Survey of Malabuyoc Geothermal

Home , Argao, Barili, Camotes Sea, Dalaguete, Ginatilan, Malabuyoc

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

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

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

c i Tamia t

CF Bl n A

Kantipase D U Cangulay c

t o

l y

Qal u u

a b

a l

BM a K t

u CF CV MF M TF o

MF Armenia S - Sorsogon

t Qal Danao r

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. Barili Formation is subdivided into Barili marl and Barili 2 Del Rosario, Pastor and Malapitan limestone members (MGB, 1985). The Barili marl is poorly 2.2 Survey Methodologies bedded, slightly sandy and with occasional thin beds of A total of 31 CSMT stations were occupied (Fig. 4). Stations limestone (MGB, 1985). The Barili limestone is generally were randomly distributed to an average areal distance of 1 dense, massive, white to buff and locally coralline. It kilometer between stations covering the Barangays of occupies the topographic high displaying karst, gorge and Montañeza, Lumbo, Armenia, Tolosa and Sorsogon all in knife-edge features. Malabuyoc and Barangays Legaspi, Lumpan and Mompeller in Alegria. Acquisition of resistivity data is through a 1.4 Thermal Manifestation receiver capable of receiving, collecting and processing Thermal manifestation occurs as two separate warm spring telluric and magnetic signals at 14 different frequencies seepages approximately 20-m apart at Montañeza River in ranging from 5,120 Hz to 0.625 Hz. Measurements of data Sitio Mainit, Brgy. Montañeza. The first thermal spring, were carried out in random style utilizing a "fixed" which is the hotter, discharges from the crevices of the lower transmitter site and a roving receiver site spaced 3-6 km limestone member at left bank upstream while the second apart. The transmitter, which is approximately 1.4 km. and o spring, diluted by the river water comes out from the pile of oriented N85 E was placed in Sitio Parale, Barangay river deposit at the middle of Montañeza River. The first Mindanao, Malabuyoc which is more or less perpendicular spring, entombed in a concrete box has a discharge to the general trend of structures in the area. Locations of the temperature of 560 C, neutral pH and flowrate of 2 l/sec proposed station in the field were verified using 1:50,000 M while second spring partly amidst river deposits has scales map, GPS and thommen altimeter. Since transmitting indeterminate temperature and flowrate due to dilution of and receiving was done on the same frequency and river water (BED, 1979; Malapitan and Del Rosario, 2002). simultaneously at a given time, the clock at the transmitter Both springs discharge crystal clear and lightly steaming and receiver sites were synchronized first before going out water with faint H2S odor. No significant hydrothermal to the field. alteration was observed at the thermal spring site.

123*20 123*21 Chemistry of the thermal water does not indicate thermal ' ' fluid mixing and the associated gas is believed due to 9*43' decomposition of the organic materials in the marine clastics LEGEND or from petroleum gas trapped in the reef limestone (BED- Receiver Station ELC, 1979; Malapitan and Del Rosario, 2002). The Transmitter Dipole A alignments of the two warm seepage possibly suggest A Profile Line Mal06 structurally controlled springs. B' Mal10 C Mal07 C' Mal02 Mal03r Mal09 1.5 Previous Works Mal08 Mal05r Mal11 Mal04r The surrounding region of the survey area is well studied for Mal30 gas and coal occurrences. Early interest in geothermal D Mal14 Mal12 prospecting was done in 1979 through the initiatives of the Mal29 Mal13 Bureau of Energy Development and ElectroConsult (BED- Mal28 Mal01r Mal23 Mal16 ELC) under the project “Regional Inventory of Philippine Mal27 Mal31

Geothermal Areas”. The effort led to the identification of Mal17 A' Mal20 Mal22 warm spring occurrences in Malabuyoc, Oslob and Catmon. Mal15 Mal26 D' In 1995, Monzon and Pendon of Department of Energy- A Mal19 Energy Resource Development Bureau (DOE-ERDB) B Mal21 Mal25 reassessed the area for direct and agro-industrial application. Mal18 In 2001, Malapitan and Del Rosario of DOE-ERDB, conducted geological/hydrological survey of the prospect Mal24 leading to the identification of the different lithologies and the hydrological mechanics of the geothermal prospect.

