## 7 Conclusions  
The Yan'an Formation is primarily characterized by three marker layers: the Baotashan sandstone, Peizhuang sandstone, and Zhenwudong sandstone. The Baotashan sandstone, located at the base of the Yan 10 interval, consists of gray, thick, blocky sandstones with prominent planar and trough cross-stratifications. The Peizhuang sandstone, present in the Yan 6 oil group, is a greenish-yellow, blocky, fine-grained sandstone with iron nodules. The Zhenwudong sandstone, present in the Yan 3 interval, is a reddish-brown to gray-white, fine- to medium-grained sandstone with noticeable scour marks at the base.  
The sedimentary grain size within the Yan'an Formation exhibits a distinct vertical pattern: coarse in the fifth section at the bottom, finer in the fourth section to second section intervals in the middle, and again coarse in the first section interval at the top. Horizontally, grain size fining occurs from the northern, western, and southern parts of the basin toward the central and eastern regions.  
The sedimentary environment of the Yan'an Formation in the Ordos Basin is primarily controlled by geological tectonic movements. Prior to the Yan'an Formation, several highlands and five ancient rivers had formed within the basin. In the Yan 10 period, these ancient rivers eroded the highlands, filling the basin. In the Yan 9 period, tectonic subsidence in the central and eastern regions led to the formation of an inland lake, transitioning the sedimentary environment from braided rivers to lake deltas. From the Yan 8 to Yan 6 periods, tectonic activity was relatively stable, permitting the lake delta environment to develop and peak. The Yan 4+5 period marked the beginning of the lake's regression. During the Yan 3 to Yan 1 periods, tectonic stability persisted, resulting in the disappearance of the inland lake and a shift from lake delta environments to meandering rivers.  
The Yan'an Formation reservoir sandbody stacking types include continuous superposition, intermittent superposition, interbedded sand-mud, and single sandbody types, with the continuous and interval superposition type being the most developed with superior reservoir performance. The oil quality in the Yan'an Formation is characterized by low density, low viscosity, low pour point, and high maturity, representing high-quality crude oil.  
**Author Contributions:** Conceptualization, T.H.; methodology, T.H. and Y.L.; software, T.H.; validation, Y.Z. ([PERSON]) and Z.Z.; formal analysis, G.C.; investigation, T.H.; resources, Y.L.; data curation, T.H.; writing--original draft preparation, T.H.; writing--review and editing, Y.L.; visualization, Z.Z.; supervision, Y.Z. ([PERSON]); funding acquisition, Y.Z. ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
**Funding:** The research was supported by the Open Fund of the Key Laboratory of Geological Safety of Coastal Urban Underground Space, Ministry of Natural Resources Key Laboratory of Ministry of Natural Resources (No. BHKF2021Z07) and the National Science and Technology Major Project of China (No. 2016 ZX05050006).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.  
**Conflicts of Interest:** Author [PERSON] was employed by PetroChina Qinghai Oilfield Company. Author [PERSON] was employed by Shengli Oilfield Company, Sinopec. The remaining authors declare that the re-search was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.