Characteristics of thrust-strike-slip faults and genesis of fractures in volcanic reservoirs in the Qionghai Uplift and its surrounding buried hills

The Qionghai Uplift and the surrounding buried hills in the Zhu III depression, Pearl River Mouth Basin, are a key oil and gas exploration target in the northern South China Sea. The volcanic rock reservoirs in this area are significantly modified by fracture development. To clarify the formation mechanisms, this study combines high-resolution 3D seismic data, well data, and thin-section observations for analysis. The results indicate that the faults in the study area developed in two stages: (1) NW–SE compression during the Early–Middle Yanshanian formed NE-trending imbricate thrust faults, which were reactivated in the Early Himalayan; (2) left-lateral strike-slip during the Late Yanshanian formed a NNE-trending fault system comprising pure strike-slip, transtensional, and transpressional segments, which were reactivated in the Middle Himalayan. Fractures developed synchronously in two stages, with the densest fracture networks occurring in the hanging-wall regions near thrust faults (e.g., WC13-I well), transpressional segments of strike-slip zones (e.g., WC19-A well), and transtensional segments (e.g., WC13-M well). Specifically, the thrust faults caused shear fracturing of the hanging-wall strata through compression, while differential stresses along various strike-slip fault segments produced distinct fracture networks. Himalayan period stress field rotation further reactivated pre-existing fractures. The resultant fracture network provides effective pathways for oil and gas migration, thereby supporting exploration strategies in this area.