## 2 Geological Setting  
The Huanghekou Depression is situated beyond the estuary of the Yellow River (Figure 1a) in eastern China and encompasses a total area of approximately 3300 km\({}^{2}\)[23; 24; 25; 26]. It shares borders with the Miaoxi Depression to the east and the Chengbei Depression and Zhanhua Depression to the west [25]. Its boundaries are defined by the Bonan Low Uplift to the north and the Laibei Low Uplift to the south (Figure 1b,c). This depression takes on a double-fault-like structure that was formed through mutual control exerted by both branches of the Tanlu Fault and an east-west strike-controlled fault [23; 24; 25; 26]. The Huanghekou Depression underwent a Palaeogene syn-rift subsidence phase, followed by a Neogene post-rift thermal subsidence stage during its Cenozoic evolution [27]. Fractures in southern Bohai Bay developed extensively during the Palaeogene syn-rift subsidence period, with common occurrences of strike-slip, normal, and strike-slip-extensional superposition faults. The Huanghekou Depression is primarily characterised by the development of two sets of fractures with near-EW and NNE strikes, which partition the basin into several secondary tectonic units: the northwest subdepression, southwest subdepression, the Central Tectonic Transformation Zone, and the east subdepression [15]. Within the four secondary tectonic units of the Huanghekou Depression, the northwest subdepression has a greater intensity than the eastern subdepression [15; 28; 29].  
The thickness of Cenozoic sediments in the Huanghekou Depression reaches a substantial 4000 m. The strata comprise the Kongdian (Ek), Shahejie (Es), Dongying (Ed), Guantao (Ng), and Minghuazhen (Nm) Formations. Lithology mainly includes conglomerate-bearing sandstone, gypsum, silty mudstone, red mudstone, oil shale, fine-grained sandstone, calcareous mudstone, grey mudstone, and grainstone (Figure 1d). The mud shale in the Huanghekou Depression is substantially thick, indicating an extensive expansion of the lake basin, while the first member of the Shahejie Formation (E\({}_{3}\)s\({}_{1}\)) was deposited. This has resulted in the development of a shallow, widely distributed lake, wherein the cap layer exhibits maximum distribution, which is primarily characterised by a combination of thick mudstone and sandy mudstone with a thickness of less than 100 m; it has a strong capping ability. The second member of the Shahejie Formation (E\({}_{3}\)s\({}_{2}\)) is characterised by the predominance of fan deltaic-shallow lake deposits and exhibits interbedding made of sandy mudstone. The third member of the Shahejie Formation (E\({}_{2}\)s\({}_{3}\)) underwent the significant development of deep lacustrine facies and fan delta deposits, characterised by thick black and dark grey shales with a wide stratigraphic distribution and substantial  
Figure 1: Map showing the regional geological outline of Bohai Bay Basin, Huanghekou Depression. (**a**) Location of study area in China; (**b**) regional tectonic units of Bohai Bay Basin, Huanghekou Depression; (**c**) structural units of study area and location of wells sampled here; and (**d**) generalised lithology, stratigraphy, and depositional environment of Bohai Bay Basin.  
thickness. The predominant lithology is mudstone, which serves as the primary cap rock for deep oil reservoirs [3; 5; 30].