The Tertiary basins of Indonesia represent three general types: extracontinental subduction basins, extracontinental median basins, and cratonic basins. In Indonesia, regressive sequences form wedge-shaped bodies of sandstone that were deposited in near-shore to open-marine environments. The North Sumatra back-arc basin is one of the medium-sized basins of Indonesia. The South Sumatra basin accounts for about 2 percent of Indonesia's oil production and about 2 percent of the nation's estimated oil reserves. Successful petroleum exploration in the Northwest Java basin dates only from the late 1960s, so the fields are relatively few and are not old by Indonesian standards. The East Java basin is of moderate size for an Indonesian back-arc basin, but it is not one of the better petroleum producers. An analysis of Indonesian oil-held production suggests that, at least in the near term, oil production will probably decline in 7 of the 11 producing basins.