The impact of the complex evaporites features’ on the seismic illumination of the underlying rocks: A case study in the Brazilian presalt

The oil industry utilizes seismic techniques to image the Earth's subsurface. The number of reflected rays generated at a single point that return to the receivers impacts directly the quality of seismic data and, in turn, seismic interpretation. Seismic illumination is used as a quality control measure to find poor coverage zones, which are commonly found in pre-salt reservoirs of eastern Brazilian marginal basins. The Santos Basin has a wide evaporite section featuring some complex structures such as overhangs, mini-basins, salt domes, salt tongues and walls more than 3 km thick. Those structures change the paths of the seismic rays and reduce the coverage of seismic imaging below the evaporite layer in some regions. In this study, a quantitative and qualitative illumination analysis of a pre-salt horizon was performed comparing four different velocity model scenarios and the behavior of the seismic ray path was analyzed. The results show that both stratification and the geometric structures existing in the salt layer have a direct impact on the imaging of the pre-salt reservoirs. Each factor influences imaging in a different way and, when associated, can generate regions of low illumination quality. Consequently, it is of extreme importance to obtain a suitable understanding of the upper salt layer as it will govern the ray paths towards the layers below the evaporites. Furthermore, the mapping of the anomalies generated by the stratification should also be considered in the creation of the velocity model.