2. BASIC PRINCIPLES AND SURVEY METHODOLOGIES 2.1 Basic Principles The CSMT method uses induced artificial electromagnetic waves to determine the subsurface electrical resistivity structure. These waves originate from a finite source (transmitter dipole), propagate in the earth and “sensed” by another set of instrument known as the receiver. The ratio of 9*43' the intersection of the “sensed” horizontal electric and magnetic fields yields the subsurface resistivity. The method Figure 4: CSMT Station and Location of Profile Lines utilizes an artificial source similar to direct current (DC) method wherein an electrical current is introduce to the 3. DATA PROCESSING AND ANALYSIS ground to produce an electrical signal. The signal travels to The time series data, written in text format are then the ground in the form of waves of varying frequencies. downloaded and processed using the CSMT 1-D inversion Low frequency waves penetrate further into the ground than software. A flowchart of CSMT data processing and high frequency waves and varying resistivity at depth could inversion is shown in Figure 5. Initial processing of the data be measured from these frequencies. starts with data sorting rejecting bad or extreme data. The sorted data are then smoothened and interpolated using the spline function. A 1-D inversion is then performed either through resistivity and phase or by phase only. It should be

3 Del Rosario, Pastor and Malapitan. noted that only the data of the far field are analyzed in the Data Coordinate inversion process. Another option is given on the program wherein both Far field data and Near field data are analyze.

In the inversion process, two options were given to the user Data Sorting to produce 1-D plot: 1) the No Input Layer Structure wherein the program automatically assigned the number of layer models/frequency and 2) the Yes Input Layer Structure Data Files Smoothing Correct Map Position wherein the user had the hand on the number of layer 1D Inversion models. Both options require station elevation and Near Field) maximum penetration depth in order to calculate the true 1D Inversion resistivity, depth and thickness of each layer. (Far Field)

Far field 1D inversion of the data was performed using both the resistivity and phase. Static effects were noted and correlated with the average resistivity curve of the area. We Depthwise allow the program to automatically assign the number of Contouring Resistivity Log Surfer layers. The maximum depth of penetration used for input for the inversion was 2,000 meters. The depth of Resistivity Section investigation is related to the square root of ground resistivity and the inverse square root of signal frequency. CSMT depth of investigation is roughly equal to skin depth Figure 5: Flow chart of CSMT data processing (δ) / √ 2. Assuming a background resistivity of 100 ohmmeter, sounding to about 2.5 Hz will get an equivalent depth of investigation of about 2,000 meters. Soundings up 3.1 Pertinent Well Information to 0.625 Hz will give an equivalent depth of investigation much deeper. Studies and practical applications have shown Three wells seeking natural gas and oil were drilled within that the maximum usable depth is usually between 2 to 3 the survey area (Fig. 6). Information derived from these kilometers. wells did not have much bearing with geothermal fluid prospecting except for information related to geology and The final product is a resistivity-phase-frequency plot of permeability. Below are brief descriptions of the wells. curve trends and distribution of data sets per frequencies. Incorporated in the plots are the average field (measured) 3.1.1 CPR-1 and calculated resistivities obtained during the measurement The well was drilled at the eastern flank of the Malabuyoc as well as the depth, thickness and true resistivities per layer. anticline for testing the middle diagonal fault and the possible anticlinal trap at the Malabuyoc anticline. The decision to reject or retest station was based on the Lithologies were the Upper Miocene to Pliocene marine results of initial processing. Station with wider range of data sediments consisting of Barili Marl, the upper, middle and set per frequency and no apparent curve trend is subject for lower members of the Barili Formation and the clastic retesting. member of the Maingit Formation. Lost circulation zones After initial processing, the data was subjected to further were encountered at depth between –117 to –257-m and – 484-m bsl respectively. CPR-1 has a total depth 1,588-m and processing to produce a more refine and acceptable data for 0 interpretation. The resistivity and phase plots of the has a maximum recordable temperature of 54 C (CPR, individual station were examined for possible error, noise 1976). and surficial effects. To determine the occurrences of near field effect and to visualize clear similarities in trend at 3.1.2 CPR-2 depth of the inverted data, correlation among different The well is located about 2.3 km SW of CPR-1 within the variables such as apparent resistivity, frequency and depth western flank of Malabuyoc anticline. It is the second well were made using MSExcel and Grapher programs. After drilled by Chinese Petroleum Corporation in January 1975 inversion, the data were processed further and was finally with a purpose of testing the NNE normal fault and the linked to Surfer, a mapping software to produce anticlinal traps on the south of the Malabuyoc anticline. isoresistivity maps and cross sections. The data are Lithologies penetrated by the well were the Lower Miocene interpolated and further smoothened during the process. to Pliocene marine sediments consisting of the upper and lower limestone member of Barili Formation, the clastic and Noise analysis, shows that majority of the stations have limestone member of the Maingit Formation, the upper and good readings in almost all frequencies. A few of the lower limestone member of Toledo Formatio and the clastic stations have scattered data particularly in the lowermost and member of the Malubog Formation. Lost circulation zone shallow frequencies. Data for 0.625 Hz are generally noisy were encountered at a depth between –17 m and –178 m and and scattered and we have some reservations of the quality between –1085 m and –1093 m bsl respectively. CPR-2 has of data at this frequency. Each station was analyzed based a total depth of 2,045 m and with maximum recordable on the compactness and closeness of the data population in a temperature of 910C (BED-ELC, 1979). particular frequency. Good data give us more confidence in the interpretation and have better fit after inversion.

