Since 1996, CO2 has been injected into the Utsira Formation above the Sleipner Øst field in the North Sea. During the injection period, the CO2 distribution in the reservoir has been monitored using eight repeated time-lapse seismic surveys. This seismic monitoring has shown that the CO2 is visible as thin bright reflectors within the Utsira Formation, interpreted to represent CO2 flooded layers. We show a detailed analysis of the thin CO2 layers, with respect to amplitude and time shift development. For a fixed location that becomes CO2 flooded, the amplitude first increased strongly and a small pull-up of the trough amplitude timing was observed. This response is attributed to interference between top and base of the layer, as long as layer thickness is beneath tuning thickness. This pull-up is an effect induced by wavelet distortion and not related to CO2 in overlying layers. With time the amplitude decreases and the pull-up is reduced to zero. This is attributed to the layer being thick enough for top and base to be separately resolved. Using our time-lapse observations we have estimated this wavelet distortion effect as a function of time of first flood, which has enabled us to correct for it, and estimate time shifts caused by CO2 in overlying layers. Thickness estimates derived from these time shifts complement previous amplitude derived estimates of CO2 thickness.

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