A Laboratory Study to Investigate Velocity Anisotropy of an Organic-rich Shale, Central North Sea, Offshore Norway

Abstract

This study investigates velocity anisotropy in a well-consolidated organic-rich shale drilled from the central North Sea, offshore Norway. Shales are known as anisotropic rocks showing vertical transversely isotropy. Quantification of anisotropy in shales attracts greater importance considering that shales comprise almost 70% of the buried rocks in sedimentary basins. Two core plugs were prepared perpendicular and at 45o inclination to the in-situ bedding planes in order to measure velocity variations in different directions by increasing the effective stress. The results show that anisotropy of measured shale is stronger for S-wave velocity. The anisotropy decreases with increased consolidation, but not significant, probably due to closing of pre-existing stress-induced fractures and microcracks in the tested sample. The outcomes of this study can contribute to indirect analyses of seal integrity of mudstones and shales from seismic velocity data.