**4. Results**

#### *4.1. Occurrence and Characteristics of Amorphous SiO2*

The shale strata of the Lucaogou Formation in the Jimusar Depression can be divided into tu ffaceous shale lithofacies, transitional lithofacies, and carbonate lithofacies [41,42]. The tu ffaceous shale lithofacies is mainly composed of feldspathic minerals including quartz and feldspar. The carbonate lithofacies mainly consists of dolomite and includes dolomite and argillaceous dolomite. The mineral composition and lithology of the transitional lithofacies is primarily a hybrid of the other two lithofacies. It can be seen by SEM that in addition to the development of authigenic quartz in the shale strata (Figure 3A,B), amorphous SiO2 is also present (Figure 3C–H). Amorphous SiO2 shows no fixed form and usually fills randomly between mineral grains (Figure 3C–E). Some of the amorphous SiO2 was wrapped in tu ffaceous components (Figure 3F), and other forms were spherical or ellipsoid shapes having varying sizes (Figure 3G,H).

**Figure 3.** *Cont.*

**Figure 3.** SEM images of quartz and amorphous silica in the Lucaogou Formation, Jimsar Depression. (**A**) Transitional lithofacies, S1 well, 3147.64 m; (**B**) tuffaceous shale lithofacies, S2 well, 3348.08 m; (**C**) transitional lithofacies, S1 well, 3147.64 m; (**D**) tuffaceous shale lithofacies, S2 well, 3343.00 m; (**E**) tuffaceous shale lithofacies, S2 well, 3348.08 m; (**F**) transitional lithofacies, S2 well, 3359.95 m; (**G**) tuffaceous shale lithofacies, S3 well, 2815.21 m; (**H**) tuffaceous shale lithofacies, S4 well, 2601.81 m; (**I**) energy spectrum analysis of point "+" in image H.

#### *4.2. Composition Characteristics of Crystalline Minerals*

Analysis of the XRD test results (Table 1) shows that the tuffaceous shale lithofacies samples exhibit the highest content of quartz-feldspathic minerals. The average content of quartz is as much as 40.26%; the average content of plagioclase and k-feldspar are as much as 16.68% and 5.26% respectively (Figure 2A). The carbonate lithofacies samples show the highest content of dolomite, reaching 63% on average. The transitional lithofacies samples present the highest content of clay minerals, which is as much as 26.14% (Figure 2A). In clay minerals, the content of the illite/smectite mixed layer is the highest, followed by illite. The average contents of the illite/smectite mixed layer in tuffaceous shale lithofacies, transitional lithofacies, and carbonate lithofacies are 41.37%, 59.86%, and 72.78%, respectively (Figure 2B). The tuffaceous lithofacies show the highest content of illite (average 37.89%), followed by transitional lithofacies (average 24.29%). The content of kaolinite, chlorite, and chlorite/smectite mixed layer is relatively low (Figure 2B).

#### *4.3. Content of Amorphous SiO2*

Analysis of the XRF test results (Table 2) shows that the tuffaceous shale lithofacies samples have the highest content of Si, reaching 34.21% on average. As expected, the carbonate lithofacies samples exhibit the lowest content of Si, only 11.51% on average (Figure 4A). Moreover, the tuffaceous shale lithofacies samples also exhibit the highest values of Si in crystalline minerals calculated by the above method, reaching 33.18% on average (Figure 4A). According to the calculations, the shale strata of the Lucaogou Formation thereby contains a small amount of amorphous SiO2. The tuffaceous shale lithofacies samples show the highest content of amorphous SiO2, reaching an average of 7.07%, and the carbonate lithofacies samples show the lowest, only 1.52% (Figure 4A). Amorphous SiO2 has a certain negative correlation with crystalline quartz (Figure 4B). During burial diagenesis, amorphous silica will gradually convert to crystalline quartz. The silica in the Lucaogou Formation is mainly derived from tuffaceous materials alteration in previous studies [17,21]. Therefore, the content of amorphous SiO2 in the tuffaceous shale lithofacies sample is the highest among the three lithofacies. The content of silica in a sample is generally definite. Hence, the higher the content of crystalline quartz, the lower the content of amorphous SiO2.


**Table 1.** Test data table of mineral composition of different lithofacies samples in the Luchaogou Formation.




**Table 1.** *Cont.*


**Table 1.** *Cont.*


