Influence of lithology and compaction on the pore size distribution and modelled permeability of some mudstones from the Norwegian margin

We have derived a permeability model which uses pore shape, pore throat size distribution and pore alignment as key inputs. The pore shape is two frustra of cones connected at their base. Both the pore shape and alignment change with increasing compaction, developing a higher aspect ratio and becoming increasingly perpendicular to the direction of maximum stress. The uncalibrated model predicts the vertical permeability of some experimentally compacted muds to a factor of three. The model is used to estimate the vertical and horizontal permeability of eleven mudstones from the Norwegian Margin for which we have also determined porosity, pore size distribution, grain size distribution and specific surface area. Samples were chosen in order to investigate the influence of both compaction and lithology on pore size distribution and permeability. Porosity is lost mainly by the collapse of those larger pores which also contribute most of the permeability. Modelled vertical permeabilities of samples buried to between 855 and 3605 m vary from 3.3 x 10(-19) to 1.2 x 10(-21)m(2) and are not simply related either to porosity or effective stress. Permeability is strongly influenced both by porosity and detailed lithology, as described for example by the % < 2 mu m particles. The modelled permeability of lithologically similar mudstones decreases logarithmically with decreasing porosity but at a single level of effective stress permeabilities of lithologically different mudstones vary by more than two orders of magnitude. At lower levels of effective stress (< 10 MPa) coarser grained (siltier) mudstones appear to have a greater range of pore radii, a much larger mean pore radius and much higher permeabilities than finer grained mudstones. Vertical permeabilities calculated using the equation derived in this paper are between three and seventy times lower than permeabilities calculated from the Kozeny-Carman equation, assuming a value of one hundred for the product of the tortuosity and shape factors in the equation. (C) 1998 Elsevier Science Ltd. All rights reserved.