The Seismic Anisotropy of Shales
The state of stress that acts on sediments during burial is anisotropic, and an anisotropic rock fabric might result. An anisotropic rock fabric can lead to seismic anisotropy, in which seismic wave velocities vary with the direction of propagation. Seismic anisotropy can result from fine-scale layering and alignment of anisotropic plate-shaped clay minerals with planes oriented approximately perpendicularly to the maximum stress direction. This chapter will discuss seismic anisotropy of shales. Chapters 4 and 7 discussed stress-induced anisotropy and fracture-induced anisotropy, respectively.
Shales are a major component of sedimentary basins and play a decisive role in fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure. Banik (1984) studied 21 data sets from the North Sea and found an excellent correlation between the occurrence of depth errors obtained from surface seismic data and the presence of shales in the subsurface. One needs to quantify shale anisotropy to obtain reliable information on reservoir fluid, lithology, and pore pressure from seismic data. In addition, failure to account for anisotropy in seismic processing can lead to errors in normal-moveout (NMO) correction, dip-moveout (DMO) correction, migration, time-to-depth conversion, and analysis of amplitude variation with offset (AVO). Shale anisotropy also affects variation in stresses in sedimentary basins, along with the containment of hydraulic fractures (Higgins et al., 2008).