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Summary This study demonstrates the use of wireline logs for the overpressure-mechanisms analysis in a field in the southwestern Malay basin. The development of overpressure means that the fluid movement in the pores is retarded, both vertically and laterally. In many Tertiary basins, overpressure is mainly generated by compaction disequilibrium caused by a high deposition rate and low permeability in shales. In the Malay basin, temperature and high-heat flow also play an important role in generating overpressure at a shallow depth, because the geothermal gradient is very high (40–60°C/km). Pore-pressure profiles and crossplots of sonic velocity/vertical effective stress and of velocity/density are used to derive the overpressure-generating mechanisms. The results obtained from the crossplots of 10 wells reveal that in the study area, overpressure is generated by both primary (compaction-disequilibrium) and secondary (fluid-expansion) mechanisms. The overpressure-magnitude analysis suggests that the overpressure generated by the secondary mechanism is very high compared with the primary mechanism. In all the wells, the Eaton (1972) method with an exponent of 3 gives good prediction when overpressure is the result of the compaction-disequilibrium mechanism, but it underpredicted the high pore pressure where the fluid-expansion mechanism is also present. However, by use of a higher Eaton exponent of 5 for the fluid-expansion mechanism, the overpressures are predicted quite well. The Bowers (1995) method, by use of the unloading parameter (U) of 6, is also used for pressure prediction and it gives a reasonably good prediction in the high-overpressure zone of all the wells.

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Quantitative predrill pore pressure prediction is very important for reducing the drilling hazards. In many Tertiary basins, generation of overpressure is mainly by compaction disequilibrium due to high deposition rate and low permeability in shale. In the Malay Basin, high geothermal gradient (i.e., 40-60°C/km) and high heat flow also play an important role in generating overpressure at shallow depth. This study describes the utilization of 3-D prestack depthmigrated seismic interval velocity for predrill pore pressure prediction in a field of southwestern Malay Basin. The quality of 3-D prestack depth-migrated seismic interval velocity was enhanced by calibration with check-shot data. Modified Gardner's equation was used to generate the 3-D density cube from the interval velocity. The Eaton and Bowers methods were used to compute and predict pore pressure values from the seismic velocity. The Eaton method with standard exponent for seismic velocity gave good prediction in the shallower zone where overpressure is caused by undercompaction mechanism, whereas underpredicted the high pore pressure at the greater depth where fluid expansion is the cause of overpressure. However, the overpressures were predicted quite well by applying correction on the Eaton method for fluid expansion mechanism.

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