Chemical Structures of Kerogen and Their Changes During Thermal Evolution Investigated by Advanced Solid-state NMR Spectroscopy

Kerogen is the insoluble fraction of organic matter in rocks and is the parent material for petroleum. Chemical structural characteristics of kerogen constrain the ability of source rocks to generate bitumen, oil, and natural gas. It is important to understand changes in kerogen structure with increasing thermal maturity to evaluate the organic-geochemical mechanisms of hydrocarbon generation. However, kerogen's complex and cross-linked geomacromolecules cannot yet be characterized fully on a molecular level. In this study, we use advanced solid- state NMR techniques to investigate structural changes of a series of Type II kerogens from the New Albany Shale across a range of maturity. Functional groups are identified and quantified by quantitative 13C NMR using direct polarization, separation of aromatic and alkyl resonances by 13C chemical shift anisotropy filtering, and CH and CH2 signal selection. 1H-13C two-dimensional heteronuclear correlation (2D HETCOR) NMR provides information on the proton fractions and nanometer-scale heterogeneity. Specific functional groups such as aliphatic CH3, aliphatic CH2, aliphatic CH, nonprotonatic aromatics, and protonated aromatics are identified and quantified in kerogen. Aromatics connected to alkyls are detected specifically in 2D HETCOR spectra with gated decoupling. In kerogen 554-1, spin diffusion equilibrates the magnetization of its aromatic and aliphatic protons within ~20 msec, proving proximity on the nanometer scale. 2D HETCOR with spin diffusion on kerogen IL3 reveals that there are more aliphatic than aromatic protons, even though the fraction of aliphatic carbons is 18%. With increasing thermal maturity, carbonyls, aromatic C-O groups, and alkyls decrease while aromatics increase in abundance and fewer connections between alkyls and aromatics are observed.