Oil, geology, and changing concepts in the Southwest Philippines (Palawan and the Sulu Sea)

The Southwest PhilippinesPalawan, its offshore shelves, Reed Bank, and the southwestern Sulu Sea encompass 350,000 km2 (126,000 mi2). This region contains the better areas to search for significant oil and gas deposits in the Philippines. The two largest oil fields yet found in the Philippines were discovered and confirmed in 1991-1992. West Linapacan field has more than 100 million barrels of recoverable oil. In June 1992, Shell Oil is reported in the Philippine press to have identified the possible occurrence of a 200 to 400 MMBO field at Camago-Malampaya about 30 km southwest of West Linapacan. The Southwest Philippines\u27 oil producing trend, about 40 km off the northwest coasts of Palawan and Busuanga is a northerly trending belt 200 km in length. Oil reservoirs in the Philippine fields are Oligocene to Lower Miocene platform limestones, reefs, and both silicic and carbonate turbidite sequences. A 540 km gap occurs between the Philippine production at Nido and the closest oil fields to the southwest in Sabah. Untested prospects and leads are present along the South China Sea coast of Palawan. The southwest Sulu Sea overlies three poorly explored Tertiary sedimentary-structural basins (Balabac, Bancauan, and Sandakan basins). Geology and oil shows suggest the possible presence of commercial oil fields. Prospects and prospect leads are numerous. One well in the Malaysia sector of the Sulu Sea (Nymphe Norde 1) suggests economic potential. Deep-water exploration targets (untested reefs and structural traps) are present in the South China Sea offshore Palawan. Large gas-condensate and oil reserves are indicated at Camago-Malampaya in 2400+ feet of water. A test of a major deep water reef prospect, Sarap-1, provided an exploration disappointment in 1991. Other major reef traps (e.g. Cliff Head) in deep water with better source rock association remain to be tested. Besides economic interest, the Southwest Philippines has been the spawning ground of several concepts and theories about the origin and evolution of the overall region. Some theories and concepts are reviewed. The "Ulugan Bay fault" is disputed and recommended for elimination from future maps based on later field work and offshore geophysical studies. A major redefinition is an alternative. Two schools of thought on the origin of much of Palawan are reviewed. Was continental crust from the South China Sea area subducted beneath Palawan, or is Palawan a complicated thrust pile composed in part of original crust types and early Tertiary sediments that have been thrust or emplaced to the northwest from the Sulu Sea region? Some future exploration areas, plays, and prospects are illustrated