Abstract

Basin analysis applied to the Sunda basin (IIAPCO Southeast Sumatra Contract) has demonstrated the influence of structure on the character and distribution of non-marine Oligo-Miocene sandstone reservoirs of the Talang Akar and Banuwati formations. Sands were deposited in piedmont fan, fluviatile, paludal, and lacustrine environments, comprising a series of depositional systems which contain interconnected source and reservoir rocks.

The best reservoirs are sandstone deposited in two fluviatile systems, comprising an earlier, predominantly bedload (braided) regime, and a younger, suspended-load (meandering) one. They contain recoverable reserves in the order of 300 MMBO in nine fields. In the south-central part of the basin, early sediments were derived from the west, and rivers flowed down the east-dipping paleoslope of a large, asymmetric graben. A series of north-oriented horsts perpendicular to the sediment supply, therefore, provided potential barriers to sediment dispersal.

After this 'basin-fill' stage, paleotopography became almost flat, and meandering rivers flowed primarily from the northeast down the graben axis. The drainage pattern was thus re-oriented at an acute angle to the north-south structural grain and deposition was controlled by growth fault activity. Rivers were frequently diverted parallel to, and on the downthrown sides of faults, enhancing sand deposition there. Lateral migration of streams away from the flanks of paleohighs and into the troughs resulted in the lateral coalescence and vertical stacking of point bars. These structurally downflank locations later became ideal sites for structural-stratigraphic traps.

The economic significance of the study of depositional systems has been to provide a predictive model for the distribution and thickness of reservoir sandstones. Interpretation of the sandstone distribution in conjunction with the effects of regional and local structural features can then yield exploratory leads, even in a well-explored basin.