Some thoughts on porosity reduction: rock mechanics, overpressure and fluid flow

A currently popular paradigm, that porosity reduction occurs as a direct consequence of the effective stress acting on the rock framework grains, is mechanistically incorrect. The commonly observed covariance between porosity and effective stress does not reflect a cause-and-effect relationship. Instead, it arises because both low effective stress and slow porosity reduction are consequences of the inability of compacting rocks to expel their pore fluids quickly enough to maintain normal fluid pressures. The process of porosity loss is here divided into sequential steps, and we argue that the expulsion of pore fluids is the rate-determining step leading to overpressuring. Thus, Darcy’s law assumes equal importance with the relationships describing the mechanical compaction of sediments. In this paper we describe how compaction can be treated as a Coulomb-plastic response that is functionally dependent on effective mean stress, deviatoric stress, and the state of compaction. In the next generation of basin models, a mechanistically correct approach is needed, combining both rock mechanics and hydrogeology.