Estimating amplitude uncertainties through illumination studies for a pre-salt reservoir

Abstract

Several discoveries were announced in pre-salt section in Brazilian Basins. There are several discussions regarding the correct way to build the best velocity model for migration purpose (algorithms, constant velocity, isotropy, FWI). These discussions look for how to build the best image only. The amplitude response seems to be a consequence of this built image. We present a method for uncertainty analysis in seismic illumination studies. Different velocity models were built in order to better understand their impact over the obtained hit-maps (ray-tracing method). The original velocity model is used as reference to compare the initial hit-map against others obtained using different velocity models. Cross-plots are used to assess areas where response values are divergent or where there is greater uncertainty in seismic illumination, and consequently in seismic amplitude response for the reservoir characterization. With original velocity model as input, we constructed four scenarios regarding the velocity of the evaporitic layer: (1) using the original velocities; (2) constant velocity for the evaporitic complex; (3) geostatistical extrapolation of wells logs; (4) conditioning the evaporitic complex velocity using amplitude as weight factor. This method can be used to investigate uncertainties and emphasize where amplitude response can be used properly for reservoir characterization.