The role of windfield, sedimentary sources, and basin structure in the formation of beach-bar deposition systems: A case study in Paleogene Shahejie formation in the Zhanhua Sag of the Bohai Bay Basin, China

Beach bar systems, as significant reservoirs in terrestrial lacustrine basins, have led to notable progress in understanding their formation mechanisms and controlling factors. However, under the complex basin setting during the fault-depression transition period, the distribution patterns, controlling mechanisms, and hydrocarbon exploration potential of different beach bar types remain poorly constrained. This study focuses on the Paleogene Second Member (Es2) in the Zhanhua Sag, Bohai Bay Basin, China, integrating well logs, core data, and 3D seismic interpretation to systematically analyze the sedimentary characteristics, dominant controls, and exploration potential of beach bar deposits. Three types are identified in the Es2: sandstone beach bars, carbonate-dominated beach bars, and clastic-dominated beach bars. Vertically, the scale of these deposits gradually increases from Part 4 to Part 1, while laterally, their distribution follows a tripartite coupling mechanism involving windfield, sedimentary sources, and basin structure: northeast-trending windfields drive wave action to form northwest-southeast-oriented beach bars; sediment supply intensity determines beach bar lithology (sandstone-dominated in high-supply zones vs. carbonate-dominated in low-supply zones); and the unique basin structure (steep slope–deep water–gentle slope) during the fault-depression transition controlled spatial differentiation. Reservoir characterization shows high porosity (25 %–30 %) but low permeability (20–50 mD), with 2–6 m thick sandy bars serving as prime exploration targets. In particular, stacked beach bars interbedded with mudstone seals in the southern gentle slope exhibit superior hydrocarbon accumulation conditions. These findings elucidate the genetic mechanisms of beach bars during a tectonic transition and their hydrocarbon enrichment patterns, providing critical geological references for lithologic reservoir exploration in analogous basins.