Correlation between chemical structures of kerogen and thermal maturity investigated by advanced solid-state NMR spectroscopy

The chemical structure of kerogen changes with increasing thermal maturity of source rocks. Proper evaluation of petroleum and gas generation potentials makes it important to understand the structural transformations of kerogen associated with thermal maturity. However, the chemical complexity and insolubility of kerogen have made it difficult to even qualitatively describe the chemical structure of kerogen during thermal maturation. In this study, we use new, advanced solid-state NMR techniques to investigate structural changes related to thermal maturity. The advanced ¹³C solid-state NMR techniques include quantitative direct polarization, cross polarization/total suppression of sidebands, ¹H-¹³C two-dimensional heteronuclear correlation NMR (2D HETCOR), ¹³C chemical shift anisotropy filtering, CH and CH₂ selection techniques for identifying specific functional groups and identification of heterogeneity in kerogen. A series of Type II kerogens with a wide maturity range from the New Albany Shale were investigated. Results show that with increasing thermal maturity, carbonyls, aromatic C-O groups, and alkyls decrease in relative abundance while aromatics increase. ¹H-¹³C 2D HETCOR spectra clearly show the weakening of connectivtity between alkyls and aromatictics with the increase of thermal maturity. In most mature kerogen, spin diffusion experiments indicate that around a mixing time of 20 ms, the magnetization of aromatics and alkyls starts to equilibrate.