Calculation of AVA Responses for Finely Layered Reservoirs

In oil and gas seismic exploration, responses of amplitude variation with incidence angle (AVA) of strata with a single thin layer have been extensively studied, thus laying the theoretical foundation for the prediction of finely layered reservoirs. However, studies on AVO responses of common shale-sand finely layered reservoirs are rare because of mathematical modeling problems involved with such complicated reservoirs. To accurately analyze the AVA responses of finely layered reservoirs, we describe such a reservoir as a transversely isotropic medium with a vertical axis of symmetry (VTI medium). We calculate parameters of the VTI medium according to the thickness ratios of gas-saturated sand layers in finely layered reservoirs and investigate the characteristics of AVA curves and gathers. The results show that the anisotropy of finely layered reservoirs will lead to large errors in actual seismic response recognition for large incidence angles or total thicknesses of sand layers close to half of the entire reservoir. The simultaneous use of - and -waves in AVA response analysis can provide better estimates of the thickness ratios of gas-saturated sand layers to finely layered reservoirs.