Makiling-Banahaw
The Bulalo (also called Mak-Ban) geothermal reservoir is located about 70 km southeast of Manila on the island of Luzon. The geothermal field lies at the foot of Mt. Makiling and within the Macolod corridor, a 40-km wide “rift-like feature” that crosses the Luzon arc and separates the northern “Bataan” and southern “Mindoro” segments of the volcanic arc. Recent and active volcanism within the Macolod corridor has formed young (<1- 2 Million years old) silicic calderas (e.g., Taal and Laguna de Bay), andesitic to dacitic stratocone and dome complexes (<2-0.02 Million years old), and maar and cinder cone fields (<0.02 Million years old).
The structural setting of the Macolod corridor is complex, but the earth’s crust in this area is “stretching” or extending due to the influence of regional strike-slip faults at its northern and southern margins. Within the Macolod corridor, the major features are NE-trending normal faults that have produced horst and graben structures and NW-trending left-lateral strike-slip faults that may bound them. There is also some evidence for older E-W directed structures. Alignment of scoria cones and maars in some areas, and andesite to dacite domes in others, also indicates that recent magmas most commonly reached the surface through NE-directed extensional structures.
The hydrothermal system developed at Bulalo is associated with two dacitic domes (Mt. Bulalo and Mt. Olila) present on the SE flank of Mt. Makiling. Mt. Makiling is a larger Quaternary volcanic complex consisting of overlapping andesitic to dacitic stratocones and domes. Bulalo and Olila domes have been dated by the 40Ar/39Ar method at <20 thousand years old, providing evidence for sustained silicic magma intrusion into the upper crust, which is the ultimate heat source of the Bulalo hydrothermal system. A NE-trending line connecting these domes has the same trend as the dominant fault and fracture directions revealed by surface and subsurface geology. The spatial association of the highly permeable and productive portion of the Bulalo reservoir with the Mt. Bulalo dacite dome suggests that permeability is related to intersections of faults with its deep conduit system. The resistivity anomaly roughly coincides with the current production area, but extends about 6 km to the north along the eastern flank of Mt. Makiling.
