In Situ Permeability and Scale Dependence of an Active Accretionary Prism Determined From Cross‐Borehole Experiments. Issue 14 (28th July 2018)

• Record Type: Journal Article
• Authors: Kinoshita, C.; Saffer, D. M.
• Is Part Of: Geophysical research letters. Volume 45:Issue 14(2018)
• Page Start: 6935
• Page End: 6943
• Publication Date: 2018-07-28
• Subjects: permeability -- pressure -- cross‐hole experiments -- scale dependence -- oceanic borehole -- Nankai; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2018GL078304
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Permeability controls fluid flow in the Earth's crust and affects a wide range of processes including advective transport and pore pressure generation. However, in situ measurements of permeability are few, especially in active tectonic settings or at scales relevant to regional flow. We analyze formation fluid pressure records from oceanic boreholes in the Nankai accretionary prism offshore southwest Japan, focusing on unexpected responses to drilling operations conducted at boreholes ~100 m to the northeast. We develop a 2‐D numerical model of transient fluid flow and conduct a parametric grid search to define hydraulic diffusivity. A value of 0.19–0.46 m 2 /s (corresponding to a permeability of 9.8 × 10 −13 to 2.4 × 10 −12 m 2 ) yields the best fit to observed pressure responses. Together with laboratory measurements on core samples and drillstrem tests reported in previous studies, our analysis indicates a strong scale dependence of permeability, likely reflecting the presence of permeable faults and fractures. Plain Language Summary: Despite the importance of permeability to a wide range of geological and geomechanical processes in subduction zones, in situ permeability at scales of tens to several hundreds of meters is generally not well constrained, particularly in deforming, active tectonic and structural system. In this work, we analyzed formation fluid pressure variations recorded in a borehole observatory in the Nankai accretionary prism offshore southwest Japan, in response to drilling at nearby boreholes. These responses provide a set of inadvertent cross‐hole experiments, and thus represent a unique opportunity to quantify formation hydraulic properties at a scale of ~100 m. Together with permeabilities reported in previous studies from laboratory experiments, down‐hole tests and regional scale numerical simulations, our results define a clear scale dependence of permeability over scales ranging from cm to > 100 m. We attribute this to the presence of permeable fractures and faults in the accretionary prism across a range of scales. Key Points: Inadvertent cross‐hole experiments enable determination of the in situ hydraulic diffusivity of the active Nankai Trough accretionary wedge Permeability of the accretionary prism at the ~100‐m scale is 9.8 × 10 −13 to 2.4 × 10 −12 m 2 Compilation of values determined using a range of methods at different measurement scales indicates a scale dependence of permeability