The Bondoc peninsula in the southeastern Luzon Province in the Philippines has numerous surface oil seeps around subsurface faulted highs, marked in green. Concentrations of hydrocarbons in surface water seepage were collected near Line B. The long wrench fault, noted on satellite magnetic mapping, passes through the Ragay Gulf.
The Philippine Islands are a colorful and complex area, both geologically and geographically. Luzon Province, in the northern island chain, is believed to have oil and gas potential, but oil exploration is still a challenge. Older onshore seismic data, acquired in tropical rainforest conditions, is generally of poor quality, and has led to shallow drilling attempts focused on surface anticlinal features.
Luzon's offshore areas are quite different, with untapped potential. In particular, the Ragay Gulf area offers the potential for more than 100 million bbl and proximity to consumer markets in Manila. The Philippine Far East region is experiencing renewed wildcat exploration as a result of improved marine seismic data and a better understanding of the prospective play types for oil and gas.
Southeastern Luzon province, south of the capital, Manila, has intriguing oil seeps, which were discovered in the 1920s and confirmed by recent re-examination. Globex is a joint venture partner in GSEC 76 (Geophysical Survey and Exploration Contract), granted in 1994, which includes 1.5 million onshore and offshore acres. The age and geologic column of the frontier Luzon Province area is similar to N.W. Palawan, because the islands share a related depositional and tectonic history. N.W. Palawan commercial discoveries are productive in carbonate reservoirs, and include Malampaya field (3 tcf gas), and Nido Field 20 MMbo) (Grotsch et. al, 2000; Mozetic, 2000) Structural styles mapped include anticlines, wrench folds, and tilted fault blocks.
New insights
Two-dimensional seismic obtained in 1995 in the offshore Ragay Gulf (Lines A and B) show a much clearer picture of the tectonic history.
The Luzon Province exploration has been hampered by three factors: (1) Onshore seismic data is generally of poor quality. (2) Many wells lack commercial quality sandstone reservoirs because of the frequent occurrence of minerals derived from erosion of volcanic ash deposits. (3) Organic-rich shale from ancient terrestrial and marine environments is widespread, but the timing of source rock maturation changes quickly from immature, mature, and post-mature among the Philippine basins.
Two major factors have improved the exploration picture. Marine seismic data presents a clearer picture of a wide range of potential structural traps than is possible with onshore data. Modern marine seismic, starting with 2D data shot by the Australian Geophy-sical Survey (AGSO) and the World Bank during the 1980s, coupled with 1990s 2D shot and re-processed by Globex and partners, shows a number of structures that could represent large-scale subsurface oil traps. Seismic shows carbonate pinnacle reefs and carbonate banks, never identified in unsuccessful onshore wells.
As a result of the analysis of GSEC 76 marine seismic, we see better resolution on structural traps and now embrace the idea of searching mainly for carbonates, a proven reservoir type in the Philippines area.
Source rock maturation appears to have been achieved in Middle Miocene marine shales buried in the offshore Ragay Gulf area. This conclusion is based on analysis of oil, collected from shallow onshore wells perforated in the Middle Miocene, and from outcrop seeps.
Satellite radar observation has detected potential oil slicks on the surface waters of the Ragay Gulf as well. The oil analysis shows that the source is terrestrial, probably from fluvio-deltaic Tertiary shales. The Australian Geophysical Survey sampled Ragay Gulf water during seismic shooting in the mid-1980s, and found high concentrations of Type I wet gas C1-C5 anomalies where subsurface faulting intersects the seafloor.
Structural provinces
The offshore Ragay Gulf is separated into the Bondoc sub-basin to the west and the Ragay sub-basin to the east. A regional, basement-cored uplift, the Alabat-Burias high, divides the area. The Ragay sub-basin is bounded by the Philippine wrench fault, a 1,200-km long left-lateral strike slip fault visible on aeromagnetic satellite imagery (Bischke, 1990). Seafloor hydrocarbon seeps on the Alabat-Burias high are marked on the map. The geologic history of the Ragay Gulf can be divided up into tectonic phases:
- Rift (Oligocene to Early Miocene)
- Drift (Early and Middle Miocene)
- Deformation (Middle to Late Miocene) related to compressive, strike-slip wrenching due to the rotation and subduction of the Philippine Sea plate in the Pacific Ocean.
Cross-section A-A' represents the onshore Bondoc peninsula. The core of the Bondoc peninsula is composed of marine shale and thin, clastic sands. Exploration (1940-1977) used surface mapping and vintage onshore 2D seismic. This effort resulted in drilling wells with non-commercial oil and gas shows.
The pinnacle reef feature, seen on a 2D seismic line, is 200 meters tall and 500 meters wide. Miocene reefs are a favored exploration target in the Far East, including Indonesia, because of their high porosity and permeability. (Inset) This 2D line shows a colored velocity model over two carbonate features and a wrench-related reverse fold (processed by GX Technology - Houston).
Seismic Line A shows the Alabat-Burias horst block at the south end of the Ragay Gulf. This uplift remained a positive feature during the Tertiary despite vertical faulting and strike-slip movement on either side. The marine 2D seismic shows a closely spaced system of nearly vertical strike-slip faults.
