The Bonan area, characterized by complex fault systems and multistage tectonic evolution, represents a critical hydrocarbon exploration target in the southern Bohai Bay Basin. Understanding the fault systems, depositional systems, and source rock distributions is crucial to hydrocarbon prospecting in this area. Here, we present a comprehensive characterization of the Cenozoic fault systems using high-precision 3D contiguous seismic data for the Bonan area. Analysis reveals, for the first time, a “three groups, three types” fault system organization that fundamentally controls the spatial-temporal distribution of depositional systems and source rocks. Specifically, three directional fault groups (NNE, NE, and EW/NWW), three fault types based on kinematic properties (strike-slip, extensional, and strike-slip-extensional), and three distinct tectonic evolution stages are identified. It is demonstrated that the early Cenozoic depositional systems were governed by the coupled interaction of EW-trending basin-controlling faults and NNE-striking-slip faults. In contrast, late Cenozoic deposition was controlled by a unified basin subsidence. These findings provide a new conceptual framework for understanding fault-controlled hydrocarbon systems in complex rift basins and offer practical guidance for exploration targeting in the Bonan area and analogous basins worldwide.

Inorganic elements in crude oil have been used in the reconstruction of the sedimentary environment and oil–oil (source) correlations; however, the effect of biodegradation on these elements has not been investigated sufficiently. In this study, 14 crude oils from the Miaoxi Sag of the Bohai Bay Basin, eastern China, were analyzed using molecular markers, trace elements, and major elements to determine the effect of biodegradation on inorganic elements. The molecular markers indicated that the oils are in the low maturity stage and are derived from similar parent materials in lacustrine source rocks. The high-sulfur oil came from a more reductive and saltier environment compared with the low-sulfur oil. The oils were subjected to varying degrees of biodegradation. The concentrations of Mg, Ca, Mn, Fe, Be, Sc, Rb, Sr, Zr, Pb, Th, and U increased significantly throughout the biodegradation process, while the concentrations of Na, K, Ti, Al, Cr, Zn, Cs, Nb, Ba, Hf, and Tl increased considerably only during the intense biodegradation stage (PM < 4). The concentrations of P, Li, V, Co, Ni, Cu, Ga, Sn, and Ta were not correlated with the level of biodegradation. The V/Ni, V/Co, Ni/Co, Cr/V, Sc/V, and Th/U ratios were affected by biodegradation when PM ≥ 4. Several ratios, including Mg/P, Ca/P, Mn/P, and Fe/P, are proposed as favorable indicators of the level of biodegradation. Differential enrichment of these elements is associated with the effects of organic acids generated by biodegradation on the oil–water–rock interactions in the reservoir.