Abstract

Four phases of Tertiary sedimentation are recognized in the northwest part of the Kutei Basin. The earliest phase of sedimentation probably began in the Early Eocene and continued through the Middle Eocene with basal deposits consisting of coarse non-marine clastics grading upwards into shallow-marine clastics. This succession is up to 1000 m thick in the Kutei Basin and onlaps and wedges out against the stable Barito Shelf. Seismic profiles over undisturbed parts of the basin margin show syndepositional faults of limited separation confined to the lower part of the basal sequence and reveal syndepositional tilting of basement blocks near the Barito Shelf edge. Sediments of this Lower and Middle Eocene sequence are part of the Tanjung Formation, which is oil productive near the town of Tanjung. Early Eocene subsidence of the Kutei Basin was synchronous with, and perhaps caused by renewed or accelerated subduction beneath the north-northwestern margin of Borneo.

A second phase of sedimentation during the Late Eocene through the Early Oligocene was characterized by deposition of claystone in deep basins where subsidence outstripped sedimentation, flanked by areas where shallow-marine claystone, siltstone, minor sandstone and some carbonates accumulated. This second phase was terminated by compressional tectonics of limited extent and magnitude, resulting in uplift and erosional truncation of some basement blocks.

The third depositional phase occurred during a Late Oligocene transgression, resulting in deposition of platform carbonates (Berai Formation) over most of the Barito Shelf and on basement highs within the Kutei Basin. Coeval carbonate slope deposits and deep-marine shales (Bongan Formation) were deposited in the basin. The Kerendan No.1 gas and condensate discovery is a large stratigraphic trap, formed by an isolated Oligocene carbonate platform on a basement high, in the Kutei Basin.

The fourth depositional phase is recorded by a thick succession of uppermost Oligocene and Miocene deltaic and non-marine deposits initially filling the basins and subsequently prograding over the carbonate platform areas. The introduction of deltaics was probably caused by erosion of areas undergoing tectonism in the northern part of the Kutei Basin and in the South China Sea area. Inversion and uplift of this part of the Kutei Basin probably occurred during the Late Miocene.