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GEOTHERMAL GEOLOGY AND REVIEW OF EXPLORATION BILIRAN ISLAND

.V . LAWLESS* R. C.

*Kingston, Reynolds, Thom and Allardice Ltd. National Oil Company

ABSTRACT GEOLOGIC SETTING Biliran Island has been the subject of a The tectonic history of Bil iran is dominated geoscientific exploration programme, and the most by its position on the Philippine Fault Zone, promising area is currently being drilled. The which as been the locus of active volcanism and first exploratory well has confirmed high tempera- tectonism since the Miocene (Alcaraz 1947). The tures at depth, although it did not encounter sig- Philippine Fault is mapped as passing to the west nificant permeability. The major thermal features of Biliran, along the coast of the Calubian of Biliran are located close to a fault, and their Peninsula of Leyte. The developed geothermal chemistry (acid-sulphate-chloride) indicates an in- field of Tongonan, and other prospective areas at put of magmatic volatiles. It is conjectured that Gaas, Burauen and Anahawan, also lie along this magmatic volatiles are rising on the fault, but zone. that away from this zone a more typical convective hydrothermal system exists. There are similarities STRATIGRAPHY to the Krafla Geothermal Field in Iceland. Apart from local ised thin deposits of reef INTRODUCTION 1 imestones and marine sediments, the surface geology of Biliran is wholly made up of volcanic Biliran Island is located imnediately to the rocks and associated volcaniclastic deposits north of the Island of Leyte, Republic of the (Table drillhole has revealed, Phil ippines. Following the successful exploration however, underlying non-volcanic formations. The and development of the Tongonan geothermal field stratigraphically lowest unit, only encountered on Leyte, Biliran has been the subject of a con- in the last of consists of sediments tinui ng geosci en ti fi c expl oration programne si and volcanic breccia, intensely sheared and then 1979, culminating in a 3-well deep drilling explor- subjected to low-grade metamorphism (upper zeolite ation programme which commenced March 1982. Explor- facies). It may be correlative to metamorphosed ation and drilling has been carried out by the sediments and vol canics of Cretaceous age, Philippine National Oil Company, with geoscientific exposed in Cebu and Samar (Bureau of Mines 1962). and drilling consultative assistance provided by Overlying this is a sequence of fossiliferous Kingston, Reynolds, Thom and Allardice Ltd., as clastic and crystal1 ine 1 imestones with inter- part of the New Zealand Government Energy calated pyritic calcareous shales, Paleontologi- ration Programme. cal dating gives an age of Late Miocene to Basal Pliocene and an outer neritic environment. Depo- Geoscientific techniques suggest the exist- sitional basins existed both to the east (Samar ence of a substantial geothermal reservoir. A Basin) and west (Visayan Basin) of Biliran during commercial resource has not yet been proven by Miocene-Pl iocene times and similar sediments to drilling, but prel iminary indications are those encountered in are now exposed on able. Of particular interest is the fact that the the adjacent islands of Leyte and Samar. more prominent surface manifestations are predo- minantly solfataric, and yet teration mineralogy A thick sequence of volcaniclastic sediments from the first drillhole and some of the surface (the Pulang Yuta Formation) occurs above the samples points to a neutral-chloride fluid as the limestones in BN-1 It is tentatively correlated alteration medium (A.G. Reyes? pers. comm.). If with similar material (the Panlahuban Formation) del drilling suceeds in proving the exposed in river valleys on the western side of existence of an exploitable neutral-chloride re- Biliran, but differs in the degree of alteration. servoir, this raises the exploration potential of other solfataric areas, The younger volcanic rocks of Biliran have been divided into a number of units based mainly on geomorphic grounds, but with some petrologic distinctions (Espiritu 1980). The volcanic format- ions of Biliran are predominantly andesitic, with minor of are therefore similar to the volcanic rocks of main- 1 and Leyte. 162

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Table 1. Stratigraphy of Biliran Island STRUCTURES The present-day topography of Biliran is made up of a series of coalescing volcanic piles and and to associated volcaniclastic debris. The degree of erosion of the volcanoes varies, indicating a and not to range of ages, from the deeply dissected composite thick. volcanic massif of Kalambis Ridge to the youthful in dacite dome of Mt. Sayao, reputed by local andesite inhabitants to have been historically active (1938). 5 The general trend of the volcanic centres of Bil iran is NW, el to the Phil ippine Fault Zone (and the Philippine Trench to the east). This volcanic ignment can be traced northward into Luzon, and southwards into Leyte. Similar NW elongation of the 1ow-resi ivity-anomal y between Vulcan and Kalambis (Figure 2) i s noteworthy and may be indirectly caused by a some deep-seated structure - perhaps a buried branch of the Phil ippine Fault.

