珠江口盆地深水区和浅水区古近系文昌组烃源岩地球化学特征对比

A comparison of geochemical features of source rocks of Eocene Wenchang Formation in the deep and shallow water zones of Pearl River Mouth Basin, SE China

袁丽平 蒋文敏 李芸 张琳 王建丰 王伟 熊永强

YUAN Liping JIANG Wenmin LI Yun ZHANG Lin WANG Jianfeng WANG Wei XIONG Yongqiang

中国科学院 广州地球化学研究所 有机地球化学国家重点实验室, 广州 510640 中国科学院 深地科学卓越创新中心, 广州 510640 中国科学院大学, 北京 100049

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, China CAS Center for Excellence in Deep Earth Science, Guangzhou, Guangdong 510640, China University of Chinese Academy of Sciences, Beijing 100049, China

珠江口盆地油气资源丰富，是我国海上油气勘探的热点区。古近系文昌组是盆内重要的烃源岩层之一，但对其地球化学特征研究还不够透彻，尤其是白云深水区文昌组烃源岩，因其埋藏深，岩心样品缺乏，目前研究大多基于易受钻井泥浆污染的岩屑样品，因而导致对其地球化学特征认识有限。因此，采用催化加氢热解技术从珠江口盆地深水区(主要来自珠二坳陷)和浅水区(主要来自珠一坳陷)文昌组的烃源岩干酪根结构中释放出键合烃，应用键合烃中生物标志化合物和单体烃碳同位素组成进行了精细的地球化学特征对比分析。研究结果表明，珠江口盆地文昌组烃源岩可划分为3种类型，包括中深湖相(WC-Ⅰ)、浅湖相(WC-Ⅱ)和藻类勃发的中深湖相(WC-Ⅲ)。中深湖相烃源岩(WC-Ⅰ)生物标志化合物组成具有低C304-甲基甾烷(4-Me/C29 < 0.2)和双杜松烷含量，较轻的正构烷烃单体烃碳同位素组成(-33‰~-31‰)特征；浅湖相烃源岩(WC-Ⅱ)生物标志化合物特征表现为极低C304-甲基甾烷含量(4-Me/C29均值为0.06)，低至高丰度的双杜松烷(T/C30H均值为1.04)，较重的正构烷烃单体烃碳同位素组成(-30‰~-27‰)特征；藻类勃发的中深湖相烃源岩(WC-Ⅲ)具有高C304-甲基甾烷含量(4-Me/C29=0.66)，重的正构烷烃单体烃碳同位素组成(-25‰~-23‰)特征。其中深水区和浅水区文昌组烃源岩均发育中深湖相(WC-Ⅰ)和浅湖相(WC-Ⅱ)，除此之外，浅水区文昌组还发育藻类勃发的中深湖相烃源岩(WC-Ⅲ)，而深水区尚未发现该类型烃源岩。通过对比珠江口盆地不同区域文昌组烃源岩键合烃的地球化学特征，发现浅水区文昌组中深湖相烃源岩可分为两种类型，为正确认识珠江口盆地不同区域文昌组烃源岩的地球化学特征提供科学依据，有助于对该区域进行油源对比。

The Pearl River Mouth Basin (PRMB) is one of the important offshore targets for oil and gas exploration in China and has great potential of oil and gas resources. The Eocene Wenchang Formation is one of the potential source rocks in the PRMB, but its geochemical characteristics are insufficiently studied, particularly for the Wenchang Formation source rocks in deep-water area of the PRMB. Due to the deep burial condition, core samples can rarely be obtained, and most samples are drilling cuttings which are easily to be contaminated by drilling mud, which makes it difficult to directly access the geochemical characteristics of Wenchang Formation source rocks. This paper aims to compare the geochemistry of the Wenchang Formation source rocks in the deep-water and shallow-water areas of the PRMB with an improved catalytic hydropyrolysis method to release the bound hydrocarbons from kerogen macromolecules. Based on the biomarkers and carbon isotopic compositions of individual n-alkanes released, the geochemical characteristics of Wenchang Formation source rocks in deep-water (mainly from Zhu Ⅱ Depression) and shallow water areas (mainly from Zhu Ⅰ Depression) from PRMB are compared. Results indicate that the source rocks of Wenchang Formation in PRMB can be classified into three types including semi-deep to deep lacustrine (WC-Ⅰ), shallow lacustrine (WC-Ⅱ), and semi-deep to deep lacustrine with algal blooming (WC-Ⅲ). The WC-Ⅰ are characterized by low C30 4-methylsteranes (4-Me/C29 < 0.2) and bicadinanes contents, and relatively light carbon isotopic composition of individual n-alkanes (-33‰ to -31‰). The WC-Ⅱ are characterized by low 4-Me/C29 ratios (average=0.06), low to high T/C30H ratios (average=1.04), and relatively heavier carbon isotope composition of individual n-alkanes (-30‰ to -27‰). The WC-Ⅲ are characterized by high content of C30 4-methylsteranes (4-Me/C29=0.66) and heavy carbon isotope compositions of individual n-alkanes (-25‰ to -23‰). The Wenchang Formation source rocks in deep-water and shallow-water areas have semi-deep to deep lacustrine facies (WC-Ⅰ) and shallow lacustrine facies (WC-Ⅱ). In addition, the semi-deep to deep lacustrine source rocks with algal blooming (WC-Ⅲ) also developed in shallow-water areas, but no such type of source rocks were found in deep-water area. This study compared the geochemical characteristics of bound hydrocarbons in source rocks of Wenchang Formation in different regions of PRMB, and found that the semi-deep to deep lacustrine source rocks of Wenchang Formation in shallow-water area can be divided into two types. It provides a scientific basis for a better understanding of the geochemical features of source rocks in different areas of Wenchang Formation in PRMB, and is helpful for oil-source correlations in this area.

广东省杰出青年基金 2018B030306006;广东省自然科学基金面上项目 2019A1515011913

https://doi.org/10.11781/sysydz202205866