ABSTRACT: Determination of Rank and Kerogen Type by High Resolution NMR Spectroscopy
Warren W. Dickinson, R. H. Newman, J. D. Collen
Nuclear magnetic resonance (NMR) spectroscopy is a nondestructive technique for measuring the chemical and structural properties of organic matter. Although used in organic geochemistry for the past 14 years, the technique has continually undergone refinement. Initially, only the aromatic and aliphatic signal areas of the carbon NMR spectrum could be measured. Carboxylic groups as well as oxygen-substituted groups on aromatic and aliphatic carbon can now be qualitatively measured.
We have examined coal and shale samples of various ranks from the Williston and San Juan basins, USA, and the Taranaki and Great South basins, New Zealand. Kerogen type can be distinguished on a plot of aromaticity versus the methylene to methyl ratio. For type III kerogens, vitrinite reflectance correlates very well with the percent of oxygen substitution on aromatic carbon and aromaticity. These parameters are excellent indicators of rank because they reflect the decrease in oxygen content and the increase in aromatic carbon as organic matter matures.
Although the initial cost of NMR equipment is high, the vast amount of chemical information on kerogen that may be obtained from a small sample and very little laboratory preparation make it a valuable tool for petroleum geochemistry.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990