Abstract

The Ardjuna Basin, which lies approximately 90 km northeast of Jakarta, is one of a series of hydrocarbon-bearing basins on the southern edge of the Sunda craton that originated during a major Eocene-Oligocene rifting event. The Ardjuna Basin is the name given to a large sag basin located over three precursor rift sub-basins that comprise the Ardjuna rift system: Northern Ardjuna, Central Ardjuna, and Southern Ardjuna sub-basins. The Ardjuna Basin as a whole covers an area of approximately 3000 km², with each sub-basin comprising an average area of 800 km². Each sub-basin is comprised of at least one half-graben system and contains, in varying amounts and facies, the primary hydrocarbon source rocks and a major reservoir facies within ARCO Indonesia's Offshore Northwest Java (ONWJ) Production Sharing Contract (PSC) area, the Oligocene Talang Akar Formation.

This paper is a review of a geological and geophysical study of the Ardjuna basin as it affected the distribution and character of the Talang Akar Formation. The study utilized seismic, well log, core, and biostratigraphic data of the Talang Akar Formation and older units. Structural depth and isopach maps are used to describe the structural history of these basins and how the timing of graben development effected the accumulation and distribution of hydrocarbon source and reservoir facies. The first prospect developed from this study, the LU-1 well located in the center of the Southern sub-basin, was spudded in February 1995 and declared a suspended oil/gas discovery in May 1995, after testing a cumulative flow of 1400 BOPD and 12 MMCFGPD from three intervals. In addition to the hydrocarbon tests, the well confirmed the presence of a thick, mature, source facies comprised primarily of coals and organic-rich fine-grained sediments in the Southern sub-basin. The presence of these mature source facies confirms that the Southern Ardjuna sub-basin was the likely source kitchen for much of the oil and gas discovered in the Ardjuna basin to date.