## 4 Results
### Characteristics of Major Element
#### 4.1.3 Well D1  
The SiO\({}_{2}\) content is 71.78\(-\)80.23%, with an average of 75.70%; the Al\({}_{2}\)O\({}_{3}\) content is 9.06-12.35%, with an average of 10.96%; the Fe\({}_{2}\)O\({}_{3}\) content is 1.00-2.47%, with an average of 2.01%; the K\({}_{2}\)O content is 2.28-3.18%, with an average of 2.77%; the Na\({}_{2}\)O content is 0.86-1.84%, with an average of 1.35%; the content of CaO is 0.26-0.91%, with an average of 0.41%; the content of MgO is 0.21-0.78%, with an average of 0.53%.  
The samples from the three wells have high SiO\({}_{2}\) content, indicating that the main material comes from quartz debris or SiO\({}_{2}\) rich minerals, and the mineral composition maturity is high. The composition maturity of Well D1 is higher than that of Well XC1 and Well D2. Al\({}_{2}\)O\({}_{3}\) mainly comes from clay minerals and feldspar. The high content indicates that the study area has a sufficient supply of terrigenous clastic materials. The K\({}_{2}\)O/Na\({}_{2}\)O ratios of Well XC1, Well D2 and Well D1 are 2.25, 3.71 and 2.22, respectively, indicating that the content of potassium feldspar or potassium\(-\)rich minerals in the sedimentary rocks is high. The samples from the three wells show a small amount of MnO and P\({}_{2}\)O\({}_{5}\) content, which is inferred to be due to the performance of heavy minerals. Compared with the Upper Continental Crust (UCC), the three wells have lower Na\({}_{2}\)O, MgO, CaO and Fe\({}_{2}\)O\({}_{3}\), as a whole. In addition, Well XC1 and Well D2 also show higher Al\({}_{2}\)O\({}_{3}\) and K\({}_{2}\)O contents, while Well D1 shows a lower Al\({}_{2}\)O\({}_{3}\) content and a higher SiO\({}_{2}\) content.  
[PERSON] et al. (1986) proposed that there is a certain correlation between geochemical elements of sandstone, and analyzed the correlation by using the Harker diagram [12]. This study found that the Harker variation diagrams of three wells showed that SiO\({}_{2}\) has an obvious negative correlation with TiO\({}_{2}\), Fe\({}_{2}\)O\({}_{3}\), Al\({}_{2}\)O\({}_{3}\), MgO and K\({}_{2}\)O (Figure 2), indicating that silicate mineral particles, such as quartz, affect the chemical properties of the whole rock. On the ratio plot of (Al\({}_{2}\)O\({}_{3}\)/SiO\({}_{2}\)) and (Fe\({}_{2}\)O\({}_{3}\) + MgO), there is an obvious positive correlation (Figure 3), indicating that the increase in ferromanganese content has a certain relationship with the supply of terrigenous clastic materials. The ratio plot of (K\({}_{2}\)O/Na\({}_{2}\)O) and (Fe\({}_{2}\)O\({}_{3}\) + MgO) (Figure 3) shows a weak positive correlation, indicating that the content of potassium feldspar and plagioclase in terrigenous clastic materials has little to do with the increase in ferromagnetic content.  
Generally, the change trend of major elements is used to reflect the overall characteristics of mineral composition in rock samples, which can further speculate the differences between sediment sources [13]. The average value of Upper Continental Crust (UCC), the average value of North American Shale Composite (NASC) and the average value of Post Archean Average Shale in Australia (PAAS) are generally used for the standardization of elements. Since the average value of the NASC and PAAS could be affected by the difference in provenance supply and provenance age, the samples in this study mainly use the UCC average value [13], with better universal applicability for standardized treatment and analysis. In the UCC\(-\)normalized major element spidergrams, the overall similarity of the trend of sample major element/UCC ratio between Well D2 and Well XC1 is good, and there is a notable difference in the composition of the major element of Well D1, indicating that the provenance of Well D1 is inconsistent with Well D2 and Well XC1 (Figure 4).  
Figure 2: Harker variation diagrams for Wudun sag samples.