COMPACTION OF CRETACEOUS MUDROCKS
OFFSHORE MID-NORWAY

Abstract

The Cretaceous mudrocks at Haltenbanken are buried 2-4 km below seafloor. Density porosity ranges over a factor of two at the same depths in different wells, with higher porosities in the west. Although the Cretaceous mudrocks are overpressured, pore pressure is laterally invariant across Haltenbanken; hence porosity differences cannot be due to differences in effective stress. Differences remain after plotting porosity against temperature. Porosity differences are somewhat reduced by adjusting depths for recent exhumation and burial. I suggest the following sequence of events: the Cretaceous mudrocks were normally compacted and hydrostatically pressured in the early Pliocene, because Tertiary sedimentation rates were low; Pliocene exhumation and deposition of Naust Formation glacial sediments occurred rapidly, without porosity loss because low-permeability overburden inhibited pore water expulsion; enhanced temperatures accelerated clay diagenesis, and lithification occurred with a laterally invariant pore-pressure profile because horizontal permeability is much higher than vertical permeability. If this interpretation is correct, the Cretaceous mudrocks at Haltenbanken exhibit chemical undercompaction, where pore water expulsion was inhibited during clay diagenesis and lithification. This finding illustrates the risk in predicting pore pressure in exploration wells using data from offset wells, and suggests a method for estimating Pliocene exhumation history on the Norwegian margin.