Petrological Identification of Multiple Heat Sources in the Bacon-Manito Geothermal System, the Philippines

Abstract

The positions of upflow zones and heat sources in the Bacon-Manito geothermal system do not necessarily coincide. Water chemistry and measured well temperatures suggest a single upflow zone, centred near wells OP-3D and OP-4D in the Botong-Pangas area. However, surface geology, subsurface alteration patterns, detailed fluid-inclusion studies in well cores and cuttings, and measured pressure gradients in 40 wells point to the existence of at least two major heat sources: Botong-Pangas, related to the major Palayangbayan upflow, and Tanawon- Cawayan south of Palayangbayan. Another localized heat source may be located 10 north of Palayangbayan, in the Manito Lowlands. Alteration and fluid-inclusion patterns suggest high temperatures in the south, where the presence of a kaipohan and young craters ka) already point to the possible existence of additional heat sources. Directional wells drilled towards these intersected temperatures of 276?C at a depth of 1400 m. Thus a heat source may remain hidden from fluid geochemistry and may reveal itself only in detailed petrological studies.