Permeabilities (k) of Lower Jurassic Whitby Mudstone samples collected from a wave-cut platform, NE England, were measured for flow of argon parallel and perpendicular to bedding across a range of effective pressures (peff) (10–70 MPa) using the oscillating pore pressure method. Petrographic analyses including X-Ray diffraction and scanning electron and optical microscopy show that samples comprise a silt-rich, clay bearing mudstone containing <2% organic matter, with measured porosities ranging between 6–9%. An anisotropic fabric is indicated by elongate clay rich lenses and oriented micas, weakened in some layers by bioturbation. Permeability parallel to this layering was measured to be 2–3 orders of magnitude higher than permeability perpendicular, suggesting increased flow-path tortuosity across the layering. Pressure cycling over the range peff = 10–70 MPa was repeated on each sample until a reproducible pattern of permeability variation with peff was observed. Cycling initially reduces the permeability by >2 orders of magnitude, after which permeability of samples parallel to the layering varies with peff according to: k = 4.1(±0.6)×10−19 exp(−0.034 ± 0.003 peff). Over this pressure range volumetric strain measurements showed that after the first pressure cycle pore volume changes are purely elastic and there is no permanent pore collapse. The technique of pressure cycling as part of the measurement of k reduces the time required to make a reliable measurement that is likely to represent the in-situ permeability and its relationship to peff. The impact of permeability sensitivity to pressure was evaluated by application of a simple reservoir model for dry gas. Unless the effective pressure dependent permeability is taken into account, substantial overestimation of gas flow rate and original gas in place will be made from well tests.

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