EXPLORATION AND GEOLOGY OF THE WEST SULU BASIN, PHILIPPINES

Abstract

The West Sulu Basin lies in the western portion of the Sulu Sea. Republic of the Philippines. It occupies an area in excess of 26,000 square miles (67,000 km2) and is bounded to the west and south by the cordilleran arc extending from the island of Palawan through Sabah and along the Sulu Archipelago to the island of Mindanao. To the north-east, the basin probably extends beyond the edge of the continental shelf in Philippine territorial waters.

The basin may be broadly divided into a western platform and an eastern deep: the latter is subdivided by northeast-trending basement ridges into three sub-basins. Sediments deposited in these sub-basins are of Tertiary to Recent age and have been affected by several orogenies and by contemporaneous movements of fault-controlled blocks. This has resulted in truncation and the development of marked erosion surfaces and onlap within the Upper Tertiary section. Many anticlinal features mapped within the basin have resulted from drape over basement highs or from penecontemporaneous growth of these highs.

Major unconformities associated with Upper Tertiary tectonic events have been recognized onshore. Extrapolation to offshore areas where these events can be seismically mapped has enabled an interpretative geologic model to be built up. Provisional identification of stratigraphic units and their nature have been made using this model.

The Upper Tertiary section within the eastern deep is expected to consist of deltaic and paralic reservoir sands interbedded with, grading into and transgressed by deeper water shale and mudstone with good hydrocarbon source potential. Some limestone lenses may be present.

The presence of Lower to Middle Miocene diapiric shale and Plio-Pleistocene intrusives coupled with data of variable quality makes seismic interpretation difficult in some areas. However, several large anticlinal features and a number of stratigraphic and combination traps have been located.

A non-commercial discovery of oil and gas has been made in the basin.