A major fault (the Vulcan Fault) running NE across Biliran can be recognised, based on topo- graphic lineation and field evidence such as intensely sheared rocks, local ised hydrothermal teration, and ignment of thermal features. Other smaller faults have also been mapped. The most intense thermal activity is located close to the line of the Vulcan Fault, and it is likely that it is acting as a channel for the upflow of hydrothermal fluid. (Figure 1). Altered ground in the Kalambis and areas i s contained within two large and 3 roughly c i rcul a r depressions, which are interpreted as volcano-tectonic collapse structures. They a r e not the focus of present-day hydrothermal activity, though weakly-flowing, dilute acid-sul phate springs occur in their vicinities. Figure 1. Geology of Biliran Island

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GEOCHEMISTRY only from Vulcan and Tenego but also at Libtong and Vulcan Gamay. Additional evidence lies in Analysis of surface thermal waters and gases high ratios (Cope 1982; Glover 1981). divides the Biliran thermal features into 3 geo- chemical types (Tables 2, 3). The chemistry of Biliran thermal features means that conventional sol geothermometers (i Hot acid-chloride-sul phate fl uids cannot be usefully appl ied. Gas geothermometers (ii) Hot or warm acid-sulphate fluids, give a wide range of results, probably indicating chloride less than 250 ppm natural variations as well as sampl ing and analytical error. (Table 3). ( ) Warm neutral -bicarbonate-chloride waters. The acid-chloride-sul phate features are con- THERMAL MANIFESTATIONS sidered to be those closest to the most actively- upflowing zones. The acid-sul phate waters are The most impressive thermal features on interpreted as steam-heated surface waters, Bil iran a r e in the Vulcan thermal area, consisting typical of the higher elevation of geothermal of boil ing pools, mudpool s and mildly superheated systems. Waters of the third type occur at lower steam vents (Figure 2). There i s a large flow of el evations and represent outflow from the gas, and much sulphur has been deposited around geothermal system, diluted with meteoric water and the vents. An unusual feature is a either derived from a neutral -chloride reservoir which emits material containing 75% sulphur with or neutralised by interaction with country rock. natroal unite and traces of gypsum and opal ine Both the spatial distribution of the thermal silica, forming lava-like flows up to 500 m long features and measured gas ratios are consistent and 1-2 m thick. with this interpretation (Figure 2). There are smaller areas of boil springs and Acid-chloride-sul phate fluids are not common steam vents at Libtong and Vulcan Gamay. The most features of hydrothermal systems, and their active thermal features of Biliran lie in a broad existence here indicates a direct input of magmatic NE belt ap roximately paralleling the Vulcan Fault. (<6 ) volatiles to the system. This is confirmed by the Warm springs occur at several locations, in presence of and in gas samples, not addition to a number of cold but mineralised seepages, some of which emit small quantities of gas. Table 2. Chemistry of Biliran Thermal Waters

AREA TEMP. CONCENTRATIONS EXPRESSED I N MOLECULAR RAT OURCE Na Na Na K Ca Mg B OF DATA a 42 7.4 125 28 52.4 38.7 0.13 2.8 121 160 163 464 7.6 290 13.3 2.05 a t 65 7.1 38 72.9 47.0 0.14 3.7 177 221 175 34 0 7.1 341 14.8 2.17 a can 90 2.1 9.4 15 53.6 23.6 0.01 6.2 375 2700 257 1.1 285 18.4 0.4 a Vulcan Gamay 89 2.2 8.9 4 32.5 17.4 0.01 3.6 7 1600 257 3.5 270 0.6 0.01 a Vulcan Gamy 85 2 301 924 b Tenego 75 1.7 50.5 21 79.4 34.9 0.2 6.3 1152 1200 231 5.0 913 56.0 2.6 a L ibtong 63 2.8 4.9 3 5.4 2.4 0.01 1.0 9 185 60 2.9 2.7 0.13 a Villavicenta 50 7.9 133 26 243 88.2 0.06 2.0 248 500 165 530 8.6 669 38.8 1.34 a 46 7.1 134 9 315 49.0 0.01 4.7 101 960 116 238 25.3 6.5 0.29 a Panamo 451 3.0 15 8 42.7 6.1 0.02 0.2 9 184 94 3.2 0.26 a Ana 351 6 213 624 can 95 1.2' 9480 b

Sources of Data: a = Gal and (1980) b = D.M. Cope, (1982)

