AmericanChemicalSociety.com

In the past year, we have used advanced solid-state nuclear magnetic resonance (NMR) techniques to continue to investigate structural changes in a series of type II kerogens, extracted from source rocks from the New Albany Shale within the Illinois Basin. Our calculated atomic H/C and O/C ratios based on NMR agree with elemental analysis.

Relationships between NMR structural parameters and vitrinite reflectance, a proxy for thermal maturity, were evaluated. Aromaticity is confirmed as an excellent NMR structural parameter for assessing thermal maturity. It is shown that 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 growing the size of the fused aromatic ring clusters significantly. The 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. In addition, we start to investigate a series of highly volatile bituminous coals (type III kerogen) with a gradient of thermal maturity near dikes. In contrast to type II kerogens from the New Albany Shale, thermal maturation of coal samples increases aromaticity by growing the size of the fused aromatic ring clusters significantly.