Publications

The early-to-middle Miocene Nurbani Reef is one of several seismically defined Batu Raja carbonate build-ups located on the western flank of the Sunda Basin, offshore sourheast Sumatra. It contains a large sub-commercial oil and gas accumulation, discovered by HAPCO in 1983 with the Nurbani-I well. Seven delineation wells were drilled, five tested oil at rates ranging from 86 to I747 BOPD.A basal transgressive platform limestone makes the substratum on which several successive carbonate build-ups, which make up the Nurbani Reef complex, grew. Three lithofacies have been recognized and mapped in the carbonate build-ups. The first facies comprises skeletal packstones and wackestones, developed along the eastern flank of the complex in a reef front environment. The second facies corresponds to a lagoonal back-reef environment developed on the westem flank of the reefal complex and comprises mudstones and marly limestones, with occasional beds of reef-derived packstones. A narrow band of ieef core, made up of in situ coral and algal boundstone, forms the third facies. This facies is interpreted to constitute the framework of the Nurbani Reef Complex.The best reservoir development is associated 'with the skeletal packstones and wackestones facies. These facies exhibit extensive mouldic and wggy porosity, together with some fracturing, and contains produceable hydrocarbons. The other facies contain large volumes of hydrocarbons which are nonproduceable because of their poor reservoir characteristics.Detailed petrological studies combined with wireline log analyses, test results, and seismic interpretations, have demonstrated facies control of reservoir distribution and performance. Therefore, a thorough undentanding of the facies distribution has been found to be essential in establishing the extent of produceable hydrocarbons in the Nurbani Reef complex.