The gas reservoirs in the northern Tarim basin in the western China are situated in the deep (4-5km) low relief structure. The complex near surface geology, e.g. rugged terrain, meandering rivers, channels and alluvial fans, results in a very complex near surface velocity structure, which has been a huge challenge for advanced anisotropic migration algorithms. The local but frequent variation due to heterogeneity or faulting in the thick overburden also has a significant impact on the structural shape and the depth accuracy of the deep reservoir. Recent drilling results show multiple cases of seismic-well depth reversal in the Hongqi gas field in Tarim basin. We explored the fundamental issues behind the reversed seismic-well depth relationship and concluded that conventional velocity building procedure could not capture the local but quick change in the overburden. We employed different velocity building and imaging strategies: (1) true ground surface or optimized ground surface using small grid smoothing as the top of velocity model and migration datum, (2) velocity for different depth levels from the near surface (0-300m), to the shallow depth (~300-~3000m), and to further deep intervals (>3000m) using different techniques and constraints, and (3) anisotropic parameters are updated during grid reflection tomography for the specific media in this field so as to enable an anisotropic PSDM. The resulting composite velocities successfully solved the seismic-well depth reversal in the Hongqi gas field.

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