The specific goal of this study was to gain a clearer understanding of the effects of stratigraphically complicated overburden and acquisition geometry on amplitudes at and close to the Tops of pre-evaporite reservoir reflectors in Karachaganak Field in Kazakhstan (Figure 1). The 3D ray-trace modeling revealed significant irregularities in illumination coverage and considerable amplitude variations at the pre-evaporite reservoir levels. The results demonstrated that these illumination irregularities can be attributed to four discrete causes: thick salt bodies varying in lateral geometry and exhibiting complex boundaries; rapid lateral velocity changes within carbonate and evaporate bodies; steep dip; and rugged topography of reservoir interfaces. The combined effects of these factors resulted in severe raypath bending and distortions. These distortions not only lead to irregularities in the fold-of-coverage and reduced presalt reflection amplitudes but also tend to create traveltime anomalies and artifacts that disrupt the continuity of desirable reflections. Understanding the origin of these distortions is needed to interpret geologic variations at the presalt reservoir target intervals properly, and hence reduce uncertainty in this complex area (Figure 2).

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