# Experimental Study on the Impact of CO2 Treatment on Different Lithofacies in Shale Oil Reservoirs
## 3 Results and Discussion  
### Lithofacies Analysis of Qingshankou Formation  
The main lithofacial types of the Qingshankou Formation are identified through core observation and XRF analysis, and the results are shown in Figure 4. The shale in the Qingshankou Formation can be divided into three main lithofacial types: foliaceous shale, laminated feisic shale and laminated diametric shale. These three main types of shale account for more than 90% of the Qingshankou formation and are the main target area for exploration and development. The photo and XRF result of foliaceous shale sample are shown in Figure 4a,d. The foliaceous shale is mainly black and dark gray, with a fine texture, high oil saturation and low interlayer difference. Its element distribution is relatively uniform. The photo and XRF result of laminated feisic shale sample are shown in Figure 4b,e. The laminated feisic shale is mainly dark gray with relatively low oil saturation and significant interlayer difference. Its element distribution is uneven with many feisic laminas. The thickness of these laminas is about 1 mm and their composition is mainly silica and iron elements, indicating a rapid alternative change of highly organic shale and lowly organic silty laminas. The photo and XRF result of laminated diametric shale sample are shown in Figure 4c,f. The laminated diametric shale is mainly dark gray with relatively low oil saturation and significant interlayer difference. Its element distribution is uneven, with many diamictic  
Figure 4: Results of core observation and XRF analysis: (**a**) photo of foliaceous shale; (**b**) photo of laminated feisic shale; (**c**) photo of laminated diametric shale; (**d**) element distribution of foliaceous shale; (**e**) element distribution of laminated feisic shale and (**f**) element distribution of laminated diametric shale. laminas. The thickness of these laminas is about 1 mm and their compositions are mainly silica, iron and calcium elements, indicating a rapid alternative change from highly organic shale to lowly organic silty and carbonate laminas. The average mineral composition of these three main types of shale is shown in Figure 5. The results show that the main minerals of foliaceous shale are clay minerals, quartz and feldspar. Clay minerals account for more than half of the total mineral content, and the proportions of quartz and feldspar are 27.4% and 10.8%, respectively. The proportions of carbonate and other minerals are very low, only 5.3% and 3.3%, respectively. For laminated feisic shale, quartz and feldspar together account for about half of the total minerals and are greater than the proportion of clay minerals, which is 41.9%. The proportion of carbonate and other minerals is very low, less than 5%. For laminated diamic shale, quartz and feldspar together account for 41.6% of the total minerals and are greater in proportion than that of clay minerals, which is 40.3%. The proportion of carbonate minerals is relatively high and accounts for more than 15% of the total content. Obviously, there are significant differences in the mineral composition among these three main types of shale samples. Due to the significant differences of mineral composition, varied micromorphologies and pore structures are observed during SEM analysis and the results are shown in Figure 6. As shown in Figure 6, after mechanical and argon ion polishing, the micromorphologies of foliaceous shale samples are smooth, flat and tight. There are some micropores, the sizes of which are less than 5 \(\upmu\)m. The main reservoir spaces are micro fractures, intergranular pores and organic pores. The number and size of these pores are relatively many and large, with some natural microfractures. As a result, the connectivity between pores and fractures maybe good, which may form seepage networks for shale oil.