

FIGURE 36. Geologic cross section B-B' (see plate 13 for location of B-B').

block showing initial N60°E fractures showing horst formed along N60° E faults

Diagram Coal Seam Fault

block showing N25°E fractures on horsted block showing rhombohedral horst block bordered by N60°E and N25°E faults

FIGURE 37. Block diagram depicting formation of rhombohediral horst blocks.

survey did not reveal any other anomalous gravity features in the vicinity of the geothermal anomaly, and more detailed gravity information was not needed at that time.

After the discovery well (B21-1) and confirmation well (B21-2) were drilled and the geologic map was completed, a tentative model of a fault-bounded reservoir was proposed. The large displacement faults shown on the geologic map (pi. 13), coupled with the significant density contrasts between different lithologic units (table 8), could allow a detailed gravity survey to locate possible reservoir boundaries. Accordingly, a 113-station gravity survey covering about 15 square miles was completed in January 1979 by an independent contractor (Dallon, 1979). There are about eight stations per square mile, and where possible the stations were located on relatively level ground rather than adhering to a strict grid system. This eliminated Hammer terrain-corrections rings B, C, and D from the topographic corrections.

A LaCosta-Romberg model G gravity meter (no. 295) was used; readings were recorded to the nearest hundredth of a milligal. All stations were surveyed with a Beetle 1600 distance meter and a Leitz T60D theodolite. The vertical elevations were obtained to the nearest foot and the horizontal control is within 5 feet. The total error in the gravity determinations due to errors in vertical elevations and horizontal control should be less than 0.1 milligal. A gravity base loop was established and all readings were taken in loops of three hours or less duration. Each loop was tied to the nearest gravity station along the base loop. A correction of 0.0635 milligals per vertical foot was added for calculation of the simple Bouguer anomaly. This value is based on the average density of 2.383 g/cm3 as determined by density logs from well 29-1. The elevation datum used is 4000 feet above mean sea level. Terrain corrections for Hammer terrain-correction rings E to J were performed manually, and a latitude correction of 1.288 milligals per mile southward was added to calculate the complete Bouguer gravity.

The complete Bouguer gravity map is shown on plate 14. The contour pattern is complex and is characterized by several small anomalies which have up to 2 or 3 milligals of relief over a horizontal distance of V4 to Vi mile. Habiger (1979) modeled the gravity data and con cluded that these small anomalies result from features above a depth of 2000 feet. Because the Tertiary volcanic sequence is present to depths greater than 2000 feet everywhere except the extreme northeastern corner of the gravity map, these small anomalies probably result from structural or lithological variations within the volcanic section.

The total gravity relief on the map is 9.4 milligals. In the southwest corner of the map the lowest value is 86.2 milligals, and the highest value of 95.6 milligals occurs in the northeast corner. There is an irregular but steady increase in gravity from southwest to northeast (pi. 14). Geologic and drill-hole information suggest that pre-Tertiary and intrusive rocks are present at shallower depths to the northeast. However, a plot of the known depths to the top of the pre-Tertiary rocks shows that the gravity map does not accurately predict the depth to pre-Tertiary rocks (pi. 14). This is partially due to complications first noted after well B23-1 was drilled. The pre-Tertiary rocks in well B23-1 are not as dense as the pre-Tertiary rocks in the other wells (table 8). Reliable compensation for this density contrast is not possible with presently available deep well data.

The gravity contour pattern which outlines the small anomalies also indicates that three main structural trends are present in the area: 1) north to north-northeast; 2) northeast to east-northeast; and 3) northwest. Plate 14, which shows these gravity trends, was constructed by drawing lineaments based on gravity contours alone. Mapped faults were added later. No single gravity trend dominates the gravity map as the north-northeast fault trend dominates the geologic map. However, the gravity survey was run over a part of the Hot Springs Mountains, which is covered mostly with sand. Outcrops are sparse and few faults were mapped in the area. Therefore, it is possible that no single fault trend predominates in the area covered by the detailed gravity study, even though the fault density there may be similar to that elsewhere in the Hot Springs Mountains.

Several north- to north-northeast-trending lineaments are shown on plate 14. These lineaments are generally parallel to mapped faults but rarely overlie them, with the exception of the Desert Queen Fault, which shows up well both on the ground and on the gravity map. Habiger (1979) reports that the Desert Queen Fault is

TABLE 8. Average densities from downhole geophysical logs and cores.

Lithologic unit Average density from Dry bulk density neutron-formation density logs Well 29-1 Well 21-2 Well B23-1

Chloropagus vesicular basalts and andésites 2.25 g/cm3 — — —

Unnamed rhyolitic unit 2.45 g/cmJ 2.55 g/cm3 Pre-Tertiary metamorphic rocks 2.65 g/cm! 2.65-2.70 g/cmJ 2.2-2.55 g/cm3

Granite — — 2.55 g/cm3 Granite fragment from well B23-1 — — — 2.56 g/cm3

one "of only two major structures detectable on the gravity map. Interestingly, the Desert Peak Fault has no expression on the gravity map, which is somewhat disturbing, since geologic evidence suggests that it may have up to 1000 feet of displacement and may be a part of the reservoir boundary system.

Several gravity lineaments which parallel the east-northeast fault set mentioned in the geology section are shown on plate 14. In fact, the longest lineament on the gravity map has this trend. However, the low gravity relief of about 2 milligals across the lineaments suggest that vertical displacement on individual faults is small.

The third trend of northwest-trending gravity lineaments has no expression in surface geology. These lineaments roughly parallel the Walker Lane (fig. 2), but the relationship between the two, if any, is conjectural. The second major structure reported by Habiger (1979) has this trend and cuts across the southwest corner of the gravity map. Although there is little geologic evidence to suggest that this lineament is a fault, thermal data shown on figure 20 and plate 12 do corroborate a fault explanation for this structure.

The most striking anomaly on the gravity map is a 2-to 3-milligal high centered in the S/2 S15,T22N,R27E. The north-south axis of this anomaly correlates well with a portion of a fault (or faults) which uplifted and exposed the unnamed rhyolitic unit (pi. 13; fig. 36). The amount of apparent separation on this fault suggests that it may be a significant feature, but it is expressed over only limited distance on the gravity map. The shape of the anomaly is more complex than would be produced by a single normal fault. Additional faulting, substantial lithologic variations, or secondary alteration or mineralization in the Tertiary volcanic rocks could all contribute to or be responsible for the formation of this anomaly.

The complete Bouguer gravity map apparently shows the effects of near-surface variations in the Tertiary volcanic pile; these variations are superimposed on a regional trend which is believed to be a result of the northeasterly rise toward the surface of basement rocks (pre-Tertiary or intrusive). Contour trends on the gravity map correlate well with the geologic fabric, although the individual fault to lineament correlation is spotty. Some mapped faults which are thought to be significant are not reflected on the gravity map, and one of the most striking gravity trends has no surface geological expression in the area. The gravity map has neither outlined the geothermal reservoir nor defined the reservoir boundaries. It has provided some amount of corroboration for mapped geologic trends, revealed some potentially significant trends unseen at the surface, and substantiated portions of a slightly perplexing temperature-distribution pattern. It should be noted, however, that in a volcanic terrain with complex surface and basement geology and limited subsurface geologic information, gravity data cannot be expected to provide simple solutions for complex problems.

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