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A combination of solid-state 13C NMR, X-ray photoelectron spectroscopy (XPS) and sulfur X-ray absorption near edge structure (S-XANES) techniques are used to characterize organic oxygen, nitrogen, and sulfur species and carbon chemical/structural features in kerogens. The kerogens studied represent a wide range of organic matter types and maturities. A van Krevelen plot based on elemental H/C data and XPS derived O/C data shows the well established pattern for type I, type II, and type III kerogens. The anticipated relationship between the Rock−Eval hydrogen index and H/C is independent of organic matter type. Carbon structural and lattice parameters are derived from solid-state 13C NMR analysis. As expected, the amount of aromatic carbon, measured by both 13C NMR and XPS, increases with decreasing H/C. The correlation between aromatic carbon and Rock−Eval T max, an indicator of maturity, is linear for types II and IIIC kerogens, but each organic matter type follows a different relationship. The average aliphatic carbon chain length (Cn‘) decreases with an increasing amount of aromatic carbon in a similar manner across all organic matter types. The fraction of aromatic carbons with attachments (FAA) decreases, while the average number of aromatic carbons per cluster (C) increases with an increasing amount of aromatic carbon. FAA values range from 0.2 to 0.4, and C values range from 12 to 20 indicating that kerogens possess on average 2- to 5-ring aromatic carbon units that are highly substituted. There is basic agreement between XPS and 13C NMR results for the amount and speciation of organic oxygen. XPS results show that the amount of carbon oxygen single bonded species increases and carbonyl−carboxyl species decrease with an increasing amount of aromatic carbon. Patterns for the relative abundances of nitrogen and sulfur species exist regardless of the large differences in the total amount of organic nitrogen and sulfur seen in the kerogens. XPS and S-XANES results indicate that the relative level of aromatic sulfur increases with an increasing amount of aromatic carbon for all kerogens. XPS show that the majority of nitrogen exists as pyrrolic forms in comparable relative abundances in all kerogens studied. The direct characterization results using X-ray and NMR methods for nitrogen, sulfur, oxygen, and carbon chemical structures provide a basis for developing both specific and general average chemical structural models for different organic matter type kerogens.

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This paper presents data on the 15N chemical shift tensor principal values in a series of 15N-enriched heterocycles. Compounds that are liquids at room temperature were frozen, and the principal values of all compounds studied were measured from static powder patterns. Four different types of nitrogen tensors are described, consisting of protonated and nonprotonated nitrogens in both five- and six-membered rings. The principal values were oriented on the molecular frame using the DFT quantum mechanical calculations of the 15N chemical shielding tensors. The agreement between the calculated and experimental principal values is adequate to make these assignments, but the relative scatters are greater than those observed in similar 13C chemical shift calculations. The largest shift component, δ11, is always oriented in the radial direction to the ring for substituted nitrogens but is tangential to the ring for the nonsubstituted nitrogens. The large variations observed in the nitrogen chemical shift tensors upon changing the nitrogen hybridization can be explained using qualitative arguments on the localization of the smallest bonding-antibonding or HOMO−LUMO gap in the molecule. The orientation of the largest shift component is always found in the plane of the molecule and is approximately perpendicular to the plane containing the bonding−antibonding or HOMO−LUMO pair of orbitals with the smallest energy gap.

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