# Sedimentary Model of Sublacustrine Fans in the Shahejie Formation, Nanpu Sag
## 2 Geological Setting
### Sequence Stratigraphy  
After years of exploration and development, abundant data and materials have been accumulated in Nanpu Sag. A large number of studies have been carried out on the geological structure, sedimentation, reservoir and oil bearing in the study area, and certain understandings have been obtained [27; 28]. The exploration has confirmed several sets of oil-bearing layers, revealing that the depression has great potential.  
The Nanpu Sag is dominated by Cenozoic deposits, with the Paleogene Shahejie Formation (Es), Dongying Formation (Ed), Neogene Guantao Formation, Minghuazhen Formation and Quaternary Pingyuan Formation developing successively from bottom to top. The maximum sedimentary thickness is 8000 m [29]. The Es and Ed developed in the Nanpu Sag. The objective of this study is to focus on the sand body of Es\({}_{1}\), with a formation thickness of about 500 m. It is currently the main target stratigraphy of the region, the main body of Palaeogene fault lake basin sedimentation in the Nanpu Sag [30], and also an important oil-bearing gas layer. The thickness of dark mudstone is 200-600 m (Figure 3). Based on the tectonic evolution characteristics, uncomformity surfaces and sedimentary filling features of Nanpu Sag, combined with seismic reflection termination relationships, seismic facies patterns, well logging response characteristics and  
Figure 2: Sedimentary facies plan of Es\({}_{1}\) in Nanpu Sag.  
lithological variations, a sequence stratigraphic framework for the Shahejie Formation in Nanpu Sag has been established. From bottom to top, the Shahejie Formation is divided into four third-order sequences, namely SQ1, SQ2, SQ3 and SQ4.  
The bottom interface of Es\({}_{1}\) is in contact with the unconformity of Es\({}_{3}\) of the interface, the formation of Es\({}_{2}\) is missing, and the local salient position can be seen to cut the underlying layer. The logging is characterized by the sudden change of acoustic curve and resistivity, the lithology shows that the siltstone changes upward into mudstone or gray mudstone, and large sections of continuous mudstone can be seen above the lithology interface (Figure 3). The bottom of the Dongying Formation (Ed) is above the top interface (SB5) of Es\({}_{1}\), which is in unconformity with the lower strata. The Es\({}_{1}\)\({}^{\text{L}}\) sequence exhibits high-amplitude, medium-to-high frequency reflection characteristics on seismic profiles, while the Es\({}_{1}\)\({}^{\text{U}}\) sequence shows moderate-to-high amplitude, medium-frequency reflection features. Localized erosional truncation of underlying strata can be observed in certain stratigraphic sections (Figure 1c). Core calibration data analysis reveals significant lithological variations at formation boundaries, characterized by transitions from siltstone to mudstone or calcareous mudstone and the presence of continuous thick mudstone intervals. These lithological changes correspond to abrupt shifts in acoustic transit time and resistivity anomalies on well logs, serving as key markers for sequence boundaries.  
Figure 3: Sequence stratigraphic analysis diagram of the study area.