Quantitative Analysis of Amorphous Silica and Its Influence on Reservoir Properties : A Case Study on the Shale Strata of the Lucaogou Formation in the Jimsar Depression , Junggar Basin , China

To establish a new quantitative analysis method for amorphous silica content and understand its effect on reservoir properties, the amorphous silica (SiO2) in the shale strata of the Lucaogou Formation in the Jimsar Depression was studied by scanning electron microscopy (SEM) observation, X-ray diffraction (XRD), and X-ray fluorescence spectrometry (XRF). Amorphous silica shows no specific morphology, sometimes exhibits the spherical or ellipsoid shapes, and usually disorderly mounds among other mineral grains. A new quantitative analysis method for observing amorphous SiO2 was established by combining XRD and XRF. On this basis, while the higher content of amorphous SiO2 lowers the porosity of the reservoir, the permeability shows no obvious changes. The higher the content of amorphous SiO2, the lower the compressive strength and Young’s modulus and the lower the oil saturation. Thus, amorphous SiO2 can reduce the physical properties of reservoir rocks and increase the reservoir plasticity, which is not only conducive to the enrichment of shale oil but also increases the difficulty of fracturing in later reservoir development.


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][2][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][7][8][9][10][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][13][14][15][16][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

Geological Settings
The Junggar Basin is located in the northwestern part of China with an area of about 1.30 × 10 5 km 2 ( 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 3 km 2 ; 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 8 t [39].
Energies 2020, 13, x FOR PEER REVIEW 3 of 22 The Junggar Basin is located in the northwestern part of China with an area of about 1.30 × 10 5 km 2 ( 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 3 km 2 ; 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 8 t [39].

Materials
The samples of the Lucaogou Formation in this study are from four cored wells (S1-S4) in the Jimsar Depression ( Figure 1B). We selected 42 samples that met experimental needs. Their lithology includes tuffaceous shale (also called siliceous shale), shale, dolomite, and dolomitic mudstone. Generally, the lithology can be divided into three lithofacies: tuffaceous shale lithofacies, transitional lithofacies (also called mixed lithofacies), and carbonate lithofacies [41,42].

Experimental Method
XRD analysis was completed at Sichuan Keyuan Engineering Technology Testing Center. XRF analysis, rock mechanics experiments, and reservoir physical properties analysis were completed at the Experimental Research Center of East China Oil and Gas Branch of Sinopec.

XRD and XRF Analysis
The mineral composition of samples was obtained by XRD, which was determined on the premise of deducting background values through the Jade 5.0 software package. The principle of XRD analysis is that different minerals show different XRD diffraction effects. Data calculated by the XRD accurately represents the relative content of each mineral. However, XRD cannot measure the content of amorphous silica because it shows no diffraction peaks.
The secondary X-rays were emitted when the X-ray irradiated on the material. Different elements show their specific secondary X-ray with certain features or wavelength characteristics. XRF analysis uses secondary X-rays to convert the data into specific elements and their abundance. Elemental Si occurs in quartz, plagioclase, k-feldspar, clay minerals, and amorphous silica.

Rock Mechanics Experiment
Samples were tested using a TAW-2000 computer-controlled electrohydraulic servo testing machine under constant confining pressure conditions. The size of test samples is 25 mm (diameter) × 50 mm (length). In the process of testing, strain rate was controlled by the DUOLI microcomputer control system, mostly 0.01-0.03, which was convenient to obtain smooth stress-strain curves. The compressive strength, Young's modulus, and Poisson's ratio can be calculated by the stress-strain curves.

Reservoir Physical Properties
The total porosity was obtained by calculating the difference between the bulk density and the skeleton density. Permeability was obtained by calculating the expansion of He with increasing pressure (5 MPa-30 Mpa) at a constant temperature. Oil saturation was measured by nuclear magnetic resonance (NMR).

