High-resolution astronomical records of shale strata in faulted lake basins and implications for the sedimentary process of laminated sediments

Abstract

Lamina structure, a typical feature of shale, has significant implications for hydrocarbon generation, shale oil and gas reservoir evaluation, and palaeoenvironmental studies. In this study, we conducted a high-resolution astronomical analysis of shale strata from the Kongdian Formation in the Cangdong Sag, Bohai Bay Basin, China, and performed macroscopic and microscopic textural characterization of core samples. The time series analysis of the G108-8 Well indicates that stratigraphic cycles of 113.2–25.3 m, 12.7–7.8 m, 4.7–2.7 m, and 2.3–1.3 m are controlled by long eccentricity, short eccentricity, obliquity, and precession, respectively. The sedimentary accumulation rate (SAR) is estimated to be approximately 20.3 cm/kyr. The core description reveals that Ek₂ primarily consists of laminated shale with individual laminae less than 1 cm in thickness. Using a polarizing microscope, the average thickness of a single lamina is approximately 250 μm, with most laminae being less than 400 μm. We constructed a time-depth model for lacustrine laminated sediments and compared it with other ancient lacustrine strata and modern lakes. The sediment accumulation rate of ancient lacustrine strata ranges from 1.3 to 20.3 cm/kyr. The sedimentation rate of shale and the thickness distribution of individual laminae provide evidence for the annual nature of the lamina couplets. Finally, we propose a simplified model to illustrate the sedimentation process, emphasizing the record of laminated sediments in semi-deep to deep facies. Our results contribute to the understanding of lacustrine sedimentary processes, laminated sedimentary records, organic matter enrichment processes, palaeoenvironments, and their potential relationships.

Keywords

Cangdong Sag / Lacustrine laminated sediments / Astrochronological record / Sedimentation rate / Varves

CRediT authorship contribution statement

Xianzheng Zhao: Writing – review & editing, Writing – original draft, Methodology, Investigation, Conceptualization. Xiaoping Liu: Writing – review & editing, Writing – original draft, Supervision, Project administration, Funding acquisition, Conceptualization. Huan Liu: Writing – review & editing, Writing – original draft, Software, Methodology, Investigation, Conceptualization. Fengming Jin: Writing – original draft, Project administration, Formal analysis, Data curation. Xiugang Pu: Writing – original draft, Formal analysis, Data curation. Biao Sun: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis. Zhannan Shi: Writing – original draft, Formal analysis, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This study is financially supported by the National Natural Science Foundation of China (Grant No. 42072150). We are grateful to the PetroChina Dagang Oilfield Company for providing basic data access.