During the Late Miocene, buried, organic-rich shale in the Bondoc wrench fault zone reached maturity and oil and gas migrated through fault zones into available reservoirs. The most prospective areas would be carbonate reefs located stratigraphically between the Middle Miocene (red horizon) to the Upper Miocene (orange horizon) on the seismic line. Heavy black faults are regional strike-slip planes. The 1,200-km Philippine wrench fault can be seen on the right side of Seismic Line A in the deepwater area.
Seismic line B is a regional look across the entire offshore Ragay area, composed of two joined 2D lines north of Burias Island. The Middle Miocene to Pliocene has an expanded depocenter on the east side of the Ragay Gulf, the result of deposition shed off Burias Island and the Bicol Peninsula, which onlaps the central Alabat-Burias horst block. Tectonic deformation is older in the west, primarily resulting in transpressional, low offset, vertical faulting, with local over-steepened reverse movement. Younger Late Miocene-to-Pliocene strike-slip faulting affected the eastern side. The Philippine fault is marked in heavy black lines. This zone is active today and was linked to the 1973 Ragay Gulf magnitude 7 earthquake (Conolly, Galloway, 1994; Bischke, 1990)
Where Middle Miocene horizons approach the seafloor, concentrations of wet gas have been detected. The shale is believed to be releasing small amounts of hydrocarbons to the surface via fault conduits. Surface water seeps are confirmation evidence that Middle Miocene shales are thermally mature for oil and gas, both in the onshore Bondoc Peninsula, and in the offshore marine area of the Ragay Gulf. The source kitchen fetch area for oil could be 400,000 acres, capable of generating 100-500 million bbl of oil.
Pinnacle reefs
This line illustrates the underlying pre-Tertiary high in the middle of the Ragay Gulf. Potential reservoirs are Miocene carbonate reefs; the source rock is Middle Miocene shale. On the right side of the seismic line, the recent Philippine wrench fault is marked with darker black fault lines.
The Ragay Gulf area has undrilled pinnacle reefs that appear similar to productive reefs in N.W. Palawan. A detail line (see inset, figure) shows an offshore 2D line in shallow water depths near the Bondoc peninsula. Correlations from onshore wells suggest that the reef is Upper Miocene in age and 220 meters. Internal seismic reflectors probably represent internal bedding inside the reef.
The pinnacle feature grew on top of a regional Middle Miocene seismic unconformity. Although the Ragay Gulf was tectonically active during the Middle-to-Late Miocene, the fault movement was slow enough that carbonate reefs were able to build up porous reef material in the shallow to marginal-deepwater setting, until drowned by deepening of the basin during the Pliocene. The overlying Pliocene is the seal for the reef prospect.
The Ragay Gulf has a variety of structural styles and prospective targets. Several 2D lines were improved by reprocessing using pre-stack depth migration. Seismic line D shows a colored seismic velocity model on a 2D depth line that has a pinnacle reef (red horizon on the left), a reverse fold (center) and a carbonate platform (blue area on the right). All these features are offshore in water depths no deeper than 150 meters.
To accomplish the reprocessing, the seismic field tapes were analyzed for velocity trends and a travel-time tomographic model was created for depth migration. The pre-stack processing improved our estimates of time-depth relationships in a frontier area with no wells, and led to predictions on lithology and porosity.
Conclusions
The Bondoc peninsula has attracted exploration attention because of numerous oil seeps and shallow wells with oil and gas shows. Previous ventures failed to find commercial fields because they targeted tight volcanoclastic-rich sands and missed carbonate reefs, a prospective reservoir. Marine seismic in the Ragay Gulf offshore area defines Upper Miocene and Pliocene carbonate pinnacle reefs and thrust anticlines in shallow water. Carbonate pinnacle features grew on top the Middle Miocene source rock creating a potentially easy vertical migration conduit into the reservoir.
The area has a favorable combination of source rock (Middle Miocene Vigo formation), depth of burial, and many seismically-identified carbonate reefs, seen as pinnacles and platforms in present-day shallow water. Hydrocarbons have been detected leaking up fault planes to the present-day seafloor.
References:
Conolly, J., Galloway, M., 1994, Evaluation Report, Ragay Gulf, report for the Department of Energy, Republic of the Philippines.
Bischke, R., Suppe, J., del Pilar, R., 1990, A New Branch of the Philippine Fault System as Observed from Aeromagnetic and Seismic Data; Tectonophysics, 183.
Grotsch, J., Mercadier, C, 1999, Integrated 3D Reservoir Modeling Based on 3D Seismic: The Tertiary Malampaya and Camago Buildups, Offshore Palawan, Philippines; AAPG Bulletin v. 83, No.11.
Mozetic, A., et al., 2000, Malampaya deepwater gas to power plants, AAPG Annual Meeting, New Orleans.
Sternbach, L., Conolly, J., 2000, Investigation of a Neogene petroleum system in the Ragay Gulf, GSEC 76: a frontier province in the Philippines, AAPG Annual Meeting in New Orleans.
Acknowledgements
Richard P. Crist, Senior Vice President, Robert Gunn, Board of Directors, and others, all of Globex Offshore, Ltd. were instrumental in the release of this paper.
About the authors
Linda R. Sternbachhas 16 years experience in exploration geology and geophysics, and has worked on geophysical interpretation of the Philippines for two years. Email: [email protected].
John R. Conolly has over 25 years experience in exploration geology. He has been evaluating the exploration potential of the Ragay Gulf since 1993. Email: [email protected].
