Chemical and nanometer-scale structure of kerogen and its change during thermal maturation investigated by advanced solid-state C NMR spectroscopy

We have used advanced and quantitative solid-state nuclear magnetic resonance (NMR) techniques to investigate structural changes in a series of type II kerogen samples from the New Albany Shale across a range of maturity (vitrinite reflectance R₀ from 0.29% to 1.27%). Specific functional groups such as CH₃, CH₂, alkyl CH, aromatic CH, aromatic C-O, and other nonprotonated aromatics, as well as “oil prone” and “gas prone” carbons, have been quantified by ¹³C NMR; atomic H/C and O/C ratios calculated from the NMR data agree with elemental analysis. Relationships between NMR structural parameters and vitrinite reflectance, a proxy for thermal maturity, were evaluated. The aromatic cluster size is probed in terms of the fraction of aromatic carbons that are protonated (∼30%) and the average distance of aromatic C from the nearest protons in long-range H-C dephasing, both of which do not increase much with maturation, in spite of a great increase in aromaticity. The aromatic clusters in the most mature sample consist of ∼30 carbons, and of ∼20 carbons in the least mature samples. Proof of many links between alkyl chains and aromatic rings is provided by short-range and long-range ¹H-¹³C correlation NMR. The alkyl segments provide most H in the samples; even at a carbon aromaticity of 83%, the fraction of aromatic H is only 38%. While aromaticity increases with thermal maturity, most other NMR structural parameters, including the aromatic C-O fractions, decrease. Aromaticity is confirmed as an excellent NMR structural parameter for assessing thermal maturity. In this series of samples, thermal maturation mostly increases aromaticity by reducing the length of the alkyl chains attached to the aromatic cores, not by pronounced growth of the size of the fused aromatic ring clusters.