Assessment of Fault Permeability Using Soil Gases in Palinpinon Geothermal Field, Philippines

Abstract

Soil gas surveying is a potentially useful method in identifying permeable faults in geothermal fields. Mapping the surface concentrations and flux of soil gases, particularly radon (222Rn), thoron (220Rn), and CO2 can delineate the structures that serve as reservoir fluid pathways which may be tapped by production wells. In this study, a grid survey was conducted in the South Sogongon and Nasuji sectors of the Palinpinon Geothermal Field to evaluate the permeability of the faults in these areas. Grid measurements were obtained using alpha spectroscopic detectors for the radioactive gases and accumulation chamber with IR detector for soil CO2 flux. The relationship of soil gases with fault permeability, as reported from well drilling and well-testing, was determined using logistic regression. The logistic regression model identified radon and thoron as the best predictors of fault permeability. Using this model, the high soil gas anomalies corresponding to good permeability identified the NW-SE and NW-SE faults as good candidates for drilling geothermal wells.