Abstract

A suite of oils was recovered from petroleum exploration wells and fields in the offshore, and onshore in Pertamina's Unit 1 area of the North Sumatra Basin. Some of the oil shows had not been recognised previously, and were identified from examination of the gas chromatograph of mudlogs. These oils were solvent extracted from the cuttings.

Detailed geochemical analyses have revealed that at least two separate oil phases have been generated. The oldest of these is now severely biodegraded and was probably expelled from the syn-rift Early Oligocene Bampo Formation. This formation probably generated significant volumes of oil but is presently overmature in the half-graben depocentres.

The regionally widespread post-rift Late Oligocene to Middle Miocene Lower Baong/Belumai Formation is the source rock for the second and final oil phase, which is characterised by specific biomarkers and is not biodegraded.

The Lower Baong/Belumai sourced oils range from condensates or light oils, with a very low abundance of aromatics and biomarkers, to 'black oils' with a high abundance of biomarker compounds. These systematic variations in hydrocarbon liquids are attributed to limited but definite variations in kerogen type, with the condensate/light oil phases also being slightly more mature than the black oils.

These geochemical data have been corroborated with geological observations and suggest that the most prospective area for future exploration is in the southwest of the North Sumatra Basin where the kerogen is expected to be more oil-prone and the reservoirs less subject to gas flushing. Stratigraphic traps within the Belumai and intra-Keutapang sand reservoirs are interpreted to be the most prospective targets.