Presentation Information
[S2-05]Permeability Tensor in the Horoman Peridotite Complex from Open Fracture Mapping and its Depth Dependence
*Yuichiro Naruse1, Yuya Akamatsu2, Tomohiro Ito3, Shuto Nakagawa1, Ikuo Katayama3 (1. Hiroshima Univ. Earth Sci., 2. JAMSTEC, 3. Kyoto Univ. Geo.)
Keywords:
Permeability tensor,Horoman Peridotite Complex,Fracture mapping,Anisotropy,Depth dependence
Permeability reflects rock pore structure, with fractures dominating fluid flow. Fracture heterogeneity (orientation, length, connectivity, aperture) controls permeability anisotropy and fluid pathways, which are crucial for seismicity, resources, and CO2 storage. Laboratory permeabilities of intact rocks are 10-21-10-14 m2, while field values are higher (10-14-10-7 m2), matching fractured samples (10-12-10-9 m2). This indicates that large fractures control field-scale flow. In this study, we mapped open fractures in the Horoman peridotite complex to evaluate the outcrop-scale permeability tensor and its depth dependence.We applied the scanline method over a 6×13 m area, using 2 m scanlines (E-W and N-S) to measure the strike, dip, and aperture of open fractures. We excluded closed mineral veins to estimate the current permeability. We then calculated the permeability tensor using Oda's theory. By factoring in the stress dependence of the aperture, we evaluated how increasing depth decreases the aperture, affects the tensor, and rotates the principal permeability directions.The fractures form two conjugate sets intersecting at 60°, suggesting simultaneous development under a specific stress field, which is harmonious with the magmatic layering plane. The calculated field-scale permeability was 10-13-10-12 m2. The maximum and intermediate permeabilities were comparable, with the maximum parallel and the minimum perpendicular to the layering plane. Permeability decreased exponentially with depth, reaching 10-15-10-14 m2 at 100 m. Assuming strong E-W compression, increasing depth enhances anisotropy and rotates the principal directions. These results indicate that the interaction between the geometric structure of the cracks and the stress state significantly influences the anisotropy of the permeability tensor.As this estimation relies heavily on aperture, there is an amount of uncertainty involved. Surface and deep permeability may also differ due to consolidation and stress. Future direct in-situ measurements are needed for validation. These findings provide fundamental data for the trapping of CO2 in ultramafic rocks, the reconstruction of paleo-stress, and the evaluation of fluid-induced serpentinization.