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Deposits, Architecture and Controls of Reefal Carbonate Margin, Slope and Basin Systems during the Cenozoic: An Example from the Sarawak Basin

Ali, Mohammad Y.*1
(1) EOR Value Assurance, PETRONAS, Kuala Lumpur, Malaysia.

The late Oligocene to late Miocene was a period of widespread carbonate deposition in Southeast Asia. In Sarawak, over 500 carbonate build-ups were developed on the Luconia Shoal that extends 250 km from the present day coastline.Large areal extent and tectonic stability provide an ideal setting for a thick shallow marine carbonate development during this period.

The morphology, types and distribution of sediments, architecture, and regional controls on carbonate margin, slope and basin sedimentation of reefs were recently studied. An integration of basin- and reservoir-scales data involving seismic, well logs, cores and outcrops was applied. Several build-ups with different ages selected from different parts of the basin were examined.

Seismic expression shows two main carbonate morphologies.
(1) Flat-top platform has a single crest with asymmetrical dipping flanks often developed on fault-bounded regional high.
(2) High relief conical-shape pinnacle has multiple crests and steeply dipping flanks associated with carbonate stringers development basinwards. These build-ups orient in NE-SW direction and gently dip towards SE and NE, coincide with the Miocene palaeo-wind directions. A new seismic-based growth model suggests that the build-ups are strongly controlled by a combination of basement topography and rates of sea-level fluctuation assuming the carbonate production is constant. Sediment distribution was also controlled by the palaeo-oceanographic parameters, sediment influx and clastic poisoning, and relative position from the shelf margin. The carbonate successions generally thicken towards the shelf margin and display layer-cake architecture of porous and non-porous layers resulted by facies variability and diagenesis.

In individual build-up, nine depositional facies were recognized to represent the carbonate margin (reef flat, reef front, fore reef), reef slope and basinal (offreef) areas of the reef systems. The facies range from coral framestone to foraminiferal grainstone / packstone / wackestone and floatstone /rudstone /bafflestone, and limestone breccia. With increasing water depths, the coral framestone changed from massive Porites to small massive and branching coral bafflestone, and argillaceous platy coral bindstone. However, the leeward and windward deposits are rather difficult to differentiate. Basinal deposits contain abundant Globigerinoides spp. and Globoquadrina spp.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California