## 5 Key Determinants of Organic Matter Formation and Pore Development in Shale
### Key Determinants of Migrated Organic Matter Development  
The formation and preservation of migrated organic matter are primarily controlled by the degree of thermal evolution, mineral composition, and the rigidity of the surrounding framework. 1. Thermal Evolution and Hydrocarbon Generation  
The source of the migrated organic matter is the liquid hydrocarbons generated during the thermal evolution of organic matter through pyrolysis. Some of the produced liquid hydrocarbons, such as oil, may in situ fill the pores and fractures formed during hydrocarbon generation, while others are expelled from the kerogen, migrating into adjacent pores where they undergo conversion into solid bitumen, thus forming migrated organic matter. Therefore, reaching the oil window during kerogen thermal evolution is a key prerequisite for the formation of migrated organic matter. Migrated organic matter also undergoes thermal evolution, and its transformation from bitumen to solid bitumen or pyrobitumen is associated with the creation of organic pores. After secondary cracking, significant quantities of dry gas are generated, leading to the formation and preservation of numerous organic pores. These pores connect isolated voids within the depositional organic matter, creating a continuous three-dimensional pore network. This network facilitates more efficient and continuous gas migration [5; 80]. Figure 14: Schematic diagram of in situ remnants from post–hydrocarbon generation. Mineral Types and Rigid Framework  
Migrated organic matter preferentially fills pre-existing intergranular pores during migration. Due to its high plasticity, the morphology of organic matter is shaped by surrounding minerals. The influence mechanisms can be divided into two types. First, they delineate the range of organic matter's plastic flow, thereby controlling its morphological distribution. Second, the types of pores and fractures in the surrounding minerals qualitatively determine where migrated organic matter will accumulate. Migrated organic matter can be classified into three types based on the mineral pore types: (1) migrated organic matter in intragranular pores, which forms when organic matter migrates into and fills intragranular pores; (2) migrated organic matter in intergranular pores, which typically has blurred boundaries and is the most common form of migrated organic matter; and (3) bitumen in microfractures, which forms when organic matter fills microfractures (Figure 15). The main minerals influencing the occurrence of organic matter in shale are quartz, pyrite, and clay minerals. Quartz can be biogenic or detrital in origin, creating a rigid, three-dimensional, grain-supported framework that preserves primary pores and generates numerous intergranular pores. These pores provide storage space for organic matter [81; 82; 83]. The accumulation locations associated with pyrite include two types: the radiolarian silica shells replaced by pyrite and the intercrystalline spaces within the pyrite crystals. Both pyrite and quartz, being brittle, form a framework with high resistance to compaction, helping to preserve organic matter and organic pores by minimizing damage during burial. Organic matter can also be stored within the platelets of clay minerals, where organic matter pores can develop. During diagenesis, clay minerals undergo dehydration, forming microfractures that are filled with migrated organic matter. However, because clay minerals are less resistant to compaction, pore development is limited, with pores concentrated in the central region and elongated along the bedding plane. Clay minerals and organic matter co-evolve during diagenesis. As maturity increases, smectite illitization causes spatial collapse, releasing adsorbed organic matter [84; 85]. However, the micro-interface interactions between clay minerals and organic matter are crucial to their catalytic role in hydrocarbon generation [86].