Pore pressure identification in mudrocks largely relies on indirect methods, including analyses of sonic velocity data. The magnitude of pore pressure changes that can be resolved from such data depends on the impact of fluid pressures on well log responses and on the velocity responses to lithological variations that could be mistaken for indications of pore pressure changes.
Analysis of relationships between sonic velocity, mineralogy, and porosity from 11 North Sea wells demonstrated porosity variations of up to 5% from the mean value in mudrocks at any given depth between 1 and 3.5 km (3300–11,500 ft). Comparisons of these wells with seven wells from the Norwegian Sea demonstrated that the sonic velocity and density variations are influenced by the content of silica with a biogenic origin, but that they can otherwise not, in general, be related to mineralogy and pore pressure variations. The sonic velocity deviations from the mean value are of the same magnitude as those that would be caused by porosity changes of ±8% in mudrocks of uniform lithology. Furthermore, the deviations are not significantly reduced when the velocity data are compensated for changes in density (porosity).
Detection limits for mechanically compacting mudrocks that are at their maximum effective stress are equivalent to static mud density changes of 0.1–0.15 g/cm3. These numbers apply to mudrocks with similar lithological variability to those analyzed in our data set but should be much higher when significant amounts of silica with a biogenic origin are present. The impact of effective stress changes on the sonic velocities of chemically compacting as well as of mechanically unloading rocks is much less because small or no porosity changes follow from the effective stress changes in such cases. The velocity responses for mechanically unloading and stress-insensitive chemical compaction can only be identified from four-dimensional (time-lapse) seismic data unless the rocks are close to the fracturing limit. These observations imply that reasonably accurate velocity-based pore pressure identification in North Sea mudrocks requires a knowledge of local mudrock heterogeneity and compaction that cannot easily be obtained. As a result, sonic velocity data should be treated with considerable caution in pore pressure evaluation.