# A New Method of Geological Modeling for the Hydrocarbon Secondary Migration Research
## 2 Characterization of 'Unit Element' for Reservoir Modeling
### Delta Front Environment  
A delta is defined as a \"discrete shoreline protrusion formed where rivers enter oceans, semi-enclosed seas, lakes or lagoons and supply sediments faster than they can be redistributed by basinal processes\" [49]. [PERSON] [50] illustrated a vertical sequence of deltaic facies that become coarser and sandier upward, corresponding to the progradation of bottomset, foreset, and topset strata (Figure 4A). The pro-delta subfacies predominantly consists of claystone with significant sediment thickness, wide distribution, and high organic content, indicating favorable conditions for oil generation [51]. Delta-front subfacies deposits include mouth bars, distal bars, and sheet sands composed of clean and well-sorted sands, indicative of favorable reservoir properties. The clays that overlay the deltaic  
Figure 3: Major geomorphologic units subdivided by [PERSON] (1978) [46] and [PERSON] (2006) [47] (**A**). Planform (**B**) and cross-section profile (**C**) illustration of a braided unit element generalized from major geomorphologic braided rivers. sand body during the transgression process, as well as the alluvial swamp deposits formed as the delta advances towards the sea, can serve as effective cap rocks. Distributary mouth bars play a significant role in the active sand deposition within modern deltas, representing a fundamental architectural element [53]. These bars form as sediment transport rates decrease basinward, resulting in deposition at the river mouth and the amalgamation of sand bodies with good lateral continuity. The size and shape of a mouth bar are influenced by factors such as the angle of plume dispersion, flow conditions, and forces acting on the river plume. In river-dominated deltas, the core of the mouth bar primarily consists of relatively pure coarse-grained sand, exhibiting a narrow band of pronounced directionality perpendicular to the shoreline (Figure 4B). Mouth bars typically have width ranging from several hundred meters to a few kilometers in length. Con  
Figure 4: (**A**) Typical example of a delta clinoform, showing topset, foreset, and bottomeset strata [50]. Bed boundaries are corrected to follow the time lines (from [PERSON] and [PERSON] [52]). (**B**) River-dominated lobes formed by mouth bar accretion (modified from [PERSON] [53]). (**C**) Dip section and strike section illustrations from mouth bar unit elements (vertical exaggeration: 10). Bottom is heterolithics of the calcareous cementation layer. versely, in wave- or tide-dominated deltas, the mouth bar sand body displays an elongated geometry, elongating perpendicular to the ancient river and parallel to the shoreline. Architectural element analysis of deltas has not progressed as extensively as that of fluvial systems, but there is a growing body of research focusing on mouth bars deposited in river-dominated delta fronts [54; 55; 56]. Mouth bars can serve as distinct physical units closely associated with terminal distributancy channels, typically characterized by depths ranging from a few meters to tens of meters, and widths spanning tens to hundreds of meters. The foresets, representing inclined sediment layers, can be observed in the cross-sectional dip profile of the delta clinoform (segment C in Figure 4A). These foresets are primarily associated with the delta front and exhibit dips ranging from a few degrees up to 13\({}^{\circ}\) (Figure 4C). Along the strike direction, overlapping mouth bars contribute to the formation of lenticular or mounded delta lobes (Figure 4B). The grain size of sandstones exhibits significant variation among different sedimentary microfacies and different parts of the same microfacies due to differing hydrodynamic forces [57]. Within the middle and upper sections of the mouth bar, the dominant sandstone compositions include medium-coarse sandstone, medium-grained sandstone, and medium-grained fine sandstone, characterized by a coarse grain size and strong hydrodynamic forces.