## 4 Pore Characteristics of Different Types of Organic Matter
### Pore Development Characteristics of Depositional Organic Matter  
There are two primary mechanisms responsible for the development of organic matter pores in depositional organic matter. The first mechanism involves the pyrolysis of  
Figure 9: SEM image of bitumen in microfractures. (**a**) Bitumen in microfractures, Y101H3–8#, 3754.94 m; (**b**) Bitumen in microfractures, Y101H3–8#, 3754.94 m; (**d**) Bitumen in microfractures, Y101H3–8#, 3759.27 m; (**e**) Bitumen in microfractures, Y101H3–8#, 3785.51 m; (**f**) Bitumen in microfractures, Y101H3–8#, 3787.78 m.  
kerogen, during which bitumen is generated and expelled into adjacent pores, leading to the formation of organic matter pores within the kerogen itself [22]. The second mechanism pertains to the formation of organic matter pores through secondary cracking of organic matter that remains in situ after the oil generation stage [55; 64]. Among the various categories of organic matter, compacted kerogen typically exhibits minimal porosity. In contrast, the in situ remnants left after the oil generation phase tend to display a greater degree of pore development. Bioclasts, with the exception of alginate--which forms angular pores--generally exhibit limited or no porosity, as is the case with zooclasts, acritarchs, and encapsulated organic matter. Both types of alginate, however, exhibit well-developed porosity. In alginite with a pure interior, the pores are predominantly irregular angular or a combination of irregular angular and circular or elliptical forms (Figure 1a). These pores range in size from tens to hundreds of nanometers and possess relatively high porosity (Figure 10a). In alginite that contains fibrous clay minerals, pores are developed between the organic matter and the fibrous clay minerals, displaying irregular angular or circular shapes. These pores are relatively uniform in size, ranging from approximately 10 to 50 nm, and the porosity is about 20% (Figure 1a). The formation of organic pores is believed to be influenced by the intrinsic biological structure and genetic characteristics of algae. The morphology, distribution, and size of these pores are primarily determined by the type and original structure of the hydrocarbon-generating organisms. In low-maturity organic matter, pores are mainly interstitial spaces between single-celled algae. Circular pores are thought to result from the early stages of petroleum generation and expulsion from algal-rich material [34; 65; 66]. The regular arrangement and elongation of elliptical pores are likely the consequence of compaction following hydrocarbon expulsion. In zooclasts, the internal structure of chitinozoan vesicles generally exhibits minimal porosity. Few pores or microfractures, typically around 50 nm in width, are observed at the interface between the vesicles and surrounding minerals (Figure 10b). Grapholite periderms also show little to no porosity, although occasional nano-sized round organic pores are observed locally, with a porosity much less than 1%. Microfractures ranging in width from 10 to 20 nm are visible along the edges (Figure 10c). The grapholite periderm features a \"cortical bandage\" structure, within which elongated nano-pores or microfractures develop between cortical fibrils, typically ranging from 20 to 70 nm in width (Figure 10d). These microfractures interconnect the pores, forming a network that serves as both hydrocarbon storage spaces and migration pathways [67]. Acritarchs and encapsulated organic matter exhibit no internal pore development. However, microfractures, typically 2-10 nm in width, are commonly observed at the interfaces between acritarchs and surrounding minerals, as well as between the organic matter and the siliceous shells of encapsulated organisms (Figure 10e,f). Compacted kerogen shows minimal porosity due to diagenetic compaction (Figure 10g). In contrast, in situ remnants from post-hydrocarbon generation display a significantly higher degree of pore development, often appearing honeycomb-like and/or alveolar upon polished examination.