Velocity Anisotropy of a Shallow Mudstone Core

Mudstones and shales are a major component of most sedimentary basins and are anisotropic because of the alignment of plate-line clay minerals. The results show that the shallow mudstone is weakly anisotropic with respect to P-wave anisotropy (Thomsen’s parameter ε) on the order of 20% but strongly anisotropic for S-wave propagation (Thomsen’s parameter γ) on an order of 64% at higher stress. The anisotropy parameter δ is observed as maximum as 42%. The magnitude of P-wave anisotropy (ε) seems to increase with increasing effective stress or depth of burial. The other anisotropy parameters (γ and δ) also increase with increasing stress upto certain stress levels (about 12 MPa mean effective stress) and than decrease gradually with increasing mean effective stress. Factors such as stress induced as well as alignment of plate-like clay minerals may causing the different anisotropic behavior for γ and δ compared to ε where shear wave polarization may play an important role. The anisotropic behavior found in the shallow mudstone core may impact seismic imaging, AVO analysis, time-depth calculations and well to seismic tie of sonic log data of deviated wells.