Analytical method to accurately estimate overpressure magnitude: The case study from Indonesia's Tertiary basins

The most widely used overpressure estimation method is empirical method such as Eaton’s method. The nature of the empirical method is that it may work well in one area, but it may not work in other areas. In this paper, we try to develop more analytical approach to estimate overpressure, with the basis on understanding of overpressure generation. We then try to apply this method in Indonesia’s Tertiary basins, and it gives sensible results. The overpressure generating mechanism is analysed by direct inspection and comparison of sonic (and resistivity) vs density log responses to overpressure. From this step, we can understand whether overpressure is caused by disequilibrium compaction, unloading, or both. In the case that overpressure is generated by disequilibrium compaction mechanism, simple velocity-effective stress relation or equivalent depth method gives good match between estimated and measured pore pressure. If disequilibrium compaction and unloading contribute to overpressure, two steps are required to estimate overpressure magnitude. The first step is to analyse log response due to disequilibrium compaction and calculate disequilibrium compaction overpressure magnitude by methods as discussed above. The second step is to calculate the contribution of disequilibrium compaction and unloading to overpressure, based on effective stress history that has ever been experienced by sediments. By applying this two steps procedure, overpressure magnitude can be estimated accurately.