f Acoustic velocities of North Sea shales

Seismic waves used for petroleum reservoir exploration spend most of their time travelling through shales. More knowledge of acoustic properties of shale will improve the quality of reservoir interpretation (e.g, corrections for shale anisotropy are important for AVO) as well as of basin evaluation and predicted safety/stability during drilling. Previous studies of velocities in shales (e.g. Jones and Wang, 1981; Johnston, 1987; Homby et al., 1994; Johnston and Christensen, 1995; Sayers, 1995) have demonstrated stress dependency, interpreted mainly as a result of deformation of the contact regions between clay particles. Anisotropy has been observed and modelled primarily on the basis of stress sensitive alignment of clay particles. Previous measurements have, however, often been performed with outcrop or shallow subcrop cores, and data are usually obtained only for a limited range of sample characteristics.