A B S T R A C TScanning and transmission electron microscopy, synchrotron X-ray diffraction, microtomography and ultrasonic velocity measurements were used to characterize microstructures and anisotropy of three deeply buried Qusaiba shales from the Rub'alKhali basin, Saudi Arabia. Kaolinite, illite-smectite, illite-mica and chlorite show strong preferred orientation with (001) pole figure maxima perpendicular to the bedding plane ranging from 2.4-6.8 multiples of a random distribution (m.r.d.). Quartz, feldspars and pyrite crystals have a random orientation distribution. Elastic properties of the polyphase aggregate are calculated by averaging the single crystal elastic properties over the orientation distribution, assuming a nonporous material. The average calculated bulk P-wave velocities are 6.2 km/s (maximum) and 5.5 km/s (minimum), resulting in a P-wave anisotropy of 12%. The calculated velocities are compared with those determined from ultrasonic velocity measurements on a similar sample. In the ultrasonic experiment, which measures the effects of the shale matrix as well as the effects of porosity, velocities are smaller (P-wave maximum 5.3 km/s and minimum 4.1 km/s). The difference between calculated and measured velocities is attributed to the effects of anisotropic pore structure and to microfractures present in the sample, which have not been taken into account in the matrix averaging.
Some rock properties show surprisingly stable relationships, to the extent that any deviation therefrom should be regarded as a signal of anomalous behavior. One of these stable relationships is the linear trend between compressional (Vp) and shear velocities (Vs), which is observed for both sandstones and mudstones. We infer that anomalous stresses, notably deviations from the basin-wide relationship between horizontal and vertical effective stresses, may cause deviations from the basin-wide Vp-Vs trends. An anomalously high or low shear velocity can therefore be indicative for an anomalous stress regime.
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