Examination of the resistivity-phase-frequency plots show that the measured and inverted resistivity values of most of the stations depict a similarity of trend. This is shown by increasing resistivities in the uppermost bands, decreasing resistivity in the middle to lower band and increasing again up to the lowermost band. 4 Del Rosario, Pastor and Malapitan

Cletom 102-29 500 Barili Upper Lst 123o 20' 123o 21' CPR-1 Barili Clastic 9o 43' LEGEN Barili Lower Lst Barili Marl CPR-2 D O il/G as Barili Upper Lst Lumpan L e WarmWell ga s Barili Clastic p i R Barili Upper Lst Maingit Clastic iv e 0 springFold Legaspi r Barili Clastic Barili Lower Lst Fault (dashed e n U D when Lanayai Barili Lower Lst cl i CSMTinferred) nt Maingit Clastic A Tum andok c Station o Samuyao y u b Abungon a U -500 al D Tape Siag M

)

s t ohm -

( er

v Maingit Clastic R i Maingit Lst meter Montaneza a

h z 200 M ne t o nt Canlaga ta an on p ez M 190 a R e iv er F Mandayang

D M ainit 180 ault -1000 Toledo Lst 170 Lomboult l Fa ona 160 D iag dle 150 Lanao Ginalayugan M id 140 U Candalag 130 D e n 120 i cl i 110 Tam ia nt A -1500 100 Kantipase D U c Cangulay o 90 t y ul u Toledo Clastic a b 80 F a h al 70 ut M o 60 Armenia S h- Sorsogon t 50 or N 40 -2000 30 20 MALABUYOC 10 S a li r in g R i v e r Parali Mindanaw Figure 6: Lithologic Profile of Well CPR-1, CPR-2 and Mahanlog Cletom 102-29 Labrador 9o 38'

Mts 0 1000 2000 3000

3.1.3 Cletom 102A-29 The well is approximately 3.3 km NNE of CPR-1 within Lumpan, Alegria. There is not much pertinent information Figure 7: Isoresistivity Contour Map at –50 m below sea level (bsl) from this well except that the penetrated lithologies were similar to that of the CPR-1 and CPR-2 wells. It has a total depth of 716 m. 4.1.2 -100 m bsl

At –100 m bsl, the Lower Lombo anomaly decrease in size while the Ginalayugan-Kantipase anomaly increased and 4. RESULTS OF THE CSMT DATA ANALYSIS practically covers western part of the survey area (Fig. 8). 4.1 Isoresistivity Plan Map The anomaly widens at the direction of MRF and middle Isoresistivity contour maps and resistivity cross sections diagonal fault while its southern edge still coincides with were prepared to determine the distribution of resistivity at trend of North-South trending fault. The widening feature depth. These maps and cross sections are based on the 1-D probably suggests the fault as possible conduit for the inversion of CSMT data and are correlated with other thermal spring in Montañeza River. relevant information such as geology and structures, well data, etc. This will serve as the basis of interpretation of the 4.1.3 -150 m bsl geothermal system present in the area. The Ginalayugan-Kantipase anomaly decreases in this horizon and is still confined at the western part of the area 4.1.1 -50 m bsl while the Lower Lombo anomaly breaks down into two At –50 m bsl, a north-south trending low resistivity anomaly segments. Conspicuous here is the intertounging feature of of <10 ohm meter was noted and termed the Ginalayugan- the 20-ohm meter contour beyond the middle diagonal fault. Kantipase anomaly (Fig. 7). This anomaly roughly coincides This perhaps suggests either the existence of a possible with the direction of North-South trending fault. Another extension of MRF beyond the middle diagonal fault or some low resistivity anomaly of <10 ohm meter termed the Lower sort of a collapse feature brought about by the movement of Lombo anomaly was noted in Lombo which seems to follow middle diagonal fault. the direction of the middle diagonal fault. These suggest structural affinity. 4.1.4 -200 m bsl The Ginalayugan-Kantipase and lower Lombo anomalies are now joined with each other forming a single anomaly that extends beyond the intersection of the middle diagonal fault and MRF (Fig. 9). This anomaly could indicate the presence of hot fluid at this depth along MRF.

