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Taking Advantage of RTM Surface Offset Gathers for Iterative Salt Modeling and Subsalt Reservoir Image Enhancement


We present the methodology adopted for updating a legacy PreSDM velocity model, in order to image steeply dipping reservoirs underneath a dual-bodies salt canopy, within a short time-frame to fulfill drilling commitments. The area of interest is in a deepwater setting and is partially covered by two overlapping salt bodies. The image quality of the sub-salt reservoir depends on the geometry accuracy of these bodies. In addition, a better delineation of the salt bodies can de-risk development wells positioning. The two operational objectives are to achieve a better control of the salt geometry through iterative salt modeling, and generate an improved image of the sub-salt reservoir. The two salt bodies were initially treated separately for top-salt horizon and salt flanks interpretation on Kirchhoff and Beam images. Salt velocity was re-assessed to take into account potassic salt and salt-flood migrations performed to interpret a first version of base-salt horizons. This formed an initial full salt body. The full salt body geometry was thereafter iteratively refined by a dedicated interpretation team. With illumination limitations and complex wavefront propagation, ray-based methods were withdrawn to the benefit of more expensive Reverse Time Migration (RTM), with generation of RTM surface offset gathers (SOG). The additional cost of using such algorithm was mitigated by the possibility, thanks to the acquisition set-up, to generate super-shot gathers, hence reducing the number of shots to migrate. In addition, a basic illumination study proved that only a sub-selection of super-shot gathers contributed to image the difficult salt area. This made re-migration/re-interpretation steps very smooth. Eventually, six iterations of salt refinement were performed to produce the final full salt velocity model. Final imaging was done with a 50Hz RTM and SOG generation by azimuth sectors. The combination of comprehensive salt model building through iterative wave-equation driven salt scenario testing and high-end algorithm imaging brought a step-change in reservoir imaging with improved continuity of the reservoir and better match with well data. It also revealed previously unseen dipping structures imaged only on the far offsets of RTM SOG. The extension of the full salt body was increased compared to the legacy model, with direct impact on the development well track. This was achieved in a fairly short operational time-frame.