Although the search for hydrocarbons in Indonesia was initiated about eight decades ago, exploration work is still at a high level.

Recent studies have resulted in a new understanding of the prolific Tertiary sedimentary basins and, especially, knowledge concerning offshore sedimentary basins has been updated significantly. More important, however, is the current knowledge on the mechanism of basin formation which seems to enhance the validity and applicability of the new global tectonics to the geology of Indonesia.

The Tertiary sedimentary basins in western Indonesia previously have been described as "idiogeosynclines," situated around the periphery of a supposed landmass of pre-Tertiary age (the Sunda Shelf). Recent exploration surveys and subsequent drilling have shown that the southern part of the Sunda Shelf actually consists of many sedimentary basins and intervening uplifts. Major faults are common throughout the area and clearly control the distribution and shapes of the basins. Block faulting appears to have broken up the periphery of the Sunda Shelf at the beginning of Tertiary time. The chief crude-oil production in western Indonesia is from the regressive and deeper transgressive sand series of Oligocene-Miocene age, except in East Kalimantan where producing zones range from Eocene to Pliocene age.

Prospects have changed considerably since oil and gas in economic amount have been proved within the interbedded limestone formation of Tertiary age and additional reserves are anticipated within stratigraphic traps.

Oil and gas discoveries within deltaic sandstones, notably in East Kalimantan, have upgraded significantly the onshore and offshore potentials of the area. Carbonate rocks are becoming a prime objective in the search for oil, especially in the East Java-Madura basinal area.

Although eastern Indonesia was chiefly the site of late Paleozoic and Mesozoic sedimentation, oil has been proved only within the strata of Tertiary age, notably in the Salawati basin. Of particular importance was the recognition of the tremendous potential that reefs, and in particular Tertiary reefs, possess as hydrocarbon reservoirs. A similar basin and environmental model is anticipated for the Bintuni basin.

Scientific cruises within the last five years have indicated the presence of several potential basinal areas between the Sunda Shelf and the Sahul Shelf.

The sedimentary basins in Indonesia can be classified into grabenlike basins, present foreland basins, and basins which are in front of the present magmatic arc.

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