Evidence for large-scale Laramide tectonic inversion and a mid-Tertiary caldera ring fracture zone at the Lightening Dock geothermal system, New Mexico
Stratigraphy in a recent intermediate depth (2100 ft) geothermal test hole is interpreted along side previous deep (7001 to 7404 ft depth) exploration holes to study large-scale structural controls for the Lightning Dock geothermal system and refine the stratigraphic and structural characterization of the shallow reservoir. Laramide compression and large-scale tectonic inversion of a northwest-trending Late Jurassic-Early Cretaceous extensional structure at the Lightning Dock geothermal site represents deformation that may exceed Neogene extensional processes in magnitude and duration and provide potential for a significant volume of shattered rock. Also, volcanic stratigraphy supports the hypothesis of Elston et al. (1983) for a midTertiary caldera ring fracture zone in the vicinity the geothermal area. Northeastand north-trending Neogene normal faults that cross cut older structures provide additional preparation of fractured ground. A north-trending Pleistocene normal fault may reopen older fractures that are favorably oriented and allow concentration and upward flow of deep-seated geothermal fluids. Because the Lightning Dock geothermal system is “blind” and has no surface manifestations, a better understanding of structures buried beneath Neogene alluvial basin fill may have application in geothermal exploration to find similar “blind” geothermal systems elsewhere in southern New Mexico and southeastern Arizona. FIGURE 1. Shaded relief topographic location map of the Lightning Dock geothermal area.
Structural Control of Warm Springs and Wells in the Hillsboro-Lake Valley-Palomas Basin Region of South-Central New Mexico
Subsurface fluid flow and heat transport in the Animas graben and horst between Hillsboro and Lake Valley and in the Palomas Basin in south-central New Mexico are evaluated using older industry and new temperature gradient data and geothermometry. Thermal manifestations include warm springs (31-38°C) and a borehole with a high geothermal gradient (80°C at 74 m) near Hillsboro. In addition, three shallow water wells drilled during the past decade near Lake Valley have encountered warm water (3138°C) at shallow depth (<24 m). These warm waters and a nearby warm spring indicate the presence of a low-temperature geothermal system north of Lake Valley along Berrenda Creek where the creek crosses the Animas horst. Three wells in the Palomas Basin tap into a 28-30°C north-striking confined aquifer at a depth of 280 m that appears to be associated with a small intra-basin graben and localized gravel deposits. Numerical hydrogeologic models of the Animas horst and graben system, using temperature as a groundwater tracer, were constructed to test the hypothesis that groundwater originating from the crest and eastern slopes of the Black Range flows eastward, circulates to depths up to 6 km to be heated, and flows upward to discharge along the western boundary faults of the Animas horst, particularly the north-striking Berrenda normal fault. Although the steady-state hydrogeologic model does produce elevated temperatures along the Berrenda fault, the temperature-depth profiles predicted by the model do not exactly match the observed data. Instead, the shapes of the measured temperature-depth profiles from the Hillsboro area are consistent with more complicated shallow and local hydrothermal processes. The maximum estimated reservoir temperatures at Hillsboro using the chalcedony and quartz geothermometers are 137°C and 161°C, respectively. The maximum estimated reservoir temperatures near Lake Valley using the chalcedony and quartz geothermometers are 83°C and 112°C, respectively. Introduction Hydrothermal systems in extensional settings are commonly associated with gravity-driven, deep circulation and heating of fluid in a high heat flow thermal regime; the heated fluids are discharged in the vicinity of faults or at intersections of multiple generations of structures (Witcher, 1988). Discharge can also occur in places where an aquifer is juxtaposed against an aquitard, above and within fractured horst blocks where aquitards have been removed by erosion, or in fractured intrusions penetrating aquitards (hydrogeologic windows; Witcher, 1988). Many of the hydrothermal systems in the southern Rio Grande rift, including Socorro (Barroll and Reiter, 1990; Mailloux et al., 1999), Truth or Consequences (Person et al., 2013), Rincon (Witcher, 1991), and Radium Springs (Witcher, 2002), have been described in the context of this type of conceptual model. The regional heat flow in the Rio Grande rift, a north-striking extensional province that stretches from central Colorado to west Texas, is high (75 to 110 mW/m2; Reiter et al, 1975; 1978; Decker and Smithson, 1975). Westerly-directed extension caused thinning of the crust, passive upwelling of the mantle and associated advection of heat starting at 36 to 37 Ma. The rift narrowed and the basins deepened significantly during the peak of extensional deformation between 14 and 16 m.y. ago; tectonic activity has decreased significantly through the last 5 Ma (Chapin and Cather, 1994). This study evaluates the geothermal potential of an underexplored region within the Rio Grande rift along the Animas horst and east-dipping half-graben, which separates the Palomas basin to the east from the Black Range to the west in south-central New Mexico (Figure 1). Structures created during three major tectonic episodes the NW-striking Rio Grande reverse fault that formed during Laramide deformation, the Emory caldera that collapsed during Mogollon-Datil volcanic field development, and northstriking, west-dipping normal faults that developed during rift extension intersect in this area (Figure 2). Thus, this area is structurally unique and is interesting from a geothermal prospecting point of view because this area shares several significant, permeability-enhancing structural elements that are also found at the high-temperature Lightning Dock geothermal resource in southwestern New Mexico (Witcher, 2008).
Assessment of geothermal reservoirs by analysis of gases in thermal waters
Gases were measured in fifty-one thermal wells and springs in New Mexico including sixteen wells in the Lightning Dock geothermal area. Correlation of gases with alkali geothermometry indicates CO/sub 2/ and He increase in abundance with reservoir temperature while N/sub 2/, Ar, NO, Kr and CH/sub 4/ show slight decreases. H/sub 2/S generally occurs in waters with reservoir temperatures >100/sup 0/C, however, H/sub 2/S is not present in all high temperature waters. Gas ratios, principally CO/sub 2//N/sub 2/, CO/sub 2//Ar, CO/sub 2//CH/sub 4/, He/N/sub 2/ and He/Ar correlate better with geothermometry than abundances of individual gases. This study indicates the gas data which could be used to identify waters from reservoirs of T > 100/sup 0/C are CO/sub 2/, CO/sub 2//N/sub 2/, CO/sub 2//Ar, CO/sub 2//CH/sub 4/, He/N/sub 2/ and He/Ar in well waters as well as CO/sub 2/, CO/sub 2//CH/sub 4/, CO/sub 2//N/sub 2/, and CO/sub 2//Ar in spring waters. A geothermometer based on CO/sub 2/-CH/sub 4/ ratio is proposed. Indications are that the CO/sub 2//CH/sub 4/ geothermometer works in temperature ranges of 50 to 300/sup 0/ and nearly as well as the other chemical geothermometer. Gases and gas ratios which correlate positively with reservoir temperature show a regularmore » zonation pattern around the hot wells in the Lightning Dock geothermal area. This agrees with other studies which indicated that the hot wells are the point of upwelling thermal waters. It is concluded that gases in waters can be a valuable exploration tool to locate favorable sites for drilling in geothermal fields.« less