Shale compaction and statistical physics

After a short historical outline of the development of the ideas on the compaction of clays and shales it is shown that the exponential porosity-depth dependence of compacted shales expressed by Athy's law can be derived using standard methods of statistical physics. The main result of the paper states that the exponential compaction law expresses the maximum-entropy equilibrium state of the pores in the rock, that is compaction is an irreversible process where clay particles tend towards a statistically defined final equilibrium. Connections with the classical theory of consolidation are pointed out.