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

We present preliminary results illustrating that overconsolidated fine-grai ned sediments without a compactional fabric but favourably aligned microfra ctures can show significant anisotropic permeability. At much reduced effec tive stress, substantial flow can occur parallel to the fabric such that an isotropy indices - calculated by dividing horizontal permeability by vertic al permeability - increase by factors above 10. SEM observations show no in tense particle alignments in the materials, but the presence of parallel or iented microfractures; mercury-intrusion porosimetry data indicate that thi s 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 pu re-size distributions. While the results, at present, are only directly app licable to the shallow subsurface, it is proposed that directional bimodal pore-size distribution needs further investigation for deeper, more consoli dated sediments in light of the potential ramifications highlighted by the results. For example, in some instances shales may not be as efficient hydr ocarbon seals as traditionally thought. (C) 2000 Elsevier Science Ltd. All rights reserved.