Geological Socitly 0/ Malo.yJio. - Circum-Pacific Council/or Fmrgy ani Mineral &!OUI'CU Tectonic FratTUwork ani Fmrgy &!OUI'CU 0/ tbe WeJtern Margin 0/ tbe Pacific l30Jin November 27 - December 2, 1992, Kuala Lumpur, Malo.yJio.

A preliminary result of the Ragay Gulf survey in the Philippines

CHAO-SlllNG LEE!, NELSON D. TRINIDAD2 AND MALcOLM C. GALLOWAY!

1Australian Geological Survey Organisation Canberra, ACT, Australia 20ffice of Energy Affairs, Makati Metro Manila, Philippines

Abstract: During March-May 1992, the Australian Geological Survey Organisation and the Philippine Office of Energy Affairs conducted a cooperative marine seismic, gravity, magnetic, bathymetry and geochemical "sniffer" survey in four offshore Philippine basins: NE Palawan, Cuyo Platform, Tayabas Bay and Ragay Gulf. The project was funded by the Australian International Development Assistance Bureau. The survey work was conducted using the Australian Government's geoscientific vessel, Rig Seismic. The cruise achieved all its objectives and was a major success. Of the four survey areas, the Ragay Gulf is of particular interest, with complex structures, multi-phase tectonism and massive sediment infill. It was in this region that many geochemical "sniffer" seeps were located. The geochemical "sniffer" data and its C-13 and C-14 isotope analyses have indicated the presence of significant source rocks. These seeps are thermogenic in origin and have reached sufficient maturity to generate significant quantities of hydrocarbons. In the near future, the geochemical data will be integrated with the seismic data to provide clues for hydrocarbon migration. The preliminary seismic interpretation have revealed the existence of five sedimentary sub-basins with 2.5 - 5 seconds of Eocene to Recent sediments. Several types of potential traps were documented. No well has been drilled in the offshore Ragay Gulf. Onshore well log information and stratigraphy have assisted in the correlation and interpretation of offshore seismic data. In addition, the importance of the carbonate and clastic reservoirs was recognised. Overall, the oil and gas potential in the Ragay Gulf area looks most encouraging.

INTRODUCTION The project has two main objectives: 1) to upgrade the knowledge of petroleum The Philippines is currently energy deficient prospectivity of selected areas in the Philippines but recent studies and exploration results (i.e. the and to promote potential opportunities for future discoveries of West Linapacan, Malampaya and Philippines-Australia joint venture exploration; Octon oil/gas fields) indicate potential oil and gas and reserves exist in offshore Philippine basins. The 2) to assist the Philippine Government to acquire Government of the Philippines has decided to the skills to obtain and interpret seismic data actively prqmote oil and gas exploration activity in and other petroleum source-related information, the country. They have identified the tectonic and use these to focus future petroleum complexity and the lack of geoscientific information exploration in Philippine waters. as contributing to the low level of petroleum A joint consultative group consisting of 15 exploration in the majority of the 13 identified Australian and 15 Philippine companies was formed offshore basins. As the result, the Philippine to provide the project with technical advice. This Government has requested the Australian group has had, and will continue to have, a Government to assist in the form of a comprehensive significant input into the technical aspects of the offshore seismic survey in selected potential oil­ project. bearing basins. During March-May 1992, the AGSO in The Philippine Marine Seismic Survey Project conjunction with OEA have conducted a geophysical is being conducted by the Australian Geological and geochemical survey in the Philippines using Survey Organisation (AGSO, formerly the Bureau the Australian research vessel, Rig Seismic (Fig. of Mineral Resources) in conjunction with the 1). This ship has a unique capability to Philippine Office of Energy Affairs (OEA) under simultaneously collect seismic, gravity, magnetic, the funding from the Australian International bathymetry and bottom-water geochemical "sniffer" Development Assistance Bureau (AIDAB). (direct hydrocarbon detection) data; the ship also

