Role of fine-scale layering and grain alignment in the electrical anisotropy of marine sediments

Electrical resistivities of seafloor sediments determined by controlled source electromagnetic (CSEM) surveys have been found to be significantly greater than those measured by electrical well logging, in some instances by a ratio of as much as 5:1. Because borehole logging techniques invariably measure electrical resistivity using currents circulating in horizontal planes and CSEM surveys are sensitive to the currents circulating in vertical planes, a possible cause of this discrepancy is strong electrical anisotropy of the sediments. We have examined electrical log data from vertical exploration and appraisal wells and deviated production wells in the North Sea. By correlating the same sedimentary units between wells, we are able to compare resistivities measured at different borehole inclination angles. As the inclination angle changes, the amount of vertical and horizontal resistivity contributing to the resistivity changes. Hence we can estimate the electrical anisotropy in the sediments. Results indicate that anisotropy ratios of 1:5 and greater are present within the shale-dominated units. We show that fine-scale horizontal layering can make only a relatively small contribution to this anisotropy, but that a model of horizontal alignment of highly oblate spheroidal grains can account for most or all of it.