A New Method for Calculating the Content of Amorphous SiO 2
In this study, a new method for quantitative analysis of amorphous SiO 2 in the Lucaogou Formation of the Jimusar Depression was established by using a combination of XRD and XRF. Through XRD analysis, the shale strata mainly consist of quartz, plagioclase, potash feldspar, dolomite, calcite, pyrite, and clay minerals (Figure 2A). Elemental Si is in quartz, plagioclase, potash feldspar, and clay minerals.
The combination of XRD and XRF can calculate amorphous silica as follows. Suppose the sample mass is M, where the mass of amorphous SiO 2 , quartz, plagioclase, K-feldspar, and clay minerals are respectively represented by m SiO 2 , m quartz , m plagioclase , m K− f eldspar , and m clay . According to XRD analysis:   According to XRD analysis: The W quartz , W plagioclase , W K− f eldspar , and W clay represent the percentage of quartz, plagioclase, k-feldspar, and clay minerals measured by XRD analysis.
According to XRF analysis: The mass percentages of Si in amorphous SiO 2 , quartz, plagioclase, k-feldspar, clay minerals, and the sample are represented by P Si−SiO 2 , P Si−quartz , P Si−plagioclase , P Si−K− f eldspar , P Si−clay , and W Si , respectively.
Placing Formulas (1)-(4) into Formula (5), thus creating Formula (6) Formula (6) can be changed to Formula (7): In Formula (7), only the mass percentage of element Si in clay minerals is difficult to determine, because the molecular formulas of other minerals are known. The molecular formulas of clay  4 . The mass percentages of element Si in these are 21.7%, 56.3%, 19.6%, and 31.1%, respectively. The P clay of the tuffaceous shale lithofacies, transitional lithofacies, and carbonate lithofacies samples can be calculated. Then, the contents of amorphous SiO 2 in these samples can be calculated by Formula (7).

Occurrence and Characteristics of Amorphous SiO 2
The shale strata of the Lucaogou Formation in the Jimusar Depression can be divided into tuffaceous shale lithofacies, transitional lithofacies, and carbonate lithofacies [41,42]. The tuffaceous 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 SiO 2 is also present ( Figure 3C-H). Amorphous SiO 2 shows no fixed form and usually fills randomly between mineral grains ( Figure 3C-E). Some of the amorphous SiO 2 was wrapped in tuffaceous components ( Figure 3F), and other forms were spherical or ellipsoid shapes having varying sizes ( Figure 3G,H).
In Formula (7), only the mass percentage of element Si in clay minerals is difficult to determine, because the molecular formulas of other minerals are known. The molecular formulas of clay minerals are variable. Therefore, the ideal molecular formulas of different types of clay minerals are applied in this research. For the mass percentage of Si in mixed clay minerals, it is calculated according to the mixed layer ratio based on XRD measurements. Molecular formulas used for kaolinite, montmorillonite, chlorite, and illite are respectively Al4(Si4O10)(OH)8, Al4Si8O2(OH)2, Al6Si4O10(OH)8, and Al4(Si8O20)(OH)4. The mass percentages of element Si in these are 21.7%, 56.3%, 19.6%, and 31.1%, respectively. The of the tuffaceous shale lithofacies, transitional lithofacies, and carbonate lithofacies samples can be calculated. Then, the contents of amorphous SiO2 in these samples can be calculated by Formula (7).

Occurrence and Characteristics of Amorphous SiO2
The shale strata of the Lucaogou Formation in the Jimusar Depression can be divided into tuffaceous shale lithofacies, transitional lithofacies, and carbonate lithofacies [41,42]. The tuffaceous 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 tuffaceous components ( Figure 3F), and other forms were spherical or ellipsoid shapes having varying sizes ( Figure 3G,H).

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).

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.

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).

Content of Amorphous SiO 2
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 SiO 2 . The tuffaceous shale lithofacies samples show the highest content of amorphous SiO 2 , reaching an average of 7.07%, and the carbonate lithofacies samples show the lowest, only 1.52% ( Figure 4A). Amorphous SiO 2 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 SiO 2 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 SiO 2 .

