Well-seismic Velocity Error Analysis and Velocity Model Establishment and Application in Weixi Exploration Area

It is very important to predict the velocity accurately before drilling in the well location demonstration of seismic exploration. There are different ways to get formation velocity:borehole velocity, seismic dense point velocity (the stack velocity picked up from velocity spectrum processed at multiple locations near the pre-exploration well. In general, the time-depth relationship can be obtained by using the seismic dense point velocity near the pre-exploration well location for pre-drilling prediction. However, in the application of complex fault block in Weixi exploration area, it is found that the seismic density point velocity is greater than the wellbore VSP velocity, which leads to a greater prediction depth, which means the prediction error will be larger. It is not possible to use seismic dense point velocity for predrilling prediction. For this reason, a 3D velocity model is established based on VSP velocity and horizon constraints, and the velocity of pre-exploration well point is extracted from the velocity model to carry out time-depth conversion. The actual drilling shows that for the complex fault block structure in Weixi exploration area, the well velocity extracted by model is closer to that of actual drilling, and the prediction error is within the allowable range. For areas with complex geological and seismic characteristics, the velocity suitable for the area should be used for prediction, so as to minimize the prediction error and reduce the exploration risk.