Implications of Anisotropy on Seismic Quantitative Interpretation: A Case Study from the North Malay Basin

The rocks of the North Malay Basin are highly anisotropic with Vertical Transverse Isotropy. The degree of anisotropy increases strongly with depth. In general, the anisotropy is characterized by high values of the Thomsen parameter  and low values of . Although it is common practice to use anisotropic migration for seismic processing, anisotropy is often ignored in other parts of the quantitative interpretation workflow. The main reason for this is that it is difficult to solve for anisotropy as part of seismic inversion. Facies-based inversions can include pre-assigned anisotropic properties for different facies, but there are usually very large uncertainties in the values to assign. Anisotropy is therefore often ignored with the expectation that the anisotropic effects are handled by the wavelet estimation process. Forward modelling of synthetic AVO seismic with a range of realistic anisotropy parameters for a sequence of rocks from a Lower Coastal Plain environment was carried out. Wavelet estimation and deterministic seismic inversion were performed under the assumption of isotropy. The delineation of gas sands was made no more challenging than if the rocks were isotropic. The predicted values of Vp/Vs and especially density, however, were not reliable which has implications for quantitative reservoir characterization.