The East Java Basin is an overpressured basin in Indonesia. The LUSI mudvolcano is one of the evidences of overpressure occurrence in the basin. Hydrocarbon drillings in the basin have been experiencing some operational problems due to the presence of overpressure such as drilling-pipe stuck, hole cavings, and kicks, leading to nonproductive drilling time. In this paper, we offer additional analytical analyses to estimate overpressure magnitude in the basin, on the basis of understanding of overpressure generating mechanism and mudrock compaction stages. The analysis shows that overpressure in the study area, at least down to the depth ∼ 3 km is caused by disequilibrium compaction, and the mudrocks has experienced transformation from smectite (S) to illite (I) at the depth ∼1.6 km. The conventional method of overpressure estimation commonly used in this basin is to use single normal compaction trend (NCT) to estimate overpressure for the entire section. This method seriously underestimates overpressure magnitude at depth. By constraining S-I transformation, we propose double NCT (smectitic NCT and illitic NCT) to estimate overpressure magnitude in the basin. We have proved that this technique can estimate overpressure reasonably accurate.


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