Ultrasonic velocities in Cretaceous shales from the Williston basin

Abstract

Compressional and shear‐wave velocities were measured in the laboratory from 1 bar to 4 kbar confining pressure for wet, undrained samples of Cretaceous shales from depths of 3200 and 5000 ft in the Williston basin, North Dakota. These shales behave as transversely isotropic elastic media, the plane of circular symmetry coinciding with the bedding plane. For compressional waves, the velocity is higher for propagation in the bedding plane than at right angles to it, and the anisotropy is greater for the 5000-ft shale. For shear waves, the SH‐wave perpendicular to bedding and the SV‐wave parallel to bedding propagate with the same speed, which is about 25 percent lower than that for the SH‐wave parallel to bedding. In general, compressional and shear velocities are higher for the indurated 5000-ft shale than for the friable 3200-ft shale. All velocities increase with in‐increasing confining pressure to 4 kbar. The 3200-ft shale exhibits velocity hysteresis as a function of pressure, whereas this effect is almost nonexistent for the 5000-ft shale. Many features of the dependence of velocity on pressure can be explained by consideration of effective pressure and the degree of water saturation. For both shales, laboratory compressional wave velocities are on average 10 percent higher than log‐derived velocities. The discrepancy cannot be explained completely, but likely contributing factors are sampling bias, velocity dispersion, and formation damage in situ.