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Use of Pore Pressure Modeling to Constrain Seismic Velocities


Pressure modeling at basin scale is used in the industry for various purposes including seal integrity studies, HC fluid phase prediction, predrill pore pressure prediction and constraining seismic velocities for seismic inversion. In Total, we have developed a workflow for pore pressure prediction that uses seismic velocities in shales constrained by sonic data from offset wells, coupled with geological pore pressure scenario modeling to constrain reservoir pressure. Shale undercompaction is the principal reason for observed overpressures and as seismic velocities are sensitive to porosity, the Eaton method can be used to quantify overpressure. However, many other geological processes are potentially responsible for observed abnormal pressures that are not manifest as porosity anomalies such as gas generation, tectonic stresses or lateral transfer, all of which can be represented by forward geological pore pressure modeling. It is frequently assumed that in basins dominated by clastic sedimentation, synclines are areas of increased subsidence with elevated sedimentation rates that lead to undercompaction and higher than normal porosities which yield lower than normal seismic velocities. In contrast, over anticlines, sediment rates are reduced leading to less or no undercompaction and higher to normal seismic velocities. However, in many Tertiary basins with high sedimentation rates we observe the opposite, a lower velocity zone above the anticlines, with velocities increasing laterally into the synclines. Using geological pore pressure modeling we can show that lateral transfer of pressure into the anticlines can generate higher overpressures in anticlines, thereby delaying compaction and resulting in lower velocities that can be less than those in the surrounding synclines. Model parameters are adjusted to match geometry and magnitude of these shallow lower velocity zones. Similar phenomena have been reported from subsalt structures where velocities are difficult to obtain and therefore geological pore pressure modeling can help to estimate velocities which can be subsequently used to help improve subsalt imaging.