Publications

The presence of overpressure in a sedimentary basin is controlled by geological factors such as burial and tectonic histories, basement configuration, thermal gradient, lithology, and mineralogical composition of the sediments. These geological factors can vary significantly across a sedimentary basin, resulting in different pore pressure regimes in the same basin. It is well known that the East Java Basin has a very complex geological history, as indicated by the occurrence of several sedimentation zones, variable depth to basement, and variations in the thickness of carbonate platforms that have built up. We have found that changes in these geological factors correlate with different pressure regimes in the onshore part of the basin. Analysis of direct pressure data, wireline log suites, drilling events, and drilling mudweight in several wells in the East Java Basin indicates that the pore pressure regime can vary significantly over short distances. It seems that basement depth and the thickness of carbonate build-up are the main controls on pore pressure regimes across the basin. Areas where the basement is relatively shallow or the carbonate build-up is relatively thick are usually characterized by low to hydrostatic pressure regimes, while areas with deep basement are characterized by moderate to high overpressure. One implication of our analysis of pressure regimes in this basin is that understanding geology is a key to successful pore pressure prediction. No matter how close an offset well may be to the location of a new well, if key geological factors such as the depth to basement are different, the pressure–depth profile in that offset well could be irrelevant for pore pressure prediction in the new well. It is possible to anticipate variations in pore pressure regime from a relatively simple understanding of the geology.