## 3 Geological Characteristics of Shales in the Yanchang Formation
### Reservoir Pore Volume  
The types of reserving space of shales in the Yanchang Formation include pores and microfractures. Compared with microfractures, the pores of shales in the Yanchang Formation are more developed. The pore types of shales are divided into inorganic intergranular pores, inorganic intragranular pores, and organic pores [22; 23]. Inorganic intergranular pores include rigid particle intergranular pores, clay debris intergranular pores, intergranular dissolution pores, and antigenic mineral intercrystalline pores. Inorganic intragranular pores include intragranular dissolution pores, fossil cavity pores, authigenic clay mineral aggregates, and pyrite aggregate intragranular pores. Organic pores are classified into kerogen organic pores and migrated solid bitumen organic pores (Figure 9). The contents of rigid elastic particles, migrated solid bitumen, carbonate cement, dissolution effects, and the development of siltstone laminations are the main factors influencing shale's pore structure and physical properties.  
Considering the applicability of different pore size testing methods, this study used both carbon dioxide and nitrogen gas adsorption methods, as well as mercury intrusion porosimetry, to test the pore structure. The obtained data were integrated to establish a comprehensive pore size distribution testing method for the Yanchang Formation shales. The experimental results show differences in pore structure among different lithofacies [24; 25; 26]. According to the pore classification of the International Union of Pure and Applied Chemistry (IUPAC), pores with a diameter less than 2 nm are micropores, pores with a diameter between 2 and 50 nm are mesopores, and pores with a diameter greater than 50 nm are collectively called macropores. Micropores are relatively well developed in the black shale, accounting for 26.1% of the total pore volume, with mesopores and macropores making up 45.8% and  
Figure 8: Histogram of TOC distribution in the Yanchang Formation shales.  
27.1% of the total pore volume, respectively. In aleurolite, mesopores and macropores are more developed, with micropores accounting for only 3.8% of the total pore volume and macropores exceeding 50%. The micropore development in laminated shale is intermediate between the other two lithofacies, with mesopores accounting for up to 60% of the total pore volume (Figure 10) [27].  
The porosity of the Yanchang Formation's shale layers ranges from 0.2% to 5.1%, with an average of 2.51%. After oil extraction using the solvent dichloromethane, the porosity increases significantly, and the average porosity can reach 3.18%. The permeability of  
Figure 10: Comparison of pore structure differences in different lithofacies of shales in the Yanchang Formation.  
Figure 9: Types of pores in the Yanchang Formation shales: (**a**) Intergranular pores in siltstone laminae. (**b**) Intergranular pores in clay layers. (**c**) Intercrystalline pores in clay mineral aggregates. (**d**) Intercrystalline pores in pyrite aggregates. (**e**) Migrated solid bitumen organic pores. (**f**) Kerogen organic pores.  
the Yanchang Formation shales ranges from 0.0043 millidarcy (mD) to 0.239 mD, with an average of 0.0133 mD, indicating that these shales generally belong to low-porosity, ultra-low-permeability reservoirs (Figure 11).