5 Del Rosario, Pastor and Malapitan.

o o o 123 20' 123 21' 123o 20' 123 21' 9o 43' LEGEND 9o 43' LEGEND Oil/Gas Well Oil/Gas Well Lumpan Lumpan L L eg eg Warm spring as Warm spring as pi pi Ri Riv Legaspi ve Fold Legaspi er Fold r Fault (dashed Fault (dashed e U D e D U when inferred) n i when inferred) n Lanaya l Lanayai l c

c t i CSMT Station t CSMT Station n

n A Tumandok A

c Tumandok

Samuyao o c

Samuyao y o

u y Abungon

b u Abungon U a b U l a D a l D Tape Siag Tape Siag a M

ohm-meter ohm-meter er iv Montaneza R er M a iv 200 on ez Montaneza R ta Canlaga an 200 M a n nt o ez e o nt Canlaga n za M an ta 190 R e on iv 190 za M er Mandayang Ri 180 Mainit Fa ver Mandayang ult 180 Fa Mainit ul Lombo t 170 ult Fa 170 Lombot nal aul 160 go l F Dia 160 ona Lanao Ginalayugan dle iag 150 Mid le D 150 Lanao Ginalayugan idd 140 M U Candalag 140 D

U Candalag 130 D e

130 D n

120 i

i n

120 t

i 110 Tamia

A i

Kantipase

110 Tamia t 100 D U Cangulay

c n

t o

100 A 90 l y Kantipase D U Cangulay

D U u

u c

a b 90 t o 80 F

l a

y l u

u a

a t

80 b 70 u F M

l o

a S 70 t 60 Armenia - Sorsogon

u M h

o t 60 50 r S Armenia o - Sorsogon

h N 50 t 40 r

40 N 30 30 20 MALABUYOC 20 10 MALABUYOC S ali ring Ri 10 ver Sa Parali lirin g R iver Parali Mindanaw

Mindanaw

Mahanlog Labrador Mahanlog 9 38' Labrador 9o 38' Mts Mts Figure 9: Isoresistivity Contour Map at –200 m below Figure 8: Isoresistivity Contour Map at –100 m below sea level (bsl) sea level (bsl) 123o 20' 123o 21' 9o 43' LEGEND Oil/Gas Well Lumpan L eg Warm spring as pi 4.1.6 -300 m bsl Ri Legaspi ve Fold r The anomaly decreases considerably in size and breaks Fault (dashed e U D

when inferred) n Lanayai l

c down into two smaller segments termed here as the Mainit i CSMT Station t

A Tumandok and the Armenia anomalies (Fig. 10). These two anomalies Samuyao c o

u Abungon b U are bounded on the east by highly resistive blocks. The a l D Tape Siag a Mainit anomaly trends NW along MRF while the Armenia M anomaly lies immediately west of the north trending fault. ohm-meter er iv Montaneza M R 200 o za nt Canlaga ne Conspicuous in this level is the looming of an almost north- an ta e on za M 190 Ri ver Mandayang Mainit Fa south trending middle resistive block that seems to coincide 180 ult 170 Lombot aul with the trend of the south Malabuyoc anticline. al F 160 gon Dia Lanao Ginalayugan dle 150 Mid 140 U Candalag