Footnote: Well discharged after steam stimulation 31/7/82. 164

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Table Chemistry of Gases, Biliran Thermal Features

CONCENTRATIONS, MOLE GAS RATIOS CALCU- SOURCE I NH3 HF TEMP LATED OF Li 93.7 84.1 3.3 0.56 0.007 0.56 0.01 0.05 0.035 9.05 1.43 25 1200 0.013 150 96 191 a L btong 88.9 91.6 6.9 0.38 0.22 0.27 0.80 13 2200 0.020 420 95 192 b can 76.1 91.1 6.1 0.53 0.12 0.16 0.07 1.81 15 759 0.750 570 64 286 b 97 83.0 3.7 0.06 2.61 0.029 8.44 2.20 23 115 a Tenego 96.0 96.5 2.5 0.002 0.01 0.46 39 48000 9600 105 186 b can 96.8 92.3 2.8 1.79 0.11 0.02 33 840 5.5 4600 292 a can 91.7 92.6 1.5 0.49 0.005 0.001 0.05 0.04 0.025 4.63 0.63 62 18000 3.8 102 220 a can 96.6 88.8 9.9 0.63 0.01 0.02 0.09 0.49 9 9000 31.5 4000 100 240 b 99.4 0.2 0.27 0.007 0.19 552 130000 0.004 500 30

lavicenta 93.5 0.02 6.37 0.0005 0.11 4670 200000 0.004 Panlahuban 63.2 0.12 36.3 0.34 300

in total sample Sources of data: * Temperatures calculated by and Panichi (1980) a = Glover (1981) using except for and b Cope (1982) Panlahuban samples.

HYDROTHERMAL ALTERAT I0N G EOPHYS Areas of intense acid teration surround the Biliran has been surveyed using a combination active thermal areas. Typical assemblages are of Schl umberger traversing, e-dip01 e, and stobal ite-gypsum-sul phur- kaol inite- i vertical electrical sounding resistivity techniques. pyrite, natroalunite, quartz, 1eucoxene and Results of Schlumberger traverses with spacing sulphate?. In addition, there are large areas of of 500 m a r e shown in Figure 3 as altered ground not related to present-day thermal contours . In general the more-deeply-penetrating activity, with generally similar mineral assemblages. techniques have tended to support the Schl umberger Occasional surface samples, however, have resul ts, but have added significant information by mineralogy more typical of neutral -chloride revealing that low-resistivity anomal i e s a r e teration (for example lonite from connected at depth beneath the fresh, unaltered the Vulcan area - T.M. Leach In part young volcanics of Mts. Sayao and Tamburok alteration must be relict, as shown by the presence (Figure 2). of high temperature phases pyrophyll ite, epidote) now occurring on the surface. The resistivity pattern is interpreted as indicating a geothermal reservoir centered approx- In contrast, cores and cuttings from the f i r s t imately on the Vulcan Gamay a r e a , and with out- deep drillhole with a total depth 2425 m), flow to the east and west. The low-resistivity reveal that acid alteration is minor and restricted anomaly between Vulcan and Kalambis may also be an to the upper portion (down to 114 m). Below this outflow, or i t may be a separate zone of upwelling. level clasts within the Pulang Yuta Formation have varying degrees and ranks of teration, but Figure examination of the groundmass and fine-grained material shows that the latest phase of alteration to produce an assemblage of chlorite- ill ite, and interlayered clays, plus calcite-quartz, with minor gypsum, hematite and pyrite and traces of epidote. This assemblage is typical of medium temperature ( alteration by a neutral -chloride fluid. However, temperatures predicted on the basis of alteration mineralogy have proved to be lower than measured downhole temperatures, and so may not be a iable indicator of present conditions in the reservoir. 165

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Figure 3. Resistivity Bil iran and Geochemistry of Thermal Manifestations

RESULTS OF EXPLORATION DRILLING Fiaure 4. BN-1 Well Data At the time of writing, June 1982, the first of 3 planned deep exploration wells, BN-1 had recently been completed to a depth of 2425 This well is located to the north of the Vulcan surface manifestations and, though hot, appears have 1 imited permeabil ity. Nevertheless it has provided useful geological information, particular, evidence for neutral -chloride teration. The major part of the well passed through a thick vol cano-sedimentary unit (the Pul ang Yuta Formation). This unit was not recognized on the surface and its thickness was unexpected. After penetrating the Pulang Yuta Formation, a gas kick was experienced within the underlying sediments indicating high formation pressures and that the relatively impermeable Pulang Yuta Formation was acting as a cap. Gas (predominantly with no distinctive magmatic component) was successfully circulated out and the well now stands with zero wellhead pressure. The 1imited permeabil i t y suggested by the lack of circulation losses during drilling is consistent with results obtained during injectivity testing a t low flowrates. However, a t higher flowrates the t y improved, possibly indicating formation breakdown. The well is s t i l l heating (maximum bottomhole temperature = a s of 3/7/82 and an attempt will be made t o in the near future). 166

