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.

Microlocal analysis of seismic inverse scattering in anisotropic elastic media

“…Macroscopically, shales exhibit anisotropy due to the orientation of laminations owing to bedding or crossbedding. Their characteristic properties have been measured ultrasonically in the laboratory [68]. Their typical symmetry is hexagonal, with the restriction that triplication of the shear wave does not occur on the symmetry axis.…”

Maslov Shear-Waveforms in Highly Anisotropic Shales and Implications for Shear-Wave Splitting Analyses

“…Jones and Wang (1981) studied Cretaceous shales from the Williston basin at depths of 3200 ft/975 m and 5000 ft/1524 m (the 5000 ft shale is actually shale B in this study). They found the 5000 ft shale with 11% porosity to be much more anisotropic (over 20% in v p ) than the 3200 ft shale with 16% porosity (12% in V p ).…”

“…Shales are of fundamental importance to the oil industry as they constitute over 75 percent of the clastic fill in sedimentary basins (Jones and Wang, 1981). They can form the source and trapping rocks for oil and gas accumulations.…”

New Results in Modeling the Phenomenon of Acoustic Hysteresis

“…According to the theory of irreversible closure of microcracks, the microcracks closed during pressurization do not reopen during subsequent depressurization (Birch 1960). After the conception of irreversible compaction of pore spaces, the pores which collapsed at higher pressures do not recover their original shapes or dimensions at lower pressures (Jones and Wang 1981). By idea of the improvement of contact conditions, the contact conditions are modified by local ductile cushions of weak, alteration materials (e.g., chlorite, sericite or serpentine) along grain boundaries and microcracks (Hashin and Shtrikman 1963).…”