Acoustic anisotropy in unconsolidated sediments and sedimentary rocks is a function of both the intrinsic character of the material and variability in the externally applied stress field. This paper reports measured intrinsic, acoustic anisotropy in unconsolidated sands from fold-thrust belts, from sub-salt settings, and for sands in extensional basin settings. In the latter, vertical effective stress has been the principal stress throughout the sand's burial history. In contrast in thrust belt settings the principal stress is non-vertical for some portion of the burial history. Similarly, in sub-salt settings horizontal stress gradients arise due to rapid changes in salt thickness. Polar and azimuthal acoustic properties were measured under isostatic stress conditions at in situ stress. Azimuthal anisotropy in thrust belt sands averaged 15% in both crestal and flank structural positions. Polar anisotropies tend to be substantially lower than azimuthal values, averaging half the azimuthal anisotropy. In sub-salt settings the degree of azimuthal anisotropy in sands ranged from 5–10%. Polar anisotropies are low, typically less than 1%. In extensional basin settings, measured azimuthal anisotropy ranged from 0–3%. In these samples, polar anisotropy is larger than the azimuthal value, and varies with the compaction state of the sands. In all cases the level of anisotropy measured in the core plugs was directly tied to textural changes observed in thin section, and is in good agreement with log measures of anisotropy. These observations suggest that a TI medium assumption may be inappropriate in fold/thrust belt and subsalt settings.
Predicting reservoir properties ahead of the bit in fold- thrust belts and in sub-salt settings must account for the action of elevated horizontal stresses that are commonly active in these settings. Elevated horizontal stresses result in enhanced compaction of reservoir sands and bounding mudrocks, although the magnitude of these effects is not well understood. One method for estimating the magnitude of the layer parallel compaction is the measurement of directional acoustic properties. This paper reports on the development of laboratory techniques for measuring acoustic anisotropy (polar and azimuthal) on single vertical plugs taken from full diameter core. The laboratory measurements were made under isostatic stress conditions. This implies that any directional differences in acoustic properties are intrinsic to the material and not related to stress state.
