## 2 Geological Setting  
Located on the eastern coast of China, the Bohai Bay Basin extends approximately 1050 km in a northeast-southwest orientation and about 440 km in a northwest-southeast direction [24], covering an area of about 200 \(\times\) 10\({}^{3}\) km\({}^{2}\). Characterized as a quintessential sedimentary basin from the Mesozoic to the Cenozoic era, it is uniquely shaped by the Tan-Lu fault zone on its eastern side and the Taihang Mountain east fault zone to its west, which together engender a dustpan-like basinal morphology [25; 26]. The basin comprises seven depressions, namely the Jizhong, Lining, Huanghua, Jiyang, Changwei, Liaodong Bay, and Lower Liaohe depressions [27]. The Bonan Sag is located in the northern part of the Jiyang Depression, is mainly controlled by a series of normal faults [24], and is characterized as a half-graben sag with a steep slope in the northwest and a gentle slope in the southeast, trending east-north, and covering an area of nearly 800 km\({}^{2}\)[28]. The Bonan Sag contains a relatively complete sequence of Mesozoic and Cenozoic sedimentary deposits from both the depression and rifting lake basin periods. The boundaries of the Bonan Sag are quite distinct: to the north, the sag is connected to the Chengnan-Chengdong Fault and the Chengdong Uplift; to the east, it is bordered by the Guxi Fault linking with the Gubei Sag and the Gudao buried hill structural belt; to the south, it transitions to the gentle slope leading to the Chenjiazhuang Uplift; and to the west, it is connected to the Yidong Fault Zone of the Yihezhuang Uplift [29; 30]. The Bonan Sag evolved as a negative tectonic domain under extensional conditions [31] and is further subdivided into four structural belts, running from south to north: the Southern Gentle Slope Zone, the Boshen 4 Step-fault Zone, the Deep Depression Zone, and the Northern Steep Slope Zone (Figure 1).  
The sediments in the Bonan Sag were primarily deposited during the syn-rift stage, spanning from the Paleocene to the Eocene, and represent two phases of sedimentation. During the initial rifting stage, the Kongdian Formation (Ek) and the lower-fourth member of the Shahejie Formation (Es4x) were deposited. In the later extensional stage, the fourth, third, and lower-second members of the Eocene Shahejie Formation were deposited. During the subsidence process, the upper second and first members of the Shahejie Formation were deposited [32]. The Paleogene Shahejie Formation is the primary oil-generation strata, owing to its considerable thickness and widespread distribution. From top to bottom, the Shahejie Formation comprises Es1, Es2, Es3, and Es4, with Es4 being further subdivided into Es4s and Es4x. The main source rocks in the Bonan Sag are lacustrine mudstones and oil shales from the upper Es4 interval, lower Es3 interval, and Es1 interval [24]. The target layer, Es4x, is characterized by depositional facies including fan delta, braided river, braided river delta, lacustrine, and alluvial fan, with an average stratigraphic thickness of about 210 m. The lower part is composed primarily of purplish-red mudstone, silty mudstone, and gray fine sandstone, while the upper part consists mainly of gray mudstone and fine sandstone, with local occurrences of coarse sandstone, gravelly coarse sandstone, pebble sandstone, and sandy conglomerate [33].