Bulletin of the Geological Society of Malaysia, Volume 49, April 2004, pp. 135 – 144
Petronas Research & Scientific Services Sdn. Bhd., Lot 3288-3289, Kawasan Institusi Bangi, 43000 Kajang, Malaysia
Abstract: The Malay Basin, offshore Peninsular Malaysia, is a large Tertiary basin that developed by extensional and strike-slip tectonics. Subsurface pressure data from the central and northern parts of the basin reveal two major overpressure compartments: one in the basin centre and another on the basin flank. The present-day depth to the top of overpressure has a convex-upward surface; hence, it is shallower at the centre and gradually deepens towards the flanks. The basinward increase in sedimentation and burial rates had resulted in a shallower top of overpressure at the basin centre. The present-day depth to top of overpressure is also influenced by the presence of regional shale seals. In the basin centre, the top of overpressure is generally between 1900 and 2000 m depth and is stratigraphically limited to within the lower part of seismo-stratigraphic unit or “group” E. The top of overpressure is shallower towards the basin flanks, and is less than 1500 m deep along the faulted, western basin margin. It appears that the top of overpressure in the basin centre is influenced by the Group F shale, and that the overpressure in the lower Group E and upper group F interval represents the overpressure transition zone. The Group F regional shale seal had effectively, therefore, deepened the top of overpressure in the basin centre. A simulation of overpressure development by disequilibrium compaction indicates that the basin-centre overpressure had developed very early, during the synrift phase (ca. 30-21 Ma), when sediment burial rates were very high (>1000 m/Ma). The overpressure that developed during this “build-up” phase, however, has been dissipating gradually since the post-rift phase began 21 Ma ago. Overpressure dissipation occurred when burial rates were reduced considerably to less than 1000 m/Ma during the post-rift phase. Hence, disequilibrium compaction as an overpressure-generating mechanism was effective only during the synrift phase of basin development. Lower sediment burial rates during the post-rift phase (generally less than 500 m/Ma) were not sufficient for overpressure to develop. Hence, the overpressure in the post-rift strata, as observed at the present-day, appears to be of secondary origin, derived from the excess pressure in the underlying synrift strata. The present-day distribution of overpressure in the basin, therefore, is not a primary feature, but is due to pressure dissipation and re-distribution during the post-rift phase of basin evolution.