## 4 Stratigraphic Characteristics
### Stratigraphy  
The first section contains the Yan 9 and Yan 10 members, which are the most resource-rich intervals for hydrocarbons and coal (Figure 3). The lithology primarily consists of gray-white and gray-yellow medium- to coarse-grained massive sandstones and pebbly sandstones. The upper layers contain carbonaceous mudstone, while the base features conglomerates (Figure 4a), with total thicknesses ranging from 0 to 120 m. In certain areas, coal seams occur at the top of Yan 10, which are characterized by distinct high resistivity and high sonic travel time on well logs, serving as an identification marker for the Yan 10 reservoir. The second section of the Yan'an Formation contains two reservoir groups: Yan 8 and Yan 7 (Figure 3). The lithology of Yan 8 primarily consists of yellow-green and grayish-white siltstone interbedded with thin layers of shale. A coal seam is exposed at the top, and the base is composed of gray-white, layered fine-to-silty sandstone, clearly demarcated from the dark mudstone of Yan 9. The Yan 7 reservoir shows a classic fining-upward sequence. The lower part consists of thick, blocky, fine-grained gray-green sandstone, transitioning from the mudstone and siltstone of the underlying Yan 8, while the upper part is dominated by gray-black and gray-yellow silty mudstone and shale. The third section comprises only the Yan 6 reservoir, characterized by gray-black and gray-green sandstone and reddish-brown mudstone (Figure 3). The middle part of Yan 6 develops the distinctive gray-green blocky Peizhuang sandstone, marked by slight cross-bedding, serving as a key marker bed for Yan 6. The lower part of Yan 6 consists of thick, blocky sandstone, clearly separated from the overlying mudstone of Yan 7. Figure 2: Field outcrop photos of marker beds from the Yan’an Formation. (**a**) Gray, massive, normally graded sandstone (Baotashan sandstone). (**b**) Large planar cross-bedding (Baotashan sandstone). (**c**) Interface between Peizhuang sandstone and overlying gray-black mudstone. (**d**) Iron nodules (Peizhuang sandstone). (**e**) Reddish-brown sandstone with parallel bedding (Zhenwudong sandstone). (**f**) Gray-white sandstone with parallel bedding (Zhenwudong sandstone). The fourth section of the Yan'an Formation comprises the Yan 5 and Yan 4 members, which are difficult to differentiate individually and are typically referred to collectively as Yan 4 + 5 (Figure 3). The lithology of the Yan 4 + 5 reservoir primarily consists of grayish-yellow to yellowish-green muddy siltstone, silty mudstone, and pure mudstone (Figure 4b), with well-developed large planar and trough cross-stratifications. The boundary is marked by the thick, blocky fine-grained sandstone at the base and the overlying gray-black mudstone of the Yan 6. The fifth section includes the Yan 1 through Yan 3 members (Figure 3). Due to erosion, these members are preserved only in the northern part of the basin and exhibit significant variation in thickness. The Yan 3 reservoir is characterized by the presence of the Zhenwudong sandstone, which is composed of gray-black and yellow-green blocky fine  
Figure 3: Sequence stratigraphy division of the Yan’an Formation in the Ordos Basin (modified from [33]). sandstone with large cross-stratifications. Distinct scour structures are observed at the base of the blocky sandstone where it contacts the underlying deep gray mudstone at the top of the Yan 4 + 5. Yan 2 typically consists of gray-brown siltstone interbedded with deep gray mudstone, with a finer grain size relative to the underlying Yan 3. Yan 1 is composed of deep gray-brown fine-grained sandstone interbedded with thin layers of deep gray-green mudstone, with iron nodules developed in the sandstone. During the early stages of the Yan'an Formation, the first section is characterized by a braided river depositional environment with ample river hydrodynamics and coarser sediment grain sizes.