**5. Discussion**

#### *5.1. Advantages and Disadvantages of the New Method*

Compared with the previous quantitative analysis methods for amorphous SiO2, the new method does not require chemical dissolution. The most important is that the cost of this method is much lower. The equipment required has already been widely used for a large-scale sample testing. This method also has some shortcomings: the ideal formula of clay mineral is used to calculate the mass percentage of elemental Si in clay minerals. Using illite as an example, its ideal structural molecular formula is Al4(Si8O20)(OH)4, and the mass percentage of Si is 31.1%. However, due to the fact that the illite in the actual sample contains impurities, its molecular formula is diverse, which introduces small errors into the calculated value.

#### *5.2. The Influence of Amorphous SiO2 on Reservoir Properties*

The silica content is mainly derived from the alteration of tuffaceous material in the shale strata. It was found through the cross plot between the calculated amorphous SiO2 content and the reservoir physical property data that amorphous SiO2 content was negatively correlated with reservoir porosity and permeability (Figure 5). The content of amorphous SiO2 is negatively correlated with the content of crystalline quartz (Figure 4B). Hence, it indicates that the higher the content of crystalline quartz, the higher the porosity and permeability of the reservoir. Alteration is an important cause of pore formation in the Lucaogou Formation because it is a process of volume reduction for the total material [43,44]. From the perspective of density, it is easy to understand this process of volume reduction. The density of volcanic ash is only 2.3 g/cm3, while the mineral density after its alteration is much higher than 2.3 g/cm3, such as quartz 2.6–2.7 g/cm3. According to the law of conservation of mass, the overall volume must decrease. In other words, a large amount of silica was released during the alteration of tuffaceous components. Some silica crystallized to authigenic quartz, which increases the physical properties of the reservoir, while some silica did not crystallize and occurs between the grains in the form of amorphous SiO2 cement, which reduces the storage space of the reservoir.

The rock mechanical parameters of the Lucaogou Formation were measured by triaxial stress experiment under given confining pressure (Table 2). The calculated content of amorphous SiO2 was positively correlated with Young's modulus and compressive strength (Figure 6A,B). It indicates that the higher the content of amorphous SiO2 was, the harder the samples were to be deformed and fractured. Amorphous SiO2 cements various grains together, making the reservoir more compacted. Amorphous SiO2 is negatively correlated with oil saturation (Figure 6D). It indicates that the existence of amorphous SiO2 is unfavorable for hydrocarbon enrichment. Previous studies suggested that volcanic

ash would lead to algal blooms, and the alteration of volcanic ash would also generate a large number of pore spaces, which provided storage space for hydrocarbon enrichment. During volcanic eruptions, a large amount of volcanic ash was deposited with particulate organic matter and well preserved in a strong reduction environment. At last, they further condensed into kerogen and became source rocks with high organic matter. The organic matter type of Lucaogou Formation shale is mainly I~II1 type, which suggests an origin of bacteria, algae, and other aquatic organisms [19]. However, the presence of amorphous SiO2 makes the tuffaceous shale lithofacies lack sufficient storage space. Furthermore, part of hydrocarbon migrated to the adjacent carbonate lithofacies. On the whole, amorphous SiO2 in Lucaogou Formation in Jimsar Depression is not high in content (Figure 4A and Table 2), which is merely the same to that of K-feldspar. Therefore, the changes in reservoir properties are likely to be caused by other factors, such as the development of laminae, the direction of stress in triaxial stress experiments, and so on. In the early diagenetic stage (Ro is 0.35%~0.5%), amorphous SiO2 has already started to crystallize to quartz in large quantities [23,24]. It can be inferred that the amorphous SiO2 should have a greater physical influence on shale samples in the earlier diagenetic stage.

**Figure 5.** Cross plot of amorphous silica content with (**A**) porosity, (**B**) permeability of different lithofacies in Lucaogou Formation.

#### *5.3. Factors Controlling the Conversion of Amorphous SiO2 into Quartz*

The conversion of amorphous SiO2 into quartz in diagenesis was affected by many factors, including temperature, properties of fluid medium, burial, and formation pressure, etc. [45–48]. It was proposed that the hydrocarbon injection and formation overpressure can inhibit the formation of authigenic quartz [46–48]. However, in the same one sample, both authigenic quartz and amorphous SiO2 occur (Figures 3 and 7), the contents of amorphous silica in the four samples (Figure 7A–D) are 6.921%, 10.484%, 11.535%, and 9.318% (Table 2). It means temperature, fluid properties, and formation pressure was not the key factor. It was found that authigenic quartz tended to develop in pores, holes, or fractures through a large number of scanning electron microscope observations (Figure 7). It was a reasonable presumption that the authigenic quartz can only grow when there was space. Without growth space, it can only be amorphous SiO2 without crystal morphological characteristics. The silica in shale strata of Lucaogou Formation mainly came from the tuffaceous material alteration. A large amount of silica was released. When these pores were filled with a large amount of amorphous SiO2, there was no room left for the growth of the authigenic quartz. Hence, the amorphous SiO2 merely existed in the amorphous state. Only when the silica-rich fluid entered one of those large pores, holes, or cracks was there enough space for silica to grow to authigenic quartz.

**Figure 6.** Cross plot of amorphous silica content with (**A**) Young's modulus, (**B**) Poisson's ratio, (**C**) compressive strength and (**D**) oil saturation of different lithofacies in Lucaogou Formation.

**Figure 7.** Scanning electron microscope of authigenic quartz in the pores, cavities, and cracks of Lucaogou Formation. (**A**) Transitional lithofacies, S1 well, 3147.64 m; (**B**) tuffaceous shale lithofacies, S2 well, 3348.08 m; (**C**) tuffaceous shale lithofacies, S3 well, 2815.21 m; (**D**) tuffaceous shale lithofacies, S4 well, 2601.81 m.
