ICE 2022


The stratigraphic deposits of the Lower Permian Maokou Formation in the south of the Sichuan Basin are located in carbonate shallow sea open platforms, and are mainly characterized by the development of intraplatform beach subfacies. At the end of the Middle Permian, affected by the Soochow Movement, the stratum was extensively uplifted, and the exposed carbonate rock suffered long-term weathering and erosion and atmospheric freshwater leaching and dissolution, and karst was very developed. In recent years, some wells have discovered karst fractured-cave systems in the syncline area (early exploration was concentrated in the anticline area) and obtained discovery of commercial gas, and exploration and development have shown that the degree of fractured-cave development is positively correlated with the level of gas production. Therefore, fracture-cave prediction is of great significance. Taking full advantage of the 3D full-azimuth and multi-wave seismic data in the YJ syncline area, the OVT domain (common offset pre-stack migration) and full-azimuth angle domain (common angle pre-stack migration) imaging of PP-wave were carried out. Based on these two kinds of gathers, fracture prediction is implemented by AVAZ method, and the PS-wave splitting is explored. Analysis showed that, compared with conventional OVT gathers, the seismic travel time and amplitude on full-azimuth common angle gathers shows more obvious anisotropic characteristics as the azimuth changes. Basic law of fractures distribution predicted by full-azimuth common angle gathers is better than that of OVT gathers, which quite conforms to the characteristics of karst reservoirs. The fast and slow shear wave separation finds that, the travel time of R component shows cosine characteristics and the T component has strong energy and polarity reversal phenomenon, meaning that the shear wave splitting phenomenon is obvious. After completing the analysis and correction of shear wave splitting in three time windows, the fast and slow shear wave time delay (indicating fracture development degree ) and polarization direction (indicating fracture azimuth) are obtained, and the predicted fracture favorable development areas are better consistent with drilling results and geological regularity than result of PP-wave AVAZ, and the fracture azimuth prediction is basically consistent with the tectonic stress field and fault distribution characteristics, indicating that the prediction result is reliable.