# A New Method of Geological Modeling for the Hydrocarbon Secondary Migration Research
## 3 A Novel Object Stochastic Modeling Method
### Unit Architecture Element Database and Distribution Model  
Stochastic modeling offers a means of characterizing fluvial heterogeneity across different scales with reduced uncertainty by defining the statistical dimensions (length, width, and height) of geobodies. Sequential Indicator Simulation, a geostatistical method, is employed in this modeling approach [64]. Instead of initially constructing a gridded framework and assigning facies or lithology codes in a stereotyped solid geometry using pixels, reservoir models are built through the simulation of geological objects with hierarchical heterogeneity. These objects are extracted from a database of reservoir bodies that can be flexibly customized, allowing for a more flexible and accurate representation of the subsurface architecture. Experimental variograms capture spatial continuity using facies indicators. Limited well data in geostatistical simulations increase uncertainty, mitigated by seismic surveys or geological proportion maps. Seismic data offer broader but lower-resolution coverage than well logs, while refined proportion maps enhance the simulations. Horizontal variograms, often inconsistent due to sparse data, can be inferred from anisotropy ratios or seismic data. Vertical variograms remain stable with dense borehole sampling. Defining variograms within a geological framework is crucial, with modeler expertise guiding reliable formulations when direct data are insufficient. Modeling each channel bed individually for every flood event would be computationally intensive and may not provide definitive conclusions. Instead, by considering the process of point bar formation and the laws of sedimentary evolution, it is understood that lateral accretion within the river system leads to the development of composite point bar patterns. These patterns emerge through the formation, erosion, and stacking of multi-stage lateral accretion bodies. In this modeling and simulation approach, hierarchical meandering bends are defined as larger-scale channel bar deposits formed during flood stages, which are interspersed with finer-grained deposits generated during falling stages and abandonment channel fills. A point bar unit element consists of stochastically generated drapes, abandonment channel fills, and lateral accretion bodies as a cohesive entity. Among these components, the finer-grained sediments known as low-flow drapes, deposited on the top of bars during falling stages, exert the greatest influence on predicting the corresponding scale of heterogeneity. To construct a unit architecture element such as a point bar, braided bar, or mouth bar, it is necessary to establish geometric parameters and define the planar or longitudinal profile morphology of the unit element. This customized unit element can then be incorporated into a new or existing database (Figure 5), which will be utilized in the subsequent stochastic modeling process. Single bars within a braided unit element are generated stochastically, taking reference from a customized non-banded reservoir body database. The geometry of the braided unit elements is carefully considered, ensuring accurate representation of the heterogeneity within hierarchical fluvial segments. Additionally, physical properties are assigned to each deposit texture. Multiple reservoir models have been constructed using this object-based approach, which effectively captures the heterogeneity and exhibits strong compatibility with subsequent migration simulations. Figure 6 provides an overview of the river mouth bar reservoir, which has been modeled with sand ratio control. In order to optimize the computational efficiency and avoid potential issues, the bottom calcareous cementation is not shown in this particular workflow step (Figure 6B), considering the size of the project area and the geometry of the reservoir body. Following the gridding process, the petrophysical probability distribution is assigned to each facies, and the flow barrier boundary is depicted in the porosity (Figure 6C) or permeability model.