## 5 Discussion
### Significance of Hydrocarbon Exploration
#### 5.2.2 Regional Source Rock  
High-quality source rocks are basic and necessary conditions for the formation of large hydrocarbon deposits. Recent studies have shown that three main sets of source rocks developed in the South China Sea and Southeast Asia, i.e., the Eocene lacustrine source rocks during the Early Paleogene rift stage, the Oligocene lacustrine and marine-continental transitional source rocks during the Middle-Late Paleogene rift stage, and the Lower Miocene marine source rocks during the Neogene depression stage [11; 32]. The source rocks in the Qiongdongnan and Zhongjiannan Basins of the Xisha sea area also have similar characteristics. The Oligocene and Miocene source rocks developed in the Qiongdongnan Basin. The Oligocene Yacheng and Lingshui Formations' mudstone constitutes the main source rock, with high thickness and wide distribution [33]. The organic matter abundance of these source rocks is high, with an average organic carbon content of 1.05%. In particular, the measured organic carbon abundance of coal source rocks is much higher, with an average of 48.4%. The kerogen types of the source rocks are II\({}_{2}\)-III, which mainly generate gas. The Miocene source rocks were rapidly deposited in the neritic-bathyal sedimentary environment, with thick argillaceous rocks and an average organic matter abundance of 0.71%, and mainly humic organic matter [14; 16; 33; 34].  
According to the analyses of paleogeography and sedimentary facies, Eocene-Oligocene lagoons, shallow lakes, moderately deep and deep lakes' facies mudstone, and the Lower-Middle Miocene neritic and bathyal facies mudstone developed in the Zhongjiannan Basin [35]. During the Eocene-Oligocene, most of the basin was in a closed, shallow lake or a deep-water environment in the prodelta zone. These environments primarily resulted in mudstone deposits and local swamp mudstones. Terrestrial sources were abundant and suitable for forming kerogen types II or II-III. The bottom of the lake was mostly in an anoxic reducing environment, conducive to the preservation of organic matter. Therefore, thick lacustrine deposits in the graben and half-graben of the basin were the primary source rocks during the Eocene-Oligocene rifting period [36; 37]. This set of source rocks can be compared with the coastal plain swamp coal seams, carbonaceous mudstones, and mudstones that developed during the Yacheng and Lingshui Formations of the Qiongdongnan Basin. Related data suggest that the average organic carbon contents are 0.47% to 1.16% in argillaceous rocks and 13.26% to 21.43% in the coal and carbonaceous mudstone. The chloroform bitumen \"A\" contents are 1.78% to 4.16%, with medium-to-good quality. The hydrocarbon generation threshold values of the source rocks are 1900-3100 m, having now essentially entered the gas-generation stage and even the mature stage of oil generation in most of the western region [38]. Another set of source rocks of the Lower-Middle Miocene developed during the continuous transgression stage. During this period, a set of neritic  
Figure 12: Hydrocarbon accumulation process of the reef and carbonate platform in the western Xisha sea area. The profile location is shown in Figure 1B.  
and metric-bathyal facies mudstones were deposited as the secondary source rocks of the basin, which developed organic matter of kerogen types I-II.