Investigation into the Effect of Heteroatom Content on Kerogen Structure Using Advanced ¹³C Solid-State Nuclear Magnetic Resonance Spectroscopy
2018
Chu, Wenying | Cao, Xiaoyan | Schmidt-Rohr, Klaus | Birdwell, Justin E. | Mao, Jingdong
To elucidate how different extreme heteroatom concentrations in oil shale kerogen may present and contribute to various structural features, three shale samples, containing kerogen with high oxygen content, low heteroatom content, and high sulfur content, were analyzed using advanced ¹³C solid-state nuclear magnetic resonance (NMR) techniques, including multiple cross-polarization/magic angle spinning (multiCP/MAS), dipolar dephasing (multiCP/DD), and 2D ¹H–¹³C heteronuclear correlation (2D HETCOR). We found that oxygen in Estonian kukersite was present mostly in aromatic C–O structures and that nonprotonated aromatic carbons bonded to oxygen and alkyl chains led to more diverse aromatic signal distributions and structures in the kukersite organic matter than were observed in the other shales. The low-heteroatom kerogen present in Australian Glen Davis torbanite had the simplest structural pattern and the lowest aromaticity, despite having a lower atomic H/C ratio than the kerogens present in the other shales. The organic sulfur-rich Ghareb marinite from Jordan contained the highest aromaticity and most diverse alkyl structures among the three shales. 2D HETCOR with ¹H spin diffusion showed that the structural heterogeneity of the Glen Davis kerogen was <1 nm, indicating the preservation of structures present in precursor organic matter. Like previous NMR studies of shales and kerogens, this analysis of organic matter in whole shale samples with unusual heteroatom content demonstrates that structural characteristics in organic matter are not necessarily captured by kerogen typing based solely on elemental ratios (H/C, O/C) or programmed pyrolysis parameters and that NMR provides deeper insights into kerogen structure.
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