Abstract

The offshore NW Java and SE Sumatra Production Sharing Contracts were signed in 1967 and 1968 respectively, and to date over 500 MMBO have been produced from Oligo-Miocene sandstones of the Talang Akar Formation in these two areas. The Talang Akar Formation is a succession of fluvio-lacustrine and fluvio-deltaic sediments which can attain a thickness of up to 7000 ft. Deltaic sediments are restricted to the upper part of the formation and river and tidal processes are argued to have been dominant in their deposition. The productive reservoirs can be divided into three categories: fluvial sandstones, distributary channel sandstones and marginal marine bar sandstones. Depending upon location fluvial reservoirs tend to be the thickest and most extensive with the best reservoir quality. The distributary channel reservoirs are generally thinner with a more limited lateral extent and sometimes lower permeabilities owing to their finer grain size and increased levels of heterogeneity caused by mudstone interbeds and drapes. Marginal marine bar reservoirs have the lowest reservoir quality owing to their generally finer grain size, extensive bioturbation and often intense later overprint by pedogenic (soil forming) processes. These bar sandstones are also characteristically thin and, although they may be fairly laterally extensive, comprise minor reservoirs at the present time. The Talang Akar Formation is shown to be a lithostratigraphic rock unit which displays diachronism during at least late Oligocene and early Miocene times. The fluvio-deltaic sediments in the upper part of the succession are retrogressively stacked in response to a regional transgression which affected the entire southern margin of the Sunda Shield.