THERMAL AREAS OF THE PHILIPPINES Ruel T. Malapitan and Alicia N. Reyes Geothermal Division, Energy Resource Development Bureau Department of Energy, Energy Center, Merritt Road Fort Bonifacio, Metro Manila Philippines Key Words - thermal areas, volcanic belts, structures, occurrences, reserves and resources 2. GENERAL GEOLOGY ABSTRACT Rocks found in the island are typical of an island arc. The disposition of the rock types are generally in accordance to the Through the years of Philippine experience in geothermal trend of the trenches responsible for the formation of the exploration, development and production, prospectors and island arc. The lithology consists of intrusive rocks (including major players in steamfield development have defined three ophiolite complexes) and their corresponding extrusive (3) major geological environment contributing to the equivalents, metamorphic and sedimentary units. development of geothermal resources. These are as follows: a) along volcanic belts; b) along major structures and other The intrusive rocks are divided into two groups: the granitic- structures, and c) along plutonic occurrences. The most dioritic and the mafic-ultramafic suites (BMG, 1982). The important among these three environments is along volcanic granitic-dioritic intrusives are predominantly diorite and are belts for high-temperature producing and potentially associated with metallic mineralization and geothermal exploitable geothermal fields are found near Quaternary resources. The age ranges from Permian to Tertiary volcanoes. (Miocene). The mafic-ultramafic suites define ophiolite complexes occurring as ridges near trenches and troughs Using available data, geothermal reserve of the country is (maybe as obducted oceanic slabs consequent to the estimated through volumetric method at 4537MWe. subduction of the oceanic plates i.e. Philippine, Manila, Cotabato and Sulu Sea). 1. INTRODUCTION The composition of the extrusive igneous rock units varies from basaltic to dacitic. The age ranges from Carboniferous to The Philippine has consistently been an important country in Quaternary. The plotted locations of Pliocene to Recent the world geothermal arena for the past twenty (20) years volcanics outline nine (9) volcanic belts (BMG, 1982) that considering the high installed geothermal capacity in the parallel the trenches believed responsible for the volcanism. country. Under the Geothermal Service Contract (GSC) system there are nine (9) high-temperature geothermal fields Metamorphic rock units found to occur in the country may be under various stages of development and production (Figure categorized as groups, which are related to structures, and to 1). Six (6) of these fields are under production: Tongonan, igneous intrusions. The metamorphics are dominated by the Bacman, Palinpinon, and Mindanao are operated by Philippine greenschist facies. Eclogites, contact and cataclastic National Oil Company-Energy Development Corporation metamorphic units, however, are also found. Their (PNOC-EDC) while Tiwi and MakBan are operated by distribution shows close affinity to major structures such as Philippine Geothermal Inc. (PGI) under an interim agreement faults and trenches. The contact metamorphic units consist of with National Power Corporation (NPC) pending sale of the marbles, skarns and quartzites are localized along the fields (Table 1). Northern Negros, Mt. Labo and Mt. Cabalian peripheries of plutons. Cataclastic units consist of cataclasites geothermal service contract areas are still under development and mylonites which are localized within fault zones. The by PNOC-EDC. These three (3) fields are scheduled to greenschists are disposed adjacent to the trenches and may produce in the next ten (10) years. represent the base of obducted oceanic slabs in the form of ophiolite complexes. Previous workers such as the Philippine Institute of Volcanology and Seismology (PHIVOLCS) (Aguila, et.al, Sedimentary rock sequences fill up depositional basins and 1987), PNOC-Energy Development Corporation (PNOC- valleys consequent to the tectonic and erosional processes EDC) and the then Bureau of Energy Development acting in the island. (BED)(ElectroConsult, 1979) have identified several prospects including the current service contract areas. The Department of Energy (DOE) currently re-assessed the country’s 3. STRUCTURAL FRAMEWORK OF THE geothermal resources and identified several other new ARCHIPELAGO prospects in the course of the inventory. The shape of the Philippine archipelago is controlled by the In this paper, the authors aim to supplement the information subduction zones bounding the island’s extremities (Figure 2). generated through the years of geothermal activities on the The northern trend of the Northern Luzon