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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Sequence Stratigraphy of Aggrading and
Backstepping Carbonate Shelves, Oligocene,
Central Kalimantan, Indonesia
By
Unocal Corporation, Brea, California
Four major Oligocene carbonate sequences were studied in the Teweh area of Central Kalimantan, Indonesia, to better understand how they might serve as reservoirs for hydrocarbons in the area. Each sequence (200-500 m thick) was delineated in outcrops and/or on seismic lines: (1) early Oligocene, (2) middle Oligocene, (3) early late Oligocene, and (4) middle to late Late Oligocene. In landward areas to the south, sequence 1 consists mainly of sandstones and shales with thin limestone beds. Isolated carbonate buildups and shales occur in basinal areas to the north in sequence 1. An erosional unconformity separates sequences 1 and 2. During deposition of sequences 2-4, carbonate shelves developed in the southern part of the Teweh area, while shales were deposited in basinal environments to the north. The carbonate shelf margin of sequence 2 was established along a structural hinge line. Boundaries between sequences 2-4 do not show onlap or erosional truncation in this area. On seismic lines, 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. Subaerial unconformities were not found in or between sequences 2-4 on outcrop, so boundaries between sequences 2, 3, and 4 were placed where strata first indicated a substantial deepening of depositional environments. Rapid rises in relative sea level (subsidence + eustatic sea level) resulted in drowning and "backstepping" of carbonate shelf margins in some locations, and stacking of shelf margins in other locations.
Internally, the carbonate shelves of
sequences 2 and 3 are characterized by
vertically building shelf margins with
landward-dipping (south-dipping),
shingled clinoforms indicating
progradation of shallow carbonate environments
from the shelf margin into
the lagoon. Sequences 2 and 3 have
well-developed transgressive systems
tracts
overlain by highstand
systems
tracts
. In outcrop, the transgressive
systems
tracts
contain interbedded large-foram
wackestones/ packstones and
coral wackestones/packstones with
poorly defined facies belts. The
highstand
systems
tracts
are characterized
by well-developed facies belts
which include from the basin shelfward:
(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 in
bioclastic packstones on the shelf edge;
(4) coralline-algae, large-foram
packstones and grainstones of back reef
flats and shelf-margin shoals; and (5)
thin-branching coral and foraminifera1
wackestones and packstones in the lagoon.
Seismic lines show the carbonate
shelf of sequence 4 as a massive buildup
which thins substantially into the basin.
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