Abstract: The boundary fault systems of rift basins are crucial in controlling the basin’s topography, reservoir distribution, and
fluid migration. Nevertheless, their formation and evolution processes are often influenced by multiple factors such as pre-existing structures, sediment competency, and the stages and directions of rifting. This leads to fault systems with diverse orientations and complex combinations, making structural analysis and hydrocarbon exploration in the basin challenging. Through 3D seismic interpretation and data analysis, this study elucidates the geometric differences in different zones of the Shinan Fault along the northern boundary of the western sub-sag of the Bozhong Sag, reconstructs the fault activity, and identifies favorable exploration areas. The analysis reveals that: (1) the fault system exhibits both inherited and new features with an increasing number of faults from early to late stages and a decrease in the size of individual faults. The pre-existing NW-oriented structures and the new EWoriented structures form a broom-like structure, while the divergent new structures form a net-like, interwoven combination; (2) The fault system exhibits vertical stratification and lateral segmentation. The deep layers are controlled by simple listric faults forming isolated half-grabens, whereas the shallow layers develop numerous adjustment faults that converge into complex Y-shaped structures. From west to east, the structure transitions from a single fault to an overstepping listric fan combination, ultimately evolving into a domino-style half-graben structure; (3) The intricate geometry of the fault system results from the composite responses to multi-phases and multi-directions of rifting events and pre-existing structures. The displacement curves of fault segments with different orientations during the same period confirm their growth linkage processes. These structures represent an overlap of NE-oriented structures from the Shahejie stage and EW-oriented structures from the Dongying stage. The segmentation points of strike are often hard linkage points of the existing faults, where large-scale deltaic sedimentary systems commonly develop.  

Key words: Bozhong Sag, fault evolution, multi-phase rift in different direction, pre-existing structure, boundary Fault