4.1.7 -400 m and –500 m bsl 130 D

e n

120 i

c i 110 Tamia t

The Mainit and Armenia anomalies remain the same. The n A

100 Kantipase D U Cangulay c

t o

middle resistive block increases in size. 90 l y

u u

a b

80 F

a l

h a 70 t

u M

60 Armenia S - Sorsogon

4.1.8 -750 m h

t 50 r

o The Mainit anomaly remains the same while the Armenia 40 N 30 anomaly increases tremendously in this level (Fig. 11). On 20 MALABUYOC 10 Sa the other hand, the middle resistive block increases in size lirin g R iver and is merged with the highly resistive block. Conspicuous Parali is the steepening of the contour at the southern edge of the Mindanaw survey area, which probably indicates manifestation of the north-south trending fault. The trend of the resistive block Mahanlog Labrador coincides with the trend of Malabuyoc anticline. 9o 38'

Mts

Figure 10: Isoresistivity Contour Map at –300 m below sea level (bsl)

6 Del Rosario, Pastor and Malapitan

o o 123 20' 123 21' W E Mal 22 9o 43' LEGEND CPR-1 Oil/Gas Well Mal 15r Mal 20 Lumpan L ohm-meter eg Warm spring as Mal 26 pi Riv Barili Marl Legaspi er Fold 200 Fault (dashed 0 Barili Upper Lst 190

e U D Member

when inferred) n 180 Lanayai l Barili Clastic c

i CSMT Station t Member 170 n

A 160 Tumandok

c Barili Lower Lst Samuyao o 150 y Member u Abungon -500 b

) 140 a U

D m Siag a Tape ( 130

M h t 120

p Maingit Clastic ohm-meter e Member 110 D er iv 100 Montaneza R Mo za -1000 200 nt Canlaga ne 90 an ta eza on 190 Ri M 80 ver Mandayang Mainit Fa 180 ult Fault North-South 70 Lombo 170 ault 60 al F 160 gon Maingit Lst Dia 50 Lanao Ginalayugan dle -1500 Member 150 Mid 40 140 U Candalag 30

130 D 20

n i

120 l 10

c i Tamia t

110 n 1500 2000 2500 3000 3500 4000 4500 A

100 Kantipase D U Cangulay c

t o

l 90 y Distance (m)

u u

a b

80 a

h a t

70 M

Armenia S Figure 12: Isoresistivity Profile along Line A-A’ 60 - Sorsogon

t 50 r o 40 N SW 30 Mal 08 20 MALABUYOC 10 500 S ali ring Ri ver Mal 27 Parali Mal 26 Mal 01r

Mindanaw 0

Mahanlog t

l u

Labrador ) o a M

9 38' -500 F (

h e

v i p e

Mts R

a z

-1000 e

t n

Figure 11: Isoresistivity Contour Map at –750 m below o sea level (bsl) M -1500 4.2 Isoresistivity Profile Four isoresistivity profiles were constructed to visualize the subsurface configuration of the survey area. These are Line 0 500 1000 1500 2000 2500 3000 3500 4000 A-A which transects the central portion of the survey area, Distance (M) Line B-B which trends NE-SW cutting MRF, Line C-C dissecting the northern edge of the survey area and Line D-D Figure 13: Isoresistivity Profile along Line B-B’ paralleling the MRF and cutting the middle diagonal fault (Please refer back to Fig. 4). 4.2.3 Line C-C This section transects the northern portion of the survey area 4.2.1 Line A-A (Fig. 14). It also runs perpendicular to the northern segment The traversed line is generally underlain by high resistive of the Malabuyoc anticline and the postulated north trending blocks (>50 ohm meter) (Fig. 12). Lying underneath were gravity fault in Lumpan area. The line is characterized by a isolated bodies of middle conductive layer (<20-ohm meter) very thick low resistivity anomaly and is bounded on the separated at near surface by middle resistive body (>20 oh east by a high resistivity block. The anomaly spreads meter). laterally towards the east capped by high resistive values. Conspicuous also on this section is the down thrusting of the 4.2.2 Line B-B resistivity contour in the east, which could be the This profile trends NE –SW passing through the thermal manifestation at depth of the postulated fault occurring in spring (Fig. 13). The line is characterized by wide and thick the Lumpan area. fault-bounded low resistive anomaly at the southwest and high resistive block in the northeast. A 10-ohm meter 4.2.4 Line D-D contour at depth of -200 to -300 m is thought to represent an This line dissects the middle portion of the survey area (Fig. aquifer that seems to be related to the hot spring. The 15). This section was done to determine the outflow of resistivity contrast indicates the presence of MRF. thermal fluid postulated to be originating east of the survey area. The section is characterized by fault bounded high resistive blocks occurring beneath and a very thick low resistivity anomaly on the NNW existing underneath. Serving as their boundary was the steep resistivity gradient postulated to be the manifestation of North-South Fault and middle diagonal fault at depth. A low resistivity anomaly 7 Del Rosario, Pastor and Malapitan. noted on the SSE side of the line was gradually increasing at structure. Foremost of these structures were the north-south depth and was overlain by high resistive capping. Probable trending fault that delineates the conductive and resistive manifestation of the Montaňeza river fault at depth was zones, the Malabuyoc anticline, MRF and the middle noted through the lateral swaying of the <10-ohmmeter diagonal fault. The result is also synonymous with the contour at shallow depth and presence of a middle postulated anticlinal structure concept of CPRS and conductive layer in between the resistive block. The middle geological mapping of Malapitan and Del Rosario (2002). conductive layer could either be a karsts aquifer or The conductive zone generally lies on the western section of containing part of the heated fluid that is channeled along the survey area trending N-S. It is characterized by a coastal the MRF. wide resistivity low from Sitio Tumandok to the north down to Armenia in the south. It lies within the intersection of middle diagonal fault and MRF and the downthrown side of the north-south trending fault. The downhrown block, being W E Mal 07 a permeable zone, provides an excellent venue either for Mal 09 Mal 10 500 hot/cold or seawater accumulation or combination of both.

