In the east-west-trending Tertiary basin of East Java (Indonesia) 132 million barrels of oil have been produced since 1888 from some 20 fields grouped around the centers of Tjepu and Surabaja.

The upper 6 kilometers of the infilling of the basin have been penetrated by the drill. Of this sequence of known Tertiary, the lowermost 4 kilometers consist almost entirely of Miocene bathyal shales and marls which may have acted as oil-mother formations. In the north of the basin the upper 2 kilometers of sediments are neritic and littoral and contain in the Tjepu and Surabaja districts the sands from which the oil of East Java is produced.

During Plio-Pleistocene the basin was folded into 3 east-west-trending zones of different tectonical character. The northern one, the Rembang zone, is formed by anticlinoria on which many minor anticlines with steep southern flanks are superimposed. Several of them are productive.

In the Tjepu area oil was trapped in the porous sands covering directly the Miocene shale series but toward west and east their reservoir properties are poor. Locally some younger sands are also productive. In Surabaja the producing horizons lie still higher (upper Pliocene, lower Pleistocene).

In East Java 3 types of oil are produced--shallow asphaltic oil from the Surabaja area, and a light, and a heavy paraffinic oil from the Tjepu area.