Thermal maturation as revealed by micro-Raman spectroscopy of mineral-organic aggregation (MOA) in marine shales with hi
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ermal maturation as revealed by micro-Raman spectroscopy of mineral-organic aggregation (MOA) in marine shales with high and over maturities Xianming XIAO
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, Qin ZHOU , Peng CHENG , Jian SUN , Dehan LIU & Hui TIAN
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School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China; State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,
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Guangzhou 510640, China Received January 29, 2020; revised April 26, 2020; accepted May 8, 2020; published online July 21, 2020
Abstract Laser Raman spectroscopy of organic matter in coals and sedimentary rocks has been increasingly investigated to assess thermal maturity. Pure organic matter such as vitrinite and solid bitumen, as well as isolated kerogen are definitely selected for this purpose, but there has been no systematic report on the Raman spectral parameters directly based on mineral-organic aggregations (MOA) in shales. In this paper, the Raman spectral characteristics and parameters of MOA in two sets of marine shale samples from southern China with either different TOC contents (TOC=0.10–4.59%) or different maturities (BRo=1.71– 4.57%) were investigated. It was found that the Raman technique is very sensitive to the organic matter in MOA. As the TOC content of the shales reaches about 0.60%, perfect Raman spectra with D and G bands derived from organic matter in MOA were obtained, and the Raman parameters of MOA are in good agreement with those of its associated solid bitumen, and can indicate the thermal maturity just like those of solid bitumen. Using the Raman parameters of MOA is a potential alternative way to estimate maturity levels of high and over mature shales, especially those lower Paleozoic and Precambrian shales that lack macerals suitable for reflectance or Raman measurements. Keywords Citation:
Marine shale, Mineral-organic aggregation (MOA), Solid bitumen, Raman parameter, Maturity
Xiao X, Zhou Q, Cheng P, Sun J, Liu D, Tian H. 2020. Thermal maturation as revealed by micro-Raman spectroscopy of mineral-organic aggregation (MOA) in marine shales with high and over maturities. Science China Earth Sciences, 63, https://doi.org/10.1007/s11430-020-9627-2
1. Introduction There are two basic forms of solid organic matter (kerogen) in organic-rich shales under an ordinary optical microscope. One is the distinguishable component, such as vitrinite, alginite, exinite, inertinite, solid bitumen, graptolite and their fragments (Teichmüller, 1986; Suárez-Ruiz et al., 2012), and another is the undistinguishable organic matter, with a very small size (80–90% of TOC content) (Yang et al., 1996; Xiao et al., 2007; Zhang et al., 2008). Figure 3 shows some typical micrographs of the shale samples with different TOC contents. The identifiable macerals in these shales are mainly solid bitumen, graptolite and vitrinite-like maceral. The shales with greater TOC contents (e.g., sample M6, Figure 3f) contain several identifiable macerals, while solid bitumen and other
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