On changes in porosity and volume during burial of argillaceous sediments

The porosity and hence volume of argillaceous sediments is determined by: (1) the magnitude of the effective stress acting within the sediment; (2) the previous stress history of the sediment; and (3) at shallow depths of burial, by features such as the mineralogy and the nature of the depositional environment. Stress paths and the critical state diagrams for a number of clays are used to investigate the range of porosities possible in argillaceous sediments as the effective stresses increase. It is found that all porosity/effective stress curves converge at large stresses. The change in porosity is strongly dependent on the mean effective stress but largely independent of the deviatoric stress, and thus is largely independent of the nature of the stress field acting on the basin (compressional, extensional etc.). Because of the dependence of porosity on the mean effective stress, no simple relationship exists between porosity and depth of burial but in the absence of overpressured pore fluids and assuming the sediment is not overconsolidated, it is possible to contour the porosity/effective stress diagram in terms of burial depths. These data should assist in recalculating stratigraphic thicknesses for basin reconstruction and stratigraphic correlation studies.