The Structural Analysis of Ancestral Mount Bao and its Implications to Geothermal Exploration in Northern Leyte, Philippines

AMFA Lagmay, AMP Tengonciang, HV Marcos, CS Pascua

National Institute of Geological Sciences, College of Science, Velasquez cor. CP Garcia Streets
University of the Philippines, Diliman, Quezon City 1101, Philippines

Abstract

Ancestral Mount Bao (AMB) is an eroded volcano located in the northwestern portion of Leyte Island, Philippines. It hosts the Tongonan Geothermal Field, which is the largest producing geothermal field in the Philippines with an installed capacity of approximately 700 MW. This field encompasses several major power plants that tap geothermal energy from the northern flank of AMB. To delineate possible exploration targets on other flanks within the 1200 km² area of the volcano, a structural model guide based on analog experiments was applied on AMB. This model provides spatial constraints on geothermal exploration to areas where more detailed investigations involving geological, geophysical and geochemical methods can be conducted. Comparisons between this model and the deformational features of AMB were used to interpret well data within the Tongonan Geothermal Field. The results of the study show that the existing producing wells in Tongonan straddle the crypto-Philippine Fault, a master fault with a trend of N34°W (azimuth 326°) inferred to traverse the base of AMB. Permeability and fluid circulation in the Tongonan Geothermal Field is fracture-controlled and consequently attributed to this master fault. This model suggests that the south-southeastern flank of AMB directly above the southern extension of the master Philippine Fault is an ideal geothermal target with potential for further exploration.