Permeability, porosity and pore geometry evolution during compaction of Neogene sedimentary rocks

Permeability k is expressed as a product of the hydraulic radius Rh (=pore volume Vp/sample surface area S) squared, porosity ϕ, and a nondimensional geometrical factors 1/G. G is often assumed constant depending on the model, partly because its measurement is difficult. We propose a method to measure G without assuming any microstructural model, and present its evolution during compaction of sedimentary rocks that are too fine-grained to observe and quantify microstructures. We measured S, k, Vp, and grain volume Vg during compaction with confining pressure up to 100 MPa of diatomaceous mudstone from Koetoi Formation (Kdm, ϕ = 0.53–0.64) and siliceous mudstone from Wakkanai Formation (Wsm, ϕ = 0.28–0.33), Horonobe, Japan. They are similar sedimentary rocks, but are different in the grade of diagenesis. Vp and S yield Rh, and thus we can estimate G. G for Kdm remains nearly constant during compaction and varies from 1.3 to 6, whereas G for some samples from less porous Wsm increases irreversibly with compaction from about 0.1 to 10. The increase in G by 2 orders of magnitude probably indicates the change in the dominant fluid conduit from concentrated flow along fractures to pervasive flow.