Overpressure in the Kutai Basin – A Radical Reappraisal

The Kutai Basin is the second largest hydrocarbon province in Indonesia and the largest for gas production. All hydrocarbons produced to date have been reservoired in Neogene strata. Overpressure is encountered in Upper Miocene strata at relatively large depths of 3–4 km in the shelfal area, where the main mechanism of overpressure generation has long been thought to be disequilibrium compaction. This interpretation necessitated an additional hypothesis that there are pore pressure discrepancies between sands and mudrocks. We have re-examined pressure and wireline log data from the shelfal area to arrive at a very different conclusion. We have found no convincing evidence of pressure discrepancies but clear evidence of unloading processes, in the form of trend reversals in well logs, and of substantial chemical compaction with mudrock densities exceeding 2.6 g/cc at the top of overpressure. Our preferred interpretation is that the mudrocks in this area are not undercompacted, but overcompacted.