TSime-lapse (4D) aalysis of legacy seismic data presents unique challenges, as neither the acquisition nor processing is designed for seismic monitoring. Two legacy seismic data sets from the Lena Field. Gulf of Mexico, are analysed for time-lapse effects. The analysis involves post-stack processing of the legacy seismic data including cross equalization and residual migration, and the definition of a new suite of 4D seismic attributes. These new attributes are used in both processing and interpretation. The time-lapse differences ard interpreted using forward modeling and production data. The 4D difference anomaly is interpreted to be the results of gas cap expansion. The identification of potentially hypassed oil based on this interpretation may affect future drilling decisions.
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