## 2 Regional Geological Background  
The HHK Depression in the study area is located in the southern part of Bohai Bay Basin, which can be further divided into the northern steep slope zone (east and west segments), the southern slope zone (east and west segments), the central strike-slip zone, and the central uplift zone (Figure 1).  
The Kongdian Formation (E\({}_{\text{1-2k}}\)), the fourth member of the Shahejie Formation (E\({}_{\text{2s4}}\)), the third member of the Shahejie Formation (E\({}_{\text{2s3}}\)), the second member of the Shahejie Formation (E\({}_{\text{3s1}}\)), the third member of the Dongying Formation (E\({}_{\text{3d}}\)3), the second member of the Dongying Formation (E\({}_{\text{3d}}\)2), and the first member of the Dongying Formation (E\({}_{\text{3d}}\)1) developed successively from top to bottom in the Paleogene in this area [31]. The E\({}_{\text{3d}}\)2 member can be further subdivided into the lower E\({}_{\text{3d}}\)2 sub-member and the upper E\({}_{\text{3d}}\)2 sub-member. The corresponding seismic  
Figure 1: Location of Bohai Bay Basin and HHK Depression (red dotted box) [64; 65].  
reflective layer interfaces for members from E\({}_{2}\)s\({}_{4}\) to E\({}_{3}\)d\({}_{1}\) are as follows: T\({}_{7}\) (E\({}_{2}\)s\({}_{4}\)), T\({}_{6}\) (E\({}_{2}\)s\({}_{3}\)), T\({}_{5}\) (E\({}_{3}\)s\({}_{2}\)), T\({}_{4}\) (E\({}_{3}\)s\({}_{1}\)), T\({}_{3}\) (E\({}_{3}\)d\({}_{3}\)), T\({}_{3}\)\({}^{\rm m}\) (E\({}_{3}\)d\({}_{2}\)-the lower E\({}_{3}\)d\({}_{2}\) sub-member), T\({}_{3}\)\({}^{\rm u}\) (E\({}_{3}\)d\({}_{2}\)\({}^{\rm u}\)-the upper E\({}_{3}\)d\({}_{2}\) sub-member), and T\({}_{2}\) (E\({}_{3}\)d\({}_{1}\)) (Figure 2).  
In terms of tectono-sedimentary evolution, the E\({}_{3}\)s\({}_{2}\) to E\({}_{3}\)d\({}_{1}\) stages of the HHK Depression are in the rifting stage of basin tectonic evolution, with an average tectonic subsidence rate ranging from 11.54 to 54.7 m/Ma. The E\({}_{3}\)d\({}_{3}\) shows a higher tectonic subsidence rate. The intensity of rifting activity gradually weakens and rifting action attenuates in the later period, which marks the beginning of the transition to a post-rift depression [64]. Macroscopically, the HHK Depression Paleogene segments have certain differences in lithology combination characteristics and sedimentary facies distribution. They also have inheritance characteristics. In the study area, E\({}_{3}\)s\({}_{2}\) to E\({}_{3}\)d\({}_{2}\) are in the fault-depression period during the development of the Bohai Bay Basin. The geomorphology tends to be flat in the E\({}_{3}\)s\({}_{1}\) period, and the overall thickness of strata in the area is thin and stable. During the sedimentary period of the Dongying Formation, the intensity of fault activity increased, and a set of thick lacustrine mudstone developed in the early stage of E\({}_{3}\)d\({}_{3}\)[65]. The HHK Sag was affected by tectonic activities during the E\({}_{3}\)d\({}_{3}\) period, resulting in fault activation and large-scale lake invasion. The HHK Depression is dominated by thick mudstone with thin sandstone and siltstone and locally developed braided river delta and beach-bar deposits. During the lower E\({}_{3}\)d\({}_{2}\) Sub-member, the basin tectonic subsidence rate gradually weakened, the paleowater depth changed from deep to shallow, and the delta sediment advanced to the lake, which was significantly affected by the transformation of lake waves. The beach-bar scale gradually increased, and braided river delta and beach-bar sediments developed. This study focuses on the beach-bar sedimentary characteristics of the lower E\({}_{3}\)d\({}_{2}\) Sub-member of the Paleogene HHK Depression (Figure 3).  
Figure 3: Sequence Stratigraphic Architecture and Depositional Infill Patterns of the Paleogene System in the HHK Depression.  
Figure 2: Generalized Paleogene Lithostrigraphic sequence stratigraphy and tectonic evolution of HHK Depression, Bohai Bay Basin, China [64; 65].