During the middle Eocene, a large extensional basin formed in Kalimantan and began to be filled by transgressive middle Eocene and lower Oligocene nonmarine and shallow marine clastics, carbonates, and deep marine clastics. This episode was followed by regressive late Oligocene-Miocene deposition. Oligocene tectonism resulted in uplift, erosion, and structural segmentation of the large Kalimantan basin into several smaller separate basins. Deltaic sedimentation began in the latest Oligocene in the upper Kutei basin and prograded eastward, so that by the end of the early Miocene, deltas were near the present Kutei coast, where deltaic sedimentation continued to the present. Lower(?)-middle Miocene deltaic sediments also filled the Barito, Asem Asem, and Pasir basins and we e probably contiguous with those of Kutei. A separate Miocene deltaic depocenter developed in the Tarakan basin (northeastern Kalimantan). Carbonate sedimentation prevailed in the shallow areas in between these deltas, and shales were deposited in the deep basins.

Eocene basin inception and Oligocene tectonism are interpreted to have resulted from plate rearrangements in southeast Asia. Eocene plate rearrangement is associated with the collision of India with Asia, and the middle Oligocene tectonism and magmatism are interpreted to be due to the collision of northern Australia with island arcs and opening of the South China Sea. Inversion of the upper Kutei basin and uplift of the Meratus Mountains started in the early part of the middle Miocene and are related to a third major plate readjustment in southeast Asia. Regionally synchronous Miocene-Pliocene tectonic phases in the region are probably related to collisions of microcontinents along Sulawesi and resulting strike-slip faulting extending into Borneo.

Original recoverable hydrocarbon reserves from the Eocene of the Barito basin are 180-200 million bbl; 5 billion bbl of oil and oil-equivalent gas may be recoverable from Miocene deltaics in the lower Kutei basin, and 200 million bbl oil from those of the Tarakan basin.