## 3 Results and Discussion
### Vertical Structural Characteristics of Sand Body in the Study Area  
The vertical structure of a sand body refers to the vertical change mode of the sand body's debris composition, sedimentary structure and logging response, that is, the sedimentary sequence or sedimentary rhythm. This is the sedimentary response to the variation of sedimentary environment factors such as provenance supply and sedimentary water capacity with time. Therefore, the vertical structural style of a sand body is obviously controlled by the sedimentary environment and sedimentary facies and is also related to the relative rise and fall of the sedimentary base level or lake level. Different sedimentary environments or facies often form sedimentary sequences with different characteristics. The vertical structural style of sand body is not only an effective way to explain the sedimentary environment but also helpful to judge the development position of high-quality reservoirs in a sand body. Based on the vertical structural style of sand bodies, this paper discusses the structural characteristics of the Chang 9 sand body.  
According to the core profile description and sedimentary microfacies analysis, combined with the shape of the logging curve and sedimentary microfacies interpretation, the longitudinal structural style of the sand body of the Chang 9 oil layer formation in the basin is roughly summarized into three basic structural types: box-shaped upward thinning, bell-shaped upward thinning and funnel-shaped upward thickening. The results are shown in Figure 11.  
Figure 11 shows that the A-type vertical structure in the sand body has a typical box logging curve and slightly upward thinning sedimentary sequence characteristics, the bottom has an obvious erosion mutation surface and the top has gradual mutation. The sand body is mainly composed of coarse-grained components, and the sedimentary structure is mainly composed of massive bedding, large-scale cross bedding and parallel bedding, which is obviously the sedimentary response to a high-energy environment and has a strong sedimentary hydrodynamic force. Among them, the continental and underwater main distributary channel sand bodies of the Chang 9 braided river delta and meandering river delta usually have this A-type vertical structure. Due to the coarse lithology and low  
Figure 10: Sedimentary facies distribution of Chang 9\({}_{1}\) in Yanchang Formation, Ordos Basin.  
shale content, it is the most favorable sand body type for reservoir development. The B-type vertical structure is characterized by the upward fine sedimentary sequence developed by the sand body. The bottom is an obvious mutation surface. The coarse-grained debris deposits accumulate on the scouring surface. The upward particles gradually become thinner, the mud content increases and the top gradually changes. The corresponding GR logging curve is usually bell-shaped. The typical feature of the C-type vertical structure is the upward coarsening sedimentary sequence developed by sand body, with a gradient at the bottom and a mutation at the top. The sediment particles coarsen gradually from bottom to top, and the mud content decreases continuously. The corresponding GR logging curve is usually funnel-shaped.