Yu Qi
Organic matter provenance and depositional environment of marine-to-continental mudstones and coals in eastern Ordos Basin, China—Evidence from molecular geochemistry and petrology
Qi, Yu; Ju, Yiwen; Tan, Jingqiang; Bowen, Leon; Cai, Chunfang; Yu, Kun; Zhu, Hongjian; Huang, Cheng; Zhang, Weilai
Authors
Yiwen Ju
Jingqiang Tan
Leon Bowen leon.bowen@durham.ac.uk
Senior Manager (Electron Microscopy)
Chunfang Cai
Kun Yu
Hongjian Zhu
Cheng Huang
Weilai Zhang
Abstract
Cyclothems, composed of interbedded mudstone, coal and sandstone layers, make up the Taiyuan and Shanxi Formations in the Late Carboniferous to Early Permian in North China under a marine-to-continental depositional environment. The cyclothems act as important fossil energy hosts, such as coalbeds, hydrocarbon source rocks and unconventional natural gas reservoirs. Organic geochemistry and petrology of mudstones and coals in the Taiyuan and Shanxi Formations in the eastern Ordos Basin were studied to reveal the organic matter sources and paleoenvironments. Total organic carbon (TOC) contents vary from 1.1 wt% (mudstone) to 72.6 wt% (coal). The samples are mainly within the oil window, with the Tmax values ranging from 433 to 469 °C. Organic petrology and source biomarkers indicate that the mudstones were sourced from a mixed organic matter input, and terrigenous organic matter predominates over aquatic organic matter. The coals are mostly sourced by terrigenous organic matter inputs. High concentrations of hopanes argue for a strong bacterial input. Some m/z 217 mass chromatograms have peaks at the hopanes' retention times as a result of high hopane to sterane ratios. These hopane-derived peaks do not interfere the identification of the steranes because the hopanes and the steranes have different retention times. Maturity-dependent biomarkers demonstrate that the samples have been thermally mature, which agree with the Tmax values. Anomalously low C29 20S/(20S + 20R) and C29 ββ/(ββ + αα) sterane ratios are present in all the samples, and are interpreted as due to the terrigenous organic matter input or the coal-related depositional environment. In addition, biomarkers and iron sulfide morphology indicate that the organic matter of the mudstones deposited in a proximal setting with shallow, brackish/fresh water bodies. With consideration of preservation of organic matter, the redox conditions are dysoxic. Redox oscillations resulted in the records of oxic conditions in some samples. Finally, the coals and the mudstones mainly generate gas and have poor oil generative potential.
Citation
Qi, Y., Ju, Y., Tan, J., Bowen, L., Cai, C., Yu, K., Zhu, H., Huang, C., & Zhang, W. (2020). Organic matter provenance and depositional environment of marine-to-continental mudstones and coals in eastern Ordos Basin, China—Evidence from molecular geochemistry and petrology. International Journal of Coal Geology, 217, Article 103345. https://doi.org/10.1016/j.coal.2019.103345
Journal Article Type | Article |
---|---|
Acceptance Date | Nov 12, 2019 |
Online Publication Date | Nov 16, 2019 |
Publication Date | Jan 2, 2020 |
Deposit Date | Nov 20, 2019 |
Publicly Available Date | Nov 16, 2020 |
Journal | International Journal of Coal Geology |
Print ISSN | 0166-5162 |
Publisher | Elsevier |
Peer Reviewed | Peer Reviewed |
Volume | 217 |
Article Number | 103345 |
DOI | https://doi.org/10.1016/j.coal.2019.103345 |
Public URL | https://durham-repository.worktribe.com/output/1276916 |
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Copyright Statement
© 2019 This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
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