渤中凹陷BZ1探区Qa构造速度异常分析

渤中凹陷BZ1探区Qa构造两口钻井在相距1.3 km的实钻过程中出现了速度异常，三维地震资料时间剖面地层深度与钻井地质分层深度出现了不一致的矛盾现象，即新钻井Qa 3井的钻井预测深度相对于本井地质分层深度更深，速度相对于Qa 2井变小，因而两口井地震速度在横向分布上出现了明显差异。分析表明，速度异常主要由沉积和构造共同作用所引起。利用地震叠加速度谱建模，通过井点速度场、趋势种子点进行校正，获取到更加准确的地下速度，并进行高精度变速成图，能够真实反映地下构造。对比单井速度预测钻井深度与单井速度场校正速度谱建模速度场预测钻井深度,并对比其误差精度，可以看出后者有效地解决了实际钻探过程中因速度异常而导致预测深度误差偏大的问题。因此钻前进行速度场建模与变速构造成图，对今后勘探井或评价井的部署及钻井深度精确预测具有重要的指导作用。

Analysis of velocity anomalies of the Qa structure in the BZ1 exploration area,Bozhong sag.

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.