Mal 03r The influence of seawater incursion is evident considering Mal 02 the trend of the resistivity anomaly relative to the sea. Montaneza warm spring seepage is within this zone and 0 ohm-meter along the trace of MRF. The presence of middle diagonal 200

190 fault is best depicted by the alignment of < 40 ohm meter t

l 180

t l

u contour at depth –250 m, -500 m and –750 m bsl

u a

) 170

m -500 F

( 160

respectively while the existence of MRF is exemplified by

h e

h t t t 150

u l

e o

u 140 t

D the deepening and widening of the 10 ohm meter contour at

S -

s 130

t r

P 120 –200 m and –250 m bsl respectively. The faulted anticlinal o

N 110 -1000 100 structure environment, which had been established by 90 80 previous studies, was proven by this study. This geological 70 concept is manifested by the plunging of the <40-ohm meter 60 50 -1500 contours in the A-A profile. In the isoresistivity map, it is 40 30 marked by the doming and “ridge effect” of the resistivity 20 10 contour roughly coinciding with the trend of Malabuyoc 2500 3000 3500 4000 4500 5000 anticline. Distance (m) Though the resistivity results reflect the subsuface structural Figure 14: Isoresistivity Profile along Line C-C’ picture of the survey area, variation in resistivity, however, did not reflect the formational boundary nor the lithological differences of the rock mapped in the survey area owing to

NNW SSE the wide range of resistivities in sedimentary rocks. The Mal 22 lithologies penetrated by the three wells were used as determinant factor in the modeling process. Near the wells, Mal 20 Mal 16 the topmost high resistive layer probably correspond to the Mal 29 Mal 01r isolated limestone beds capping the Barili Marl while the 0 middle conductive layer (<10 ohm meter) could either be the Barili Marl or the Barili Upper Limestone member. The ohm-meter presence of clay makes the Barili Marl highly conductive 200 and the low resistivity signature in the limestone could be -500 190 180 due to its water-saturated vug and cavities. The middle )

m 170 ( resistive bodies at depth of –1000 m bsl probably correspond h 160 t p

M e 150

N to the clastic and limestone member of the Maingit

D i

o 140

t l

e -1000 h 130 Formation while at lower depth it might be the lower

D S 120

u limestone member of Barili Formation. This assertion is g 110

n 100

F a made possible by the fact that the lower limestone member

a l 90

l a 80

u penetrated by CPR-1 is harder, denser and more crystallized

l 70 -1500 t 60 than the upper limestone member. 50 40 30 20 6. GEOPHYSICAL MODEL 10 Due to absence of mappable igneous rocks that will warrant 2000 2500 3000 3500 4000 4500 Distance (m) existence of a possible heat source that will trigger convection, the system in the area was modeled to be similar to a basement aquifer beneath sedimentary basins as shown Figure 15: Isoresistivity Profile along Line D-D’ by their remarkable resemblance.