Geol. Soc. MalaY.lia, Bulletin 33, November 1993; pp. 289-301 290 CHAO-SHING LEE, NELSON D. TRINIDAN AND MALCOM C. GALLOWAY has the capacity to conduct various forms of Basement geological sampling. The basement consists of hornblende schists A total of 2,950 km. of seismic and magnetic with scattered quartz veins. These are locally data was obtained during a 45-day cruise in the foliated. Alteration to chlorite and epidote is Philippines. In addition, some 5,000 km of common. No estimation of thickness has been geochemical, gravity and bathymetry data were possible. Age is pre-Miocene to possibly pre-Tertiary. obtained. The work was undertaken in four areas: the NE Palawan Shelf, Cuyo Platform, Tayabas Unisan Formation Bay and Ragay Gulf (Fig. 2). Unconformably overlying the basement schists Overall the cruise achieved all its objectives is the Unisan Formation. It consists of volcanoclastic and was a major success (Lee and Ramsay, 1992). carbonate breccia composed of fine-grained The seismic records indicate good quality data was porphyritic volcanics and tuffs together with obtained in most areas. The Ragay Gulf is of penecontemporaneous tuffaceous silty carbonates particular interest, with complex structures and containing rich foraminifera. The thickness of the sedimentary deposition centres of up to 5,000 m unit is unknown. In Katumbo Creek well, it shows thick. It was in this region that many geochemical close petrographic afimities in composition, texture "sniffer" seeps were located (Bishop et al., 1992; and alteration to the outcrop samples from northern Evans et al., 1992). Bondoc (BED, 1986). The age is late Eocene to late The newly acquired data are being processed Oligocene, probably in range P15-P19. at AGSO, integrated with existing seismic and well data, and interpreted and analysed for petroleum Panaon Limestone potential. The final analysis, together with basic The Panaon Limestone unconformably overlies data, will fIrst be presented in the Philippines and the Unisan Formation. It consists of limestone Australia and later internationally, to promote which varies from dense crystalline with coralline further exploration. This report is one of the relics to completely recrystallised limestone. Toward promotional papers and deals with the preliminary the south of the Peninsula this carbonate unit, if results of the Ragay Gulf survey. present, is deeply buried and beyond economically drillable depths. However its age equivalent, the STRATIGRAPHY Malakawang Limestone, crops out over Burias and Templo Islands. It is, therefore, highly probable The Ragay Gulf is bounded to the east by the that the facies extends along, and adjacent to, the Bicol Peninsula and to the west by the Bondoc Alabat-Burias Ridge, under Ragay Gulf, at Peninsula (Fig. 3). The stratigraphy of Bondoc economically drillable depths. Peninsula has the greatest relevance to the Thickness is estimated to be 500-1,000 m. It is sequence underlying Ragay Gulf and Tayabas Bay. thought to be highly variable in thickness and locally Our knowledge of the stratigraphy is based on lenticular. It was deposited in shallow marine limited number of maps and reports from oil and conditions in the Late Oligocene to Early Miocene mineral companies as well as data from the wells N3/P22-N7. drilled in the peninsula. Despite 25 wells having been drilled on the peninsula, 16 are less than Vigo Formation 1,000 m deep, only five exceed 1,500 m and two The Vigo Formation (Gumaca Formation of exceed 2,000 m deep. Just four wells, Katumbo Antonio, 1961; Yap, 1972) outcrops along the length Creek 1, San Francisco 1, 2 and Mayantok 1, have of the axis of Bondoc Peninsula. It has been a complete suite of logs. Two wells, Aurora 1, subdivided into upper and lower parts. Relations 2,088 m and Aurora 2, 1,103 m, which were drilled with the underlying Panaon Limestone vary from in 1970, probably had a modern suite of logs run conformable to unconformable. Geochemical but no logs or reports on either well have been analyses (BED, 1986) of well cuttings and outcrop located. These wells, together with San Francisco samples show the Vigo to have significant 1 and 2, were drilled less than 1 km. apart and hydrocarbon source potential. would have provided a good measure of The Lower Vigo Formation consists stratigraphic lateral continuity. predominantly of alternating sandstone and shale From the lithological descriptions of the field with, especially near its base, lenses of limestone outcrops (Antonio 1961; Yap, 1972; Poblete and up to 10 m thick, conglomerates and volcanic Ferrer, 1990) and the information from the World interbeds apparently in up to five separate horizons. Bank Report (BED, 1986), a description oflithology Locally, carbonaceous silts, shales and coal seams has been assembled. The stratigraphy is occur. The conglomerates contain pebbles of schists, summarised in Fig. 4. volcanic rocks and limestone. A PRELIMINARY RESULT OF THE RAGAY GULF SURVEY IN THE PHILLIPPINES 291

Figure 1. Australian geoscientific research vessel, Rig Seismic.

