Estimating Anisotropy from Deviated Wells Offshore Malaysia

The rocks in many of the Asia Pacific region have been measured to have strong anisotropy. Typically shales are anisotropic with vertically transverse isotropy (VTI). The process of estimating anisotropy can be done by comparing sonic velocities in vertical and deviated wells. Since anisotropy is expected to be a function of clay content (volume of shale) and to vary as a function of depth due to mechanical and chemical compaction in shales, it is important to ensure that the anisotropy estimation is made from rocks of equivalent shale content and level of compaction. The anisotropy estimated from a field offshore Malaysia indicates high Thomsen epsilon but with a low delta parameter. In such cases the impact on velocity and reflectivity is low for small angles. The anisotropy is found to be not a linear function of shale content but decreases rapidly with an increase in sand content. Although the AVO analysis suggests that the impact of anisotropy of these rocks on the reflectivity is low over standard acquisition angles, anisotropy acts to improve the reflectivity contrasts between sands and shales.