# A New Method of Geological Modeling for the Hydrocarbon Secondary Migration Research
## 2 Characterization of 'Unit Element' for Reservoir Modeling  
Traditional sedimentary facies analysis and sedimentary architecture analysis share the common goal of studying the origin and spatial distribution of sedimentary units. However, they differ in terms of scale and the focus on interfaces. Traditional sedimentary facies analysis typically categorizes facies into the following three levels: facies, subfacies, and microfacies [27]. It should be noted that planar facies distribution often does not reach the scale of individual microfacies. In such cases, multiple distributary channels are often considered as a distributary channel microfacies [28]. In contrast, sedimentary architecture studies concentrate on the distribution within a single unit, with the minimum scale being the accretion body in it. For fluvial deposits, sedimentary microfacies maps usually depict the distribution of channel sand bodies, while architecture analysis further subdivides composite channel sand bodies into different single-channel sand bodies. Additionally, it breaks down individual microfacies (e.g., point bars) within a single-channel sand body and further divides lateral deposits within a single point bar [19]. Regarding facies boundaries, sedimentary facies analysis typically employs statistical boundaries in planar sedimentary facies maps, where one dominant facies encompasses the interior while others are included. On the other hand, architecture analysis necessitates determining the isochronous physical interfaces of a single facies unit in space, such as the top and bottom of channel sand bodies [28].  
[PERSON] [19] introduced a subdivision order that aligns with the scale hierarchy in the study of fluvial facies architecture, where the first order represents the lowest level. In this study, we have adopted the latter scheme to describe the characterization and modeling methodology of architecture elements, specifically focusing on the third to fifth interfaces, which define the actual reservoir architecture units. In fluvial deposits, architecture elements bounded by the fifth-order interface generally correspond to the scale of sedimentary microfacies combinations, such as river channels. Architecture elements defined by the fourth-order interface typically represent a single microfacies, such as an individual point bar or crevasse splay. Lastly, architecture elements, defined by the third-order interface, are roughly equivalent to constituent units within a single microfacies, such as the lateral accretion body within a point bar.  
The primary modeling approach employed in this study revolves around assigning each unit reservoir architecture element, hereafter referred to as a 'unit element', which is primarily bounded by the fourth or fifth interface. Each unit element is equipped with a default external geometry and associated size parameters that can be modified by the modeler, instead of relying on stereotyped solid geometry created by pixels. Stochastic methods are utilized to generate internal architectural elements bounded by lower hierarchy interfaces (third or fourth interface). The default geometry of distinct unit elements deposited in various sedimentary environments is determined based on the understanding of the formation process and evolutionary laws of sediments.  
In this study, three widely recognized unit reservoir bodies predominant in three sedimentary facies environments are comprehensively summarized and characterized. These findings serve as the foundational components of the 3D subsurface reservoir modeling methodology.