oa The Overpressure History of the Malay Basin, Offshore Peninsular Malaysia

The Malay Basin, located offshore Peninsular Malaysia, is a large Tertiary basin that developed by crustal 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. In both cases, the overpressure is sealed by laterally extensive, regional shale units. In general, the present-day depth to the top of overpressure has a convex-upward surface; shallower at the centre and gradually deepening towards the flanks. This phenomenon is found to be related to the varying sediment burial rates from basin centre to flanks; a higher sedimentfburial rate produces a shallower top of overpressure. In detail, however, 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 limited stratigraphically 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 lower Group E down to Group F represents the overpressure transition zone. Modelling studies indicate that overpressure had developed very early, during the synrift phase (ca. 30-2 1 Ma), when sediment burial rates were very high (>1000 rn/Ma). The overpressure developed during this “build-up” phase, however, has been dissipating gradually since the post-rift phase began 21 Ma ago, when burial rates were reduced considerably below 1000 rn/Ma. This indicates that disequilibrium compaction, resulting from high sediment burial rates, was effective only during the synrift phase of basin development. Low sediment burial rates during the post-rift phase (generally less than 500 rn/Ma) are not enough for overpressure to develop. Hence, the overpressure in the post-rift strata, as observed at the presentday, 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.