## 3 Geological Characteristics of Shales in the Yanchang Formation
### Shale Oil and Gas Occurrence Characteristics and Contents
#### 3.6.2 Characteristics and Contents of Shale Oil and Gas  
In the Yanchang Formation shales, hydrocarbons exist in diverse states, including adsorbed and free-state shale oil, as well as adsorbed, free-state, and dissolved shale gas [28; 29; 30; 31; 32]. Some shale oil and gas exist in a free state within pores and fractures, while others are adsorbed onto the surfaces of organic matter and clay minerals. Additionally, some shale gas dissolves into the shale oil under formation temperature and pressure conditions. When observed under a scanning electron microscope, shale oil can appear as droplets filling pores or existing in microfractures (Figure 13), or as thin films adsorbed onto solid particle surfaces.  
The contents of shale oil can be approximated using parameters such as chloroform asphalt \"A\" and the pyrolysis parameter S\({}_{1}\). Chloroform asphalt \"A\" refers to the organic matter extracted from shale using chloroform, reflecting the content of soluble organic matter in the shale. S\({}_{1}\) refers to the hydrocarbons volatilized from a rock sample when heated to no more than 300 \({}^{\circ}\)C during rock pyrolysis analysis, representing the content of free hydrocarbons that can be extracted from the rock. The chloroform asphalt \"A\" content in the Yanchang Formation shales mainly ranges from 0.31% to 1.15%, with an average of 0.73%. The S\({}_{1}\) values in the Yanchang Formation primarily range from 1.0 to 7.0 mg/g, with an average of 3.9 mg/g. Among the Yanchang Formation, the sh  
Figure 12: Graph showing the mass percentage changes of various hydrocarbon components at different evolutionary stages.  
Figure 13: Liquid hydrocarbons filling in (**A**) pores and (**B**) fractures under a scanning electron microscope.  
has the highest S\({}_{1}\), with an average of 4.1 mg/g; the shale in the Chang 8 member is second, with an average of 3.6 mg/g; and the shale in the Chang 9 member has the lowest S\({}_{1}\), with an average of 2.9 mg/g (Figures 14 and 15). The mobility of shale oil is characterized by the oil saturation index (OSI). In the Yanchang Formation shales, the OSI mainly ranges from 25 to 200 mg/g TOC, with an average of 86.13 mg/g TOC. Among the lithofacies, aleurolite has the highest OSI, followed by sandy laminated shale, with black shale having the lowest. The total gas content of the Yanchang Formation shales, as determined through core gas content analysis, ranges from 2.2 to 3.8 m\({}^{3}\)/t, with an average of 2.6 m\({}^{3}\)/t [33,34].  
The retained shale oil in the Yanchang Formation shales significantly affects the gas content. On the one hand, the retained shale oil restricts the adsorption capacity for methane, reducing the amount of adsorbed shale gas. In isothermal adsorption experiments on shale, comparing methane adsorption before and after Soxhlet extraction using chloroform, the methane adsorption amount before extraction was on average reduced by 45% after extraction. On the other hand, under formation conditions, shale oil can dissolve part of the shale gas, which can account for 15% to 20% of the total gas content.