• A typical basement aquifer occurring beneath 5. DISCUSSION OF THE RESULTS sedimentary basin is characterized by the presence of a To understand the geological significance and geothermal highly permeable aquifer within or near the top of the implications of the CSMT result, the modeled isoresistivity basement covered by a sequence of younger maps and profiles were correlated with the existing sedimentary rocks of low permeability (Hochstein and geological and well studies done within the survey area. Yang, 1988). Similarly, serving as the basement for the Malabuyoc system was the resistive lower limestone Results of correlation show that the resistively structure of member of the Barili formation and the clastics and the thermal prospect was largely controlled by geological limestone member of the Maingit formation while the aquifer is the karstic low resistive upper limestone 8 Del Rosario, Pastor and Malapitan

member of the Barili Formation. It is in turn covered by coincident with MRF. Possible hot water can be extracted low permeable highly resistive limestone capping the from permeable karst aquifer between depth –100 to –300 m Barili Marl. bsl. Correlatively, these are the lost circulation zones encountered from CPR-1 and CPR-2 wells. • Basement aquifer system occurs beneath the flank of the basin or in anticline and constitutes a low Given the above-mentioned results, the just concluded temperature resource and fluids with temperature survey in Malabuyoc has delineated possible presence of an ranging from 500 C to 650C. Correlatively, resistivity aquifer at shallow-medium depths. The real nature of this results depict a faulted anticlinal structure that is aquifer has to be established by drilling. consistent with the recently concluded geological 123*20' 123*21' mapping conducted by Malapitan and Del Rosario and 9*43' LEGEND

to the drilling and seismic studies of Chinese Petroleum Oil/Gas Well Lumpan Warm spring L Corporation. Montañeza warm springs occur at the eg as pi Ri flank of the postulated Malabuyoc anticline having a Fold Legaspi ver 0 Fault (dashed when inferred)

discharge temperature of 56 C. It is a low temperature e U D

n Lanayai l

CSMT Station c i resource with subsurface temperature ranging from 61- t n

A 0 Tumandok Inferred Fluid Flow c 68 C. Chemistry of the fluid probably derived from Samuyao o

u Abungon b U a marine sediments. l D Tape Siag a Ohm-meter M • Basement aquifer system was relatively shallow 200 er 190 iv Montaneza M R o za occurring from 0.5 – 1-km depth. Relatively, the nt Canlaga ne 180 an ta ez on a M Riv 170 er Mandayang aquifer of the Malabuyoc system was shallow occurring Mainit Fau 160 lt Lombo ult between the 200 – 400 m depth below the surface as 150 l Fa ona iag 140 le D shown from the generated isoresistivity map and Lanao Ginalayugan idd 130 M profiles. Likewise, lost circulation indicating permeable 120 U Candalag D

110 e n

zones was encountered several times between the depth i l

100 c i Tamia t

–117 to –257 m bsl from the drill logs of CPR-1 and 90 n A

Kantipase D U Cangulay c

80 t o

l y

between the depth –17 to –178 m bsl of the drill log of u u

70 b

a l

h a CPR-2. 60 t

u M 50 o Armenia S - Sorsogon

h 40 t r

Having established the nature and type of the Malabuyoc 30 N thermal system, it can be inferred that the aquifer system is 20 10 MALABUYOC situated within the middle diagonal fault and MRF as 5 S ali 0 ring Ri outlined by 10-ohm meter contour at depth –250 m bsl. The ver heated fluid probably originates within the center of the Parali basin at the eastern portion of the survey area, was Mindanaw channeled along the middle diagonal fault and MRF and later emerges as warm spring seepage at Montaňeza River. Mahanlog Labrador The source of an abnormal thermal gradient that trigger 9*38' forced convection can be postulated as a trapped heat either due to the relatively high thermal conductivity of the Mts basement rock or due to deep burial or extensive sedimentary veneer. Figure 16: Geophysical Model of Malabuyoc Thermal System (at –250 m below sea level) If a well is to be drilled in the area for the purpose of extracting possible hot water for spa and resort development, the best target site is an area bounded by steep resistivity 8. ACKNOWLEDGMENT gradient coincident with MRF. The depth of the well should be within the range of –100 m to –300 m bsl to be able to hit The authors would like to thank the following persons who the permeable aquifer zone. contributed to the success of the survey:

