Topography- and fracture-driven fluid focusing in layered ocean sediments

Upward fluid flow is often invoked to explain the occurrence of methane hydrate in ocean sediments, whereas one-dimensional compaction models predict downward flow relative to the seafloor. Explaining the presence of upward flow requires a more complete compaction model. We develop a two-dimensional model of compaction-driven flow to quantify the focusing of pore fluids by topography and fractures when sediments have anisotropic permeability. We use a bulk anisotropic permeability to capture the effects of lithologic layering when the grid spacing is too coarse to resolve individual layers. Even small slopes (10 degrees) in bedding planes produce upward fluid velocity, with focusing becoming more effective as slopes increase. Additionally, focusing causes high excess pore pressure to develop below topographic highs, promoting high-angle fracturing near the crest. Magnitudes of upward pore fluid velocity are much larger in fractured zones, particularly when the surrounding sediment matrix is anisotropic in permeability. Enhanced flow of methane-bearing fluids from depth provides a simple explanation for preferential accumulation of hydrate under topographic highs. Citation: Frederick, J. M., and B. A. Buffett (2011), Topography-and fracture-driven fluid focusing in layered ocean sediments, Geophys. Res. Lett., 38, L08614, doi:10.1029/2010GL046027.