An analytic formulation is obtained that relates mapped time-lapsed seismic amplitudes and time-delays directly to the sum of reservoir pressure and saturation changes weighted by the principal groups of controlling parameters from the petroelastic model. Application to data from the North Sea and West Africa provides insight into how the petroelastic model controls the seismic response and the relative interplay between pressure and saturation changes for each reservoir type. These data, together with published literature examples, allow the identification of regions in the reservoir where the saturation-driven 4D signature dominates over the pressure-driven, or vice-versa. These observations are used to determine a simple constraint on the relative magnitude of the controlling groups of petroelastic model parameters. It is concluded that the evaluation of 4D seismic as saturation- or pressure-dominated is a key measurement for our understanding of the in situ petroelastic model.


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