Characterization of kerogen using solid-state nuclear magnetic resonance spectroscopy: A review

Heterogeneity, complexity, and insolubility of kerogen have presented a big challenge in its characterization. Among the various analytical methods applied to kerogen characterization, solid-state nuclear magnetic resonance (NMR) spectroscopy becomes increasingly the method of choice due to its capacity to provide comprehensive structural information nondestructively. This review first summarized the structural studies of oil shale and kerogen using solid-state NMR spectroscopy, including the applications in the comparative studies of kerogen and its source rocks (oil shale), evaluation of oil shale resource, determination of kerogen types, and assessment of thermal maturity. Then conventional solid-state NMR techniques used in the literature and limitations of 13C cross polarization/magic angle spinning (13C CP/MAS) technique were critically reviewed. Finally, advanced 13C NMR techniques were presented and their applications discussed. The conventional technique used for kerogen study was primarily 13C CP/MAS, which was sometimes combined with dipolar dephasing. The drawbacks of 13C CP/MAS technique include sidebands, baseline distortions, and poor quantification. The problems with deconvolution as an attempt to resolve overlapping peaks were also discussed. Possible solutions to these problems were provided. Recently, significant advances have been made in solid state NMR toward more structural details, as well as more reliable quantification. These advanced NMR techniques include spectral editing techniques, two dimensional 1H–13C heteronuclear correlation spectroscopy, 1H spin diffusion, etc. Their applications allow for identification and quantification of specific functional groups, detection of connectivities and proximities of specific functional groups, examinations of domains and heterogeneities as well as estimation of aromatic cluster size for systematic characterization of kerogen.