Physical
Properties in Shales and Mudstones Versus Depth and
their Implications for Basin Analysis
Øyvind, Marcussen1, Christer
B. Peltonen1, Nazmul Haque Mondol1, Knut Bjørlykke1,
Jens Jahren1 (1) University of Oslo, Oslo, Norway
Changes in physical properties of
fine-grained sediments during burial are the result of complex compaction
processes. Well log data from the Norwegian continental shelf, together with
mineralogical and geochemical data, have been used to study the compaction
behaviour for shales and mudstones with different composition. In the North Sea, smectite rich
mudstones of Eocene and Oligocene age have lower velocities and bulk densities
compared to mudstones consisting mostly of kaolinite, chlorite and mica. The
difference in velocities can be in the order of 500-700 m/s at a burial depth
less than 2 km. This is consistent with results from experimentally compacted
clay minerals which show that smectitic clays have a low compressibility
compared to kaolinitic clays. The low velocities and densities in smectitic
rich sediments may also partly be due to a reduced effective stress caused by
overpressure in the low permeability smectitic clays. In basin analysis simple
exponential or linear compaction curves are often applied for shales. Well log
data from the Norwegian continental shelf show that shales and mudstones are as
different as sandstones with respect to their physical properties during
burial. Changes in lithology and mineralogy can be related to provenance and
sedimentary facies also for mudstones. Different types of shales and mudstones
have different compaction trends. Using simple standard exponential curves may
introduce large errors in basin analysis and seismic interpretation.