23 / 05/16 Figure 2. Survey areas of the Philippines Marine Seismic Survey Project. 292 CHAo-SHING LEE, NELSON D. TRINIDAN AND MALCOM C. GALLOWAY Canguinsa Formation The Canguinsa Formation overlies the Vigo Formation with a marked angular unconformity. It is subdivided into an upper and lower part (Lopez and Hondagua Formations respectively of Antonio 1961; Yap, 1972). BICOL The lower part of the Lower Canguinsa PENINSULA Formation consists of cross-bedded sandstone and poorly sorted conglomerate with interbedded grey shale, siltstone and dark coloured sandstone. Carbonaceous streaks occur throughout. The conglomerate contains quartz, schists, igneous rocks, limestone, some hard siltstone and sandstone with finer grains of quartz schist and igneous rock fragments. In the upper part, the conglomerate is lighter in colour, calcareous and contains mica flakes but little carbonaceous materials. Maximum recorded thickness is 1,490 m in Bondoc 3 but similar thicknesses have been measured in the field in northern Bondoc. As with the Vigo Formation the presence of limestones and thin coals and carbonaceous silts and shales indicate a varying depositional environment from shallow I..,.' ...... marine to littoral-paralic. Age is Late Miocene and Water depth (km) ranges NI6-NI7. The Upper Canguinsa Formation overlies the Seismic line Lower Canguinsa with no angular unconformity. A Sniffer line time break disconformity is suggested by the absence • Type I anomalies of thin shelled megafossils in the Lower Canguinsa • Type II anomalies and their common occurrence in the Upper L....------TGiS092,;sBiw/ Canguinsa. South of Buenavista, on the east side of Figure 3. Seismic and geochemistry lines in the RagayGulf. the Peninsula, there is an obvious angular Location of geochemical "sniffer" anomalies. unconformity in the field as well as changes in San Francisco 1 and 2. Faunal changes in the cuttings Thickness of the unit in its incomplete section have been recorded in Katumbo Creek 1, San is 2,840 m but the total thickness was estimated at Francisco 1, 2 and Mayantok 1. Poblete and Ferrer 3,000-4,000 m (Antonio, 1961). It was deposited in (1990) believe the provenance of the Canguinsa an environment that varied from shallow marine, Formation was from the Tayabas-Marinduque area. as evident from the common limestone throughout, Lithologically it consists predominantly of to coastal paludal swamps, as is evident from the indistinctly bedded calcareous and micaceous, buff thin coal seams and carbonaceous silts and shales. coloured siltstone interbedded with thin mudstone Age is earliest Middl.e Miocene N8-N9. layers and some limestone. There are conspicuous Upper Vzgo Formation shows a marked increase thin shelled megafossils throughout. in the proportion of conglomerate-sandstone as The maximum thickness claimed by Antonio compared to sandstone-shale of the Lower Vigo (1961) exceeds 1,025 m but is believed to thicken to Formation. Individual conglomerate-sand beds are the southeast. The unit appears to have been commonly over 1 m thick. Limestone lenses are deposited exclusively in shallow marine conditions scattered throughout the section. Volcanic during the Pliocene NI8-N21. interbeds are known from several locations but are restricted to the northwest near Pitago-Unisan. In Malumbang Formation the Southern Bondoc Peninsula, the formation is The Malumbang Formation (Catanauan less sandy and has been termed the Vigo Shale Limestone of Antonio, 1961; Yap, 1972) (Hashimoto and Matsumaru, 1981). Maximum unconformably overlies the Canguinsa and all older thickness was measured at 1,870 m. It was units as a relatively flat drape. It consists deposited in similar conditions to the Lower Vigo predominantly, in the lower part, of chalky white, Formation in the Middle Miocene NI0-NI4. porous, dense, crystalline megafossiliferous APRELIMINARY RESULT OF THE RAGA Y GULF SURVEY IN THE PHILLIPPINES 293

rU'GIIIIII"""GI 'ullauu.... MAJOR wCI) 4111 .... - ., :J: .!! : STRATIGRAPHY STRUCTURAL STRUCTURAL OW ::E ~ ~~~a.;: '" 4( ~>- 0 STAGE -go!!--:- STAGE EVENTS ~~g ~ .&.! E a: 'D CI) 0 w ",0 g W 0 D.. ~OQ w (!J ~D.. < ;: W ( 1) (2) (3) ~~~ (5) (6) (6) W:J: (!J < .. 111- a:... < D.. w- rf A..I,)"(- r--- r'1-.s...:...... : __ II_ __1:",/_ • I. I I 770+ STAGE IV Major sinistral 1025+ strike-slip 0 WRENCH In FAULT motion along Tf J I lower I 1490+ I'~c STAGE Philippine 0 u Fault System E Tt 2 I Upper LL I 1870+ I STAGE III 0 I 0" :> lower I I 2840+ Tt,

BURDIGALIAN I 124+ Ponoon I limestone BLOCK Te 5 FAULT AQUITANIAN I N4 I 0 I Globorofolia kugleri 23.8-- 0 STAGE

Globigerina ciperoensis CHATTIAN I .-- I Block- 30 =:::1101 r I I_I 1["4" faulting 30 ---l P21 !-=-l Globorotalia opima ~ initiating horst RUPELIAN and IPI'''"I_m _I~ I~~ ~Q graben features Unison Fm I 43+

• Tb 40 • ___ :.-_.-"_'__ J_ I I STAGE II

P12 Marozovella lehneri

I TO J Globigerap'sis LUTETIAN P11 subconglobofo PLATFORM 50_£11 1,"1.-.1 I P10 Hanfkenina oraganensis STAGE 52 P9 Acarinina pentacamerota Morozovella aragonensis P8 ~ T0 Basement YPRESIAN / Moro7ovello fDrmD.~D "- 2 Schists

