Geothermal Exploration Using Soil CO2 Flux Surveys in Arid Environments

Abstract

A 20 km2 soil CO2 flux survey was conducted at the San Jacinto-Tizate geothermal power project, Nicaragua. Conditions were dry and hot for the entire survey period and the results have negligible interference from biological gas flux. The survey showed a broad area of low CO2 flux surrounding the central production area, which probably represents the capping formation; the cap prevents gas flux from reaching the surface. An area of relatively high CO2 flux occurs NNW of the steamfield in close association with i) a magnetic high anomaly, previously interpreted to result from unaltered material, ii) MT resistivity high, and iii) watershed catchment boundaries. The area of high CO2 flux is hydrologically isolated from the central production area, and may lack a reservoir. This interpretation is supported by a recent reinjection well drilled into the area; early testing showed the well has massive permeability with no pressure connection to the production reservoir. Results suggest CO2 gas flux surveys are well suited to dry environments (e.g. Atacama Desert, Basin and Range), or in areas with a pronounced dry season (Central America, East Africa). Under these conditions, interference from biological activity is greatly reduced, and CO2 flux surveys can identify the clay cap as a zone of relatively low CO2 flux. This is a key goal of geothermal exploration for well targeting.