New Mexico Geological Society Annual Spring Meeting — Abstracts

When geothermal fluids rise, they commonly boil giving off their dissolved gases, especially carbon dioxide, the predominant vapor phase (other than water vapor) in geothermal systems. The CO₂ is likely to rise along the same faults that act as pathways for the rise of the geothermal fluids. By measuring soil CO₂ flux, it is possible to detect the CO₂ plume associated with a fault and is therefore useful in locating faults that may be buried. By mapping out the location of faults, drill targets are easier to identify and can save money when exploring for geothermal fluid reservoirs. The Lightning Dock Known Geothermal Resource Area (LDKGRA) is located southwest of Lordsburg, New Mexico and has no known surface expression. Shallow near boiling geothermal waters are used to heat greenhouses and raise fish fingerlings. Silica and alkali geothermometers indicate a shallow reservoir temperature of about 165^o C. Drilling to find this resource is difficult because faults in the LDKGRA are covered by hundreds of meters of alluvium, and are difficult to locate by geophysical methods. Over four hundred fifty soil CO₂ flux measurements at 50 to 100 meter East-West spacing were made near the main surface heat anomaly of the LDKGRA to develop and test the soil CO₂ technique as a method of geothermal exploration. Measurements were made by the accumulation chamber method using a LI-COR CO₂ analyzer and an aluminum box with volume of 0.01 m³ placed on the ground away from vegetation. The fluxes of CO₂ into the box were measured over at least 200 seconds and showed a range from a low value of 0 g m^-2 day^-1 to a high value of 12 g m^-2 day^-1. The sites of the highest CO2 flux are believed to mark buried faults. One such probable fault lies on line with a fault hypothesized from geological data by previous workers. Previous well water analyses show that the shallow ground waters trending southwest of the discovery well have anomalous CO₂. Soil CO₂ flux measurements varied at control points within acceptable ranges.