60

I P3 :~I "u"v'vw'nuJ eVJmv ITo, Morozovena uncmata ~ C;"hhflfmfl f, mirlrlrl"nr",,, DANIAN c - ~, fill S I' _._-":1-.... - --:1--"'- r 1. Berggren al al. (1985a. b). Burger Qn prep •• a & b) 4. Adams (1984), Adams at al. (1986), Chaproniere (1981, 2. Blow (1969, 1979), Berggren (1969) 1983). Jenkins et aI. (1985) 3. Balli (1957a & b, 1966), Stainforth el aI. (1975), 5. Antonio (1961), Yap (1972), OEA (1988) Balli & Premoli·Silva (1973), Caron (1985) 6. BED (1986) Figure 4. Stratigraphy of Bondoc Peninsula. 294 CHAO-SHING LEE, NELSON D. TRINIDAN AND MALCOM C. GALLOWAY limestone. In the middle part there is a sequence 3) Siltstones and fine to very fine sands could not of silty to shaly calcareous sandstone with limestone be differentiated, presumably due to the high interbeds. In the upper part it consists of porous, clay, basic rock fragments and Fe-Mg mineral gently dipping limestone. In the south, this three­ content. fold subdivision is not evident. The Malumbang 4) No coals were identified which was presumed Formation is generally a very shallow marine to be due to coal and coaly intervals being carbonate complex with the lithology varying from thinner than resolution of the logging tools. reefal limestone to detrital carbonates and some In the Canguinsa interval of San Francisco 1 clastic facies. and 2 and the Canguinsa and Vigo intervals of Maximum recorded thickness is about 770 m Katumbo Creek 1 and Mayantok 1, thin limestone and age is Pleistocene, N22-N23. was evident from spikes on the resistivity and DT but little other lithological differentiation could be LITHOLOGY IDENTIFICATION FROM recognised. This was thought to be due to the high clay and carbonate content. This was despite, in WELL LOGS the case of the -Canguinsa section of the San There is a major problem in interpreting the Francisco wells, the mudlog clearly showing fine­ original logs because of the high proportion of clay, medium and fine-coarse sandstone in 10-60% of volcanogenic rock fragments and Fe-Mg minerals each logged interval. together with the lack of quartz and radioactive minerals. This results in the gamma ray only RESERVOIRS varying in the range 15-45 API units with no consistent pattern of variation between sands, Evaluating the reservoir potential of the shales and coals. Only minimal variation is recorded sequence is essentially conjectural as only four fully on the resistivity logs and FDC-CNL logs do not logged wells were available. Both carbonate and follow the pattern on variation which characterises clastic reservoirs with suitable porosities are to be rocks derived from a more mature granitic or expected beneath Ragay Gulf. Outcrop geology of quartzose provenance. Attempts to match calliper, Bondoc Peninsula shows that carbonate sequences gamma, SP, DT, FDC and CNL responses with of significant thickness are present in the Panaon lithology using conventional methods has shown no Limestone, the base of the Lower Vigo Formation apparent consistency. and in the Malumbang Formation. Thinner In an attempt to identify lithologies from the carbonates are present in the Vigo and Canguinsa logs, the logs were digitised and redisplayed at Formations. Clastic reservoirs are possible in the magnified scales as follows: Lower Canguinsa and Lower Vigo Formations. the calliper was displayed at 6-16 inches Carbonate Reservoirs together with the gamma ray which was displayed at 1-50 API units; 1) The Panaon Limestone occurs overlying the the induction or short normal was displayed at basement outcrops near Lian Point of the Northwest logarithmic 0.6-10 or 0.5-20 together with the Ragay Gulf and the Panaon equivalent, the DT displayed at 260-60 or 250-50, and; Malakawang Limestone, is known to exist on Burias the NPHI was redisplayed in porosity units and Templo Islands. Seismic evidence shows a and at 0.5-0.1 together with the FDC which high amplitude event at shallow to moderate depth was also redisplayed in porosity units 0.65- along the length of the Alabat-Burias Ridge and 0.05. adjacent. The Panaon-Malakawang Limestone can This allowed identification, especially in the be followed throughout much of Ragay Gulf where San Francisco wells, of: it is characterised by a high amplitude event that 1) Limestones and carbonate cemented sands underlies the comparatively bland Vigo interval. which showed low gamma values, high The Panaon event is variable in thickness but resistivity, high sonic, and low neutron and consistently characterised by a high amplitude density porosity. The carbonate cemented sands event. It is found at 0.8-1 second depth along the were particularly evident in the Vigo Formation crest and 1-1.5 seconds down the flanks of the of the San Francisco wells. Alabat-Burias Ridge where it is at readily drillable 2) Non-carbonate cemented sands showed low depth. Careful analysis of the seismic, including gamma, moderate resistivity, very low neutron specialised processing may produce events that porosity and low density porosity. The relatively reflect variation in porosity within the carbonate. lower neutron porosity was thought ·to be due 2) The Vigo Formation carbonates that are to the high content of clay, unstable basic around 10 m thick in outcrop may also occur offshore igneous material and Fe-Mg rich minerals. but have not yet been identified with confidence on I A PRELIMINARY RESULT OF THE RAGAY GULF SURVEY IN THE PHILLIPPINES 295 the seismic. identified major horizons. However, the quality of 3) The Upper Canguinsa Formation carbonates, data greatly diminishes in areas where diapirism although very young, and not overlain by seals has taken place and in zones of extensive faulting. onshore, may provide sealed reservoirs offshore In both, seismic reflectors are extremely poor if not where the thickness of the Canguinsa increases completely obliterated. Similar conditions can be and is overlain by up to a half a second of sediments. observed onshore where available seismic and radar These have optimum attraction in deeper parts of imagery data suggest a northwest-southeast trend the trough where the thickness of cover is greatest. of fault-related flower structures in Bondoc Peninsula. Clastic Reservoirs No offshore well has been drilled in Ragay Gulf. From the outcrop geology and well data, clastic The horizons were based on the general reflector reservoirs are likely to be found in the Lower characteristics and were interpreted across major Canguinsa and Lower Vigo Formations. Despite structures such as faults, grabens and horst blocks outcrop descriptions of quartz rich sands, well data which are the predominant features in the east indicates these are generally volcano-clastic Fe-Mg Ragay Gulf. Six horizons were identified and tied rich reservoirs with only about 10-15% quartz. with the onshore stratigraphy. Furthermore where encountered in the San The Blue horizon, a fairly consistent deep event, Francisco wells, they were all too often carbonate appears as strong, low frequency and continuous cemented with high resistivity and sonic values. In reflection. Although generally mappable throughout Mayantok 1, maximum sand grain size is only very the gulf area, this event is barely recognisable in fine to fine but Mayantok did not test the whole reflection-free diapir zones in east of Ragay Gulf. Vigo section and may not have drilled into the This event is initially described here as equivalent Lower Vigo. If the source ofthe Vigo and Canguinsa to top of Unisan volcanics of probable Late Eocene sediments was from the west, then beneath Ragay to Early Oligocene age. Gulf, cleaner sands may be present. However, The Green horizon, above the blue, is equivalent from the geochemical evidence, the numerous seeps to the top of the Late Oligocene-early Miocene and the many fluorescing side wall cores in the Panaon Limestone encountered in the onshore