# A New Method of Geological Modeling for the Hydrocarbon Secondary Migration Research
## 2 Characterization of 'Unit Element' for Reservoir Modeling
### Sandy Braided Environment  
While significant research has been conducted on meandering fluvial systems, there has been less consensus and recognition regarding the geomorphological elements, stratification, and depositional facies sequences in braided rivers [31; 43; 44; 45]. Braided rivers are characterized by wide and shallow channels and bars, with key sedimentary processes such as bar formation, channel-floor dune migration, low-water accretion, and overbank sedimentation [28]. To address these challenges, [PERSON] [28] proposed a categorization of lithofacies types and facies associations using six vertical profile models, which have undergone revision over time. However, these lithofacies codes are descriptive and interpretive, and many facies types can occur in multiple environments within a river. As the interest in 3D reservoir modeling grows, there is a pressing need for a more definitive subdivision and unified nomenclature for the elements of the braided reservoir architecture (Table 1). [PERSON] [46] developed a comprehensive facies model for the sandy braided South Saskatchewan River, incorporating the following four key geomorphological elements: channels, slipface-bounded bars, sand flats, (vegetated) islands, and floodplains. Furthermore, based on morphological classification, [PERSON] [46] also identified ripples, sand waves, and dunes as bedforms in lower flow regimes. [PERSON] et al. [47] further refined the architectural subdivision of sandy braided rivers, delineating unit bars, compound bars, cross-bar channels, and dunes. Compound bars consist of amalgamated unit bar deposits, often intersected by cross-bar channels filled with dunes and rippled sands. In this study, we introduce the concept of a \"braided unit element\" as the fundamental unit for stochastically filling braided reservoirs. We address concerns regarding the consistent preservation of fluvial deposits in one dominant direction and the tendency to substitute lenticular reservoir bodies. Based on our understanding of braided river sand bodies, the braided unit element exhibits a lenticular cross-section, resembling the geomorphology of river depressions, with an elliptical planfor \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reservoir Architecture** & **Scale** & **[PERSON] [28]** & **[PERSON] [46]** & **[PERSON] [47]** & \begin{tabular}{c} **Geomorphological** \\ **Features** \\ \end{tabular} \\ \hline \begin{tabular}{c} Fall-siltseams \\ Fine-grained sediment \\ Unit bar \\ \end{tabular} & 4 th-order & \begin{tabular}{c} low-water accretion \\ 4 th-order \\ 4 th-order \\ 4 th-order \\ 4 th-order \\ \end{tabular} & \begin{tabular}{c} none \\ overbank sedimentation \\ limquoid or transverse \\ bar \\ \end{tabular} & \begin{tabular}{c} none \\ floodplains \\ bar, cross-channel bar, \\ unit bar \\ \end{tabular} & \
\begin{tabular}{c} none \\ floodplains \\ \end{tabular} &
\begin{tabular}{c} top of braided channel \\ top and inside of unit bar \\ long, low, \\ slipface-bounded \\ large areas of sand \\ accumulation \\ convex downward and \\ separated by bars \\ actual topographic \\ depressions \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Variables Considered for the Model. deposits in this braided unit element are characterized by both singular \"unit bars\" and individual channel fills. The braided unit element possesses a flat bottom and a convex top, and is superimposed within the convex downward lenticular braided channel sand under varying hydrodynamic and topographic conditions. We do not model complex \"compound bars\" or \"sand flats\" as independent elements that can be stochastically simulated through the amalgamation of unit bars. Additionally, the recognition of compound bar deposits poses challenges [48], making dimensional estimation problematic. Bedforms such as dunes and ripples are all upscaled and represented as channel fills within our modeling approach.