## 2 Geological Conditions of the Study Area  
Located in the south of the Bohai Sea, the Laizhou Bay Sag is sandwiched by the east and west branches of the Tanlu strike-slip fault and is a Cenozoic sag developed on the Mesozoic basement. Adjoining the Huanghekou Sag, the Qingdong Sag, and the Maoxi Sag, this sag extends to the Ludong Uplift in the east, the Weibei Uplift in the south, the Kendong Uplift in the west, and the Laibei Low Uplift in the north, with a total area of about 1780 km\({}^{2}\)[19; 20]. The study area is located in the south of the Laizhou Bay Sag, with an area of about 920 km\({}^{2}\). The sag mainly includes five secondary structural units: the eastern strike-slip zone, the northern depression, the central structural zone, the southern subdepression, and the southern gentle slope belt (Figure 1). The basin-controlling faults developing in the south of the Laizhou Bay Sag mainly include the fault system composed of the east branch of the Tanlu strike-slip fault and its branches, the Laibei Number 1 main fault zone with a steep slope in the north, and the Laibei low uplift fault on the west side of the northern depression. The basin-controlling faults in the south of the Laizhou Bay Sag are mainly composed of secondary faults and tertiary secondary faults, which form the Lainan fault system (Figure 2) [21; 22]. These faults are mainly characterized by high activity intensity and long activity time, which greatly affects the internal pattern of the whole sag. The Mesozoic, Paleogene Kongdian Formation (Ek), the Shahejie Formation (Es), the Dongying Formation (Ed), the Neogene Guantao Formation (Ng), the Minghuazhen Formation (Nm) and the Quaternary Plain Formation (Qp) are successively developed from bottom to top in the Bohai Bay Basin. Among them, the Shahejie Formation can be divided into four sections, the Dongying Formation can be divided into three sections, and the Guantao Formation and the Minghuazhen Formation can be divided into upper and lower sections. There are volcanic rocks in the Paleogene of Dongying Sag, the Huimin Sag and the Liaoxi Sag in the Bohai Bay Basin. A large number of volcanic rocks were found in the Cenozoic of Bozhong area of the Huanghekou Sag and the D structure area of the Laizhou Bay Sag. Compared with the Qingdong Sag and the Miaoxi Sag, the distribution range of the Shahejie formation in the Laizhouwan Sag is large and the thickness is thick, which is in unconformity contact with the underlying Kongdian  
Figure 1: Study area geographical location. Formation. The development range of the second member of Kongdian Formation is small, and the fourth member, the third member, the second member and the first member of Shahejie Formation are successively developed from bottom to top. According to the drilling and seismic reflection characteristics, the fourth member of Shahejie Formation is divided into two parts: the upper sub-member and the lower sub-member. The fourth member of the Shahejie Formation (E2s4) is a set of alluvial fans to saline lake and shallow lake deposits. Coarse clastic rocks with mudstone and blue-gray mudstone are developed. Gray-white salt rock and gypsum layers are mainly intercalated with dark gray argillaceous dolomite and a small amount of gray and purple mudstone. Gray, dark gray, and brown-gray mudstone are mainly intercalated with sandstone and oil shale of carbonate rocks. The mudstone strata in the middle and upper parts are regionally stable source rocks. The third member of the Shahe Formation (E2s3) is a set of deep lake and semi-deep lake to shore-shallow lake delta deposits; a large set of dark mudstone, oil shale with a small amount of sandstone, and the upper sand and mudstone are interbedded, which makes a good reservoir. The third member of the Shahe Formation and the underlying strata are cut unconformably at the slope. The second member of the Shahe Formation (E2s2) is a set of shore-shallow lake delta and river marsh facies deposits.