# Research Status of and Trends in 3D Geological Property Modeling Methods: A Review
## 2 Research on Reservoir Grid Generation Method  
Building a 3D reservoir grid model is a key intermediate process and importantly affects the subsequent reservoir property modeling and numerical simulation based on geostatistics. In modeling a reservoir by using geostatistical tools, the gridding process essentially determines how to characterize the macroscopic homogeneity of the reservoir. Since geostatistics are implemented on logical grids, a grid with excessive and excessively large distortion can easily cause the variogram to become distorted, thus affecting the accuracy and reliability of the modeling of properties, such as sedimentary facies and pore permeability [13; 14]. Additionally, the shape and resolution of the grid affect the accuracy and speed of the numerical simulation of the reservoir [14; 15]. Research on reservoir grid models has been carried out for many years, resulting in many grid types. [PERSON] [16] conducted comprehensive gridding and outlined the general procedure of grid generation. Reservoir grids are categorized mainly into structured grids and unstructured grids. Structured grids are typically generated using hexahedra and are organized in an orderly manner with i, j, and k, such as Cartesian grids and corner point grids, while unstructured grids include tetrahedral grids and perpendicular bisector (PEBI) grids. In stratified deposits such as oil sand, often, the Cartesian grid is transformed to a stratigraphic grid using a non-linear coordinate transformation, which is referred to as unfolding. [PERSON] [17] compared the characteristics of the s-grid, faulted s-grid, and pillar grid as well as these grids' performance differences in property distribution, grid coarsening, and numerical simulation. As show in Figure 1, For the s-grid, faulted s-grid, and pillar grid, the main difference is that they contain different fault treatment approaches. Three different examples are extracted in Figure 1a-c, and the conceptual models are drawn, as show in Figure 1d, to show the different fault treatment approaches. S-grid is also known as stair-step grid; the sides of this kind of grid cell remain vertical and are controlled by an orthogonal footprint [17], as show in Figure 1a. Faulted s-grid is known as Jewel Grid; the grid cells are split exactly at the local position of the fault plane, as show in Figure 1b [17]. Pillar grid is widely used in petroleum engineering, and the most common grid is the corner point grid. For the position of faults, the pillar that controls the grid cells is parallel with the fault lines. Thus, the fault should be simplified when the fault is too complicated, as shown in Figure 1c. [PERSON] et al. and [PERSON] et al. [18; 19] divided the grids into \"geological grids\" for geostatistical property modeling and \"fluid simulation grids\" for numerical simulation based on the different needs of reservoir modelers and digital modeling engineers for grid use. In the many grid studies, the corner point grid represented by the pillar grid has been the hotspot of research, with the most mature application. Pillar grid generation involves three main steps [20; 21]: (1) a fault model is formed by simulating the fault using pillars; (2) gridding is carried out by combining fault pillars into a 3D grid framework; and (3) vertical stratification is conducted by generating initial layer surfaces on the grid framework for geological stratification. The pillar grid is characterized by its grid orientation along fault lines, boundary lines, or pinch-out lines, indicating that the grid can be distorted, thus overcoming the inflexibility of the orthogonal Cartesian grids and allowing for easy simulations of faults, boundaries, and pinch-outs [22; 23]. With the development of technology and the continuous improvement in reservoir simulation accuracy, the defects of the pillar grid are gradually exposed as follows. First, due to the limitation of the geometric location of pillars, it is necessary to simplify the fault when using pillars to construct a complex fault network.