# 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  
The first mineral to emerge from the solution was montmorillonite [70]. In the Fengcheng Formation, this part of montmorillonite was gradually transformed to form the Aemon mixed layer. With the further reaction, the salinity and alkalinity of the interlayer solution rose to the range of feldspar precipitation, and the feldspar precipitated was associated with clay minerals, followed by zeolite and quartz precipitation. The early zeolite was unstable, and with a further increase in temperature, it further generated zeolite and feldspar and released calcium ions, and the surplus calcium ions promoted the formation of calcite [61; 71]. Under the conditions of temperature and pressure, and the abundant magnesium and iron ions in the solution, part of the montmorillonite was transformed into chlorite. Along with the hydrolytic alteration of volcanic glass materials, magnesite minerals, such as pyroxene, hornblende, biotite, and feldspar phenocrysts, crystal chips, and potassium feldspar in pyroclastic materials also have some chlorite (Figure 8). At the same time, the antibgenic minerals precipitated under such environmental conditions are as follows: (1) bicarbonate minerals (carbonaceous calcium stone, trona, carbonaceous sodium magnesite, etc. ); (2) salt minerals (sodium silicate, salt, etc. ); (3) aluminum silicate minerals (potassium feldspar, alibite, zeolite, etc. ); (4) calcium and magnesium carbonate minerals (dolomite, calcite, etc. ); (5) siliceous minerals (quartz, opal, etc.) [72]. The alteration process of the Fengcheng Formation can be classified as follows: (1) hydrolysis of volcanic glass, generation of montmorillonite, generation of zeolite minerals and related transformation, albitization; (2) chlorite influenced by magnesium and iron ions; (3) feldspar alteration, formation of chlorite and calcite, rest albite. In particular, hydrolysis occurred in the early diagenetic A stage. Dolomite formation can be divided into three stages, most of which were in early diagenetic stage B. Salt minerals were formed in the same generation. There are three phases of dissolution, which are scattered from the late early diagenetic stage to the middle diagenetic stage B (Figure 8). Alkaline Sedimentary Evolution and Diagenetic Model of Fengcheng Formation Shale Series in Maltu Sag  
Based on core slices from wells FN1, FN14, AK-1, and MH7 in the Mahu sag, combined with the literature data, the sedimentary evolution and the diagenetic model of the Fengcheng Formation in the alkaline lake environment were established from the aspectsof climate, provenance, diagenetic environment, and underground fluids (Figure 9), which can be roughly divided into four stages: before the deposition of the Fengcheng Formation, during the deposition of the Fengcheng Formation, the low-temperature alkaline burial diagenetic stage, and the hydrothermal-related medium-high-temperature alkaline diagenetic stage. Figure 8: Sequence diagram of the diagenetic evolution of the Fengcheng Formation in Mahu sag. Figure 9: Alkaline sedimentary evolution–diagenesis model of the Fengcheng Formation (the faults shown are schematic faults [24; 39]. (**a**) before the deposition of the Fengcheng Formation (**b**) during the sedimentation of the Fengcheng Formation. (**c**) low-temperature alkaline burial diagenesis. (**d**) medium-high-temperature alkaline diagenesis. Before the deposition of the Fengcheng Formation, in the arid/semi-arid climate, the lake water was evaporated and concentrated [41], resulting in a gradual increase in the alkalinity of the water body [57; 62; 63]. In addition, due to active volcanic activity, a large number of volcanic rocks developed at the bottom of the lake basin, and the low-temperature unstable minerals contained in the volcanic rocks were hydrolyzed, which provided Na\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\), and Fe\({}^{2+}\) for the water body, which laid the material foundation for diagenesis and alkaline lake formation (Figure 9a).