Structural evolution of southern Sichuan Basin (South China) and its control effects on tectonic fracture distribution in Longmaxi shale

Abstract We aimed to reveal the structural styles and evolution sequences of the superimposed folding zone in the southern Sichuan Basin, South China, and to predict the distribution rules of tectonic fractures in the Longmaxi shale. The field survey and geological section analysis showed that the structural pattern is characterized by the superposition of near-N–S-trending folds and near-E–W-trending compressive structures. The two structural systems experienced multistage development, and the finalization of the E–W structures was later than the N–S ones. By combining these findings with the back-stripping method and fluid inclusion analysis, the burial, thermal, and structural evolution histories were reconstructed. The results indicate that the E–W-trending compressive structures were finalized at the end of the Late Cretaceous whereas the N–S-trending folds were formed by the Oligocene Epoch. A comparative analysis was conducted to reveal the relationship between tectonic fractures and fold structures. The analysis revealed that the Longmaxi shale in the core zones of the N–S-trending anticlines and the cores and near-core zones of the E–W folds is more likely to show a well-developed natural fracture system, which was verified by fracture observations on shale cores and outcrops. These places have potential for the exploration and development of shale gas resources.