Porosity Modification Under Soft-Sediment Shear: Application of Magnetic Techniques

Rob Butler, Ernie Hailwood, Adriana Del Pino Sanchez, and William McCaffrey
University of Leeds, Leeds, United Kingdom

Shear deformation in semi-cohesive materials aligns elongate rigid grains and so creates anisotropic porosity structure. For samples of naturally sheared sediments the bulk anisotropy of the porosity can be measured using the anisotropy of magnetic susceptibility (AMS) of samples injected with a magnetic fluid (the Magpore method).

This approach has been used to link depositional grain alignment and permeability anisotropy in turbidite sands of the Britannia reservoir. It is now being extended to investigate the influence of deformational kinematics on porosity anisotropy in sediments from this reservoir that have experienced post-depositional creep. Shear directions and gradients in selected units have been determined from deformed dewatering structures and the porosity anisotropy of these units is currently being investigated by the Magpore method. Initial results show significant modification of pore structure related to downslope movement. The processes involved in this modification and the effect on porosity and its anisotropy are now being investigated in detail and results will be presented in this paper.