Compaction of Cretacous Mudstones across Haltenbanken and Implications for Pore Pressure Estimation

Abstract

Summary Mudstones of the Lange and Kvitnos formations at Haltenbanken are diagenetically mature and overpressured with a laterally consistent pressure—depth profile. Density logs show that porosities vary by a factor of two at 2700 m depth, with greater porosities in the west. Exhumation is not responsible because these Cretaceous mudstones are at their maximum burial depths across most of the area. Nor are lateral variations in geothermal gradient high enough for diagenesis to be the main cause. X-ray diffraction and grain size analyses show no significant lithological variations. We infer that the lateral differences in compaction trends developed because pore water escape was more inhibited in the west during recent rapid burial by glaciogenic sediments, and that associated lateral variations in overpressure have since decayed. We conclude that chemical compaction of diagenetically altered mudstones can only proceed if pore water escapes, which implies that chemical compaction is not independent of effective stress. Mechanical compaction also needs to be accounted for in pore pressure estimation. We illustrate the difficulties by applying Eaton’s method using the sonic log. The mixed results show that the normal compaction trend has to be selected correctly according to the maximum effective stress experienced by the mudstones.