Publications

The field under study is located in the Malay Basin with 13 exploration/appraisal wells, including side tracks, drilled. A marine 3D seismic data acquired over the area in 2002 was reprocessed via PSDM in 2009. The area is characterized by near-surface velocity anomalies caused by shallow gas sand bodies of different sizes. These anomalies resulted in pull-down effects on the time structure and needed to be addressed by the time-depth conversion. The stacking & PSDM velocities from the reprocessing were both used for time-depth conversion, of which the results were then compared.A velocity model was constructed using the stacking velocity independent of the PSDM velocity. The available residual move out (RMO) stacking velocities were picked on a 500m by 100m grid that should allow at least the larger near-surface velocity anomalies to be captured. However the increasing noise with time of the velocities masked the small near-surface anomalies in the stacking velocity field at reservoir level, which needs to be captured for more accurate time-depth conversion. One way to address this problem is the introduction of a constant surface in depth below the near-surface gas clouds. The time conversion of this surface with the stacking velocities revealed the pull-down effects of the velocity anomalies nicely. This time surface was included in the final velocity model and marks the boundary of the upper-most 'artificial layer' to capture the low velocities of the shallow gas anomalies. It is important to use the unsmoothed stacking velocities for the first 'artificial' layer and strongly smoothed stacking velocities or else, well velocities for all other layers of the velocity model.A comparison of the results of the velocity model with the depth surfaces based on the PSDM velocities showed surprisingly small differences.A depth error analysis of both models (stacking velocities & PSDM velocities) using the 'blind well test' showed that the PSDM based model delivered a slightly smaller standard deviation for the deep horizons.In conclusion, the superiority of the PSDM velocities for time-depth conversion remains unchallenged. However in many situations, a velocity model based on a time variant filtering of the stacking velocities may deliver results similar to the PSDM velocity model.