Assessment of Thermal Evolution of Kerogen Geopolymers with Their Structural Parameters Measured by Solid-State ¹³C NMR Spectroscopy

Abstract

The sidebands in the solid-state 13C NMR spectra of 13 polycyclic aromatic hydrocarbon compounds associated with kerogen structure were suppressed with cross polarization (CP), magic-angle spinning (MAS), and total sideband suppression (TOSS). The chemical shift values of these model compounds were obtained under various chemical circumstances, which were subsequently used to determine the chemical shifts of aliphatic and aromatic carbons in kerogen structure via CP/MAS/TOSS 13C NMR measurements. Dipolar dephasing (DD) was used to obtain the spectra of nonprotonated carbon, discriminating protonated and nonprotonated carbons in the aromatic cluster. Consequently, the structural parameters for different carbons were characterized. High-resolution solid-state 13C NMR measurements were conducted on a suite of kerogen macromolecules isolated from source rocks originated from the Kuqa depression, Awati River, and Kapusaliang River of the Tarim Basin in northwestern China to develop an NMR-associated method for monitoring the thermal alteration of kerogens. These rocks covered a broad maturity range, as indicated by the Ro values of 0.52%−1.81%. Our results suggest that the aromaticity (fa) and aromatic cluster size (χb) are effective parameters for assessment of the thermal evolution of kerogens that resulted from the geological heating process undergone in the ancient sediments.