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CONCEPTUAL MODELS OF THE B I L I R A N SYSTEM CONCLUSIONS Two alternative models have been developed t o Geoscientific exploration and drilling has described the Biliran geothermal system. They can not yet proven the existence of an exploitable geo- be sumnarised as follows: thermal resource on Biliran but indications are promising. The presence of a magmatic component 1) Thermal features in Biliran are the result of i n outflow from surface thermal features i s a cause the direct upflow o f magmatic volatiles, mixing f o r concern, as it may indicate the existence o f with groundwater t o produce a hot, acid fluid. acid conditions at depth. However, by analogy The centre of upflow i s near the Vulcan thermal with other explored geothermal is area. Outflow o f mixed fluid and condensate, probable that acid conditions are restricted to and hydrothermal teration by the outflowing localised upflow zones. Investigation of the area fluid, produce the observed low- resistivity i s continuing. anoma1y . ACKNOWLEDGEMENTS 2) A magma chamber o r intrusive at depth ively heats meteoric water, giving rise to a This paper has drawn on a number of unpublish- convective hydrothermal system. Within this ed PNOC, KRTA and DSIR reports, and their contri- system, high- permeability channels along faults bution is gratefully acknowledged. Thanks are due permit the local upflow o f magmatic volatiles, to PNOC for permission to publish exploration data producing restricted zones of acid fluid, How- and support i n the preparation and presentation of ever, the bulk of the system consists of this report. Our appreciation i s also extended to chloride fluid, as is connnonly encountered i n t h e many colleagues i n PNOC and KRTA who have other convective hydrothermal systems, and this collaborated on the Biliran Project. is responsible for the observed low resistivity. REFERENCES If the first o f these hypotheses i s correct, then the area may be underlain by an acid, cor- Alcaraz, A.P. 1947. The major structural lines rosive fluid, a similar situation to that of the Philippines The Phili ine encountered a t Tatun geothermal field, Taiwan Geologist, 1 (2): (Chen 1970). In this case the area could not be exploited using conventional geothermal Armansson, H.; Gislason, G.; Hauksson, T. 1981. Magmatic gases i n well fluids aid the If the second model applies, then there i s a mapping o f the flow pattern in a geother- much better chance of developing an exploitable mal system. Geochemica e t Cosmichimica resource. There may be acid zones within the Acta ( i n press). field which will have to be avoided, but the rest Bureau o f Mines 1962. Geological map o f the of the reservoir can be drawn upon. A similar Phi1 ppines, scale 1:1000000 situation exists i n Krafla field, Iceland (Armansson e t where magmatic volatiles Chen, C.H. 1970. Geology and geothermal power arise along a 1 inear structural feature producing potential of the Tatun volcanic region. acid conditions, but are surrounded by a (Geothermics 2 (2): 1134. chloride reservoir, C.M. 1982, Further studies on Biliran aas chemistry Proceedings o f 4th Annual On balance, the second model o f Biliran is EDC Geoscientific Conference, favoured for the following reasons: April 1982. - evidence of neutral- chloride alteration in D'Amore F., Panichi, C. (1980). Evaluation of some surface samples. deep temperatures o f hydrothermal systems by a new gas geothermometer Geochemica e t - evidence o f neutral chloride teration Cosmochimica Acta 44: 549-556. most o f BN-1. Espiritu, D.D. 1980. Geology o f Biliran Island - an indication on interpreted (Report 2). PNOC-EDC unpublished report, sections of 2 separate low- resistivity layers: a shallow one presumably correspond- Galia, Clemente, V.C. 1980. Geochemistry ing to acid condensate, and a deeper one of Biliran gas and water samples and i t s which may represent the neutral reservoir geothermal significance PNOC-EDC (Figure 3). unpubl ished report. - restriction of acid- chloride water i n over, R. B. 1981. Geochemical report: DSIR surface features to those along the line Mission t o the Philippines. Unpublished of the Vulcan fault. DSIR report, Vasquez, N.C., Tolentino, B.S. 1972. The Geology o f the Tongonan Geothermal Field. COMVOL Letter VI