Depth Conversion Problems of the Skua Field

Before oil volumes and economics can be calculated for an oil field, the seismic time map must be converted to a depth map. The Skua Field, located in Permit AC/P2 in the Timor Sea, has proved particularly difficult to depth convert. Velocity anomalies and inconsistencies in the seismic times, termed ‘lags’, have created distortions in the seismic time map which require compensation.

Beneath a velocity anomaly, both seismic undershoot and increased velocity, which are difficult to determine, must be compensated for during depth conversion. The current depth map was produced by smoothing through the pull-up regions on the time map, which required judgement, then depth, converting using a regional average velocity field.

The seismic lag, which is the difference between the seismic time and an ideal vertical path travel time, can only be measured at the wells and appears to be unpredictable. The seismic lag between Skua-4 and Skua-5 was assumed to change linearly in order to produce the depth map. Large lags can be introduced into the data in the common depth point stack stage of data processing. The stacking velocity with the largest stack response does not necessarily result in the smallest lag error.

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