## 2 Geological Setting and Samples  
The Subei Basin is a sedimentary basin with origins dating back to the continental Mesozoic and Cenozoic periods. It formed upon the foundation of the Cretaceous-period bedrock. Encompassing a vast expanse of approximately 3.5 \(\times\) 10\({}^{4}\) km\({}^{2}\), the Subei Basin is located in the eastern region of China [7]. The basin is delimited by the Sulu uplift in the northern direction, the Suman uplift in the southern direction, the Tanlu fault zone in the western direction, and the South Yellow Sea Basin in the eastern direction. The Gaoyou Sag, located in the Subei Basin, underwent a comprehensive process of rift-basin evolution spanning from the Late Cretaceous to the Cenozoic. This evolution can be characterized as a sequence of stages; namely, early depression, fault depression, fault depression, and late depression. During the sedimentary period of the second member of the Tai-zhou Formation, which corresponds to the first member of the Funing Formation, the basin exhibited characteristics indicative of an early stage of rift basin evolution. The development of solely boundary faults and the presence of weak activity suggest that the basin was in a state of depression. During the sedimentary period encompassing the second and fourth members of the Funing Formation, there was a notable escalation in the degree of basin rifting and an intensification of tectonic activity. Currently, the basin has transitioned into the phase of fault-depression evolution. During the depositional phase of the Dainan Formation, the basin underwent a period characterized by fault depression. The formation of the thick Dainan Formation depression's basic structural pattern was influenced by the common control of the first and second faults of the depression. During the sedimentary phase of the Sanduo Formation, the basin underwent a transition period characterized by a rift depression, leading to the subsequent subsidence of the basin and the formation of a multi-level fault system. The denudation was a result of the uplift of the entire basin during the late Sanduo Formation. The Neogene and Quaternary basins have transitioned into the late depression stage of evolution. The cessation of tectonic activity resulted in the deposition of the Yancheng Formation and Dongtai Formation strata throughout the basin, exhibiting a uniform distribution. The Gaoyou Sag, with an area of 3500 km\({}^{2}\), is one of the most important oil-bearing depressions in the southern part of the Dongtai depression of Subei Basin. It is bounded by the northward dipping Zhenwu Fault in the south, the Lingtangqiao low bulge and Zhe Tuo low bulge in the north, the southern-fault step zone, the central deep-depression zone and the northern slope zone in turn, from south to north, and the overall NE-SW trend, which has the characteristics of a steep fault in the south and a gentle fault in the south and an overlap in the north [5,7]. (Figure 1). The Gaoyou Sag demonstrates a sequential geological progression from the lowermost stratum to the uppermost stratum, encompassing five significant formations: the Taizhou Formation from the Cretaceous period, the Funing Formation from the Paleogene era, the Dainan Formation, the Sanduo Formation, and the Yancheng Formation from the Neogene epoch, as depicted in Figure 2. The stratigraphic characteristics are concisely outlined as follows. The Taizhou Formation exhibits a thickness ranging from approximately 100 to 300 m. The spatial distribution of lithology exhibits significant variability. Within the Taizhou Formation, the lower section displays a consistent occurrence of interbedded (dark) brown sandstone and mudstone throughout the entire region. Additionally, the sedimentary rhythm within this formation is characterized by a lack of distinct patterns. The eastern region of the Gaoyou Sag exhibits a significant abundance of dark mudstones, serving as a prominent source rock within the Subei Basin.