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