ANALYSIS AND PREDICTION OF ABNORMAL VELOCITY IN PALEOGENE MUDSTONE OF STRUCTURE BZ19-6 IN OFFSHORE BOHAI BAY BASIN

The structures of BZ19-6 and BZ21/22 are located in the southwest of Bozhong Depression of the Bohai Bay Basin, where commercial oil and gas have been discovered in the glutenite layers of Early Paleogene, the Archean buried hill at structure BZ19-6 and the Paleozoic buried hill at structure BZ21/22. Drilling has revealed that abnormal high pressure generally occurs in the Paleogene mudstone, with pressure coefficient changing between 1.2 and 2.0. The pressure is so high that traditional velocity prediction methods are always invalid for undrilled blocks. Accurate formation velocity prediction is critical important to depth prediction of the top boundaries of the deep glutenite layer and buried hill reservoirs, and to safe drilling. According to the forming mechanism of overpressure, it is found that the velocity abnormals in the area are mainly caused by undercompaction and hydrocarbon generation. Comparing the pressure and velocity affected by the change in undercompaction and hydrocarbon generation, it is found that as the buried depth increased to certain level, the influence of undercompaction on velocity will gradually become stable, and the change of velocity will be dominated by hydrocarbon generation process in the overpressured Paleogene mudstone. Accordingly, the quantitative relationship between velocity and influencing factors is established. Based on this method, the predicted depth of the top of buried hill is in good agreement with the actual drilling data, and the safety of drilling operation is also improved.