Configuration of the Geothermal Prospects in the Leyte Geothermal Reservation (Philippines) and Implications of a Volcano-Tectonic Framework in Exploration

Abstract

The island of Leyte in The Philippines hosts the largest developed liquid-dominated geothermal system in the world. The Leyte geothermal production field is located within the Leyte geothermal reservation along with several other geothermal prospects that are adjacent to areas with Neogene to Pleistocene period of volcanism (Bayrante & Palma, 1987). This field is located in the northern flank of the eroded Ancestral Mount Bao (AMB) volcano (Lagmay et al., 2003). The other prospects southeast of LGPF are Alto Peak, Lobi, Mahagnao and Bato Lunas, all located within the traverse of the Philippine Fault System.
Several exploration well drilling in Alto Peak, Lobi, and Mahagnao have proven the existence of high temperature volcanic systems with different physical and chemical reservoir characteristics. Alto Peak hosts an immature geothermal system related to a young volcanic center. Lobi and Mahagnao, on the other hand, have encountered high temperature systems but the reservoir is within the basement ultramafic complex that is inherently impermeable. These geothermal prospects have yet to fully develop for power generation.
Recent analysis of volcano-tectonic structures and analog sand cone experiments indicate that the existing producing wells in LGPF straddle the crypto-Philippine fault, a master fault that induced fracture-controlled permeability within the geothermal field. The structural model suggests that the southern flank of the AMB directly above the southern extension of the master Philippine Fault is an ideal geothermal target (Lagmay et al., 2003). The drilling of a deep well in Lobi in 2003 did not reach this postulated extensional area so the above structural model has not been confirmed. However, considering these latest structural analyses, the area south of LGPF within the concave side of the identified sigmoid structure may warrant further detailed geoscientific studies for geothermal exploration.