## 5 Key Determinants of Organic Matter Formation and Pore Development in Shale
### Key Determinants of Depositional Organic Matter Development  
The diversity of depositional organic matter is primarily attributed to variations in hydrocarbon-generating precursors, compaction processes, as well as the properties, evolutionary stages, and characteristics of the surrounding minerals. 1. Hydrocarbon-Generation Precursors  
All organic matter originates from biological organisms, and precursors play a fundamental role in shaping the development characteristics and hydrocarbon generation potential of organic matter [71; 72]. The influence of hydrocarbon-generation precursors on depositional organic matter is particularly evident in zooclasts, which can be further subdivided based on their preserved biological structural characteristics. Organic matter in marine shale is primarily derived from marine planktonic algae and bacteria. The diversity of organisms and their degradation states are key factors contributing to variations in organic matter. Alginite results from the humification of planktonic algae in marine environments and inherits the soft tissue structure of planktonic algae, exhibiting smooth edges and loose surface structures, which lead to irregular angular pores. It appears as amorphous particles due to degradation, and maintains high hydrogen and heteroatom content (e.g., N, S) lowers activation energy. After biological degradation, it develops sponge-like or flocculent characteristics [73], with well-developed pores, enhancing its hydrocarbon-generating potential. Zooclasts primarily originate from graptolites and chitinozoans, most of which are planktonic animals, with a few benthic organisms. The fibrous \"cortical bandage\" structure of graptolites retains the periderm tissue, which is characterized by low aliphatic content and high aromaticity. As a result, the graptolite periderm, perpendicular to bedding, has little or no porosity, though nanometer-scale pores have been observed in the cortical fibrils at the edges of graptolites resulting in low hydrocarbon-generating capacity, contributing to better preservation. Acritarchs likely originate from certain algae and unicellular organisms. Encapsulated organic matter, particularly from radiolarians and sponge spicules, is characterized by siliceous shells, with organic matter filling the cavities. These shells are resistant to dissolution providing physical protection, isolating the organic matter from microbial decay [62; 74; 75]. Depositional organic matter precursors primarily include four sources: planktonic plant fragments, zooplankton remains, zooplankton excrete, and bacteria (Figure 12). 2. Compaction and Adjacent Mineral Properties  
The impact of diagenetic compaction on depositional organic matter is most evident in compacted kerogen. During shallow burial, organic matter remains in an immature state, where ductile kerogen, under pressure from overlying strata, is compressed and squeezed into the spaces between rigid particles. This leads to a direct reduction in intergranular pores, causing a decrease in porosity and triggering dehydration. As compacted kerogen deforms under stress, it comes into closer contact with surrounding detrital particles, increasing the contact area. The contact transitions from point contact to line contact and, ultimately, to concave-convex contact. Due to the low thermal maturity at this stage, organic pores are typically underdeveloped, with only a few primary biological tissue structure pores remaining. At this point, compaction dominates as the primary diagenetic process, and intergranular pores between inorganic minerals continue to decrease as compaction intensifies (Figure 13). Non-siliceous alginite and encapsulated organic matter are also affected by compaction. Non-siliceous alginite, like compacted kerogen, undergo compressive compaction during burial, resulting in plastic deformation as they are squeezed between rigid mineral grains. Meanwhile, the disappearance of intergranular porosity prevents the minerals from undergoing cementation.