mainland is aspect of geological environment, new area for exploration influenced by the orientation of the East Luzon Trench in the and current geothermal resources estimate. east and the Manila Trench in the west. The northwest orientation of the islands south of Northern Luzon extending to Mindanao is very well defined by the attitude of the 1395 Malapitan, R.T. and Reyes, A.N. Philippine Trench on the east. The skews on the western side in Philippine geothermal areas which generally provide the of the Visayas and Mindanao region are due to the differing major source of permeability in the wells (Reyes, 1990). trends of the Negros, Sulu and Cotabato trenches. The development of these trenches was attributed to the collision Through the years of Philippine geothermal energy of the Philippine archipelagic landmass with the Eurasian plate exploration, development and production has classified the in the west and the oceanic Philippine Sea Plate. The trenches present systems to be related to volcanism, plutonism and are located where the dense oceanic basins – the South China tectonism (Figure 2). In this report, the regional distribution of Sea, Sulu Sea and Celebes Sea basins in the west and the Philippine thermal areas have been adopted and modified from Philippine Sea in the east – are subducting underneath the Troncales (1979). archipelago as a consequence of inter-plate collisions. 4.1 Along Volcanic Belts Consequent to movements in these trenches, a major fracture running 1,300 km. along the length of the archipelago is These refer to regions of active volcanism where geothermal developed as a response to the oblique compressional forces of activities are dominantly confined along foothills or the base of the subduction at the trenches (David, 1996). The fracture is a younger Quaternary volcanoes. Along these volcanic centers left-lateral transcurrent fault called the Philippine Fault. The there are deep tectonic fissures responsible for the trapping of same mechanism holds for the development of the Mindanao magma reservoir and ascent of high-temperature magma fluid. fault in response to the compressive forces of subduction along the Cotabato Trench. Geothermal manifestations more commonly occur in andesite- dacite volcanic terrains compared to basaltic ones because a It is interesting to note that thrust faults mapped in the country higher rate of heat flow into the country rocks is attained with are situated mostly along the archipelago’s western and eastern increased viscosity of the intruding magma. Also because extremities. The thrust fault locations coincide very well with basalts rarely make shallow magma chambers. Moreover, the the occurrences of ultramafic/ophiolite sequences which are progressive concentration of volatiles in the liquid phase of evidences of obducted oceanic slabs in a collision/subduction andesite-dacite magma at its outer shell could cause an zone. Therefore, these thrust faults and ophiolitic zones are explosive discharge of gases accompanied by successive also due to the above-discussed tectonism. episodes of rock fracturing necessary in the formation of secondary permeability. In effect, these fractures/faults provide good conduits for geothermal fluid ascent to the surface facilitating formation of 4.1.1 Santa Ana Volcanic Belt - Luzon Central Cordillera several hot springs in the country, the heat emanating from the Belt - Central Luzon Belt - Cuyo Belt (Vasquez, N.C. different volcanic centers and young plutonic occurrences et.al. 1997) spread widely in the country. The Manila Trench located west of Luzon with several Consistent with the structural trend, fold patterns also follow associated onshore and offshore volcanic belts bound this belt. the orientation of the compressional forces brought about by The Santa Ana Belt originated in Batanes Group of Islands the plate collisions along the trenches. The fold pattern is terminating at the northernmost tip of the Central Cordillera. dominantly trending N-S in Northern Luzon and Southern From the central northern Luzon, the belt passes through the Mindanao, NW-SE in Southern Luzon to Northern Visayas, central portion of the Northern Luzon Cordillera (Luzon and NE-SW in Central Visayas to Western Mindanao. Central Cordillera), and swings westerly traversing western Luzon Island and then abruptly terminated by the Tablas Lineament in the south to include Marinduque Island (Luzon 4. Distribution of geothermal resources Central Belt). The Philippine archipelago is a complex assemblage of island This belt consists of a heavy concentration of active and arcs, which has been accreting between the two opposing inactive volcanoes hosting geothermal resources. In the north major tectonic plates - the Eurasian and the Philippine Sea