Proceedings World Geothermal Congress 2005 Antalya, Turkey, 24-29 April 2005 Resource Potential of the Southern Leyte Geothermal Prospect, Philippines: A Geologic Evaluation Josephine B. Rosell and Maribel C. Zaide-Delfin PNOC Energy Development Corporation, Energy Center, Merritt Road, Fort Bonifacio, Taguig, Metro Manila, Philippines [email protected] Keywords: Southern Leyte, geothermal resource, geologic BI LI RAN I SLAND MT. NALIWATAN model MT. SUIRO Calubian Biliran GTGF Tacloban ABSTRACT JANAGDAN ALTO PEAK Ormoc MT. LOBI L e L E Y T E y The Southern Leyte Geothermal Prospect is located at the MT. MAHAGNAO te Camotes Sea MT. BATO G u P l H f southeastern tip of Leyte island in the Philippines. Mt. I Ba y b a y L . F A U L Cabalian and Mt. Cantoyocdoc comprise the Quaternary T MT. CABALIAN volcanoes in the area. Maasin 050 KI LO METERS Pa na o n Is. CAB3 D-6 Surface exploration studies indicated a viable geothermal C 10°25' F a a t u m Le yt e Gu l f H resource possibly centered beneath Mt. Cabalian. In 1997, l o Hinunangan t in Z n o . - R d n B y the first exploration well SL-1D was drilled on the eastern i a Bu g h o a e s s- F a a a g Liptong y D An-An flanks of Mt. Cabalian. However, it was non-productive u a lt n Sa b a n g S due to poor permeability, and measured low temperatures o Z P Mt. Catmon . o Hinundayank a H g r n n im of only ~240°C at –1870 m MSL. To better define the o e Mt. Tamar C ia b n d u a la Am b ao n n na ns configuration of the deep geothermal reservoir, a more g Pa a o Pa n a s B R Mahayag detailed magnetotelluric (MT) survey was conducted in . Sa g b o k a R Mt. Cantoyocdoc y y 2000. In contrast to the pre-drilling model, results of MT a R. Cabac g la n a Libagon . ahMa halo a F R Nava M surveys point to an upflow centered near the western flanks F a a St . Be rn a rd s b Anahawan u a lo n S c Pa n i a n u of Mt. Cantoyocdoc. Hence, in 2003, the second l g t it o a H Magcasac M Amagusan exploration well SL-2D was drilled towards this direction. g Sa n J u a n Well SL-2D encountered better permeability and higher o Himbangan Agay-ay 10°15' Su a d Hindagan temperatures in comparison to the non-productive well SL- Magaupas C a 1D. Based on fluid inclusion and alteration mineralogy Le p a n t o b B Gac at a Na ilong li a a (i.e., garnet, actinolite, euhedral epidote), well SL-2D will n Ba produce hot neutral brine with temperature of ~280°C at – y Marangay y ' 5 1840 m MSL. 1 Him a ya ng a n ° 5 2 Tabugon 1 LEG END: Results of exploration drilling and MT surveys showed that 0 5 Lilo a n Vo lc a n i c Pe a k an active geothermal system exists in Southern Leyte whose KILO METERS Municipality hottest part is likely centered near the western flanks of Mt. Other Communities PANAON ISLAND Cantoyocdoc. Transport of fluids across the reservoir is achieved through permeable faults and stratigraphic contacts. Major fluid outflows gave rise to surface thermal Figure 1: Location map of Southern Leyte geothermal manifestations mapped east and west of the field. Based on prospect MT anomaly, the size of the potential geothermal resource in Southern Leyte is ~7.5 to 12 km2. To target center of This paper will discuss results of surface exploration and this resource, the next wells should be drilled from a new drilling activities conducted in the area. A geologic model pad located north of well SL-2D. From the existing pad of of the SLGP geothermal system will be presented based on well SL-2D, another well may be drilled towards the these exploration studies. northwest to delineate southern boundary of the geothermal resource. 2. EXPLORATION ACTIVITIES 1. INTRODUCTION 2.1 Surface Exploration The Southern Leyte Geothermal Prospect (SLGP) is located Initial surface exploration studies were done in Mt. at the southeastern tip of Leyte Island in the Philippines Cabalian area by Bureau of Energy Development (BED) (Fig. 1). Mts. Cabalian and Cantoyocdoc comprise the and Electroconsult of Italy (ELC) in 1979. An inventory of Quaternary stratovolcanoes in the area. These two thermal springs yielded an estimated reservoir temperature stratovolcanoes form part of the eastern Philippine volcanic of 221°C based on chemical geothermometry (BED-ELC, front (Datuin, 1982) which runs from Camarines Norte 1979). down to Davao del Norte, and is aligned sub-parallel to the PNOC-EDC started reconnaissance geoscientific surveys in Philippine Trench and the Philippine Fault. In Leyte island, SLGP in early 1980’s. Favorable results of these the 723-MWe Leyte Geothermal Production Field (LGPF) investigations led to a detailed geoscientific investigation lies along the