## 5 Discussions
### Weathering and Sedimentary Characteristics in Provenance Area  
The chemical composition of clastic sedimentary rock is mainly affected by the chemical composition of its provenance area, but the later geological factors such as weathering, transportation, sedimentation and diagenesis will also have a significant impact on its chemical composition [16]. The analysis of the above factors has a certain indicating significance for identifying the source of sediments in different well areas. The main changes of chemical weathering of rocks in the provenance area are the decomposition of feldspar minerals and the formation of clay minerals, which are manifested in the gradual loss of alkali metal elements (such as Ca, Na, K, etc.) and the relative enrichment of Al\({}_{2}\)O\({}_{3}\) in clastic sedimentary rocks. Therefore, [PERSON] and [PERSON] (1982) put forward the chemical alteration index (CIA), which is used to quantitatively judge the intensity of chemical weathering in the provenance area, with a certain indication for paleoclimate [17]. Previous CIA studies on sediments propose that the CIA value of clastic sedimentary rocks without chemical weathering is close to that of feldspar minerals (50), and that of clastic sedimentary rocks with complete chemical weathering is close to that of kaolinite (100). A greater CIA value may cause greater chemical weathering intensity in clastic sedimentary rocks. Moreover, a hot and humid climate will also cause a higher CIA value. The content of non\(-\)clay minerals decreases and the content of clay minerals increases during the sedimentary cycle of clastic sedimentary rocks. Clay minerals have higher Al\({}_{2}\)O\({}_{3}\) content than non\(-\)clay minerals. Therefore, [PERSON] et al. (1995) proposed the composition variation index (ICV) to reflect the composition maturity of rocks. Clastic sedimentary rocks with high ICV value indicate an active tectonic environment. Clastic sedimentary rocks with low ICV value are considered to come from mature sedimentary provenance areas containing a large amount of clay minerals, indicating deposition in a passive tectonic environment [18]. It can be seen from the ICV\(-\)CIA plot [15] (Figure 8) that the CIA values of all samples range from 68.26 to 83.23, indicating that all the samples are subject to a certain degree of chemical weathering. Among them, the CIA values of samples from Well D1 are significantly different from those of Wells XC1 and Well D2. The ICV value of Well D1 samples is between 0.60 and 0.83, and the CIA is less than 75, showing that the original rock has medium maturity and weak chemical weathering. The ICV values of samples from Well XC1 and Well D2 are between 0.46 and 0.78, and most of the CIA values are greater than 75, indicating that the maturity of the original rock is relatively high and the chemical weathering is strong. There are also some trace element discrimination methods that can be used to distinguish the weathering and sedimentary characteristics of the provenance area. The influence of different geological factors can be explained by using the characteristics of geochemical behavior of trace elements. The Th element is almost insoluble and stable, while the U  
Figure 8: ICV\(-\)CIA plot for the J\({}_{2}\)z\({}_{1}\) Formation of Wudun sag. element is easy to dissolve and lost in chemical weathering and sedimentary recycling. Therefore, the ratio of Th/U can be used to distinguish chemical weathering and sedimentary recycling. Previous studies on the Th/U ratio show that the ratio of Th/U greater than 3.8 usually indicates that the sediments have experienced weathering or sedimentation for shale [14]. On the other hand, the sediments that have been affected by weathering with the ratio of Th/U greater than 3.0. Both theories support that the ratio of Th/U increases with the increase in kaolinite content during weathering [19].