# Eocene Sedimentary-Diagnetic Environment Analysis of the Pingtai Area of the Qaidam Basin
## 7 Conclusions  
The clay minerals of the Lulehe Formation are mainly composed of high smectite, chlorite and illite content without illite-smectite mixed layers or kaolinite; the Xiaganchaigou Formation is dominated by high illite-smectite mixed layer, chlorite and illite content without smectite or kaolinite. Chlorite and illite contents gradually decreased, smectite disappeared and illite-smectite mixed layers appeared in the Xiaganchaigou Formation. These findings show that the sedimentary basin was in a cold and dry climate environment during the early Eocene, and that the climate gradually changed to dry and wet alternately in the late Eocene.  
The diagenetic medium was mainly alkaline, poor in K+, rich in Mg\({}^{2+}\), Na\({}^{+}\) and Ca\({}^{2+}\) and leaching and chemical weathering were weak. Both the Lulehe Formation and the Xiaganchaigou Formation show obvious characteristics of high CaO and Na\({}_{2}\)O contents. Xerophilic oxides, such as SiO\({}_{2}\) and Na\({}_{2}\)O, are more enriched in the Lulehe Formation, whereas hygrophilic oxides, such as K\({}_{2}\)O, MgO, TiO\({}_{2}\) and Fe\({}_{2}\)O\({}_{3}\), are more enriched in the Xiaganchaigou Formation. In addition, CaO content fluctuates more. This indicates that the overall climate was dominated by aridity during the deposition period and gradually became relatively humid in the later period. The V/(V + Ni) values of the Lulehe Formation and the Xiaganchaigou Formation have little variation, ranging from 0.69 to 0.80 with an average value of 0.75. In the Lulehe Formation, the average Cu/Zn ratio is 0.43, the average &U is 2.24, the average Th/U ratio is 3.91 and the average Ce/La ratio is 1.86. In contrast, in the Xiaganchaigou Formation the average Cu/Zn ratio, &U, Th/U ratio and Ce/La ratio values are 0.35, 3.12, 3.36 and 1.97, respectively. These results indicate that the Eocene sedimentary period was dominated by an anoxic reducing environment and the climate was cold and dry. Compared to the Lulehe Formation, the Xiaganchaigou Formation had a wetter climate, deeper water body and stronger reduction conditions in the sedimentary period. In terrigenous sedimentary environments, stronger reducibility and deeper water bodies are not conducive to the development of high-quality reservoir rocks. This shows that the reservoir of the Lulehe formation is better than that of the lower Ganchaigou Formation, which provides a basis for reservoir prediction and future oil and gas exploration in the Pingtai area.  
In the early Eocene, global tectonic activities intensified and the Tibetan Plateau and Kunlun, Atun, and Qilian Mountains continued to rise which blocked water vapor from the Indian Ocean and the Pacific Ocean, making the Qaidam Basin a closed inland basin with a cold and dry climate. During the middle and late Eocene, the global climate became wetter, and the climate of the Qinghai-Tibet Plateau changed accordingly. Weak leaching and weathering also reflect the characteristics of cold, arid and near-source deposition in the enclosed inland basin.  
Conceptualization, G.S., S.Z. and Y.W.; Methodology, G.S., S.Z. and Y.W.; Formal Analysis, G.S., Y.W. and Y.L.; Investigation, G.S., H.G. and S.B.; Writing--Original Draft Preparation, G.S., S.Z., Y.L. and Y.W.; Writing--Review and Editing, G.S., S.Z., Y.W., Y.L. and Y.W. All authors have read and agreed to the published version of the manuscript.  
This work was financially supported by the National Natural Science Foundation of China (Grant No. 41872145), and Key Laboratory Project of Gansu Province (Grant No. 1309 RTSA041).  
Not applicable.  
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.  
All analysis data used to support the findings of this study are available from the corresponding author upon request.  
We would like to thank J.-L.Q. for their technical assistance with chemical analyses.  
The authors declare no conflict of interest.