Abstract

The Sunda Basin contains large thicknesses of elastics and carbonates deposited in a back-arc basin setting and is a proven prolific hydrocarbon producer. Rama field is one of the largest oil producing fields in the basin. Production comes mostly from the Early Miocene Baturaja Formation which includes a number of carbonate reservoirs, all deposited in reefal and near-reefal environments. Five main lithofacies subdivisions have been recognized, mainly as a result of a detailed study of 1117 ft (340.5 m) of Baturaja core taken from fourteen Rama wells. These are: 1) skeletal wackestones, 2) mudstones and minor coal, 3) foraminiferal packstones, 4) coral boundstones, and 5) coral/skeletal packstones. Each was formed in a specific reefal environment depending on water depth and the amount of mud in the system. Log responses, porosity/permeability ranges and reservoir characteristics are described for each lithofacies. Porosity is mainly intraskeletal and vuggy/moldic, with some interparticle porosity and probable cavernous porosity within karsted zones. Dolomitization is important in some facies and is related to the diagenesis of clay-rich carbonates. The secondary vuggy/moldic porosity is restricted to the packstones which are debris tongues of bioclastic material derived from the main reef. These packstones commonly constitute good reservoir zones capable of testing at flow rates of 2000-3000 BOPD. The relatively minor amount of in-situ reef present is tightly cemented and displays poor reservoir quality.