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The application of current knowledge of clay colloid chemistry and mineralogy to the question of how water escapes from muddy sediments suggests two conclusions. (1) The alteration of montmorillonite to illite takes place after deep burial and involves the transfer of large amounts of bound water from montmorillonite surfaces to interparticle areas where it is normal water. This water transfer has an important bearing on the porosity, permeability, abnormal fluid pressure, and initial release of hydrocarbons from mudrocks. (2) In deposits of primary illite, water is compacted out of clay soon after burial, before the formation of hydrocarbons comparable with those found in reservoirs.
These points are important in petroleum exploration in new areas because it appears that the development of a shale source rock requires the initial deposition of a montmorillonitic organic mud, and its subsequent alteration to illite after deep burial. Abnormally high fluid pressures may easily be caused by a volume increase associated with the desorption of the last few monomolecular layers of water from montmorillonite during its diagenesis to illite. This understanding of mineralogical characteristics makes it possible to make meaningful interpretations of data on the bulk properties of compacting mudrocks.
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