Evaluation of structural permeability in Mahanagdong geothermal field, Philippines using soil gases (220Rn, 222Rn, CO2 flux)

Abstract

The use of soil gases as a geologic tracer has long been elaborately demonstrated in several volcanic, seismic, and geothermal studies. Carbon dioxide (CO2) is an abundant geothermal gas while radon (222Rn) and thoron (220Rn) are radioactive gases with a half-life of 3.8 days and 55 seconds, respectively. In theory, anomalous radon, thoron, and CO2 gas emissions can be indicative of rapid vertical transport through permeable structures that serve as pathways of degassing. These gas anomalies can indicate structures, reservoir changes and processes. The application of soil gas mapping to permeability exploration was tested in production, reinjection, and undeveloped areas in Mahanagdong Geothermal Field, Leyte, Philippines. Most soil gas anomalies were observed at structural intersections and segments that were reported by drilling to exhibit good permeability. The concurrence of soil gas anomalies at fault traces indicated the presence of highly permeable structures that can be targeted for drilling exploration or make-up/replacement wells.