**1. Introduction**

The success of shale gas exploration and development in North America has promoted the development of the shale gas industry around the world. At present, successful exploration and development of shale gas in China is mainly concentrated in the Sichuan Basin and surrounding areas, such as the Weiyuan, Zhaotong, Zhengan, and Jiaoshiba areas [1–3]. The shale sections containing commercial scale gas in these areas are located at the top of the Wufeng and the bottom of the Longmaxi Formations, corresponding to the 2-3 graphitic biozones of the Wufeng Formation and the 1-4 graphitic biozones of the Longmaxi Formation [4,5]. These high-quality shale sections contain high content of silica: as much as 60% [6–11]. Although there are different opinions about the evidence of biogenesis, most researchers consider that the silica in these high-quality shale sections has biogenic sources [12–17]. Shale oil sources are mainly concentrated in basins in China, where lacustrine shale is widely developed, such as the Ordos, Songliao, and Bohai Bay Basins. Shale oil exploration has been particularly successful in the second member of the Kongdian Formation in

Cangdong Depression of Bohai Bay Basin, where commercial-scale oil has been obtained in several wells [18]. This quartz-feldspathic shale exhibits good quality, high TOC content, and high hydrocarbon potential [18]. The tuffaceous shale sections of the Lucaogou Formation shale strata in the oil reservoir of the Malang Depression contain high total organic carbon and exhibit high hydrocarbon generation potential. These tuffaceous shales are also mainly composed of quartz and feldspar [19]. In both marine shale gas and or lacustrine shale oil reservoirs, silica is an important component, having a significant impact on shale reservoir properties, organic matter enrichment, shale oil and gas accumulation, and fracturing potential [14,15,20–25]. Hence, silica is a hot spot in shale reservoir research at present.

Studies on silica diagenesis have shown that the end members are amorphous SiO2 and crystalline quartz. The quartz can be further divided into authigenic quartz formed during diagenesis and detrital quartz from deposition. During diagenesis, amorphous SiO2 will gradually change from the amorphous state (opal-A) to the cryptocrystalline state and finally to the fully crystalline state (α-quartz), which is authigenic quartz. Amorphous SiO2 can be from biological organisms or an abioticearly diagenesis stage. Some studies sugges<sup>t</sup> that amorphous SiO2 has already transformed into crystalline quartz during early diagenesis (Ro is 0.35%~0.5%) [23,24]. Others sugges<sup>t</sup> that the conversion of amorphous SiO2 to crystalline quartz in shale reservoirs may be much later, because amorphous SiO2 has been seen in the middle diagenetic stage A (Ro is 0.5%~1.3%) [21]. During clay mineral conversion, a large amount of silica is generated, and its content is closely related to mineral composition, crystallinity, and thermal conditions; it also affects the physical properties and brittleness of the reservoir [26–28]. The influence of amorphous SiO2 on reservoir properties can make a large difference in different evolution stages. From the beginning of diagenesis to the cryptocrystalline state, formation porosity has been shown to be reduced from about 45% to less than 25%, and the permeability declines to be difficult to be measured [29]. In the authigenic quartz stage, reservoir physical properties and brittleness increases instead, which improves reservoir fracturability [29]. Thus, it can be seen that amorphous SiO2 also plays a grea<sup>t</sup> impact on reservoir properties. If the influence of amorphous SiO2 on reservoirs can be clarified, it will be of grea<sup>t</sup> significance for evaluating shale oil reservoirs and fracturing potential, especially for immature lacustrine shale oil reservoirs.

Accurate calculation of amorphous SiO2 content is the key problem to understand the influence of amorphous SiO2 on reservoir properties. There are currently four methods for the quantitative analysis of amorphous SiO2 in heterogeneous systems. The first is chemical dissolution, which means removing minerals other than amorphous SiO2. However, chemical dissolution incudes crystalline, which affects the accuracy of quantitative analysis. The second is quantitative analysis using XRD as proposed by Lin (1997) [30]. Although the method is correct in theory, human error enters into in the quantification [31]. Thirdly, Chu (1998) proposed a new quantitative XRD method based on the increment method proposed by Popovi´c et al. (1983) [32,33], but this method required preparation of a standard sample having a known mineral composition and proportions; the error was relatively large in the actual experiment. Fourth, Huang et al. (2015) established a calculation method for amorphous SiO2 in the Yanchang Formation shale of the Ordos Basin by using XRD combined with QEMSCAN analysis [34]. However, this method has two disadvantages. Firstly, it is too expensive to conduct large-scale tests. Secondly, the mineral composition obtained by QEMSCAN analysis can be understood as a volume percentage. Hence, it needs to be converted into a mass percentage, but the density of minerals was not determined in Huang et al. (2015) [34].

In view of the shortcomings of previous methods for calculating the content of amorphous silica [30–34], a new quantitative analysis method for amorphous silica content was established in this research, based on XRD and XRF analysis of core samples from the Lucaogou Formation in the Jimsar Depression. Through the analysis of the relationship between physical parameters, rock mechanical parameters, oil saturation, and amorphous silica content in shale strata, the effect of amorphous SiO2 on reservoir properties and its geological significance was determined.

## **2. Geological Settings**

The Junggar Basin is located in the northwestern part of China with an area of about 1.30 × 10<sup>5</sup> km<sup>2</sup> (Figure 1A); it is geotectonically located at the intersection of Kazakhstan, Siberian, and Tarim plates. The Jimusar Depression is in the southeast of Junggar Basin, covering an area of 1.278 × 10<sup>3</sup> km2; it is surrounded by the Shaqi Uplift to the north, the Guxi Uplift to the east, the Fukang faults zone to the south, and the Santai Uplift to the west (Figure 1B). The periphery of the Jimsar Depression is bounded by six faults (Figure 1B). The Permian Lusaogou Formation has a thickness of 200~350 m and is in conformable contact with the lower Jingjingzigou Formation and in unconformable contact with the upper Wutonggou Formation (Figure 1C). The Lucaogou Formation is mainly composed of deep and semideep lake facies formed of fine-grained, mixed sedimentary rocks [35,36]. It was formed in an intracontinental rifted saline lake basin environment, accompanied by volcanic eruptions and hydrothermal activity [37,38]. Since September 2011, J25, J23, J28, J30, and other exploration and evaluation wells have been successively drilled in the Jimusar Depression, oil testing shows industrial potential, and shale oil was discovered in the Lucaogou Formation. After nine years of development, the calculated reserves of shale reservoir have reached 11.12 × 10<sup>8</sup> t [39].

**Figure 1.** Diagrams showing (**A**) Junggar tectonic units and location of the Jimusar Depression, (**B**) structure and well location map of the Jimsar Depression, and (**C**) the stratigraphic sequence from Upper Caboniferous to Lower Cretaceous in the Jimsar Depression (modified from [40]).

#### **3. Materials and Methods**
