Publications

The North Sumatra Basin is well known as a highly overpressured basin in Western Indonesia. Most of the sedimentary basins in western Indonesia, including the North Sumatra Basin, have similar histories of basin development. The basement of Paleozoic and Mesozoic sedimentary and metamorphic rocks is overlain by a syn-rift sequence, then a post-rift sequence associated with basin subsidence, and finally a sequence of shallow marine–fluvial deposits. Overpressured strata are commonly encountered in the post-rift sequence because it is dominated by fine-grained sediments of low permeability. In the North Sumatra Basin, the post-rift sequence comprises the Baong Formation. We have analyzed direct pressure measurements, wireline logs, drilling mudweight, and drilling events in Pertamina’s area of the North Sumatra Basin. The analysis shows that top of overpressure coincides with the top of the Baong Formation. The overpressure profile is characterized by a zone of low overpressure zone in the Upper Baong, decreasing to lower or even hydrostatic pressure in the sand-prone Mid Baong, a zone of high overpressure in the Lower Baong, and hydrostatic pressure in the underlying carbonate build-ups beneath the Baong Formation. The sonic log through the Baong Formation exhibits a strong reversal, which is the classic response at a steep transition zone into high overpressure. A density-sonic cross-plot indicates that overpressured shales in the Upper Baong Formation fall on the compaction trend for smectite-rich mudrocks, while the overpressured shales in the Lower Baong Formation lie on the compaction trend for illite-rich mudrocks. It is inferred that overpressure in the Upper Baong is caused by the disequilibrium compaction mechanism, and that high overpressure in the Lower Baong may be caused by smectite–illite transformation, resulting in unloading of the sediments (i.e., reduction in vertical effective stress). The difference in mechanism implies that different methods should be applied in estimating pore pressure in the Baong Formation in order to accurately predict overpressures throughout the formation.