Horizontal flow of pore fluids

Sir-Hanor, in his calculation1 of the density-driven subsurface movement of pore fluids near a salt dome on the Louisiana Gulf Coast, derived horizontal velocities of the order of 10 m per year at depths exceeding 1,000 m. These velocities are large for a sedimentary basin - Bethke et al.1 cite values measured in cm per year - even in the geologically disturbed environs of a salt dome. In reality, the vertical permeability in deep sedimentary basins, evaluated over thick sequences of alternating sands and shales, is not large, even next to a salt dome, as Hanor himself has more recently implied'. [...]convective fluid overturn, which is typical of densitydriven fluid movement1, will almost always be accompanied by a significant component of vertical flow, especially near an upward-tilting boundary such as a dome. The model is not based on the hydrostatic assumption, and, moreover, recognizes that subsurface systems characteristically display highly anisotropic permeability behaviour when taken over thick sand/shale sequences, featuring very different values for permeability in the horizontal and vertical directions.