Geochemical Characteristics and Developmental Models of the Eocene Source Rocks in the Qiongdongnan Basin, Northern South China Sea

Seismic data show that a set of Eocene lacustrine source rocks exists in the Qiongdongnan (QDN) Basin, northern South China Sea (SCS). The high content of C-30 4-methylsteranes detected in the crude oil suggests the existence and the effectiveness of the lacustrine source rocks, and these compounds were not found in the Oligocene and Miocene source rocks. The above date indicates that the Eocene source rocks are effective and oil-prone. Lacustrine source rocks in faulted period have high organic matter abundance and better organic matter types in the SCS and other typical passive continental margin basins in the world. Lacustrine source rocks in the Eocene source rocks (E(2)w) from the Baiyun Sag of the Pearl River Mouth Basin have relatively high total organic content (TOC) values, which can be explained by moderate to high paleoproductivity and anoxic environment. Therefore, it can be speculated that the Eocene source rocks have good hydrocarbon potential in the QDN Basin. The developmental pattern of Eocene source rocks is revealed as follows: In mid-deep lake environment, high-quality source rocks could be formed, because of high productivity and better organic matter preservation conditions, and the central depression belt is the most favored zone beneficial to Eocene source rock formation, because of the relatively large-scale mid deep lake facies. In shore-shallow lake and delta environment, high-quality source rocks could not be formed, because of the poor organic matter preservation conditions.