Title:

Exploration and Targeting of the Socorro, New Mexico Direct Use Geothermal Exploration Well, a Gred III Project

Authors:

Richard M. Baars, Lara Owens, Harold Tobin, David Norman, William Cumming, Greg Hill

Geo Location:

Socorro, New Mexico

Conference:

Stanford Geothermal Workshop

Year:

2006

Session:

Field Studies

Language:

English

Paper Number:

Baars

File Size:

601KB

View File:

Abstract:

The Socorro Peak uplift in central New Mexico is the center of a local zone of high heat flow indicated by shallow thermal gradient wells that encountered heat flow as high as 490 mW/m2 and measured temperatures of 43?C at 60 m depth. Aqueous geochemistry and mixing relationships in warm springs (32?C) suggest that high Cl fluids at depth have temperatures of over 92?C. A variety of geochemical and geophysical studies, particularly magnetotelluric soundings and soil geochemistry profiling, have identified a drilling location in the Rio Grande basin about 1300 m east of Socorro Peak to target a >60?C geothermal reservoir at <1000 m depth. The exploration hole will be drilled in 2006 to determine if geothermal reservoir with sufficient temperature and capacity to provide direct use heating for the New Mexico Tech campus.

High-resolution magnetotelluric traverses, conducted at a 100 m station spacing, were employed to characterize the distribution of resistivity across the range bounding fault on the east side of the Socorro Peak uplift. One and two dimensional inversions of the data show a steeply dipping range-bounding fault juxtaposing resistive footwall Precambrian and Paleozoic rocks and Tertiary volcaniclastics against conductive hanging wall fanglomerates. Within the hangingwall block, the MT profile data identify a 400 m thick aquitard that separates shallow aquifers from deeper target thermal waters.

Selective extraction geochemistry and pH analyses were performed on soil samples collected at 50-100 m intervals along transects paralleling the geophysical surveys in the geothermal resource area. Patterns of volatile trace elements, including V, As, Br, Se and Mo, were typical of those observed over the top of epithermal mineral deposits just east of the area with highest heatflow near Socorro Peak. Rare earth elements exhibit apical anomalies paralleling the major projected range-bounding fault. Soil pH analyses indicate H+ accumulation within the range front (pH 5.5-6.5) compared to background levels (~9.3) observed with the basin, supports a strong presence of geothermal fluids within the mountain block due to secondary permeability. pH signatures correspond well with certain selective extraction detected elements also derived from oxidation cell geochemistry, namely ore-forming metals such as Zn and Cu. These geochemical surveys indicates a focused thermal source at approximately 500-1000 m depth to the east of the range bounding fault with fluid leakage along a highly-fractured range-front fault system.

We chose the drilling target based on a synthesis of thermal gradient data, the geophysical and geochemical information, geologic structure, hydrologic models, and economic factors. The slimhole well will be on the eastern side of the Socorro Canyon fault near the area of highest measured heat flow, and near the intersection of the Socorro caldera fracture system, a transverse shear zone, and the Rio Grande rift bounding fault, and close to the New Mexico Tech campus. Fluid leakage up or across the range front fault system is a likely host for deep >60?C thermal waters. The borehole will penetrate both the faulted structural conduit as well as permeable sediments below the aquitard within the hanging wall block to evaluate multiple hypotheses for the source of the heat flow anomaly.
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