Direct measurement of the aliphatic and aromatic carbon structures of the total organic matter in whole rocks is provided by NMR techniques known as cross polarization with magic-angle spinning. The proportion of organic carbon present in aliphatic structures correlates with the oil-generating potential of a source rock. Alteration during catagenesis can also be evaluated. Carbonaceous shales from the Retort and Meade Peak phosphatic shale members of the Permian Phosphoria Formation. Retort shales have a high potential to generate oil, but the Meade Peak shales, having already undergone thermal alteration, have a low potential to generate additional oil.--Modified journal abstract.