Abstract

A model for the deposition of the early Middle Miocene Ngrayong Formation in the East Java Basin is presented which emphasizes palaeogeographic controls and lithofacies variation. The study highlights the distribution and the geometry of reservoir facies in the undrilled portions of the East Java Basin, where there is the potential for trapping large quantities of hydrocarbons.

The Ngrayong Formation represents a complete regressive-transgressive sedimentary cycle which ranges from coarse-grained sandy clastics in the lower part grading to fine-grained clastics and limestones towards the top. Within the study area, the Ngrayong is shown to comprise five principal sedimentary facies. These are tidally-influence cross-bedded sandstones, sandy turbidites, contourites, hemipelagic mudstones and bioclastic carbonates which are locally reefal.

Cross-bedded sandstones, capped by thin bioclastic carbonates are widely distributed in the shelf and upper slope area that outcrops in the northern part of the study area. The contemporaneous turbidites, contourites and hemipelagic mudstones were deposited on the lower slope and in the deep basin to the south. The distribution of the turbidites is localized being found mainly in submarine fans and channels.

The regional pre-Tertiary basement architecture controlled the Oligocene-Miocene palaeogeography and strongly influenced the deposition of the Ngrayong sequence.

Good to excellent reservoir characteristics are found in the quartzose cross-bedded shelf sandstones and sandy turbidites.

The sandy turbidite facies have been the most productive and forms the primary exploration target.

The cross-bedded sandstone facies has produced gas in the northwestern part of the study area, however, no hydrocarbons have been produced over most of the remaining area. The main reason is due to destruction of traps by extensive exposure and erosion. The deep marine carbonates within the contourite facies which tested hydrocarbons in the Tuban Block form a secondary target.