Abstract

Ramba and Tanjung Laban are the first commercial oil discoveries in the Corridor Block, South Sumatra, in almost fifty years. Discovery wells for these two fields were drilled by Asamera/Bow Valley/Pertamina in June and August 1982 on structurally similar seismically mapped closures. Oil with some associated gas is present in Early Miocene Talang Akar sands and Batu Raja carbonates in both fields.

In the Corridor Block, north-south trending horst and graben tectonics controlled Eocene-Oligocene Lemat basinfill sedimentation over a variety of basement types and topography. Continental to marginal marine Talang Akar clastic sediments filled basins during the Late Oligocene and Early Miocene. These clastic wedges locally contain excellent reservoirs. The marine transgression continued in the Early Miocene with a thin Pendopo marine shale deposited over all but island highs. A quiet tectonic period followed during which a relatively thin Batu Raja shelfal patch-reef complex developed on seaward platform areas.

Configuration of these platform areas was determined by Eocene-Oligocene, north-south tensional faulting, but the morphology of the patch reef complexes themselves was determined by subtle topography and wind/current directions during deposition. Strong Plio-Pleistocene convergent tectonics structurally altered and raised Ramba and Tanjung Laban into closures on the upthrown sides of high-angle reverse faults. The Tanjung Laban and Ramba-Lilin fields contain structural and stratigraphic trapping elements in both clastic and carbonate reservoirs.

Understanding the geological history in this part of the South Sumatra basin has been a key factor to exploring for other new oil-productive carbonates and fluvial sands in a relatively mature exploration province.