## 4 Results
### Pore System and Pore Size
#### 4.3.2 Organic Pores  
In addition to inorganic pores, organic pores (OPs) are also common in the Yanchang shale (Figure 10). These include spongy, bubble-like, slit-like OPs, modified Ops, and shrinkage seams in the original organic matter and migrated solid bitumen [21; 36; 56].  
Figure 9: Histograms of pore sizes measured from SEM images. (**A**) Pore size distribution of inorganic pores of sand-silt intervals. (**B**) Pore size distribution of OPs of sand-silt intervals. (**C**) Pore size distribution of inorganic pores of mudstones. (**D**) Pore size distribution of OPs of mudstones.  
OPs are rare in most of the primary organic matter (POM) in the Yanchang shale (Figure 5A-E). However, in some parts of the POM, bubble-like, spongy pores and shrinkage seams can be observed (Figures 6F and 10D,J). Circular, subcircular, polygonal, and irregular spongy OPs are relatively common within the MSB of the Yanchang shale, with pore sizes generally less than 100 nm (Figure 6H-L and Figure 10A-C,F,J). Circular, subcircular, and oval bubble-like OPs in the MSB have significantly larger pore sizes compared to those of spongy OPs (Figure 10). Most of these bubble-like pores have diameters greater than 50 nm, and pores with sizes in the hundreds of nanometers are also relatively common (Figure 10D-F,J). Additionally, some bubble pores and spongy pores are interconnected, forming a complex organic pore network (Figure 10C,F,J).  
The organic matter in sand-silt intervals is primarily characterized by spongy OPs, with a few bubble-like organic pores also present. Due to the shielding effect of rigid particles, the OPs within the OM hosted in interP and dissolution pores of rigid particles have not undergone deformation as a result of compaction (Figure 6K,L and Figure 10A-C). Some elongated organic pores can be observed within clay platelets or clay aggregates hosted in interP pores of rigid particles (Figure 10G,H). These pores are primarily developed  
Figure 10: SEM photographs showing organic pores in POM and MOM. (**A**) Spongy OPs and cracks (yellow arrows) in MSB filled in interP pores, well W3. (**B**) Spongy OPs in MSB filling interP and intraP pores, well Y10. (**C**) Spongy (red arrows) and bubble (yellow arrows) OPs in MSB, well Y8. (**D**) Bubble OPs in POM, well W1. (**E**) Bubble OPs in POM, well C3. (**F**) Spongy and bubble OPs in MSB, well Y8. (**G**) OPs developed along the orientations of clay minerals, well W3. (**H**) OPs developed along the orientations of clay minerals and interconnected to form complex pores, well W3. (**I**) Elongated OPs in POM aligned the laminations of shale, well Y8. (**J**) Spongy and bubble OPs in MSBs, well Y8. (**K**) Spongy and bubble OPs in MSBs, well Y8. (**L**) Spongy OPs and cracks in POM, well Y11.  
in sand-silt intervals. However, a few can also be observed in the mudstones. In the sand-silt intervals, most of the OPs have diameters less than 200 nm, accounting for 91.9% of the total counted pores, with a median pore size of approximately 87 nm. OPs larger than 300 nm account for 2.8% of the total (Figure 9B).  
In mudstones, organic pores are primarily characterized by bubble-like and spongy OPs (Figure 10B,D,F,K-L). Some of these pores may deform due to compaction, with the long axis oriented parallel to the bedding planes (Figure 10I). Additionally, shrinkage seams are commonly observed within the organic matter as a result of contraction (Figure 6H-L and Figure 10A,L). Compared to sand-silt intervals, the size of the OPs in mudstones is relatively smaller. Most OPs have diameters less than 150 nm, accounting for 92.6% of the total counted pores, with a median pore size of approximately 65 nm. OPs larger than 300 nm account for about 2.0% of the total (Figure 9D).