The estimates of undiscovered conventionally recoverable petroleum in Indonesia at probability levels of 95 percent, 5 percent, and statistical mean are for oil (in billions of barrels): 5, 35, and 16; and for gas (in trillions of cubic feet): 13, 94, and 42.
In Indonesia, petroleum occurs in five types of basins: forearc, back-arc (foreland), median, inner-arc, and downwarp into-small-ocean basins. The back-arc, median, and downwarp basins have significantly greater petroleum potential than do the fore-arc and inner-are basins. The latter two types are expected to yield only small discoveries; also, significant portions of such basins lie in water depths in excess of 1,000 m. The back-arc basins have been the most petroleum productive, but they also have been the most extensively explored. The greatest undiscovered petroleum potential is estimated to lie in the downwarp and median basins.
In Indonesia, six general types of geological settings or plays have been identified as being favorable for petroleum accumulation. They are transgressive clastic sequences, regressive clastic sequences, deltas, carbonate platforms, pinnacle reefs, and fractured igneous and volcanic rocks. The multiple-pay transgressive clastic reservoirs in the back-arc basins have produced most of Indonesia's crude oil. In several basins, carbonate platform porosity has become a primary exploration target, but each reservoir (reef, bank, or bioclastic zone) tends to be restricted in size.
Typically, Indonesian oil is of medium gravity with a paraffin base and has a moderate to high pour point, a low sulfur content, and a relatively low gas to oil ratio.