Effective Stress And Minimum Velocity Trends Available to Purchase

An almost universal assumption in current pore pressure prediction methodologies is that effective stress, the stress upon which P-wave interval velocities depend, is equal to differential stress, the mean stress minus the pore pressure. If that assumption were correct, then for overpressures due to disequilibrium compaction P-wave interval velocities would remain constant at all depths below the formation of a pressure seal for lithostatic loading. Analysis of sonic logs from marine Tertiary siliciclastic basins shows an increase in the minimum mudstone and shale P-wave velocities, which to first order can be fit as a linear increase of velocity with depth. This trend of increasing velocities in highly overpressured lithostatic and near lithostatic pore pressures is direct evidence that pore pressures are unable to entirely offset confining stress (due primarily to overburden) in maintaining lowered P-waves velocities after seal formation, and hence that effective stress is not equal to differential stress in mudstones and shales. Evaluation of a revised effective stress should lead to more accurate pressure calculations from P-wave velocities.