Stress-dependent seismic anisotropy of shales

A simple theory for the stress-dependent seismic anisotropy of shales can b e obtained in terms of a second-rank tensor and a fourth-rank tensor that d epend on the orientation distribution of contacts between clay platelets Th e theory allows the normal and shear stiffness of the contact regions betwe en clay platelets to be obtained as a function of stress from measurements of seismic P- and S-wave velocities for shales. The ratio of the normal-to- shear compliance, B-N/B-T, Of the contact regions between clay particles is found to be sensitive to the saturation state of the shale. Inversion of v elocity measurements for fully saturated shales shows a low value of B-N/B- T when compared with measurements on air-dry shales, consistent with the ex pected reduction in normal compliance in a fluid-saturated, low-permeabilit y rock. For all shales considered, B-N/B-T is found to be less than unity. The contacts between clay particles art therefore more compliant in shear t han in compression.