Microscopic Characteristics and Formation of Various Types of Organic Matter in High-Overmature Marine Shale via SEM  
[PERSON]  
1 Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China; [EMAIL_ADDRESS] (M.Z. ); [EMAIL_ADDRESS] (Z.S. ); [EMAIL_ADDRESS] (Q.Z. ); [EMAIL_ADDRESS] (T.Z. ); [EMAIL_ADDRESS] (L.Q)  
[PERSON]  
2 National Energy Shale Gas Research and Development Experimental Center, Beijing 100083, China 23 National Elite Institute of Engineering, CNPC, Beijing 100096  
generation, offering essential insights into the evolution pathways and hydrocarbon generation processes of shale reservoirs [3; 4; 5]. Organic matter is highly heterogeneous [6; 7], and current research primarily focuses on its variations in abundance, maturity, and pore structure across vertical, horizontal, and planar dimensions [8; 9; 10; 11]. The microscopic characteristics of organic matter significantly influence its hydrocarbon generation and evolution. Organic matter pores result from the generation, diffusion, and expulsion of gases during the thermal evolution of source rocks. These pores serve as key reservoirs for free gas in shale and act as primary adsorption sites for adsorbed gas [12; 13; 14; 15; 16]. Organic matter pore development is largely controlled by the properties of the organic matter itself [17; 18]. During thermal evolution, organic matter pores develop within the organic material itself [19; 20; 21; 22; 23; 24], significantly impacting shale gas enrichment and production. Previous studies have classified organic matter based on factors such as macroals, distribution patterns, occurrence forms, and genetic dynamics to identify shale organic matter components and their pore development characteristics [25; 26; 27; 28] (Table 1). But the current research faces the issue of oversimplification regarding the organic matter as a hydrocarbon source. Furthermore, marine shale contains a relatively low amount of vitrinite and inertinite, with smaller particle sizes, making it difficult to classify macroals. There is still no unified classification scheme based on the distribution patterns of organic matter, and it remains challenging to identify the hydrocarbon source material. From a genetic perspective, the classification of shale organic matter remains underdeveloped, with no comprehensive classification system established. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Classification Criteria** & **Classification of Organic Matter** & **Characteristics** \\ \hline \multirow{7}{*}{Maceral groups [12]} & \multirow{2}{*}{Virtinite} & Originating from higher plants, these particles are dispersed within shale matrices and generally lack cellular structure. \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \hline \multirow{2}{*}{Maclasts} & \multirow{2}{*}{Decolved organic matter} & \multirow{2}{*}{Derived from fragments of planktonic or benthic organisms, distinguishable by their microscopic morphology.} \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \hline \multirow{7}{*}{Mode of occurrence [28]} & \multirow{2}{*}{Infilling organic matter} & \multirow{2}{*}{Filling clay-sized pores, bedding planes, or interlaminar fissures, serving as a cement for detrial grains, with indistinct boundaries between the organic matter and surrounding minerals.} \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \hline \multirow{7}{*}{Distribution pattern [29]} & \multirow{2}{*}{Particulate organic matter} & \multirow{2}{*}{Includes two types: one characterized by interbedded organic-rich layers with mineral layers, and the other by elongated organic matter dispersed within a mineral matrix.} \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \cline{2-3}  & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of organic matter classification. Organic matter pores are typically smaller than 1 \(\upmu\)m [30].