Grain Fabric Control on Directional Permeability in Deep-Marine Sandstones
Jaco H. Baas1, William. D. McCaffrey1, E. Hailwood2, and M. Kay1
1 University of Leeds, Leeds, United Kingdom
2 COREMAGNETICS, United Kingdom
Grain orientation constitutes a fundamental control on the direction of pore fluid flow within deep-water sandstones. It is often assumed that long axes of sand grains are oriented parallel to the main flow direction of the associated sediment gravity flow. This would imply that the preferred path of fluid flow parallels (paleo)flow direction as well. A new systematic study of grain fabric in deep-water turbidites shows that this concept is oversimplified and possibly wrong. Statistical analysis of a large set of fabric data, acquired using enhanced anisotropy of magnetic susceptibility (MAGPORE), yielded the following relationships between fabric type and type of sedimentary structure in turbidites: (1) Grains in ripple cross-laminated divisions have a single mode of preferred orientation parallel to the paleoflow direction. (2) Grains in plane-parallel laminated divisions have two modes, a flow-parallel mode and one in which long-grain axes are oriented perpendicular to flow direction, suggesting a contribution of rolling grain transport. (3) Grains in massive, “structureless” divisions have many orientations, yet 3 modes prevail. In addition to flow-parallel and flow-perpendicular orientations, statistically significant, but hydrodynamically enigmatic, flow-oblique orientations (mean deviation angle: ~45°) have been found as well. This large variety of fabric types suggests a similar variety in preferred paths of fluid flow within turbidites, controlled to at least some degree by depositional process. This is confirmed by independent laboratory measurements of permeability anisotropy (using gas permeametry) on samples for which grain fabric data is available.