Journal of Petroleum Geology, vol. 22(2), April 1999, pp. 141-165

BASEMENT LITHOLOGY AND ITS CONTROL ON SEDIMENTATION, TRAP FORMATION AND HYDROCARBON MIGRATION, WIDURI-INTAN OILFIELDS, SE SUMATRA

P. C. Tonkin* and R. Himawan**

The Widuri-Intan oilfields produce from late Oligocene sandstones of the Talang Akar Formation, which were deposited in a fluvial-to-deltaic setting on the NW side of the Asri Basin, offshore SE Sumatra. The Asri Basin is of rift origin and formed during the early Oligocene, with its axis oriented in a NE-SW direction. Approximately 310 million brls of oil have been produced from the fields within the 12-by-12 mile (20-by-20 km) study area. The oil occurs in a series of structural and stratigraphic traps within slightly sinuous to meandering channel sandstone bodies. The reservoir sequence (sandstone interbedded with minor mudstone and coal) overlies basement rocks, which are predominantly Cretaceous in age. Forty-nine well penetrations have shown that the basement is composed of one of four lithologies: (1) hornblende granodiorite; (2) metamorphic rocks, mainly mica schist; (3) plugs of metabasalt and related volcanic rocks; or (4) dolomitic limestone.