Abstract

Faced with insufficiency of local data, regional sequence stratigraphy and depositional models were developed to predict the geometry and orientation of reservoirs within the Massive and Main Intervals of the Upper Cibulakan Formation in the L and LL fields, Offshore Northwest Java (ONWJ).

Various regional data, in the form of well logs, conventional and sidewall cores, cuttings, and 2-D seismic were integrated. 3-D seismic amplitude anomalies and modern analogues were observed to infer the geometry and preferred orientation of the reservoirs.

The Massive and Main sediments, deposited in deltaic and shelf environments, can be divided into a number of relatively small-scale sedimentary sequences. Since the highstand system tract (HST) and the lowstand system tract (LST) were difficult to differentiate, a sequence has been initially defined as a package of sediments that simply consists of a Regressive System Tract (RST) and a transgressive system tract (TST). The RST and TST are separated by a transgressive surface of erosion (TSE) with the possibility of being co-planar with the pre-existing lowstand surface of erosion (LSE).

Sandstone reservoirs within the sand-rich Massive Interval were found within both the RST and the TST. On the other hand, sandstones in the shale-rich Main Interval are predominantly present within the TST, as most of its RST are shaly.

As an example for the TST, two productive sandstone reservoirs were selected as the focus of further discussion, namely LL-40A of the Massive and LL-32 of the Main.

These sandstones are interpreted to have been deposited mainly as tidal sand ridges trending north-northeast with rare possibility of being lowstand channel fills trending relatively west to east. Net reservoir of the LL-32 zone is 3-15 m thick and 0.5-1 km wide, whereas the LL-40A zone is 5-10 m thick and 0.5-1.5 km wide. The two zones can be as long as 10 to 15 km.