西湖凹陷K区烃源岩有机质碳同位素倒转及地质意义分析

Carbon Isotope Inversion of Organic Matter and Geological Significance Analysis in Source Rocks of K Area, Xihu Sag

孙鹏

SUN Peng

中海石油（中国）有限公司上海分公司研究院，上海　200030

Research Institute of Shanghai Branch of CNOOC (China) Limited, Shanghai 200030, China

东海盆地古近系烃源岩是该盆地的主力烃源岩，K区烃源岩的碳同位素分布呈现出普遍的倒转现象，但是目前对这一现象尚没有进行系统分析和解释。该文利用碳同位素分析的方法对K区烃源岩的碳同位素进行分层、分区研究，并对发生倒转现象的原因进行综述，对碳同位素倒转所表明的地质意义进行阐述。研究结果表明，K区烃源岩碳同位素的分布以局部倒转为主，从宝石组到平上段，碳同位素的分布表现出总体正常→芳香烃倒转→非烃倒转→饱和烃-芳香烃双倒转的特征；碳同位素的倒转分布指示了该区总体处于海陆过渡的环境，有机质来源于经细菌强烈降解的高等植物和低等水生生物的混合贡献，并且从斜坡高带到西次凹，沉积有机质中高等植物的贡献逐渐减小，而水生生物的贡献则逐渐增加。该文的成果对邻区或相似地区有机质来源和沉积环境的研究具有一定的借鉴意义。

As the main source rocks in East China Sea Basin, the Paleogene source rocks of K area shows a general inversion phenomenon, which has not been paid attention to and explained at present. In this paper, the stratification and zoning of carbon isotopes in the source rocks of K area are studied by means of carbon isotope analysis, and the causes of inversion are summarized. The geological significance of carbon isotope inversion is described. The results show that the distribution of carbon isotopes in the source rocks in K area is dominated by local inversion. From Baoshi Formation to the upper section of Pinghu Formation, the distribution of carbon isotopes shows the characteristics of global normal → aromatic inversion → non-hydrocarbon inversion → saturated hydrocarbon and aromatic hydrocarbon double inversion. The inversion distribution of carbon isotopes indicated that this area was generally located in a transitional environment between sea and land, and the organic matter came from the mixed contribution of higher plants and lower aquatic organisms strongly degraded by bacteria. From the slope high belt to the western subdepression, the contribution of higher plants in sedimentary organic matter gradually decreased, while that of aquatic organisms gradually increased. The results of this paper have certain reference significance for the research of organic matter sources and sedimentary environment in adjacent or similar areas.

“十四五”重大科技项目之项目4“海上深层/超深层油气勘探技术”课题2“弧后盆地深层/超深层油气成藏条件与成藏机制研究-西湖凹陷”（KJGG2022-0402）。

https://doi.org/10.3969/j.issn.1008-2336.2023.03.008