northern portion of this volcanic front, while undertaken by PNOC-EDC in 1989. This included the SLGP is situated in the southernmost part (Fig. 1). geologic mapping (1:50,000 scale), geochemical sampling and analyses of thermal waters and gases, and 1 Rosell and Zaide-Delfin Schlumberger Resistivity Traversing (SRT) and Vertical postulated in the pre-drilling model (Rigor et al., 2001). Electrical Sounding (VES) surveys (PNOC-EDC, 1989). The proposed center of the resource also coincided with a high-gravity anomaly interpreted to be a large intrusive Results of these surveys indicated an upflow of hot, neutral- body beneath the two Quaternary volcanoes Cabalian and chloride fluids (minimum temperatures of 190-205°C) Cantoyocdoc (Catane and Apuada, 1998). associated with Late Pleistocene summit eruptions of Mt. Cabalian. Two low-resistivity anomalies defined the major The second exploration well SL-2D was drilled in 2003 to fluid outflows to the east emerging as Mahalo-Mainit test the revised MT model. It was spudded on Site C (Fig. thermal springs, and to the west towards Nava and Tabunan 4) on the western side of Mt. Cabalian, and targetted springs (Fig. 2). To test this model, two exploration wells towards the western flanks of Mt. Cantoyocdoc. The well were recommended, one targeting the postulated upflow reached a total vertical depth of 2362 m (Table 1). zone beneath Hugpa crater (Site A), and another intersecting major fluid outflow towards Mainit-Mahalo Well SL-2D encountered hotter temperatures and better sector (Site B). permeability in comparison to the first well SL-1D (Zaide- Delfin et al., 2003). Drilling losses and abundant drusy In preparation for exploration drilling scheduled in 1997, veins attest to the good permeability of faults intersected by PNOC-EDC conducted additional geoscientific surveys in SL-2D; while alteration mineralogy indicates ~280°C September 1996 starting with detailed microtectonic and neutral-pH fluids near bottom hole at -1840 m MSL. structural mapping with repeat geochemical and isotopic Results of well SL-2D therefore confirmed that the hot sampling. Regional gravity and magneto-telluric surveys region in SLGP lies near the western flanks of Mt. (MT) were also conducted to characterize the subsurface Cantoyocdoc, and not beneath Mt. Cabalian as previously geometry of the geothermal reservoir in Mt. Cabalian. postulated. These studies further strengthened the exploration model of an active geothermal system related to intrusive bodies 3. STRATIGRAPHY AND PERMEABILITY beneath Mts. Cabalian and Cantoyocdoc (Fig. 3). In addition to the two drilling sites, a third (Site C) was 3.1 Reservoir Rocks recommended to target the northeastern sector of the The two deep exploration wells in Southern Leyte were geothermal resource (Leynes et al., 1996). both spudded on Late Pleistocene eruptives of Mt. Cabalian which belong to the stratigraphic unit termed as Quaternary 2.2 Exploration Drilling Volcanics (QV) (Leynes et al., 1996). The QV is ~660 In September 1997, the first exploration well in Southern meters thick in well SL-1D, but reaches ~1085 meters in Leyte (SL-1D) was drilled from Site B on the eastern flanks well SL-2D (Fig. 6). It can be further subdivided into an of Mt. Cabalian (Fig. 4). Well SL-1D was deviated towards upper and a lower member (Table 2). The upper member is the northwest to intersect deep outflowing fluids towards composed of oxyhornblende clinopyroxene andesite lavas the eastern portion of the postulated geothermal resource. and tuff breccias. The lower member, on the other hand, Table 1 shows the basic well data of SL-1D which reached consists of dacitic to andesitic lavas and tuff breccias with a total vertical depth (VD) of 2398 m. minor clasts of basalts and sedimentary rocks. Separated by an unconformity, the QV is underlain by Table 1: Basic well data of SL-1D and SL-2D Middle Miocene limestones and clastic rocks belonging to the Tertiary Clastics (TC). Both wells were terminated Well SL-1D Well SL-2D within the TC that reached a thickness of ~1740 meters in Elevation 526 m 517 m SL-1D and ~1280 meters in SL-2D (Fig. 6). The TC is Pad Site B Site C made up of a thick sequence of fossiliferous limestone, Throw azimuth 337° 360° calcareous sandstone and breccia with lesser amounts of Kick-off point 214 m 842 m carbonaceous siltstone and claystone. Total vertical depth 2398 m 2362 m Total throw 1178 m 624 m Based on surface geologic mapping, the TC is expected to be underlain by a thinner sedimentary sequence of Early- Surface Casing 152 m 100 m Middle Miocene age termed as Tertiary Limestones (TL).