Publications

The middle Miocene Upper Cibulakan Formation (Main interval) of Offshore Northwest Java (ONWJ) comprises up to 1500 ft of interbedded shales, sandstones, siltstones and limestones deposited in an overall paralic setting under transgressive conditions. An interpretation of the Main interval using sequence stratigraphic techniques was carried out in order to aid exploration and development efforts in these hydrocarbon- bearing intervals. This approach was made possible by a recent, extensive coring program, including 750 ft of continuous core from the U-11 well, and subsequent detailed sedimentological studies. In this model, the best reservoir quality rocks occur in one of two settings: sandstones derived from a fluviodeltaic dominated shoreline during periods of relative sea-level lowstand, and sands that have been reworked during periods of transgression. Unlike many other sequence stratigraphic interpretations that were developed in other basins having well-defined shelf margins such a shelf-slope break is not observed in these middle Miocene deposits. In addition, there is no evidence for the development of any shailow marine shoreface sequences, nor of surfaces of subaerial exposure. A characteristic of the Main interval is the organisation of the major sandstones into 50-100 ft thick packages separated by thicker sections (up to 250ft) of tuffaceous, grey-green, high gamma-ray, marine shales. Each package reflects the abrupt introduction of coarse clastic material into a relatively quiescent marine setting by the reworking of delta front sands at times of relative lowstand. Firmgrounds at the basal contacts imply an episode of subaqueous erosion prior to deposition of the sandstone. The complex stacking of sandstones and mudstones within the packages reflects the interplay between largely autocyclic processes (e.g. lobe switching), subtle variations in basin topography, sediment supply and relative sea-level fluctuations. Regionally extensive limestones at the top of the packages evidence relatively rapid transgression. The sediments deposited by this process (the ",maximum flooding", event) are regionally correlatable throughout the study area. The use of the sequence stratigraphic model, constrained by sedimentological observations, has resulted in more accurate reservoir distribution models, and an overall better understanding of the ONWJ basins.