exploration wells, it would seem highly likely that Katumbo Creek well. Reflections are characterised any porous sand with an effective seal could be oil by a thin chaotic event in the central ridge and a or gas filled. Any drilling should be done with moderately divergent pattern which progressively muds that are least likely to react with the unstable thickens towards the deep trough. In zones of mineralogy of the clastic sands so that porosity is extensive step-faulting (near south tip of Bondoc not occluded. Peninsula and in north of Ragay Gulf), the event is obscured. SEISMIC INTERPRETATION The Orange horizon, a distinct event above the green, is clearly evident in some of the new AGSOj Tying the stratigraphy to the onshore seismic OEA data. Where it is mappable such as in the and then linking with the offshore seismic of Ragay deeper part of the Ragay Gulf, this sequence is Gulf was done using the Mayantok 1 well and the prognosed to be the equivalent to the top of the 1988 Ran Ricks seismic. The well data was tied to Middle Miocene Lower Vigo Formation. the NE-SW seismic line (B-105) that passed through The Red horizon, a major unconformity, the Mayantok 1 well location, then correlated to separates the Middle Miocene Upper Vigo from the the set of NW-SE tie lines that ran to the northwest overlying Late Miocene Lower Canguinsa toward the Bigol River area. The quality of the Formation. This unconformity differentiates two seismic was such that the outcrop geology had to be depositional sequences which show similar internal constantly plotted on the seismic sections to ensure stratal configurations although the red event the validity of the seismic interpretations. There overlies a discontinuous sequence which becomes are five SW-NE cross lines to which the seismic progressively continuous toward the troughs. As picks were transferred then, where possible, with the underlying green event, the red event transferred to the offshore seismic by skip onlaps the blue in the central ridge, close to where correlation. the sediments outcrop as shallow reefs and islets. Several vintages of lines were used in the initial The Yellow horizon represents the internal offshore data interpretation, including preliminary boundary between the Upper and Lower members stacks of the newly acquired AGSOjOEA lines. The of the Canguinsa Formation but there is little overall data quality can be described as fair to distinction between the two sequences. Both are good, specially the recent lines which clearly show characterised by very continuous, high amplitude better internal reflector characteristics of the and medium to high frequency reflections although 296 CHAO-SHING LEE, NELSON D. TRINIDAN AND MALCOM C. GALLOWAY the lower event appears less continuous in some Oligocene to Early Miocene. Carbonate build-ups part. This event is conspicuously absent over appear to have been developed within the Panaon structural highs. Its absence farther south of the Limestone, as is evident from the reef-like patterns. Bondoc Peninsula suggests that the formation is In the deeper part of the trough, there exists a confined to Ragay Gulf and onshore Bondoc. series of diapir structures composed mainly of low The top Pink horizon which is confined in the velocity materials (Le. clay or mud). Distinguishing deep Ragay Gulf separates the Canguinsa from the the low velocity reflectors from the sea bottom recent sedimentary accumulation with materials multiples in velocity analysis for processing is probably derived from the regional uplift and erosion difficult and nearly all the processing techniques during the Pleistocene. This is interpreted as the done so far have failed to identify and remove the top of the Upper Canguinsa and the base of multiples. Malumbang Formation. The Burias Sub-basin, a relatively smaller basin northeast of the Burias Island, has maximum OFFSHORE SEDIMENTARY BASINS sediments of up to 2 seconds. The Oligocene Panaon Limestone varies in thickness and depth with Five sedimentary sub-basins have been probable carbonate build-ups observable within. identified (Fig. 5), occurring on both sides of the North of Ragay Gulf, the Capuluan Sub-basin Alabat-Burias Ridge that passes northwest­ extends onshore with maximum measured thickness southeast through the middle of Ragay Gulf. This of up to 2.5 seconds. Complex tectonics have ridge is bounded on both sides by the Philippine resulted in a series of tilted fault blocks in the sub­ fault system. The Capuluan, Ragay and Burias basin. The interpreted structural high, possibly a sub-basins form a major elongate depositional sub-branch of the Philippine fault system, separates terrace west of the central ridge. On the other side, it from the Peris Sub-basin to the west. Tectonics the shallow Peris sub-basin joins the much deeper and sediment infill are similar in both. Wrench Bondoc sub-basin to the south, forming the other related flower structures are present onshore where major depositional centre that extends to onshore the Peris sub-basin truncated. Bondoc Peninsula. The entire Ragay Gulf was classified as a back­ The Bondoc Sub-basin contains possible Eocene arc basin modified by strike-slip wrenching along to Recent sediments with up to 3 seconds of fill the Philippine fault system which cuts through its south of Alibijaban Island. This sedimentary basin central portions (BED, 1986). It consists of elongate thickens onshore toward the southeast part of the NW-trending ridges and sub-basins formed by Bondoc Peninsula and pinches out in area adjacent several phases of tectonic evolution. Early studies to the wrench fault-related flower structures along done on the area have indicated a two phase the major Philippine fault system passing through tectonism for Ragay Gulf: block faulting in the the peninsula. These flower structures can be Oligocene to Middle Miocene and wrench observed in radar imagery data (ARCI, 1988) deformation in the Late Miocene and younger. The although not clearly visible in the Landsat 5 satellite seismic interpretation supports a more complicated imagery data. Here the Panaon Limestone (green geohistory . horizon) is very thick (Fig. 6) which is suggestive of considerable subsidence during the Oligocene time GEOCHEMISTRY RESULTS The Malumbang Formation (pink horizon) is missing, indicating a major period of uplift and Field observations of geochemical "sniffer" erosion during the Pliocene-Pleistocene time when program on board the R/V Rig Seismic showed movement along the Philippine fault system had minor oil/gas seeps over the NE Palawan Shelf and shifted to the south (Bischke et al., 1989; Sarewitz major ones in Ragay Gulf (Lee and Ramsay, 1992). and Lewis, 1991). Those in Ragay Gulf appear to be related to major The entirely offshore Ragay Sub-basin covers faulting and/or deep diapiric structures. Some of more than 150 km of the deep trough along the east these structures (Le. Alibijaban, R-1 and R-2 of the Alabat-Burias Ridge. To the north, it is prospects) were originally identified in previous separated from the Capuluan Sub-basin by a ridge reports by exploration companies and in the World and to the south, it pinches out near the edge of the Bank Report (BED, 1986), while new structures Bicol Peninsula. Sediments thicken to about 5 and seeps have been identified by the current seconds towards the middle of the deep trough. program (Fig. 3). Here the interpreted Oligocene Panaon Limestone Two types of geochemical anomalies have been is relatively thin compared to that in Bondoc sub­ observed (Bishop et aI., 1992). Type I is basin (Fig. 7), suggesting a major shift in regional characterised by high concentrations of methane, tectonics and subsequent sedimentation during late ethane and propane with traces of butanes and A PRELIMINARY RESULT OF THE RAGAY GULF SURVEY IN THE PHILLIPPINES 297

