**6. 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 tu ffaceous 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 SiO2, the lower the porosity of the reservoir. Moreover, amorphous SiO2 was found to be inversely proportional to the compressive strength, Young's modulus, and oil saturation of the reservoir. It indicates that amorphous SiO2 reduces the physical properties of the reservoir, increases the plasticity, and increases the di fficulty of fracturing during development for hydrocarbon extraction. The lack of growing space is the key factor affecting the conversion of amorphous SiO2 into crystalline quartz. Thus, the existence of amorphous SiO2 is harmful to shale reservoirs in many ways and has economic impact deleterious to oil and gas exploration and development.

**Author Contributions:** Conceptualization, Q.C.; methodology, K.S.; software, K.S. and C.C.; investigation, K.S.; resources, Q.C. and G.C.; writing—original draft preparation, K.S.; writing—review and editing, K.S., Y.L., G.C. and C.C.; supervision, Q.C. and G.C.; project administration, Q.C. and G.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Science and Technology Major Project Foundation of China (2016ZX05006-003-002; 2016ZX05014-002); National Nature Science Foundation of China (41802157); Fundamental Research Funds for the Central Universities, China University of Geoscience (Wuhan) (102-162301202627); China Postdoctoral Science Foundation (2016M592265).

**Conflicts of Interest:** The authors declare no conflict of interest.
