Relationships between porosity and effective stress in shales

In order to predict overpressures by basin modelling, it is essential to understand the process of mechanical compaction and its effect on the porosities and permeabilities of sedimentary rocks. Two fundamental issues for modelling overpressure history are the functional relationships between porosity and effective stress, and between permeability and porosity. This article is concerned with the first of these relationships in shales, since they comprise the majority of rocks in sedimentary basins and their low permeabilities restrict the rate at which excess fluid pressure can be dissipated. My key point is that porosity reduction during mechanical compaction of a sediment depends on the mean effective stress, not the vertical effective stress. This must be recognized if we wish to understand the origin of overpressures observed in sedimentary basins.