What is the porosity of a mudrock?

Several types of porosity (total or physical, advective flow, advective and diffusive transport, and geochemical) can be distinguished in a mudrock. Each may have a different value. Physical porosity is the volume not occupied by mineral grains. Interconnected physical porosity can be found using injection techniques and water-content measurements. Advective and diffusive flow and transport porosities are used when describing the movement of fluids and solutes. Advective porosity relates the average linear velocity of a fluid to its Darcy flux (velocity), and in mudrock is relevant principally to fractures. Diffusive porosity relates diffusion in rock to diffusion in pure water and depends on the diffusing substance and the medium. In mudrock, water diffusion porosities equal water content porosities but solute diffusion porosities are smaller because solutes do not have access to the entire water content of a rock. Inaccessible water may be in pores too small to admit solutes, or present as interlayer or surface-layer water. Geochemical porosity is required to model pore-water composition and reactive transport. It represents the fluid volume in which reactions occur, and for mudrocks, seems equivalent to solute-diffusion porosity. Review of studies of several mudrocks shows halide geochemical porosities from 0.3 to 0.7 of other porosity values. Uncertainties of this size are unimportant for transport calculations, but lead to significant uncertainties in modelled pore-water compositions. To make acceptably accurate calculations of water-rock reactions and transport in mudrock, it is important to define the geochemical porosity.