Evaluation of Structural Characteristics of Huadian Oil Shale Kerogen Using Direct Techniques (Solid-State 13C NMR, XPS, FT-IR, and XRD)

A sample of kerogen isolated from Huadian oil shale was studied using a combination of solid-state 13C NMR, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR), and X-ray diffraction (XRD) techniques to evaluate its structural characteristics. 13C NMR results indicate that the carbon skeletal structure of this kerogen is mainly composed of a fairly high fraction of aliphatic carbon (86.1%), with a very low aromaticity (fa) of 9.7%; methylene (CH2) carbons dominate in all types of aliphatic carbons and the majority of them exist as many long straight chains but not saturated alicyclics. The average methylene carbon chain length (Cn) is between 12 and 24. There are only one fused aromatic ring (e.g., naphthalene) or two single aromatic rings (one benzene ring and one penta-heterocycle) per 100 carbon atoms. However, aromatic rings in this kerogen have a very high value of substitutive degree (δ = 0.42–0.75). Furthermore, XRD analysis suggests that most methylene straight chains and aromatic carbons can not form crystalline but amorphous structure and are linked to each other by various bridge bonds and methylene (CH2) chains. FT-IR, XPS, and 13C NMR results show that organic oxygen in the kerogen exists as mainly three types of oxygen functional groups. Both XPS and 13C NMR results agree on the same ordering of their respective contents: C–O and C–OH groups are dominant, followed by O═C–O, and C═O or O–C–O groups. The 13C NMR results further suggest that more oxygen of C–O and C–OH groups is bound to aromatic carbons. XPS shows that over half the total amount of organic nitrogen in Huadian kerogen exists as aromatic heterocycles, which concludes pyrrolic nitrogen richest in total organic nitrogen, pyridinic, and protonated-pyridinic forms. A relatively high content of amino nitrogen over 30 mol % also is present in this kerogen, which is much higher than that of other same type kerogens. Organic sulfur is distributed in this kerogen as aromatic and aliphatic sulfur, sulfone, and sulfoxide in the order of the relative mole fraction.