Mature hydrocarbon provinces are increasingly being re-explored to extend the productive lives of existing fields by testing outlier or deeper objectives for additional reserves. In recent years the introduction of affordable 3-D pre-stack depth migration (PSDM) has allowed very complex geology to be properly imaged to more fully assess hydrocarbon potential in many of these mature areas. This paper discusses a 3-D seismic depth reprocessing program undertaken in a complex U.S. Overthrust area to assess the validity of a deep structure (approx. 5500m) identified on 3-D time-processed data in this mature prolific gas producing province. The 1998 vintage seismic 3-D acquisition method did not allow a seismically derived velocity model capable of accurately representing the subsurface to be defined in this older/fast rock regime. A 3-D pre-stack depth migration velocity model-building solution is presented that overcame the limited sensitivity to velocity of these 1998 3-D seismic data. By collaborating with client geologic expertise to integrate key geologic insights into the 3-D velocity model-building process and tightly constraining the seismically derived velocity with available well control, a subsurface velocity depth model was obtained that provided an accurate depth image of the deep target zone in this geologically complex area.


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