Implications of VTI Anisotropy for Quantitative Seismic Interpretation of Clastic Reservoirs in a Sedimentary Basin

Abstract

Abstract This study investigates the importance of anisotropy of vertically transversely isotropic (VTI) media for quantitative seismic interpretation (QSI) of lithofacies of clastic reservoirs in a sedimentary basin. We emphasize the role of complex geologic processes and how their interactions control this importance. We look into two clastic stratigraphic intervals A and B of Mesozoic and Paleozoic age respectively that are part of the same sedimentary basin. For each interval, we model the distributions of elastic properties and Thomsen parameters of VTI anisotropy of lithofacies and we calculate reflection coefficient of P-wave as a function of angle. The results reveal that sandstone and shale have close values of acoustic impedance in interval A unlike sandstone and shale of interval B which show high separation of acoustic impedance values. Therefore, obtaining S-wave related attributes such as VP/VS ratio is more critical for interpreting lithofacies by QSI in interval A than in interval B. The values of Thomsen parameters of shale are higher in interval A than in interval B which can be attributed to differences in the mineralogical composition. When we calculate P-wave reflection coefficient as a function of angle for both isotropic and VTI anisotropic media, the discrepancy between the curves of isotropic and VTI anisotropic media is much higher in interval A than interval B. In summary, the importance of VTI anisotropy for QSI of lithofacies of clastic reservoirs is dictated by the geologic processes of deposition, burial and diagenesis. The separation of acoustic impedance values of sandstone and shale depend on the degree of consolidation of sediments driven by burial and diagenesis. Thomsen parameters depend on mineralogical composition that results from deposition and diagenesis. Interval A is an example of a scenario in which VTI anisotropy is crucial for QSI and seismic reservoir characterization.