Phytoplankton as main organism in the Eocene organic-rich turbidites of Jiyang Depression, China: Implication for organic matter accumulation mechanism

ABSTRACT

Persistent anoxia and still water condition is traditionally regarded as the prerequisite to deposit organic-rich sediments. Recent studies found that turbidity current can also carry and deposit terrigenous organic matter, forming organic-rich turbidites. However, it is still unclear whether the organic-rich property of turbidites can form by depositing phytoplankton. Here we found organic-rich turbidites (avg. 2.86 wt% in total organic carbon) in the Eocene Jiyang Depression, China, with the organic matter dominated by phytoplankton. Investigations show turbidity current and algae blooms coupled with each other, both of which induced by warm-humid climate. The turbidity current involved the living phytoplankton during motion, and then phytoplankton deposited along with turbiditic minerals. Meanwhile, the high depositional rate of turbidites prevents organic matter degradation by bottom-water oxidation. Organic-rich turbidites, phytoplankton as dominating organisms, consequently form in the lake basin, acting as a new mechanism for organic matter accumulation. This mechanism shows that turbidites can also act as source rock, as phytoplankton is ideal parent material for hydrocarbon.

KEYWORDS:

• Organic matter
• Turbidites
• Algae blooms
• Lake
• Jiyang Depression

Acknowledgments

The study was co-funded by the China National Key Research Project (2017ZX05009-002) National Natural Science Foundation of China (41772090, 41802130) . We are grateful to the Geological Research Institute of Shengli Oil Company for permission to access to the in-house database.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [2017ZX05009-002];Foundation from Shandong Key Laboratory of Depositional Mineralization & Sedimentary Mineral, Shandong University of Science and Technology [DMSM20190024];China National Key Research Project [2017ZX05009-002];

Notes on contributors

Jianguo Zhang

Jianguo Zhang is an assistant professor at China University of Geosciences (Beijing). He received the Ph.D from China University of Geosciences (Beijing) in 2017. His research focuses on shale sedimentology, sequence stratigraphy, shale diagenesis, and shale reservoir quality prediction

Zaixing Jiang

Zaixing Jiang is a professor of sedimentary geology at China University of Geosciences (Beijing). He received the Ph.D. from the Graduate School of Chinese Academy of Sciences in 1997. He is currently leading a group working on reservoir sedimentology and sequence stratigraphy.

Siqi Wang

Siqi Wang is a Ph.D student at China University of Geosciences, Beijing 100083, China. Siqi, once an engineer in Schlumberger, was responsible for well log analysis of shale gas. Her research focuses on deposition and diagenesis of shale.

Xiangxin Kong

Xiangxin Kong is a Ph.D student at China University of Geosciences, Beijing, 100083, China. He is involved is shale reservoir prediction.