New Mexico Geological Society Annual Spring Meeting — Abstracts

When geothermal fluids rise they often boil giving off their dissolved gases especially carbon dioxide which is the predominant vapor phase 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 more easily identified 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. The geothermal waters are used to heat greenhouses used to grow long-stem roses and raise fish fingerlings. There is also interest in exploiting the water as a renewable source of energy for the region.

Geothermal waters rise convectively, then commonly flow laterally near surface. The plume of thermal water resembles mushroom cloud. Faults in the LDKGRA are covered by hundreds of meters of alluvium, and have been difficult to locate by geophysical methods. Even when faults are located, it is difficult to identify which ones carry geothermal fluids. Over two hundred soil CO₂ flux measurements at 50 to 100 meter spacing were made near the main surface heat anomaly of the LDKGRA to develop and test the soil CO₂ technique as method of geothermal exploration. Measurements were made by the accumulation method using an aluminum box with volume of 0.01 m³ which was placed on the ground well away from any vegetation. The fluxes of CO₂ into the box was measured over 10 minutes and showed a range from low value of 0.01 ppm.s^-1 to a maximum of 0.51 ppm.s^-1. The sites of the highest CO₂ flux are believed to mark buried faults trending NE-SW. These faults may carry the geothermal fluids from a source in the SW to the main heat anomaly. Previous well water analyses show that the shallow ground waters trending SW of the discovery well have anomalous CO₂. Soil flux measurements were repeatable over several days time. Future CO₂ flux measurements will be used to refine the location of these faults.