# A New Method of Geological Modeling for the Hydrocarbon Secondary Migration Research
## 2 Characterization of 'Unit Element' for Reservoir Modeling
### Meandering Fluvial Environment  
The architecture and facies heterogeneity of fluvial point bars and braided bars, as significant reservoir bodies within classical channel patterns, have garnered considerable attention and collaboration among scientists [18; 29; 30; 31]. A point bar represents a bank-attached element that forms as a result of lateral accretion or downstream-oriented accretion during the growth of a fluvial meander. Geologists have studied the migration of channel meander bends extensively for several decades [31; 32; 33; 34; 35]. During individual flood events, erosion takes place along the outer bank of the channel, leading to the retreat of the cutbank, while deposition occurs along the inner bank of the bend. This process results in the deposition of a new layer of sediment on the point bar, forming a gently dipping bed towards the outer bend, often referred to as a \"lateral accretion bed\" [9]. These laterally accreted sediments, also known as scroll bars, provide a record of the incremental growth of the meander over time. A point bar typically consists of multiple groups of genetically related scroll bars. In Figure 1A, the channel (in yellow) and the scroll bar (in orange) show the incremental growth procedure. The lateral accretion sand bodies developed in the growth procedure are separated by shale, which is called shale drape. The cross section is shown in Figure 1B. Figure 1: The conceptual model of meander bends migration (**A**) and cross-section (**B**) view of a point bar unit element. The deposits of many point bars are typically rich in sand, making them favorable reservoirs. However, the significant architectural heterogeneity of point bars is evident, both in the planform and in cross-section view (Figure 1). In the 1980s, meandering facies models were notably recognized as \"Inclined Heterolithic Stratification\" (IHS) [36; 37; 38]. The IHS model comprises inclined packages of alternating sand-prone and mud-prone beds or laminations. This lithologic heterogeneity is mainly driven by the varying proportions of mud- and sand-prone sediment. In actual deposits, the depositional beds exhibit internal complexity, recording the deepening of thalweg areas and the development of bar tops during rising flood stages, followed by the accumulation of sands in channels and mud deposition on the bar tops during falling flood stages. Meander shape refers to the morphology of a river bend, which exhibits high variability and poses challenges when attempting to quantify it using simple shape parameters. The assessment of meander shape variations is crucial for predicting the distribution of small-scale heterogeneity [20]. The fundamental modes of meander bend transformations, including expansion, translation, rotation, and their combinations, are widely acknowledged [14; 39; 40]. Understanding these modes helps in comprehending how the variations in bed topography and sedimentary textures vary as a river develops (Figure 2A). However, there remains a relatively limited understanding of the relationship between the migratory behavior of a meander bend and the resulting accumulated sedimentary architecture [41]. Conceptual or qualitative process-based descriptions have inherent limitations when used as guidance for object modeling methods. [PERSON] et al. [42] introduced an approach for comparing fluvial geometries by examining both the meander shape and the surface expression of the associated scroll-bar accretion direction. This methodology involves subdividing the upstream and downstream limbs  
Figure 2: (**A**) A route demonstration of a meander migration into a preserved planform (modified from [PERSON] et al. [42]). (**B**) A classification of the meander shape and key characteristics by which each type is geometrically defined [42]. at the meander apex. However, to prevent bias or preference, this study will not cover all of the parameters, but instead focus on understanding the potential range of meander shape variability.