The oil fields of the Tertiary basins of Sumatra, Java, and Borneo have produced more than one billion barrels of oil to the end of 1940. The proved fields still represent a considerable reserve and many undeveloped areas of the Archipelago have excellent possibilities.

The oil produced to date has been derived almost exclusively from sands of Miocene and Pliocene age. Shortly before the Japanese invasion, however, commercial accumulations were discovered in beds of Eocene age. The Boela field, in Ceram, probably produces Triassic oil, accumulated in overlapping sands of Plio-Pleistocene age.

Rather gentle to moderately steep anticlines are the traps for the accumulation of the oil. Stratigraphic traps are undoubtedly present but they are not yet explored.

From the point of view of Tertiary geotectonics the East Indian Archipelago is an area of great complexity and of special interest. Two major geosynclinal belts, folded into orogenes by subsequent mountain-building phases, meet in this area. These are the circum-Asiatic geosyncline, a part of the world-wide Tethys geosyncline, and the Australo-Pacific geosyncline. The Tertiary basins of Borneo are outside these two belts. The latter basins lie within, although near the edge of, the Asiatic continent, constituting intra-continental geosynclines.

An accompanying geotectonic sketch map shows the location of the oil-producing areas of the East Indian Archipelago.