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

摘要

Permeability k is expressed as a product of the hydraulic radius R_h, (=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, V_p, and grain volume V_g 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. V_p and S yield R_h, 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.