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

The Widuri-intan oilfields produce from kite Oligocene sandstones of the Talang Akar Formation, which were deposited in a fluvial-to-deltaic setting on the NWside ofthe 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. A combination of drill cuttings, sidewall and conventional cores and FMS/FMl images has been used to identify and map the distribution of basement rock type. The basement was subjected to exposure and deep weathering prior lo the formation ofthe Asri Basin, as evidenced by the zones of surface alteration encountered during drilling. The basement palaeotopography had a strong influence on the later distribution of major fluvial channels and sand pinch-outs. Several major faults appear to be. controlled by basement lithology, especially at the boundaries ofgranodiorite and.metabasalt intrusives. n important shear zone, oriented NW-SE, appears to have offset the basement between he main Widuri and Intan fields, and was subsequently the site of silicification ofthe ica schists in the basement. The Lidya field is situated where the reservoir pinches out nto eroded areas of basement silicification along this shear zone. Palaeocurrents in the upper 34-2 and 34-1 channel sandstones in the Widuri field were controlled hy the orientation of this basement feature. Drape and compaction ofOligocene Taking Akar Formation sediments over eroded volcanic plugs have defined or enhanced a number of structural/stratigraphic plays, including the Widuri and Cliesy fields. From seismic and well evidence, the reservoir sequence at the Indri field is underlain by dolomitic limestone and exhibits a series of unusual karst-reluted sinkhole and collapse structures. These are circular to slightly elliptical in shape, and extend from basement level to over 900 ft vertically into the overlying Talang Akar Formation.