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46373-G2
New Insights into Molecular Structural Changes of Kerogen During Thermal Evolution Investigated by Advanced Solid-State NMR Spectroscopy
Jingdong Mao, Old Dominion University
In the past year, we have used advanced solid-state nuclear magnetic resonance (NMR) techniques to investigate structural changes in a series of type II kerogens. Type II kerogens in this study represent source rocks from the New Albany Shale within the Illinois Basin. Samples were selected to encompass a wide range of maturity, from the least mature samples collected at the eastern border of the Illinois Basin (Indiana) towards higher maturity in the center of the Illinois Basin (Illinois). Six samples: 472-1, 634-1, 548-1, 554-1, IL-6 and IL-3, were selected and their Ro equaled 0.29, 0.50, 0.63, 0.63, 0.94, and 1.27%, respectively. Variations among these samples are predominantly due to different maturities, whereas facies variability is negligible. Advanced solid-state NMR techniques included spectral-editing techniques such as dipolar dephasing, 13C chemical-shift-anisotropy filter, CH and CH2 selection, two-dimensional 1H-13C correlation NMR (2D HETCOR), 1H-13C recoupled long-range dipolar dephasing, and 2D HETCOR with spin diffusion. Specific functional groups such as CH3, CH2, alkyl CH, aromatic CH, aromatic C-O, and other nonprotonated aromatics have been identified and quantified. 1H-13C two-dimensional heteronuclear correlation (2D HETCOR) NMR provides information on the proton fractions, connectivities and nanometer-scale heterogeneity. Strong signals of alkyl-connected (nonprotonated) aromatics are detected specifically in 2D HETCOR spectra with gated decoupling. In kerogen 554-1, 1H spin diffusion equilibrates the magnetization within ~20 ms, proving the nanometer proximity of aromatic and alkyl structures. 2D HETCOR NMR with equilibration by 1H spin diffusion reveals that there is more 1H in alkyl than in aromatic structures of kerogen IL-3, even though only 18% of all carbons are in alkyl structures; this is consistent with the CHn quantification from 13C NMR. Dipolar dephasing confirms that only 20% of the 82% aromatic carbon atoms are protonated. 1H-13C long-range dipolar dephasing technique was used to probe the fused-ring carbons in kerogens and their changes with thermal maturity. It is shown that thermal maturation increases aromaticity by reducing the length of the alkyl chains attached to the aromatic cores, not by growing the size of the fused aromatic ring clusters. The cluster size is probed in terms of the fraction of aromatic carbons that are protonated and the average distance of aromatic C from the nearest protons in long-range C-H dephasing, both of which do not increase with maturation, in spite of a great increase in aromaticity. The long-range recoupled dephasing shows that the aromatic rings do not form large polycondensed aromatic domains, with clusters consisting mostly of two or three fused rings. Evidence for the reduced length of alkyl chains is provided by increased contact between alkyl C and aromatic H in HETCOR NMR. Relationships between 13C-NMR structural parameters and vitrinite reflectance, a proxy for thermal maturity, were evaluated. With increasing thermal maturity, the aromaticity and the abundances of protonated and nonprotonated aromatic fractions as well as the ratio of aromatics to aliphatics increase while other NMR structural parameters, including the aromatic C-O fractions decrease. At the same time, higher maturity reduces the associations between aromatic and alkyl groups whereas alkyl groups experience branching and closer association with aromatic structures. Aromaticity is confirmed as an excellent NMR structural parameter for assessing thermal maturity.
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