The northwestern oil basin of Borneo (East Indian Archipelago) is one of the marginal troughs of the Sunda shelf, and was formed during the Tertiary cycle of deposition and orogenesis.
The present basin came into existence at the beginning of the Eocene, during which a thick series consisting of mostly sandstone and shale was deposited. During the Oligocene, deposition continued in the basin itself, while along the rim folding and uplift caused local nondeposition. The same process repeated itself in the Mio-Pliocene—in the center continuous sedimentation of thick marine sediments grading upward into more nearshore and deltaic sedimentation and nearer to the rim tectonical movements causing erosion and unconformities.
Folding, which is strong in the interior of Borneo, diminishes basinward where narrow anticlinal zones are separated by wide gentle synclines.
Oil ranging between 170 and 370 A.P.I. gravity is produced from the Miri field (since 1910, total production 73 million bbls.) and the Seria field (since 1929, total production 285 million bbls.).
Although no typical oil mother rocks have been found, the oil is thought to have originated in the marine Miocene sediments of the basin center. From there it migrated upward and toward the rim of the basin and was trapped in the lowermost covering sands. The accumulation seems to be mainly structurally controlled as both producing fields and the seepage areas are connected with faulted anticlines.