Role of zooclasts in the kerogen type and hydrocarbon potential of the lower Paleozoic Alum Shale

• Published in: International journal of coal geology Vol. 248; p. 103865
• Main Authors: Zheng, Xiaowei, Sanei, Hamed, Schovsbo, Niels H., Luo, Qingyong, Wu, Jia, Zhong, Ningning, Galloway, Jennifer M., Goodarzi, Fariborz
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-12-2021
• Subjects: Alum shale; Graptolite periderm; Hydrocarbon generation potential; Kerogen type; Lower Paleozoic; Maceral; Solid bitumen; Kerogen type; Lower Paleozoic; Graptolite periderm; Hydrocarbon generation potential; Alum shale; Solid bitumen; Maceral
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/j.coal.2021.103865

This study investigates the role of zooclasts in the bulk organic matter composition, kerogen type, and hydrocarbon generation potential of the lower Paleozoic marine Alum shale in Baltoscandia, northwestern Europe. The results show that graptolite periderm is composed of non-granular cuticles and granular filling. The graptolite cuticle is non-fluorescing and has a well-polishing measurable surface, while the graptolite granular fraction is semi-translucent and has brownish fluorescence, suggesting remaining hydrocarbon generation potential. The graptolite granular fraction shows similar optical characteristics to fluorescing diagenetic solid bitumen. Both the graptolite granular fraction and diagenetic solid bitumen have low intensity yellow to brown fluorescence and contribute to generative potential (Rock-Eval S2). The sum of semi-quantitative zooclastic cuticle (non-granular graptolite cuticle, chitinozoan cuticle, and vitrinite-like) and diagenetic solid bitumen content, obtained from maceral point-counting, correlates with the non-generative organic carbon content. Enrichment of zooclastic cuticle in the samples is displayed as enrichment of inert organic carbon, and hence bulk geochemical kerogen of type III. This refractory carbon-rich maceral enrichment results represents autochthonous ‘refractory organic carbon dilution’ and will lead to Hydrocarbon Index (HI) underestimation. Confined hydrous pyrolysis was used to compare the hydrocarbon generation process of a zooclastic cuticle-lean sample (0.2 vol%) and a zooclastic cuticle-rich (1.6 vol%) sample. Results indicate that there are four stages of hydrocarbon generation in the artificial maturation of unconventional source rock of the Alum Shale. The zooclastic cuticle-lean sample has higher hydrocarbon generation potential and more oil-prone than the zooclastic cuticle-rich sample, which is gas-prone and generates more CO2. This difference in hydrocarbon generation is attributed to differences in the organic constituent composition.
•The influx of the primarily non-generative organic carbon in graptolite cuticle in the lower Paleozoic Alum shale led to the dilution of the autochthonous refractory carbon in marine kerogen.•The dilution of the autochthonous refractory carbon in marine zooclast-rich sedimentary rocks lead to kerogen type Ⅲ with lower HI than expected for a marine type kerogen.•The granular graptolites have variable fluorescence and show oil generation potential.