A Study On Low Amplitude Structural Of An Oil Field In The Eastern South China Sea

Studying on velocity is of great importance to geophysics, since the velocity-field-built method can exert significantly impact on the accuracy of the structural mapping, trap definition, yield analysis and regional economic value assessment. Formation velocity plays a significant role in the whole process of the oil and gas exploration. With the development of petroleum exploration, exploration interest and activity gradually move to deep basin and complicated zone, which results in complicated seismic data acquisition conditions, dramatic lateral variation of velocity and difficulty in accurately requiring formation velocity. Thus, it is critical to obtain accurate seismic velocity field under the condition of strong heterogeneity, low amplitude structure and vertical and lateral variation of lithology and mineral.Enping Sag is located at the eastern South China Sea. The regional reservoir rocks and cap rocks are developed well,forming favorable source-reservoir-cap assemblages with juxtaposed source rocks. However, the tectonic movements and deposition environment resulted in low amplitudef structure in this target and uneven distribution of calcareous mineral, which leads to the dramatic vertical and lateral change of velocity. Also, lacking drilling data (only 4 wells) at the beginning of exploration in this target makes it challenge to require velocity field accurately.This study employed various methods to conduct depth-time conversion and comparative analysis on regional structure, including pre-stack depth migration, building velocity field with calibration velocity and VSP velocity, layered-model-based conventional variable velocity mapping and depth-profile-analysis-based variable velocity mapping, etc.,and thereby,proposed a method on studying structure with velocity for this target.Also, this study put forward a workflow for velocity field correction. Firstly, the vertical trend of tomography is corrected, which was determined by processing pre-stack time/depth migration with VSP velocity. Furthermore, the distribution of calcareous mineral is predicted with geostatistical inversion and log data. Finally, reasonable vertical and lateral velocity field was realized by compensating the corrected vertical trend of velocity field with the impact of mineral contents on velocity, while mineral contents from wells can be determined by forward modeling of rock physics.Based on methods mentioned above, interval velocity curve extracted from velocity field near well is in accordance with the VSP curve from log, and the lateral variation of velocity follow local geological rules, which is favorable for defining traps and clearly mapping the boundary of the traps and lays a good foundation for future reserves calculation and economic prospect evaluation.