Abstract

Twenty-five years of drilling in the oil prolific Sunda and Asri Basins of offshore Sumatra and Java have established the existence of non-marine strata dating from the early Miocene to at least as far back as the late Paleogene. These clastic formations were deposited during the widespread extensional tectonics of the early to late Paleogene and have to a large degree escaped Pliocene basin inversions that characterize many of the onshore Sumatra and Java basins.

Recent investigations of early basin history have defined different geologic histories than previously proposed for the Sunda and Asri Basins. The Sunda basin data-set contains adequate well control and good to poor quality seismic data. The Asri basin, on the other hand, has very limited well control but good to excellent seismic data quality. Integration of exploration efforts between the two basins has revealed a nearly symmetric, fault bounded extension in both basins for their early history followed by a shift to a more classic asymmetric rift. The early basin fill of the Sunda Basin consists of the Banuwati Formation and the earliest section of the Zelda Member of the Talang Akar Formation. The Banuwati Formation of the Sunda Basin records an overall transgressive event and culminates in the widespread deposition of the Banuwati Shale member which is the recognized source rock in the Sunda Basin.

A thorough study of the well log sequence stratigraphy and available whole core of the Banuwati Formation in the Sunda Basin has identified alluvial fan, fluvial, and shallow lacustrine facies within the non-marine Banuwati Formation. The application of the Sunda Basin's Banuwati sequence stratigraphic model to the Asri Basin gives a predictive model which has proved successful and the resulting exploration has found many similarities in the facies of the early fill of each basin.

The sequence stratigraphy of the lower Zelda Member was also examined as the lower Zelda sandstones represent the earliest non-Banuwati carrier beds for the migration of hydrocarbons out of the basins.

This paper describes part of on ongoing investigation into the early basin history of the Sunda, Asri and Hera basins.