Abstract

Recent expansion of ARCO Indonesia's exploration strategy into the deeper part of the Northwest Java Basin area (NWJ) has resulted in a refined stratigraphy and better understanding of the early evolution of rifting in the area. Two phases of syn-rift sedimentation have been recognised and are dominated by fluvio-lacustrine and volcanic deposits.

A series of cored intervals from the OC-2 and TY-1 wells illustrate the major sedimentological characteristics of these two phases. The cored intervals are subdivided into six facies ranging from pebbly planar cross stratified sandstone, palaeosols, mudstone, crystal/lithic tuffs and interlaminated tuff/algal fines. Reservoir facies are restricted to fluvial channel-fill pebbly sandstones, and interbedded within the sequence are lacustrine mudstones with good source potential.

Locally very good reservoir quality occurs in the pebbly cross-stratified fluvial sandstone facies, as a result of well developed, primary intergranular pores and lesser secondary dissolutional porosity. This locally good reservoir potential, however, is laterally juxtaposed with well cemented, poorer quality facies. Extensive bioturbation and subsequent pedogenesis of stacked sandstone-siltstone facies has also formed thick intervals of lower reservoir potential. This essentially restricts reservoir targets to somewhat unpredictable, laterally confined, mineralogically immature fluvial channel-fill sandstones.

Good to excellent source rocks occur within the fine grained lacustrine deposits of both the phase I and II syn-rift sections, with TOC values up to 9% and initial calculated hydrogen indices greater than 700.