Advantages and Disadvantages of the New Method
Compared with the previous quantitative analysis methods for amorphous SiO 2 , 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 Al 4 (Si 8 O 20 )(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.

The Influence of Amorphous SiO 2 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 SiO 2 content and the reservoir physical property data that amorphous SiO 2 content was negatively correlated with reservoir porosity and permeability ( Figure 5). The content of amorphous SiO 2 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/cm 3 , while the mineral density after its alteration is much higher than 2.3 g/cm 3 , such as quartz 2.6-2.7 g/cm 3 . 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 SiO 2 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 SiO 2 was positively correlated with Young's modulus and compressive strength ( Figure 6A,B). It indicates that the higher the content of amorphous SiO 2 was, the harder the samples were to be deformed and fractured. Amorphous SiO 2 cements various grains together, making the reservoir more compacted. Amorphous SiO 2 is negatively correlated with oil saturation ( Figure 6D). It indicates that the existence of amorphous SiO 2 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~II 1 type, which suggests an origin of bacteria, algae, and other aquatic organisms [19]. However, the presence of amorphous SiO 2 makes the tuffaceous shale lithofacies lack sufficient storage space. Furthermore, part of hydrocarbon migrated to the adjacent carbonate lithofacies. On the whole, amorphous SiO 2 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 SiO 2 has already started to crystallize to quartz in large quantities [23,24]. It can be inferred that the amorphous SiO 2 should have a greater physical influence on shale samples in the earlier diagenetic stage. 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.

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/cm 3 , while the mineral density after its alteration is much higher than 2.3 g/cm 3 , such as quartz 2.6-2.7 g/cm 3 . 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

Factors Controlling the Conversion of Amorphous SiO 2 into Quartz
The conversion of amorphous SiO 2 into quartz in diagenesis was affected by many factors, including temperature, properties of fluid medium, burial, and formation pressure, etc. [45][46][47][48]. It was proposed that the hydrocarbon injection and formation overpressure can inhibit the formation of authigenic quartz [46][47][48]. However, in the same one sample, both authigenic quartz and amorphous SiO 2 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 SiO 2 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 SiO 2 , there was no room left for the growth of the authigenic quartz. Hence, the amorphous SiO 2 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 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.

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][46][47][48]. It was proposed that the hydrocarbon injection and formation overpressure can inhibit the formation of authigenic quartz [46][47][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.

Conclusions
The amorphous SiO2 in the shale strata of the Lucaogou Formation of the Jimusar Depression had no specific form and was usually mounded among mineral grains. XRD analysis measured the percentage of crystalline minerals, while XRF measured the percentage of elemental Si. Therefore, a new quantitative analysis method for calculating the percentage of amorphous SiO2 was established by combining the two methods. The content of amorphous SiO2 in the tuffaceous shale lithofacies of

Conclusions
The amorphous SiO 2 in the shale strata of the Lucaogou Formation of the Jimusar Depression had no specific form and was usually mounded among mineral grains. XRD analysis measured the percentage of crystalline minerals, while XRF measured the percentage of elemental Si. Therefore, a new quantitative analysis method for calculating the percentage of amorphous SiO 2 was established by combining the two methods. The content of amorphous SiO 2 in the tuffaceous shale lithofacies of the Lucaogou Formation was the highest, with an average of 7.07%.
The calculation confirmed that the higher the content of amorphous SiO 2 , the lower the porosity of the reservoir. Moreover, amorphous SiO 2 was found to be inversely proportional to the compressive strength, Young's modulus, and oil saturation of the reservoir. It indicates that amorphous SiO 2 reduces the physical properties of the reservoir, increases the plasticity, and increases the difficulty of fracturing during development for hydrocarbon extraction. The lack of growing space is the key factor affecting the conversion of amorphous SiO 2 into crystalline quartz. Thus, the existence of amorphous SiO 2 is harmful to shale reservoirs in many ways and has economic impact deleterious to oil and gas exploration and development.