Figure 5. Structural framework of Ragay Gulf showing location of 5 sedimentary sub-basins and the Alabat-Burias ridge. Bendoc Peninsula well names are: Amoguis, A; Bondoc, B; Maglihi, M; San Francisco, SF; Sapa, S; Yeeban, Y. Bicol Peninsula well names: Pili, PI; Sto Domingo, D; Sto Nino, N.

o 10 20 30 km I I

-500- Water depth (m) RAGAY ·GULF

23 / 05/2 4 pentanes. Hydrocarbon wetness values (% wetness= geochemical anomalies. Type I, increasing wetness (C2+C3+C4)/ (C1+C2+C3+C4) x 100) are 1-1.5%. with increasing methane, suggested the These are referred to as wet gas anomalies. Type hydrocarbons are sourced from gas-condensate rocks II, known as dry gas anomalies, are characterised or accumulations. Type II, decreasing wetness with by high concentrations of methane, with only traces increasing methane, are indicative of either dry of ethane. Hydrocarbon wetness values are <0.5%. thermogenic gas or biogenic hydrocarbons. The differences between these two types of In order to help clarify the "origin", whether anomalies are shown in cross-plots of percent thermogenic or biogenic, 25 gas samples were sent hydrocarbon wetness versus methane concentration to the New Zealand National Institute of Water (Fig. 8). The trends in these plots may be used to and Atmospheric Research Limited for C-13 isotopic predict the hydrocarbon "source" (i.e. dry gas, gas­ analyses and subsequently 8 of them were selected condensate and liquid prone), which produce the for additional C-14 analyses. 298 S N YELLOW

RED ..-.. --(f) ill 1 ~ GREEN ~

>-c:x:: S BLUE I 2 0 S ~

LINE 109/62 AGSO 921883BW 2KM

Figure 6. Seismic interpretation in the Bondoc Sub-basin, Line 109/62. Pink:Pliocene Upper Canguinsa Fm equivalent; Yellow:Late Miocene Lower Canguinsa Fm equivalent; Red:Middle Miocene Vigo Fm equivalent; Green:Late Oligocene-Early Miocene PanaonLst equivalent; Blue:Eocene­ Early Oligocene Unisan Fm.

