Publications

Accurate pore pressure prediction is important for well planning, not only because of safety and cost considerations, but also to ensure that the target depths can be reached. Understanding which mechanisms have been responsible for generating overpressure is essential for successful prediction. Overpressure in Indonesias Neogene basins, such as the Lower Kutai, North Sumatra, and East Java basins, has been attributed by previous workers to disequilibrium compaction associated with rapid burial. However, an important factor that needs to be considered when evaluating overpressure generating mechanisms is that these basins are warm, especially where the water is shallow (e.g. temperature = 31Z + 30 °C in the shelfal area of the Lower Kutai Basin, where Z is depth in kilometers). The warm conditions promote other overpressure generating mechanisms, such as gas generation and clay diagenesis, categorized as unloading mechanisms because they cause reductions in effective stress.Pressure analysis in warm basins based on the disequilibrium compaction concept can lead to underestimation of overpressure. An example from the Lower Kutai Basin shows that overpressures estimated in mudrocks from sonic logs by the Equivalent Depth Method and Eatons Method with exponent 3 are lower than measured overpressures in adjacent sands by as much as 20 MPa.Further analysis shows that gas generation and clay diagenesis, which are mostly temperature-driven, play an important role in generating overpressure in this basin. This example shows that temperature is an important factor in overpressure development in warm Neogene basins. Unloading mechanisms may be the main causes of overpressure in other Indonesian Neogene basins, such as the East Java and North Sumatra basins which may also be categorized as warm basins. Detailed analysis of well data is required to determine the overpressure generating mechanisms in those basins, so that pressure prediction can be performed as accurately as possible.