Title:

Surface Geochemistry in Exploration for a Buried Geothermal System, Socorro, New Mexico

Authors:

Gregory T. Hill, Lara Owens, David I Norman

Geo Location:

Socorro, New Mexico

Conference:

Stanford Geothermal Workshop

Year:

2006

Session:

Geochemistry

Language:

English

Paper Number:

Hill

File Size:

663KB

View File:

Abstract:

Selective extraction soil geochemistry is being used, along with geological, geophysical, and hydrological methods, to define a potential buried geothermal system in Socorro, New Mexico. Geophysical studies, along with projection of mapped geology suggest that a buried geothermal reservoir may occur approximately 2000-3000 feet below the surface. Soil geochemical patterns at the surface indicate the presence of a zone of thermal water at depth, as well as the presence of several important fault zones that may act as fluid conduits.

Two geochemical methods, enzyme leach (EL) and terrasol leach (TS), are being utilized to selectively dissolve specific Mn and Fe oxide and oxyhydroxide coatings on soil grains, without dissolving the bulk of the mineral substrate. The leach solutions are analyzed by ICPMS and up to 60 elements are reported with detection limits in the low parts-per-billion and parts-per-trillion levels. By effectively lowering background for most elements to levels that are well below those achievable by conventional extractions, subtle halo anomalies and linear highs in the ppb or ppt range can be resolved. At these and higher levels of concentration, many weakly bound elements contained on and within mineral coatings produce distinctive geochemical responses can be used to effectively target subsurface geothermal waters. In many cases, the anomalies are too subtle to recognize through conventional extractions such as aqua regia or four acid digestions.

The mechanisms of formation of surface anomalies detectable through selective extraction geochemistry are controversial and complex. At least two major processes are involved in forming the surface distributions: (1) upward migration of elements from
the subsurface and (2) reconfiguration of elements already at the surface into forms that are soluble by the selective extraction being applied; in some circumstances, a small number of elements can be reconfigured into phases that are insoluble by some selective extractions. Each of these two components contributes varying amounts to the anomalous patterns depending on a large number of factors including depth of burial, climate, type of overburden, and fault and fracture density, to name a few. Electrochemical cells are key features that affect the element distributions. They are observable above buried geothermal systems, mineral deposits, and oil fields through measurements of pH, SP, conductivity, CO2/O2 fluxes, and trace and major element distributions. The advancement in understanding of these electrochemical cells enhances the ability to discover and explore geothermal systems.

An exploration well will test a high quality target developed by an integration of geological, hydrological, geophysical, and geochemical data.


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