Abstract:  The velocity anomalies occurred in two wells 1.3 km apart, drilled in the Qa structure of the BZ1 exploration area in the Bozhong sag. There was a contradiction between the depth of stratum from the time section of the 3D seismic data, and drilling depth from geological stratification. The predicted depth for well Qa3 (a new drilling well) was deeper than the geological layer depth, while the velocity of well Qa3 is lower than that of well Qa2. Thus, these two wells clearly showed anomaly in the transverse distribution of the seismic velocity. The main cause of this velocity anomaly was both sedimentary and structural. Using seismic stack velocity spectrum modeling and correction of well point depth and trend seedpoint, more accurate speed was obtained, and the resulting high precision variable speed mapping reflected the true underground structure. In the paper, single well velocity method was compared with velocity spectrum modeling corrected by single well velocity for predicting drilling depth. The latter method achieved higher precision in depth prediction by effectively reducing errors caused by velocity anomaly in the actual drilling process. Therefore, velocity model building, combined with variablevelocity mapping before drilling, should play an important role in future deployment of exploration and appraisal wells, and prediction of drilling depth.

Key words: velocity anomaly, velocity field modeling, seismic velocity, variablevelocity mapping