Abstract

The Sulu Sea Basin is one of 14 depocenters in the Philippine Archipelago, a collage of island arcs, continental fragments and tectono-stratigraphic terranes of various origins. The 260,000 sq. km. Sulu Sea Basin is located between the islands of Palawan, Panay, Negros, Western Mindanao, the Sulu Archipelago and Sabah on the Northeastern coast of Borneo. It consists of several Northeast trending sub-basins; the Northwest and Southwest Sulu Basin; and three comparatively smaller sub-basins; namely, the Balabac, Bancauan, and Sandakan of the Northeast shelf of Sabah. These smaller sub-basins which have been collectively called the Western Sulu Basin has been the site of active exploration for oil and gas during the past two decades.

The Western Sulu Sea Basin occupies an area of approximatley 80,000 sq. km., and contains an infill of Tertiary marine sediments aggregating in excess of 7,000 meters, on a complexly structured Cretaceous-Paleogene basement. The stratigraphic section from the 21 offshore wells drilled in this basin reflect a sequence of geologic events involving basin subsidence during most of the Neogene subsequent to basin structuring events which commenced during Cretaceous-Paleogene time; the development of an extensive fluvio-deltaic system through most of the Miocene and Early Pliocene, and a regional marine transgression dominated by carbonate sedimentation during the Late Pliocene and the Pleistocene. Volcanic activity during the Miocene is also recorded in the stratigraphic sequence.

The principal source rocks are shales and siltstones of the Miocene sequence where Total Organic Carbon (T.O.C.) values range from .03 to 3%. Unequivocal evidence of hydrocarbon generation in the basin is given by the successful flow of oil and gas from Miocene deltaic sands in one of the wells in the Sabah sector, and shows of oil and gas in those drilled in Philippine territorial waters.

Seal is provided by intraformational fine clasitcs in the Neogene section, and by faults and impermeable barriers imposed by igneous bodies associated with Neogene volcanism.

Good reservoir quality rocks are present in deltaic sequences of Miocene age and carbonate buildups on volcanic mounds and synchronous highs.

Basin structuring during the depocenter’s inception in Late Paleogene time and subsequent tectonic movements has produced a variety of traps or trap situations favorably positioned with respect to suspected generative depressions. These include anticlines, wrench-fault-related folds, listric fault associated closures stratigraphic pinchouts, carbonate build ups on synchronous highs and subseafans.

Only 21 wells have been drilled in the Sulu Sea Basin in an area approximately 80,000 sq. km. In terms of drilling density this translates to one (1) well per 3,800 sq. km. The basin must therefore be considered as only partially explored.