Pore Pressure Regime in the South Sumatra Basin, Indonesia

Although the South Sumatra Basin is one of the most prolific hydrocarbon basins in Indonesia, no synthesis of the basin-wide pore pressure regime has previously been published. Where the basement is shallow, pore pressure is hydrostatic throughout the overlying Palaeogene–Neogene sediments. In the deeper parts of the basin, two distinct overpressure zones can be identified, a zone of low‒medium overpressure at intermediate depths, and a deep zone of high overpressure. We have analysed density and sonic logs through the shale sections to infer the overpressure generation mechanisms. Disequilibrium compaction is interpreted to be the sole mechanism of overpressure generation in the upper part of the zone of low‒medium overpressure. However, an unloading mechanism can be inferred from the sonic-density crossplots in the deeper part of this zone, and clay diagenesis is implicated. There is a steep pressure ramp into the zone of high overpressure, starting at depths where the temperature has reached 130 °C. This temperature corresponds to the threshold for gas generation, so we infer that gas generation is principally responsible for the high overpressures in the deep zone, although disequilibrium compaction and clay diagenesis must also have contributed to the observed overpressures.