Determination of molecular structure of kerogens using 13C NMR spectroscopy: I. The effects of variation in kerogen type
A "carbon type" analysis for kerogens and coals based on solid state 13C NMR is reported here. Standard CP/MAS NMR spectra and dipolar dephased spectra are acquired. Chemical shifts of appropriate carbon types, stored in a computer-based reference library, are fitted to the dipolar dephased spectrum using computer software. This step is then repeated on the difference between the standard CP/ MAS spectrum and the dipolar dephased spectrum. The method has been applied to a suite of eight immature kerogens and coals from sediments which vary from lacustrine, marine, to deltaic in origin, in order to investigate differences in molecular structure with variations in organic input. Distinct differences are observed in the distribution of oxygen-containing, aromatic and aliphatic carbon types depending on the type of organic matter contributing to the sediment (algal/bacterial or terrestrial) and the depositional setting. These differences are systematic and can be explained in the light of the chemical composition of the organic matter which originally contributed to the sediments.