Abstract

Offshore southern Kalimantan (Borneo), Indonesia, the Sunda Shelf is bounded on the south by the east-west-trending Java-Madura foreland basin and on the north by outcrops of the granitic core of Kalimantan. Major northeast-southwest-trending faults created a basin and ridge province which controlled sedimentation at least until early Miocene time. Compressional folds are restricted to two areas, and compaction folds over buried basement features are common elsewhere, as are horsts and tilted fault blocks, with contemporaneity of faulting and deposition.

Rocks below the pre-Tertiary angular unconformity are considered to represent economic basement. The Tertiary intervals discussed are rock units, although certain ones may approximate time-stratigraphic units. Just above the unconformity, the oldest pre-“CD Limestone” clastic strata are fluviatile and lacustrine, the remainder consisting largely of shallow-marine, calcareous shale with interbeds of fine-grained, quartzose sandstone. Continued transgression led to deposition of the massive “CD Limestone,” which typically is a nummulitic wackestone with zones of intraclastic and bioclastic packstone. These low-energy carbonate rocks were deposited on moderately shallow shoals or banks with restricted circulation.

A flood of terrigenous detritus—Kudjung unit 3—resulted from post-“CD Limestone” uplift, and is more widely distributed. Unit 3 consists largely of fluviatile sandstone interbedded with shale and mudstone, grading upward to marine clastics with a few thin limestones near the top. Deepening waters of this transgression inundated most of the area, and because of the reduced size and relief of land areas, only fine detritus was supplied. The resulting Kudjing unit 2 is largely a shallow-basinal deposit, comprising thin, micritic limestones interbedded with calcareous shale and mudstone. Infilling of the basins was nearly complete by the end of Kudjung unit 1 deposition. Bank and basinal facies similar to those described constitute most of the unit and, on the south, patch reefs are abundant In both. A barrier-reef fades was formed in an open-marine basin on the north and a buildup with reefal affinity along a shelf edge on the southeast.

Eastern equivalents of Kudjung units 1 and 2 are known as the Berai limestone interval (comprising bank, reefal, basinal, and open-marine limestones, and marl); these units cannot be delimited from outcrop data. The Berai marl overlies these thick carbonate rocks and those of unit 1 and is thinner over positive areas, indicating incipient uplift although the principal deformation occurred later.

Of the three oil fields in the area, two are shut in, but one has produced nearly 100 million bbl. Gas shows were recorded in most wells of the area, but the maximum flow was 1.8 MMcf methane/day, although larger flows with high percentages of carbon dioxide and nitrogen were reported. In the southern part, several oil shows were encountered, and more than 2,000 BOPD was tested from two thin unit 2 limestones.

Fine-grained clastic strata of unit 3 are continuous with those farther south, where geochemical data indicate good source and hydrocarbon-generating potential. Sandstones with reservoir capability are present in the clastic intervals, and several carbonate facies have sporadically developed porosity. A variety of structural and stratigraphic traps is present.

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