## 4 Pore Characteristics of Different Types of Organic Matter
### Pore Development Characteristics of Migrated Organic Matter  
The bitumen that migrates outside of kerogen can undergo secondary cracking, resulting in the formation of organic matter pores. Migrated organic matter typically develops nanoscale pores and microfractures, with the pores often exhibiting ellipsoidal or irregular shapes. Organic matter pores within the migrated organic matter in intragranular pores are well-developed. Under scanning electron microscopy (SEM), organic matter filling the interior of fossilized skeletons--such as radiolarians or sponge spicules--replaced by pyrite, predominantly displays a spongy morphology. These pores are generally circular or elliptical, with diameters ranging from tens to hundreds of nanometers, and the porosity is approximately 50% (Figure 11a). Pyrite, which forms concurrently with bacterial sulfate reduction, can replace the siliceous skeletons of radiolarians. This material is subsequently transported to the sedimentary basin, where its internal cavities become filled with migrated organic matter. The confinement of the migrated organic matter within these biological cavities results in a distinct microstructural appearance under microscopy. In intergranular pores, nanopores are well-developed within the migrated organic matter (Figure 11b-e). Pores within the organic matter located between clay mineral lamellae are predominantly elliptical, with directional elongation, and exhibit diameters ranging from approximately 500 nm to 1 \(\upmu\)m. Contraction fractures are also evident. The porosity in these areas is approximately 5-10% (Figure 11b,c). Within the intergranular pores of framboidal pyrite, the organic matter primarily forms granular pores, most of which have diameters in the tens of nanometers. Occasionally, moldic pores are observed, with diameters ranging between 1 \(\upmu\)m and 2 \(\upmu\)m. The porosity of the migrated organic matter in these intergranular pores is estimated to be around 10-30% (Figure 11d). Organic matter within authigenic microcrystalline quartz primarily develops spongy or honeycomb-like pores, with diameters ranging from tens to hundreds of nanometers. Additionally, a small number of relatively large, randomly shaped organic matter pores are observed, which are primarily alveolar in shape and range in diameter from 1 \(\upmu\)m to 2 \(\upmu\)m. The porosity in these areas ranges from 50% to 70% (Figure 11e). During the late stages of burial, cracking of the migrated organic matter generates significant quantities of dry gas, leaving behind spongy organic matter pores. The organic components found between euhedral mineral particles consist of solid bitumen or pyrobitumen, the result of further cracking of the oil generated within the formation.  
Bitumen within microfractures along bedding planes typically does not form pores but often exhibits contraction fractures (Figure 11f), which play a significant role in the transport system. During the thermal maturation of kerogen, bitumen migrates into microfractures, filling them in the direction of extension. This bitumen is frequently associated with fragments of the surrounding rock or framboidal pyrite, suggesting that the microfractures primarily develop during the kerogen degradation process, which is linked to overpressure generated during hydrocarbon generation.  
This study demonstrates that alginite, in situ remnant from post-hydrocarbon generation, along with migrated organic matter in intragranular and intergranular pores, generally exhibits well-developed porosity. This is indicative of favorable hydrocarbon potential and suggests enhanced reservoir quality. In contrast, zooclasts, acritarchs, encapsulated organic matter, and compacted kerogen typically exhibit poor porosity development. Reservoirs enriched with these materials may exhibit reduced hydrocarbon generation capacity, thus potentially lacking favorable prospects for hydrocarbon accumulation.