## 1 Introduction  
The beach-bar system represents a typical sand-rich sedimentary body in coastal zones and shallow lacustrine environments. Geologists have systematically investigated its genetic mechanisms [1, 2] and depositional processes [3, 4]. With the advancement of hydrocarbon exploration, global clastic petroleum exploration has progressively entered the phase of exploring subtle reservoirs. It is now recognized that lacustrine beach-bar sandbodies, widely developed in faulted lacustrine basins, constitute potential high-quality reservoirs. For instance, breakthroughs have been achieved in the hydrocarbon exploration of beach-bar sands within the Green River Formation of the Uinta Basin (USA), and in the Bohai Bay, Junggar, and Qaidam Basins (China) [5]. This success establishes the beach-bar system as a critical target for lithologic reservoir exploration in mature oilfields, demonstrating broad development prospects. As a significant reservoir type in continental lacustrine basins, research on the beach-bar depositional system still exhibits notable discrepancies in classification schemes and depths. Classification criteria vary considerably as scholars have proposed similar or divergent schemes [6] based on lithological characteristics (e.g., carbonate versus clastic beach-bars), planar positions (e.g., shore-attached, nearshore, offshore, and isolated beach-bars), etc. From the perspective of sandbody thickness, the system can be categorized into thick-bedded (\(>\)10 m average single-layer thickness) and thin-bedded types. The thick-bedded type typically forms composite sedimentary bodies tens of meters thick via vertical stacking, with well-developed internal interbeds, and can be sub-divided into progradational and retrogradational stacking patterns. Conversely, the thin-bedded type exhibits frequent sand-mud interbedding, with bar sands averaging 3 to 8 m and bench sands \(<\) 2 m per layer [7; 8]. Whilst the thick-bedded beach-bar, characterized by favorable reservoir properties, is a primary exploration target, the thin-bedded variant poses significant challenges due to its limited thickness and rapid lateral variability. These technical constraints result in comparatively limited research on its genetic mechanisms in complex tectonic settings [9; 10; 11]. The beach-bar system within the Paleogene of the HHK Depression, Bohai Bay Basin, exemplifies an atypical thin-bedded system. It features limited sand layers (average single-layer thickness: 1 to 8 m) but exceptional lateral continuity and stability. Production data indicate that this reservoir exhibits high stability, productivity, and significant oil column heights, making it a key target for reserve growth. However, unlike typical thin-bedded beach-bars with frequent sand-mud intercalations, this deposit comprises thick mudstone sequences interbedded with laterally persistent sandstone layers. Furthermore, it developed under a strike-slip extensional tectonic framework, implying a complex genetic mechanism [12; 13; 14]. Consequently, research on such thin-bedded yet laterally extensive systems remains scarce. Studying this system not only enriches beach-bar depositional theory but also facilitates a strategic shift in subtle reservoir exploration from \"thick-bedded targets\" to \"thin-bedded, widespread plays,\" offering new pathways for lithologic trap exploration in complex tectonic zones [15; 16]. Developing under the synergistic control of multiple factors (paleoclimate, paleo-water depth, sediment provenance, and tectonic activity), the beach-bar system serves as a vital hydrocarbon reservoir in lacustrine basins. Conventional genetic studies rely primarily on lithological identification, well-log facies analysis, and three-dimensional seismic interpretation [17; 18]. Although effective in delineating macroscopic sandbody distribution, these approaches inadequately constrain paleoenvironmental parameters quantitatively and exhibit limited precision in provenance tracing within complex tectonic settings.