Waveform-based AVO inversion and AVO prediction-error

Abstract

Abstract A practical approach to linear prestack seismic inversion in the context of a locally 1-D earth is employed to use amplitude variation with offset (AVO) information for the direct detection in hydrocarbons. The inversion is based on the three-term linearized approximation to the Zoeppritz equations. The normal-incidence compressional-wave reflection coefficient R0 models the background reflectivity in the absence of hydrocarbons and incorporates the mudrock curve and Gardner's equation. Prediction-error parameters, ΔRsh and ΔRρ, represent perturbations in the normal-incidence shear-wave reflection coefficient and the density contribution to the normal incidence reflectivity, respectively, from that predicted by the mudrock curve and Gardner's equation. This prediction-error approach can detect hydrocarbons in the absence of an overall increase in AVO, and in the absence of bright spots, as expected in theory. Linear inversion is applied to a portion of a young, Tertiary, shallow-marine data set that contains known hydrocarbon accumulations. Prestack data are in the form of angle stack, or constant offset-to-depth ratio, gathers. Prestack synthetic seismograms are obtained by primaries-only ray tracing using the linearized approximation to the Zoeppritz equations to model the reflection amplitudes. Where the a priori assumptions hold, the data are reproduced with a single parameter R0. Hydrocarbons are detected as low impedance relative to the surrounding shales and the downdip brine-filled reservoir on R0, also as positive perturbations (opposite polarity relative to R0) on ΔRsh and ΔRρ. The maximum perturbation in ΔRsh from the normal-incidence shear-wave reflection coefficient predicted by the a priori assumptions is 0.08. Hydrocarbon detection is achieved, although the overall seismic response of a gas-filled thin layer shows a decrease in amplitude with offset (angle). The angle-stack data (70 prestack ensembles, 0.504–1.936 s time range) are reproduced with a data residual that is 7 dB down. Reflectivity-based prestack seismograms properly model a gas/water contact as a strong increase in AVO and a gas-filled thin layer as a decrease in AVO.