7. SUMMARY AND CONCLUSION Mr. Vicente M. Karunungan, our Division Chief; Director Antonio Labios, Gerald Cabizares, Richard Russel and Enric Modeling of resistivity results depicts a faulted anticlinal Padayao of DOE-Visayas Field Office; Mayor Lito Creus of structure. Resistivity anomalies of the prospect were largely the Malabuyoc town for logistic support; Ms. Alicia Reyes controlled by fault structures with the Montañeza warm for her geophysical comment and edition of this report; and spring occurring at the western flank of the north trending to the Department of Tourism and Department of Health for Malabuyoc anticline. However, resistivity variation did not their continued trust and believe to ERDB-DOE technical. reflect the formational boundary nor the lithological differences of the various rocks mapped in the survey area. REFERENCES As such the lithologies penetrated by the nearby wells were Bureau of Energy Development. 1979. :Inventory of the used in showing the geothermal implication of the resistivity Thermal Areas in the Philippines, Philippine-Italian results. Malabuyoc system was categorized as a basement Technical Cooperation on Geothermics Stage II, DOE aquifer beneath sedimentary basins with the heated fluid Internal Report, Pp 2 probably originating at the center of the basin located east of the survey area. The fluid was channeled along the middle Del Rosario, R.A., Pastor, M.S., Malapitan, R.T., Papasin, diagonal and Montaňeza river faults and emerged at R.F. and Domingo, F.G. 2002.:Comparative Study of Montaňeza River as warm seepage. the 1999 and 2001 Controlled Source Magnetotelluric Surveys Done in Manito Lowland Geothermal The best target site to drill a well for spa and resort Resource, Manito, Albay, DOE Internal Report, 61pp. development is the area bounded by steep resistivity gradient 9 Del Rosario, Pastor and Malapitan.

Del Rosario, R.A., Papasin, R.F. and Domingo, F.G. 2000.: Mines and Geosciences Bureau. 1982.: Stratigraphy of Controlled Source Magnetotelluric Survey of Mabini Southern Cebu. Geology and Mineral Resources of the Geothermal Prospect, Mabini, Batangas. DOE Internal Philippines Vol.1 Report. Pendon, R.R. and Monzon, B. J. 1996.: Cebu Geothermal Hersir, Gylfi Pall and Bjornsson, Axel. 1991.: Geophysical Prospects, DOE Internal Report Exploration for Geothermal Resources Principles and Application, UNU Geothermal Training Program Philippine Bureau of Mines. 1956.: Geology and Coal Report 15, 88 pp. Resosurces of the Argao-Dalaguete Region, Cebu, Special Project Series. Publication No. 7, 50 pp Hochstein, M. P. 1990.: Classification and Assessment of Geothermal Resources. UNITAR/UNDP, p 31-57 Robertson Research International Ltd. 1977.: The Philippines: An Evaluation of Coal Resources, Atlas. Hsieh, S. H. and Hsu, C. H. 1974.: Well Completion Report Vol 2 of the CPR-1 Well Alegria-Malabuyoc Anticline, Cebu, Philippines, Chinese Petroleum Corporation, 39 pp Santos-Ynigo,L. 1951.: Geology and Ore Deposits of Central Cebu, Bureau of Mines Internal Report Hsieh, S. H. and Hsu, C. H. 1975.: Well Completion Report of the CPR-2 Well Alegria-Malabuyoc Anticline, Cebu, Wardell, A. 1985.: Report on Coal Resources of the Philippines, Chinese Petroleum Corporation, 50 p. Philippines, RP-UK Coal Resource Study Hsieh, S. H. and Hsu, L. M. 1977.: Well Completion Report Zonge, K. L. and Hughes, L. J. 1992.: Controlled Source of the Malabuyoc- Well Alegria-Malabuyoc Anticline, Audio-Frequency Magnetotellurics, Society of Cebu, Philippines, Chinese Petroleum Corporation, 25 Exploration Geophysicists, Investigation in Geophysics pp No.13, p 713-807. Malapitan, R.T. and Del Rosario, R. A. 2002.: Geology and Zonge, K. L. 1992.: Introduction to CSAMT, Northwest Hydrology of Malabuyoc Geothermal Prospect, Mining Association, Practical Geophysics II for the Malabuyoc, Cebu, DOE Internal Report. Exploration Geologists, p 440-49