Proceedings World Geothermal Congress 2005 Antalya, Turkey, 24-29 April 2005 Controlled Source Magnetotelluric (CSMT) Survey of Malabuyoc Thermal Prospect, Malabuyoc/Alegria, Cebu, Philippines R.A.Del Rosario, Jr., M.S. Pastor and R.T. Malapitan Geothermal and Coal Division, Energy Resource Development Bureau, Department of Energy, Energy Center, Merritt Road, Ft. Bonifacio, Taguig, MM, Philippines [email protected], [email protected], [email protected] Keywords: Controlled Source Magnetotelluric, Malabuyoc, From Cebu City, the town of Malabuyoc is about 123 kms MRF or 4-hour drive via Car-Car-Dalaguete-Ginatilan Provincial Highway or about 3-hour via Car-Car Barili Road going ABSTRACT south to Ginatilan. From Malabuyoc proper, the thermal spring is located north-northwest in Brgy. Montaneza. It The Controlled Source Magnetotelluric (CSMT) survey could be reached via barangay road leading to Sitio Mainit, conducted at Malabuyoc geothermal prospect aims at where the thermal spring is situated. establishing the presence and extent of hot water to a medium depth of 300-500 meters for non-electrical applications of geothermal fluids. The resistivity anomalies in the prospect are largely controlled by geologic structure (faulted anticlinal structure). VISAYAN SEA P h The Malabuyoc system is categorized as a basement aquifer i l ip P p HI i beneath a sedimentary basin with the heated fluid probably Y n Tacloban City L A e I P N F P A Iloilo I a NE originating at the center of the basin east of the survey area. P u Basin l t Z The fluid is channeled along the Middle Diagonal and Iloilo City o n U e S Montañeza River Faults (MRF) and emerged along the Bacolod City B E E A C stretch of Montaneza River as warm seepages. LEYTE S Cebu City O R Visayan G E Basin N The best site to drill for a well that could extract possible L O H O hot/warm water for spa resort development is the area B bounded by steep resistivity gradient coincident with MRF. Tagbilaran Survey A E Area S Possible hot water is at permeable karst aquifer located at M O A in d N depth between the –100 to –300 m below sea level (bsl). a A n D a IN o M L in Butuan e a 1. INTRODUCTION m e n t The Controlled Source Magnetotelluric (CSMT) survey was Dipolog conducted over the Malabuyoc Geothermal Prospect from Cagayan de Oro May 16, 2002 - June 8, 2002 covering the towns of Malabuyoc and a portion of Alegria, in Cebu Province. The Figure 1: Tectonic Scenario of Cebu Province survey is the Department of Energy's (DOE's) commitment in the Health Spa Resort Development Project, an inter- 1.2 Regional Geologic and Tectonic Setting agency collaboration between the Department of Tourism The whole island of Cebu, together with parts of Negros and (DOT), Department of Energy (DOE) and Philippine Bohol Islands, lies within the Visayan Basin, a N-S trending Institute of Traditional and Alternative Health Care- structural basin consisting of horst and graben and several Department of Health (PITAHC-DOH). The project small-interconnected sub-basins (BED, 1985) (Fig. 1). It is objective is to promote identified geothermal areas for health bounded on the east by the Philippine Fault and on the west spa resort development while the survey is to establish the by the probable extension of Mindanao Lineament. Cebu presence and extent of hot water at a medium depth between Island is situated near the center of the Visayan Basin 300-500 meters for non-electrical applications. This forming a structural high separated from Negros and Bohol technique has been used successfully in the exploration of by downfaulted blocks. Structural trends are generally shallow groundwater resources in the United States (Zonge, northwesterly in the southern part and northerly in the 1992). northern part. 1.1 Location and Accessibility Forming the backbone of Southern Cebu is the Late The Malabuyoc Geothermal Prospect Area (MGPA) is Cretaceous to Paleocene Pandan Formation consisting of located in Cebu Island (Figure 1). Cebu Island is situated metamorphosed limestone, shale and conglomerate with between the islands of Negros in the west, Bohol and Leyte lenses of lava flows and coal (Barnes, 1956; Hashimoto and on the east, Siquijor on the south and Masbate on the north. Balce, 1977; Alcantara, 1980; BMG, 1981) (Fig. 2). Cebu City is the capital of the province located midway on Intruding Pandan Formation were several fine to medium the eastern coast of the island. It is accessible by air, land grained andesite sills and dykes (Barnes, 1956). The and sea. intrusions are generally concordant with Pandan but in places cut through the sedimentary rocks as dykes. Pandan Formation is unconformably overlain by the Argao Group 1 Del