Resource Potential of the Southern Leyte Geothermal Prospect, Philippines: A Geologic Evaluation

Abstract

The Southern Leyte Geothermal Prospect is located at the southeastern tip of Leyte island in the Philippines. Mt. Cabalian and Mt. Cantoyocdoc comprise the Quaternary volcanoes in the area.
Surface exploration studies indicated a viable geothermal resource possibly centered beneath Mt. Cabalian. In 1997, the first exploration well SL-1D was drilled on the eastern flanks of Mt. Cabalian. However, it was non-productive due to poor permeability, and measured low temperatures of only ~240?C at -1870 m MSL. To better define the configuration of the deep geothermal reservoir, a more detailed magnetotelluric (MT) survey was conducted in 2000. In contrast to the pre-drilling model, results of MT surveys point to an upflow centered near the western flanks of Mt. Cantoyocdoc. Hence, in 2003, the second exploration well SL-2D was drilled towards this direction. Well SL-2D encountered better permeability and higher temperatures in comparison to the non-productive well SL-1D. Based on fluid inclusion and alteration mineralogy (i.e., garnet, actinolite, euhedral epidote), well SL-2D will produce hot neutral brine with temperature of ~280?C at -1840 m MSL.
Results of exploration drilling and MT surveys showed that an active geothermal system exists in Southern Leyte whose hottest part is likely centered near the western flanks of Mt. Cantoyocdoc. Transport of fluids across the reservoir is achieved through permeable faults and stratigraphic contacts. Major fluid outflows gave rise to surface thermal manifestations mapped east and west of the field. Based on MT anomaly, the size of the potential geothermal resource in Southern Leyte is ~7.5 to 12 km2. To target center of this resource, the next wells should be drilled from a new pad located north of well SL-2D. From the existing pad of well SL-2D, another well may be drilled towards the northwest to delineate southern boundary of the geothermal resource.