Publications

The stratigraphy and facies of hydrocarbon reservoirs in the West Natuna Udang and Gabus Formations are poorly understood, being viewed as being intertidal due to their content of brackish water foraminifera, or marine, or lacustrine.This study applies a comprehensive biofacies approach using cores and cuttings analysis combined with sedimentology to construct a detailed stratigraphic model of the North Belut field reservoir interval. The study involved foraminiferal and palynological analysis of cored intervals, followed by analysis of cuttings, from the Barat, Udang and Gabus Formations. Cored intervals were studied using closely spaced samples, from which the biostratigraphic signature of each lithofacies was determined. Cuttings were analysed using regular sample spacing, through both uncored and cored intervals, allowing judgement as to how noncored intervals could be interpreted based on results from the cores.Fourteen biofacies were differentiated from the core studies. These reflect distal to proximal lacustrine, distal and proximal lower coastal plain, and upper coastal plain facies, each in both brackish water and wholly freshwater settings. Shales were differentiated as either allocyclic or autocyclic. Allocyclic shales are considered to be geographically widespread, to have potential as regional seals and are of three types: a) freshwater lacustrine, b) brackish/fresh/brackish and c) wholly brackish. The brackish/fresh/brackish allocyclic shales required a special setting for their formation, involving a silled-basin that was intermittently filled with brackish water, then freshwater, and then successively breached and returned to marine base level.A total of 15 cycles, each capped by allocyclic shale and interpreted as 4th-order sequences, have been identified through the upper part of the Gabus and the Udang Formations. These high frequency packages can be differentiated into three groups based on successively increasing degrees of brackish influence, coupled with the biofacies and lithofacies succession. These broad cycle groups are thought to reflect 3rd-order sequences.