Abstract

Play concepts in the Northwest Java Basin, especially in the Pertamina working area, have only recently focused on syn-rift and post-rift sediments, whilst in the past they concentrated on carbonate buid-ups on structural highs and the Jatibarang volcanics. The area chosen for this study is the Cipunagera Low (Figure 1), which has a complete sequence of rifting events. Although most of the wells penetrated only the objective target, there are several wells that reached basement; therefore, a basin fill history could be identified. Seismic sequence stratigraphy, depositional sequence from wells, and petrophysics analysis are applied for this study using Landmark software.

The sedimentary succession in the Cipunegara Low can be divided into four sequences: sequences A, B, C and D. Sequence A consists mainly of syn-rift and early post-rift sediments, whereas sequences B, C, and D comprise of post-rift and sag phase sediments (Figure 2). Depositional setting for sequence A ranges from fluvio-deltaic to shoal carbonate, which is part of the major transgression in this region. The base of sequence B is characterized by karstification, unconformably overlain by transgressive middle shoreface facies and shallow water carbonate. The upper succession of sequence B is composed of prograding tide dominated middle shoreface to offshore sediments, which covered almost the entire region. Although detailed well correlation across this region indicates that sequences C and D have a similar cycle to that of sequence B, the type of facies and their distribution are different (Figure 3).

Seismic interpretation and sequence analysis identify differences between the syn-rift and the post-rift sedimentation patterns. Syn-rift deposits are characterised by fluvial dominated deltas, whereas the post-rift deposits are more tide-dominated (Figure 4). The fan delta orientation of syn-rift deposits is strongly controlled by segmentation of the normal fault alignment, which in the Cipunegara Low has a NE-SW orientation. In syn-rift play concepts, the focus is on delta plain channels and delta front sand bars, which has been proven as hydrocarbon reservoirs in WNJ-1 and CMR-9. Play concepts for post-rift settings are prograding middle shoreface to offshore and agrading/prograding shelf margin carbonates. The orientation for those features is relatively NW-SE, similar to the regional paleoshoreline to the north. The differences between these two rifting events have implications on play concepts, especially in locating the best reservoir for hydrocarbons, and also in deciding which target to focus upon.