4D pre-stack inversion workflow integrating reservoir model control and lithology supervised classification

We have run a 4D pre-stack inversion on seismic data acquired over a giant field located in deep water offshore Angola. The objective was to obtain dynamic information from 4D seismic data. The 4D inversion workflow started with a prestack 3D inversion of the baseline seismic survey. Using the relative P-wave velocity variations computed by warping, the initial impedance model of the baseline was updated in order to build the initial impedance model for the monitor survey. The update was done through a 4D mask which defines where impedance variations are allowed between the baseline and monitor impedance volumes. Due to the poor impedance discrimination between shales and water-bearing sands, where 4D effects may occur because of salinity differences between injected and aquifer water, reservoir model information was introduced in the mask in order to locate water-bearing sands. Ranges of relative impedance variations computed by the inversion were limited by 4D constraints derived from reservoir simulations before first oil and at the time of the monitor survey. 4D inversion brought sharper images compared to other 4D attributes. The high quality of the 4D inversion results evidenced by quantitative quality controls has opened the way to quantitative applications in reservoir management.