Well Log and Seismically Derived Impedance of Clay-Rocks for Higher Resolution of Pore Pressure Prediction

In this paper we discuss how acoustic impedance can be utilized for pore pressure prediction. Acoustic impedance, the product of velocity and density, of normally pressured shales behaves in a predictable manner of increasing impedance with increasing burial depth as shale compacts. Overpressure in clay rocks is noted by a deviation from the normal compaction trend to lower values. The minerology of clay rocks affects the impedance in a manner similar to other log responses of velocity, resistivity and density and must be considered when evaluating for pore pressure. Previous applications of seismically derived impedance for pore pressure have used it to modify migration-derived velocities to improve resolution. In this application we use a calibrated inversion of seismic data for shale impedance and directly convert to pore pressure. This process reduces the effects from other rock types, improving resolution and potentially accuracy. This is in contrast with seismic migration velocities which include an average property of all rocks over relatively thick sections. This negatively affects pore pressure predictions since almost all pore pressure calculations are based on the predictability of shale and clay rocks, not sandstone, siltstone, marl, limestone and other rock types.