Abstract

Oligocene sedimentary sequences were studied in the Teweh Block area of Central Kalimantan to better understand carbonates that might serve as hydrocarbon reservoirs in the area. Four major Oligocene depositional sequences, each 200-500 meters thick, were delineated in outcrops and on seismic lines: (1) latest Eocene to Early Oligocene (34-38 Ma), (2) Middle Oligocene (29.7-32 Ma), (3) early Late Oligocene (28.2-29.7 Ma), and (4) middle to late Late Oligocene (N3). In landward areas to the south, sequence 1 consists mainly of sandstones and shale with thin limestones. Isolated carbonate buildups and shales occur in sequence 1 in basinal areas to the north. An erosional unconformity is observed between sequences 1 and 2 outcrops and on seismic lines.

During deposition of sequences 2-4, carbonate shelves developed in the southern part of Teweh Block, while shales were deposited in basinal environments to the north. The carbonate shelf margin of sequence 2 was established along a structural hinge line. In seismic data, boundaries between carbonate sequences 2-4 are defined by surfaces of renewed carbonate growth (mounding and/or downlap) on the shelf immediately above the sequence boundary. No unconformities were found between sequences 2-4 in outcrops, rather boundaries between sequences 2-4 are recognized by features characteristic of abrupt deepening. Deepening events sometimes resulted in drowning and "backstepping" of the carbonate shelf margin. Internally, the carbonate shelves in sequences 2 and 3 are characterized by vertically-building shelf margins with landwarddipping (south-dipping), shingled clinoforms indicating progradation of shallow carbonate environments from the shelf margin into the lagoon. Seismic lines show the carbonate shelf of sequence 4 as a relatively massive buildup which thins substantially into the basin.

Outcrops of sequence 2 contain a well-developed shelf-to-basin profile with a transgressive systems tract (TST) overlain by highstand systems tract (HST). The TST has interbedded large-foram wackestone-packstones and coral wackestone-packstones with poorly-defined facies belts. The HST is characterized by well-developed facies belts with (1) shale and carbonate debris flows deposited on the lower slope, (2) argillaceous large-foram wackestones on the upper slope, (3) discontinuous coral wackestones and boundstones surrounded by bioclastic packstones on the shelf edge, (4) corallinealgae large-foram packstones and grainstones on the shallow shelf-rim, and (5) thin-branching coral and miliolid-rich foraminiferal wackestones and packstones in the lagoon.

In some circumstances, the definition of sequences apparently requires more flexibility than given in Van Wagoner et al. (1988). Sea level may never drop below the shelf in carbonate systems during times of rapid subsidence and low-amplitude sea-level fluctuations; hence, unconformities are not present to separate depositional sequences. The Haq et al. (1987) sea-level curve may also require modification, at least with regard to magnitudes and rates of sea-level rise. Deposition of the carbonate shelves in Central Kalimantan spans the large mid-Oligocene (29.5-30 Ma) sea-level drop of Haq et al. (1987). Shallowing and subaerial exposure of the carbonates would be expected during such a large sea-level drop, however, none was observed. Instead, deepening and local drowning of carbonate shelves were observed at 29.5-30 Ma.