关键词: 南海北部, 始新统, 成烃生物, 湖相烃源岩, 生物标志化合物

Abstract: The Eocene lacustrine source rocks in the northern South China Sea constitute the principal source of discovered crude oils in the region. Previous studies have generally inferred that the organic matter in these lacustrine source rocks is predominantly derived from planktonic algae, with C₃₀ 4-methylsteranes commonly employed as an indicator of algal input. However, with continued exploration, relatively low abundances of C₃₀ 4-methylsteranes have been identified in some high-quality source rocks, highlighting the limitations of this proxy in identifying high-quality lacustrine source rocks and quantifying their contributions, and thereby constraining investigations into their formation mechanisms and the development of evaluation frameworks. Based on pyrolysis, micropaleontological, and molecular geochemical analyses of the Eocene semi-deep lacustrine source rocks from major hydrocarbon-rich sags in the northern South China Sea, three dominant algal assemblages are identified. The Eocene source rocks in Zhu Ⅱ and Zhu Ⅲ depression of the Pearl River Mouth Basin are dominated by acritarchs and green algae. Green algae are dominated by a single source of Pediastrum in Zhu Ⅲ depression, while both Pediastrum and Botryococcus in Zhu Ⅱ depression. In contrast, the Eocene source rocks in the Zhu Ⅰ depression of Pearl River Mouth Basin and the Beibuwan Basin exhibit more complex algal compositions. In addition to acritarchs, dinoflagellates generally contribute more than green algae. Controlled by the dominant algal assemblages, high-quality Eocene lacustrine source rocks in both the Beibuwan Basin and the Zhu Ⅰ depression of Pearl River Mouth Basin generally exhibit high abundances of C₃₀ 4-methylsteranes. By contrast, source rocks in Zhu Ⅲ depression are characterized by high abundances of C₃₀ tetracyclic polyprenoids (TPP) and are nearly devoid of C₃₀ 4-methylsteranes indicative of dinoflagellate input. In the Eocene source rocks of Zhu Ⅱ depression and their derived oils, neither C₃₀ 4-methylsteranes nor TPP are prominent. However, botryococcane is clearly identified in some samples. It is therefore proposed that integrated consideration of C₃₀ 4-methylsteranes, TPP, and botryococcane contents can enhance the accuracy of identifying high-quality source rocks and predicting their distribution. On this basis, relatively high-quality Eocene semi-deep lacustrine source rocks are inferred to be developed in the Baiyun sag of Zhu Ⅱ depression.

Key words: northern South China Sea, Eocene, hydrocarbon-generating organism, lacustrine source rock, biomarker