Laplace Inversion for Obtaining Relaxation-Chemical Shift Correlation

Kerogen is the most abundant organic matter that is dispersed in the earth’s formation and is the source of fossil fuels such as oil and gas. Large amounts of kerogen exist in the form of oil shale which is not favorable for extraction.

The largest accumulation of oil shale is located in the Piceance Basin, Colorado. The so called ‘Green River oil shale’ was deposited in a lacustrine environment and contains type-1 kerogen as an organic resource. Even though the depositional environment is known there are details of the molecular structure of type-1 kerogen which still have to be identified.

We used 13C and 15N high resolution solid state benchtop NMR spectroscopy as one of the primary methods to elucidate the structure of kerogen through measurements of chemical shift, spin-lattice relaxation time, variable contact time, and dipolar dephasing rates. Using single or double exponential fitting methods, one can extract structural information from the variation of peak intensity with mixing times used in CP/MAS or dipolar dephasing experiments.

Here we introduce a new data processing method that uses a Laplace inversion algorithm to process a set of CP/MAS NMR analyzer data with different mixing times to generate 2D NMR spectrum that gives a chemical shift in one dimension and relaxation time in the second dimension.

The relaxation-chemical shift 2DNMR can be used to determine 13C (or 15N) chemical shift, proton spin-lattice relaxation time, cross polarization time, and dipolar dephasing time constant as well as their distributions for mixture samples such as oil shale. It also improves the accuracy of deriving structural parameters of macromolecules using both 13C or 15N chemical shift and relaxation cutoffs.