w E

PINK

YELLOW ..-.. (f) RED 1 ill ~ ~ GREEN «>- S I 0 2 S BLUE I-

LINE 109/43 1 KM AGSO 92/882BW

Figure 7. Seismic interpretation in the Ragay Sub-basin, Line 109/43. Pink:Pliocene UpperCanguinsa Fm equivalent; Yellow:Late Miocene Lower Canguinsa Fm equivalent; Red:Middle Miocene Vigo Fm equivalent; Green:Late Oligocene-Early Miocene Panaon Lst equivalent; Blue:Eocene-Early Oligocene UnisanFm. A PRELIMINARY RESULT OF THE RAGAY GULF SURVEY IN THE PHILLIPPINES 299 The C-13 analyses showed that most samples 1. The oil and gas potential in the Ragay Gulf are of a thermogenic origin, ranging from -57 to - area looks encouraging, with many of the 38% (Evans et al., 1992). According to the Bernard petroleum occurrence criteria having been model (Bernard et al., 1977; Rice and Claypool satisfied. 1981; Claypool and Kvenvolden, 1983), all Type I 2. From seismic interpretation, we have observed anomalies (8 samples) were found to be thermogenic several types of potential traps: in origin and the Type II indicate thermogenic (7 structural traps (i.e. B-1 prospect), samples), mixed thermogenic-biogenic (9 samples) stratigraphic traps (i.e. R-1 and B-2 prospects), and biogenic (1 sample). The C-14 measurements combined structuraJjstratigraphic traps (Le. have further supported the evidence for a "fossil" R-2 and Alibijaban prospects), and hydrocarbon source. carbonate reefal build-ups (in Ragay and According to the Tissot and Welte model (1984), Burias sub-basins). both the molecular and isotopes data suggested 3. The geochemistry sniffer results and the isotopic that the geochemical anomalies may be "sourced" analyses from the Ragay Gulf seeps indicated: from gas or condensate-prone source rocks (Evans the presence of significant source rocks, et al., 1992). It is interesting to note that the the source rocks are of thermogenic origin, adjacent onshore wells, San Francisco1 and they probably have reached sufficient Bondoc3, have indicated a terrestrially derived, maturity, and waxy kerogen source (BED, 1986). Further analyses the geochemical data will be integrated with of probable source rocks would help to clarify the the seismic data to provide clues for actual kerogen type and composition of the source. hydrocarbon migration. 4. From onshore well information, we have CONCLUSIONS suggested that the target for reservoir rocks: • the carbonate sequences are almost Processing of the recently acquired AGSOjOEA certainly the primary target for reservoirs, seismic data is ongoing. However, the preliminary the clastic sands, although a secondary interpretation of stack sections and previous target, may be cleaner beneath Ragay Gulf industry data in conjunction with the geochemistry and should not be overlooked, especially as data have led to following conclusions: gas reservoirs.

Methane v. %Wetness

2.5

• / Analytical noise 2 0

• i} g 0 1.5 11 ~ 0- eD •• 0 oS Type I anomaly trend 0 ~ • 0 0 0 ~ 0 0 ~ aD 0 1 .dib 0 tfI' •• 0 0 0

·1i~~1.""~Ij;! c 0.5 ." Ii'!: iIb-. r~ 0 Q,,~ ~':I~~li~g :~...... Type II anomaly trend ...... -.. "I If". •••• • ••• •• • • • • • • II• • 0 0.00 10.00 20.00 30.00 40.00 SO.OO 60.00 70.00 80.00 90.00 ppm Methane

