Our current knowledge of clay colloid chemistry and clay mineralogy was applied in an investigation of the mechanisms by which water escapes from muddy sediments. The alteration of montmorillonite to illite after deep burial involves the transfer of large amounts of water from bound positions on montmorillonite to interparticle areas where it has an important bearing on the porosity, permeability, abnormal fluid pressure, and the initial release of hydrocarbons from mudrocks.

In contrast to montmorillonite deposits, water is compacted out of illitic deposits very soon after burial, before the formation of hydrocarbons comparable to those found in reservoirs. The development of shale source rock requires the initial deposition of a montmorillonitic organic mud, and its subsequent alteration after deep burial to illite. These requirements are of decisive importance in the exploration for oil in new areas.

A considerable reduction in density accompanies the desorption of the last few monomolecular layers of water from montmorillonite during its diagenesis to illite, and the associated volume increase could easily account for the abnormally high fluid pressures so often encountered in drilling operations.

The new compaction theory enables us to give meaningful interpretations to data on the bulk properties of compacting mudrocks.

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