Clay mineral diagenesis has a considerable effect on the physical properties of siliciclastic mudstones. The dominant clay mineral reaction, the conversion of smectite to illite, involves a series of dissolution and reprecipitation reactions which results in a significant change in the orientation of the whole clay fabric; not only smectite-illite but also kaolinite and chlorite.

We have found clear evidence for chemically enhanced compaction (porosity loss/sediment volume reduction) associated with illitization of smectite in Lower Cretaceous mudstones at Halten Terrace, offshore mid-Norway from measured physical, textural, and mineralogical properties, and from log responses. We also see continuing compaction of diagenetically mature, illitized mudstones with increasing effective stress in the Central Malay Basin.

These results are evidence that tends to refute the stress-independent chemical compaction model for diagenetically altered mudstones at temperatures above ∼100 C. Compaction in these diagenetically-altered mudrocks appears to be driven mainly by mechanical processes, although porosity loss is enhanced by small-scale fabric destabilization as a result of clay diagenesis.


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