Anisotropic permeability and bimodal pore-size distributions of fine-grained marine sediments

Abstract We present preliminary results illustrating that overconsolidated fine-grained sediments without a compactional fabric but favourably aligned microfractures can show significant anisotropic permeability. At much reduced effective stress, substantial flow can occur parallel to the fabric such that anisotropy indices — calculated by dividing horizontal permeability by vertical permeability — increase by factors above 10. SEM observations show no intense particle alignments in the materials, but the presence of parallel oriented microfractures; mercury-intrusion porosimetry data indicate that this enhanced flow is due to microfractures opening when favourably oriented. Such an effect is displayed as a distinctly bimodal pore-size distribution when intruded by mercury parallel to the fabric; identical samples with the fabric oriented perpendicular to intrusion direction show only unimodal pore-size distributions. While the results, at present, are only directly applicable to the shallow subsurface, it is proposed that directional bimodal pore-size distribution needs further investigation for deeper, more consolidated sediments in light of the potential ramifications highlighted by the results. For example, in some instances shales may not be as efficient hydrocarbon seals as traditionally thought.