Rosario, Pastor and Malapitan. (Barnes, 1956) of Late Oligocene to Early Miocene age 1.3 Local Geology and Structures consisting of three members: Calagasan Formation, Bugtong The survey area is located along the western flank of an 18- Limestone and Linut-Od Formation. Alcantara (1980) added kilometer long and 3 kilometer wide NNE trending plunging a fourth member, the Mantalongon limestone. Calagasan asymmetrical anticline. (CPRS, 1974 and BED-ELC, 1979) Formation is composed of conglomerate, sandstone and (Fig. 3). This anticline is separated into two by the NW-SE shale with limestone and coal interbeds while Bugtong trending strike slip fault at the vicinity of Legaspi River Limestone is massive to thin bedded, white to light brown namely: the Alegria structure in the north and the and yellowish gray, fine to medium grained relatively dense Malabuyoc structure in the south. On the surface the crystalline to shaly and sandy limestone deposited in Malabuyoc Anticline a length of 6 kilometer and a width of shallow warm seas during Early Miocene. The two are 3 kilometer trends NNE. A diagonal fault runs across the overlain by the Linut-Od Formation consisting of Lower middle part of this anticline with the northern block forming Miocene coal-bearing sequence of clastic rocks similar in an upthrown structure favorable for trapping hydrocarbon lithology to Calagasan. In turn, the Linu-Od is overlain by (CPRS, 1976). The northern block was offset left-laterally Mantalongon Limestone, a Miogypsina and lepidocyclina by another WNW trending fault called by the present study bearing limestone blocks in Argao-Dalaguete region. Argao as the Montañeza River Fault (MRF) (BED-ELC, 1979). Group is equivalent to Naga Group of Santos-Ynigo and MRF pass through the Montañeza River and was manifested Cebu and Malubog Formations of Corby (1951) and BED in the field by abrupt topographic break and alignment of (1985) in Central and Northern Cebu. Widespread in the thermal spring along the Montañeza River (Malapitan and area is the Barili Formation (MGB, 1981) of Late Miocene Del Rosario, 2002). South of the thermal area, a North- to Pliocene consisting of lower limestone member and upper South normal fault was noted running parallel with the marl member. The limestone is light brown, hard, corraline, southern extension of the Malabuyoc anticline. This fault locally porous or sandy, richly fossiliferous while the marl is was intersected by Malabuyoc-1 well (CPR, 1976). poorly bedded, generally brown, slightly sandy, fossilferous with thin limestone interbed. Together with Mt. Uling 123*20' 123*21' Limestone, Toledo and Carcar Formation, Barili formation 9*43' LEGEND was initially included in the Balamban group of Barnes Oil/Gas Well Warm spring Lumpan L eg (1956) and Santos-Ynigo (1967). Occupying the lower as pi Barili Marl Riv Legaspi er flanks of the ridges and covering almost all of the coastal Barili Limestone Carcar Formation areas is the Pleistocene Carcar Formation. It is a shallow e U D n Quaternary Alluvium Lanayai l c marine, porous, coralline, massive to poorly bedded Geologic Contact Bl i t C n Fold A limestone deposit that is rich in mollusk, coral stems, algae Tumandok Samuyao c o Fault (dashed when inferred) BM y and foraminifera. u Abungon b CSMT Station a U l D Tape Siag a M BM CF Daanbantayan er iv Bantayan R Qal Montaneza M a o z Island nt Canlaga ne CF an ta e on za M CF Ri ver Mandayang Mainit Fa ult Qal Lombot aul NBM al F Qal gon Dia Lanao Ginalayugan dle Mid U Candalag Bm D e BM n i l c i Tamia t CF Bl n A Kantipase D U Cangulay c t o l y Qal u u a b F a l h BM a K t u CF CV MF M TF o MF Armenia S - Sorsogon h t Qal Danao r o CF N CF MF CV X Compostela MF Qal K CF CAMOTES SEA MALABUYOC CV BM X Qal S ali Mactan Island Qal ring CF MF Riv t K er i Cebu City CF a Parali r Qal t S Naga Bl n BM o CF Mindanaw n a T Carcar LEGEND Qal CF Bl K Pandan Formation CV Cansi Volcanics Mahanlog Bm X Diorite Labrador Bl it 9*38' a r t MF Malubog Formation S Toledo Formation Bl Argao TF l o Bl Barili Limestone h Mts o Bm Barili Marl B Dalaguete CF Carcar Formation Bl Qal Alluvium Geologic Contact Figure 3: Geology of Malabuyoc Thermal Prospect Bm CF Fault The prospect and its surrounding environs are basically Fold underlain by the Mio-Pliocene marine sediments consisting CF of Barili Formation and Quaternary alluvium (CPR, 1976; MGB, 1985; Malapitan and Del Rosario, 2002) (Fig. 3). Barili Formation is the most dominant rock type in the Figure 2: General Geology of Cebu Province survey area while the Alluvium consisting of detrital sediments, flood plain debris, coral reefs and gravel deposits abounds the coastlines, riverbanks and lowland areas.