## 6 Conclusions  
1. The fracture development in the Chang 7 member of the Yanchang Formation indicates that the majority of fractures are structural, predominantly occurring as shear fractures. Observations of rock cores indicate that the majority of fractures are high-angle or vertical, with fewer instances of horizontal or bedding fractures. The length of these fractures ranges between 20 and 30 cm, and their aperture is generally concentrated  
Figure 16: Mechanical stratigraphy classification by rock mechanics parameters (the red, orange, and purple colors in column 8 indicate the first, second, and third categories, respectively, while white indicates the fourth category).  
between 0.05 and 0.5 mm. Most fractures are either filled or partially filled, which is advantageous for subsequent oil and gas accumulation. Microscopic examination of rock thin sections and scanning electron microscopy shows the presence of intragranular, intergranular, and transgranular fractures, which facilitate later dissolution and transformation processes for oil and gas. Imaging logging reveals a predominance of dark colors over bright colors, suggesting that most fractures in the study area are unfilled.
2. A comprehensive analysis of fracture response characteristics resulted in the selection of acoustic logging, natural potential logging, and caliper logging, which were assigned weights of 0.5, 0.3, and 0.2, respectively. The comprehensive probabilistic index \(P_{f}\) effectively identified the fracture development sections at 2135-2140 m and 2173-2181 m, demonstrating high recognition accuracy.
3. The thickness of rock layers exhibits a negative correlation with fracture linear density, following an exponential relationship. When the thickness of the exposed layer exceeds 5.2 m, the amplitude of variation in fracture line density becomes minimal. Additionally, the brittleness index influences fracture development; both the length and linear density of fractures are positively correlated with the brittleness index, while the fracture aperture shows a negative correlation. The inclination angle of fractures displays a weak positive correlation with the brittleness index.
4. Fractures are most extensively developed in fine sandstone and sandstone, whereas they are absent in mudstone. Additionally, the development of fractures is influenced by the rock mechanics layer; fractures are most pronounced in the first, second, and third types of rock mechanics layers, while the fourth type of rock mechanics layer exhibits no developed fractures.  
Conceptualization, P.C., S.Y. and X.C.; methodology, P.C. and S.Y.; validation, S.Y. and X.C.; formal analysis, P.C. and X.C.; data curation, P.C., Z.L. and X.C.; writing--original draft preparation, P.C. and X.C.; writing--review and editing, P.C., C.S. and H.Q.; visualization, P.C. and Z.L.; supervision, C.S. and H.Q. All authors have read and agreed to the published version of the manuscript.  
This work was supported by National Major Science and Technology Projects of China (grant number: 2016 ZX05046-003).  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
We express our gratitude to PetroChina Company Limited for providing the necessary data.  
The authors declare no conflict of interest.