Combined High-Resolution Solid-State 1H/13C NMR Spectroscopy and 1H NMR Relaxometry for the Characterization of Kerogen Thermal Maturation

Abstract

A key factor for the petroleum potential of source rock is the degree of chemical and physical structure evolution of its kerogen fraction through a range of maturation processes. In this study, various high-field, solid-state NMR methods have been applied to a series of kerogen isolates (type I) over a defined maturity range (vitrinite reflectance R₀ from 0.98 to 1.86%). Results obtained from ¹³C MAS NMR show that the sp²/sp³-hybridized carbon ratio of kerogen, here defined as the aromatic/aliphatic ratio, increases with increasing maturity. ¹H MAS NMR spectra contain partly overlapping aliphatic and aromatic resonances with distinct transverse relaxation behavior. In Hahn-echo experiments, the aromatic signal decays more slowly than the aliphatic signal, indicating that for these systems, transverse ¹H relaxation is rather controlled by local distances between hydrogen atoms than by molecular mobility. Similar relaxation differences are also found in static (nonspinning) ¹H Hahn-echo NMR experiments, here used to discriminate between phases with different proton mobilities and/or densities in the kerogen samples and, ultimately, between aromatic and aliphatic fractions. The distributions of the static transverse relaxation time (T₂), extracted from the Hahn-echo decays, are characterized by a short-T₂ peak (∼10 μs) and a long-T₂ peak (∼100 μs). The ratio between these two peaks correlates well with the aliphatic-to-aromatic signal intensity ratios in MAS NMR spectra of the corresponding kerogen samples, suggesting that a net decrease in kerogen proton density—occurring during maturation—is also reflected by ¹H NMR relaxation. For the investigated kerogen isolates, the long-T₂ peak in the T₂ distribution can be considered an indicator of aromatic content, which can be directly detected by measuring ¹H T₂ relaxation.