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Petroleum Geoscience

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Overpressure development in rift basins: an example from the Malay Basin, offshore Peninsular Malaysia

PETRONAS Research & Technology Division, Bangi Research Centre, Lot 3288–3289, Kawasan Institusi Bangi, 43000 Kajang, Malaysia (e-mail: mazlamd@petronas.com.my)

The Malay Basin is a Tertiary transtensional rift basin located in offshore Peninsular Malaysia. A study of the subsurface pressure data has revealed at least two major overpressure compartments that are sealed by regional shale units. The main, basin-centre overpressure compartment has a domal shape and, in profile, shows a convex-up top-of-overpressure surface. In the basin centre, the top of overpressure is generally between 1900 m and 2000 m depth, and is confined mainly to a particular stratigraphic unit, the Middle Miocene unit E. It appears that the top of overpressure is influenced by the underlying regional shale seal in unit F, and that the mild overpressure encountered in unit E represents the overpressure transition zone. Due to the convex-upward top-of-overpressure surface, overpressure is generally encountered at shallower depths in the basin centre compared to the basin flanks. A smaller overpressure compartment is also identified on the northeastern flank of the basin. This compartment is sealed by the onlapping, transgressive shale of unit L (Lower Miocene) and occurs at a depth of 2600–3000 m. Hence, regional shale seals have a strong influence on overpressure distribution.

Disequilibrium compaction is believed to be the primary causal mechanism for the overpressure in the basin centre. Overpressure development is the consequence of high subsidence and sedimentation rates during basin extension. Low subsidence and sedimentation rates on the basin flanks do not generate overpressure. The domal shape of the top-of-overpressure surface is thus a result of different rates of subsidence and sedimentation across the basin.

The basin's overpressure history is simulated by means of a simple model. Modelling results indicate that the basin overpressure developed very early, during the syn-rift phase (c.30–21 Ma), when sediment burial rates were very high (>1000 m Ma^–1). Overpressure build-up occurred rapidly during the syn-rift phase but has started to dissipate gradually since the post-rift phase began 21 Ma ago, when sedimentation rates were well below 1000 m Ma^–1. This suggests that disequilibrium compaction as an overpressure-generating mechanism was effective during only the syn-rift phase of basin development. Sedimentation rates during the post-rift phase (generally less than 500 m Ma^–1) were not high enough to generate overpressure. The pre-existing overpressure is thus able to dissipate through the sedimentary column, causing the build-up of pore pressure in the post-rift section. Thus, the overpressure in the post-rift strata is probably of secondary origin, derived from excess pressure dissipated from the underlying syn-rift strata. Overpressure generated by disequilibrium compaction during the syn-rift phase has been modified and re-distributed as the basin evolved through the post-rift phase.

KEYWORDS: overpressure, disequilibrium compaction, overpressure compartment, basin modelling, rift basins