# Diagnosis of the Permian Fengcheng Formation in the Mahu Sag, Junggar Basin, China
## 5 Discussion
### Diagenetic Evolutionary Sequence of Fengcheng Formation Shale Series in Maltu Sag
#### 5.2.2 Diagenetic Mineral Precipitation Sequence
During the deposition of the Fengcheng Formation, there were Cl\({}^{-}\), CO\({}_{3}\)\({}^{2-}\), SO\({}_{4}\)\({}^{2-}\), and other anions in the lake basin. During the process of lake basin shrinkage and alkalization, the concentration of ions gradually increased, collided, and combined with each other, and a large number of Mg-Ca carbonate minerals and alkaline minerals with anions of CO\({}_{3}\)\({}^{2-}\) appeared. In the presence of Mg\({}^{2+}\), Na\({}^{+}\), and Ca\({}^{2+}\) plasmas, the order of binding with HCO\({}_{3}\)\({}^{-}\) and CO\({}_{3}\)\({}^{2-}\) is Ca\({}^{2+}\), Mg\({}^{2+}\), and Na\({}^{+}\)[73], with CaCO\({}_{3}\) precipitation before MgCO\({}_{3}\). After calcite precipitation, the lake water was stratified under the action of evaporation and concentration, which promoted the development of argillaceous sediments rich in organic matter [28]. The different types of organic matter were mainly from that organic matter [74; 75], and their decay products were conducive to the large-scale development of carbonate minerals, forming lamellar carbonate mineral layers (mainly calcite) and argillaceous sediment layers. At this time, due to the large consumption of Ca\({}^{2+}\), the Mg\({}^{2+}\)/Ca\({}^{2+}\) ratio in the water body increased, and the Fe\({}^{2+}\) was contributed by the volcanic tuff material. The microcrystal dolomite was also deposited [76]. The extensive development of carbonate minerals consumed a large amount of Ca\({}^{2+}\) and Mg\({}^{2+}\), while the content of Na\({}^{+}\) increased relatively, and the consumed CO\({}_{3}\)\({}^{2-}\) was continuously supplemented by volcanic activity. With the further shrinkage of the lake basin water body, alkalinity reached the maximum amount, Na\({}^{+}\) and CO\({}_{3}\)\({}^{2-}\) tended to be saturated, and Na\({}^{+}\) containing alkaline minerals such as trona and sodium-calcium carbonate was formed successively. Finally, the climate became hot and humid, and the water level rose. Under the supply of terrigenous detrital materials, materials such as mud and sand were deposited, and the sedimentary strata of the Fengcheng Formation were deposited (Figure 9b). In the low-temperature alkaline diagenetic stage, the residual marine brine in the Carboniferous and Jiamuhe Formations underlying the Fengcheng Formation from the mid-Perman to the mid-Jurassic period [67], entering into the pores and fractures of the Fengcheng Formation along the fault under tectonic compression, it brought abundant elements such as Fe, Mn, and Mg [73] and cemented around the early calcite or calcite with a low degree of dolomitization, and the local distribution of fine-grained iron dolomite with clear ring was formed. With the surging of residual marine brine, Na\({}^{+}\) was also present. In addition, the two necessary conditions of alkaline formation water and the rising temperature due to the formation's burial were also present. The dissolution of quartz occurred, and the \"salt effect\" of Mg\({}^{2+}\), K\({}^{+}\), and Ca\({}^{2+}\) in formation water further promoted the dissolution of quartz [52]. In this stage, in addition to the formation of iron dolomite and quartz dissolution, the cementation of laumontite also occurred under the background of alkaline diagenesis, low temperature, and the supply of Mg\({}^{2+}\) and Fe\({}^{2+}\) (Figure 9c). The hydrocarbon source rocks of the Fengcheng Formation did not enter the oil generation threshold until the end of the Permian period [77], and kerogen began to transform into oil and gas. This process was accompanied by the formation of oxalic acid and acetic acid [13], where local pore fluid pH dropped and became acidic, resulting in the dissolution of early turbidite, iron dolomite, and alkaline minerals. The acidic fluid generated by oil and gas was neutralized by alkaline formation water and alkaline minerals, and the whole presented an alkaline diagenetic environment.