Figure 8. Methane concentration versus % wetness in the Ragay Gulf. 300 CHAO-SHING LEE, NELSON D. TRINIDAN AND MALCOM C. GALLOWAY ACKNOWLEDGEMENTS BLOW, W.H., 1969. Late Middle Eocene to Recent planktonic foraminiferal biostratigraphy. In: P. Bronniman and The enthusiasm, skill, and cooperation of the H.H. Renz (Eds.), Proceedings of the 1st International masters and crew of the Rig Seismic and African Conference on Planktonic Microfossils. Geneva, 1967, I, Queen are gratefully acknowledged. Published with 199-422. permission of the Director of the Australian BLOW, W.H, 1979. The Cainozoic Globigerinida. A study of the morphology, taxonomy, evolutionary relationships and the Geological Survey Organisation, Canberra, ACT, stratigraphical distribution of some Globigerinida (mainly Australia. Globigerinacea). E.J. Brill, Leiden. Bow, H.M., 1957a. The genera Globigerina and Globorotalia in REFERENCES the Paleocene-lower Eocene Lizard Springs Formation of Trinidad, B.W.I. In: Studies in Foraminifera. United ADAMS, CG., 1984. Neogene largerforaminifera, evolutionary States National Museum Bulletin, 215, 61-81. and geological events in the context of datum planes. In: BoLU, HM., 1957b. Planktonic foraminiferids from the N. Ikebe and R. Tsuchi (Ed.), Pacific Neogene Datum Oligocene-Miocene Cipero and Lengua Formations of Planes: Contributions to Biostratigraphy and Chronology, Trinidad, B.W.I. In: Studies in Foraminifera. United Univ.ofTokyo Press,47-67. States National Museum Bulletin, 215, 97-123. ADAMS, CG., BUITERIN, J., AND SAMANTA, B.K., 1986. Larger BOLLI, HM., 1966. Zonation of Cretaceous to Pliocene marine Foraminiferida and events at the Eocene/Oligocene sediments based on planktonic foraminifera. Boletin boundary in the Indo-West Pacific region. In: C Pomerol Infonnativo, Associacion Venezolan de Geologia, Mineria y and I. Premoli Silva (Eds.), Tenninal Eocene Events, 237- Petroleo, 9, 3-32. 252. Elsevier, Amsterdam. BOLLI, H.M., AND PREMOLI SILVA, I., 1973. Late Cretaceous to ANTONIO 1.5.,1961. Geological report on PEC 68, 73, and 102 Eocene planktonic foraminifera and stratigraphy of the Northern Bondoc Peninsula Quezon Province, Luzon Leg 15 sites in the Caribbean Sea. In: N.T. Edgar, J.B. Island, Philippines, Fourth Calendar Year. Maremco Saunders, et al. (Eds.), Initial Reports of the Deep Sea Mineral Corp. and Republic Resources and Development Drilling Project, 15, 475-497. U.S. Government Printing Corp. 53p. Unpublished. Office, Washington. ARO,1988. Radar image interpretation to assess the hydrocarbon CARON, M, 1985. Cretaceous planktonic foramnifera. In: H potential offour sites in the Philippines. Arkansas Research M. Bolli, J.B. Saunders and K Perch-Neilson (Eds.), Consultants Inc., 3 volumes, 142p. Planktonic Stratigraphy, 17-86. Canterbury University BED, 1986. Sedimentary Basins of the Philippines - their Press. Geology and Hydrocarbon Potential. World Bank financed CHAPRONIERE, G.CH., 1981. Australian mid-Tertiary larger Petroleum Exploration Promotion Project, prepared by foraminiferal associations and their bearing on the East the Bureau of Energy Development Office of the Indian letter Classification. BMR Journal of Australian President, under the supervision of Robertson Research Geology and .Geophysics, 6, 145-151. (Australia) and Flower Doery and BuchanPty Ltd. 1986. CHAPRONlERE, G.CH., 1983. Tertiary larger foraminiferids BERGGREN, W.A., 1969. Cainozoic chronostratigraphy, from the north western margin of the Queensland Plateau, planktonic foraminiferal zonation and the radiometric Australia. Bureau ofMineral Resources Australia, Bulletin time scale. Nature, 224, 1072-1075. 217,31-57. BERGGREN, W.A.,KENr,D.V.ANDFLYNN,J.J., 1985a. Palaeogene CLAYPOOL, G.E. AND KVENVOLDEN, KA., 1983. Methane and geochronology and chronostratigraphy. In: N.J.Snelling other hydrocarbon gasses in marine sediment. Ann. Rev. (Ed.), The Chronology of the Geological Record. Earth Planet Sci. 11, 299-327. Geological Society of London, Memoir, 10, 141-195. EVANS, D., HEGGIE, D.T., BISHOP, J.H., REYEs, E.N. AND LEE, CS., BERGGREN, W.A., KENT, D.V., AND VAN COUVERING, J.A., 1985b. 1992. Light Hydrocarbon Geochemistry in Bottom Waters The Neogene: Part II. Neogene geochronology and of the Philippines Continental shelf, Part A: Ragay Gulf chronostratigraphy. In: N.J. Snelling (Ed.), The and Tayabas Bay. Australian Geological Survey Chronology of the Geological Record. Geol. Soc. of Organisation, Record 1992/92,350. London, Memoir, 10, 211-250. HASHIMOTO, W. AND MATSUMARU, K, 1981. Geological BERNARD, B., BROOKS, J.M. AND SACKETI, W.H., 1977. A significance of the discovery of Nummulites fichteli geochemical model for characterization of hydrocarbon (Michelottz) from the Sagada Plateau, Bontoc, Mountain gas sources in marine sediments. 9th Annual.Offshore Province, Northern Luzon, Philippines. Contributions to Technology Conference, Houston Texas, OTC 2943 (May the Geology and Paleontology of Southeast Asia, CCXVI, 1977),435-438. University of Tokyo Press, 75-192. BISCHI

POBLETE, RG. JR., AND FERRER, A.P., 1990. Regional geological STAINFORTH, RM., LAMB, J.L., LUITERBACHER, H., BEARD, J.H., study of the total Canguinsa section. Internal report, AND JEFFORDS, R.M., 1975. Cainozoic planktonic Lomar Logistic/Marketing Philippines Inc., 12p. foraminiferal zonation and characteristics of index forms. Unpublished. University ofKnnsas Paleontological Contributions, Article RICE, D.D. AND CLAYPOOL, G.E.,1981. Generation, accumulation 62,1-425. and source potential of biogenic gas. Am. Assoc. Petrol. TISSOT, B.P. AND WELTE, D.H., 1984. Petroleum formation and Geol. Bull., 65, 5-25. occurrence. Springer Verlag, Berlin, 2nd Ed., Fig. 11.6.7, SAREWITZ, D.R AND LEwIs, S.D., 1991. The Marinduque intra­ p.210. arc basin, Philippines: Basin genesis and in situ YAP, A.L., 1972. Geological Report on PEC 68 Bondoc development in a strike-slip setting. Geological Society of Peninsula, Quezon Province. Maremco Mineral America Bulletin, 103, 597-614. Corporation. 14p. Unpublished.

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Manuscript received 12 March 1993