## 3 Classification of Organic Matter in Shale and Its Morphological Characteristics
### Depositional Organic Matter
#### 3.1.1 Bioclasts  
In highly overmature marine shale, bioclastic organic matter can be classified into four main types: alginite, animal fragments, uncertain sources, and encapsulated matter. 1. Alginite: Alginite typically exhibits irregular shapes and is often closely associated with inorganic minerals, with well-defined boundaries and no secondary minerals in between (Figure 1). Microscopic examination reveals two main characteristics: one where algal debris contains clay minerals arranged in a fibrous distribution, showing some structure and orientation (Figure 1a), and another where the algal mat is pure, free from clay minerals (Figure 1b-d). During burial, alginite fragments can develop channels and fissures, where fluids interact with the rock to form fibrous illite [46], resulting in alginite containing fibrous illite. * The identification features of alginite under scanning electron microscopy (SEM) include three key aspects:
* Large organic matter particle diameters: in the diagenetic stage, mineral particles are typically around 5 \(\upmu\)m, insufficient to support interparticle pores larger than tens of micrometers, indicating that alginite is a type of depositional organic matter, rather than migrated organic matter. * Absence of secondary mineral enlargement at organic matter edges: secondary enlargement of quartz at pore edges is common during diagenesis, which further confirms that alginite is depositional organic matter. * The pores are mostly angular and vary in size. 2. Zooclasts: Zooclasts primarily include gaptolite periderm, chitinozoan vesicles, and scolecodonts, with gaptolite periderm being the most common, and chitinozoan vesicles occasionally present. Gaptolite periderm fragments typically appear as flat carbonaceous films parallel to bedding, exhibiting a banded or vein-like distribution with strong anisotropy (Figure 2a-c). These fragments show clear contact interfaces with the surrounding matrix, sometimes developing microfractures at these interfaces (Figure 2b,c). The cavities within the gaptolite periderm often display segmented structures (Figure 2a-c) and are characterized by a \"cortical bandage\" structure (Figure 2d). Gaptoilites were primarily planktonic, though some were benthic and sessile [57], feeding on fungi, algae, and other microorganisms. After death, they were buried in sediments, forming flattened carbonaceous films preserved in black shales [58]. High gaptolite abundance in shales often indicates deep-water, reducing environments [59]. Gaptolite periderm has distinct morphological characteristics, and its microscopic identification features mainly include the following:  
Figure 1: SEM image of alginite. (**a**) Alginite containing fibrous clay minerals, Y206#, 3851.82 m; (**b**) Alginite with a pure interior, Y206#, 3864.75 m; (**c**) Alginite with a pure interior, Y206#, 3861.36 m; (**d**) Alginite with a pure interior, Y206#, 3861 m.  
Banded or Vein-Like Habit: the organic matter exhibits a banded or vein-like structure, likely derived from graptolite fossil fragments. * Segmented Structure: the periderm develops a segmented structure resembling complex skeletal morphology. * \"Cortical bandage\" Structure: the periderm is composed of layered structures with cortical fibrils. * Poorly Developed Pores and Marginal Microfractures: under SEM, graptolite periderm displays poorly developed pores, with microfractures often present at the contact interface with matrix minerals. Chitinozoan vesicles are typically vase-shaped but can also appear conical, cylindrical, or in other forms (Figure 2e,f). Some vesicles are connected in chains and usually exhibit symmetrical structures (Figure 2f). The cavities within these vesicles are often filled with inorganic minerals. In high-overmature marine shales, chitinozoan vesicles show strong anisotropy and generally range from several tens of micrometers to over 100 \(\upmu\)m in length. Key microscopic features of chitinozoan vesicles include the following:  
Figure 2: SEM image of zooclasts.