Chemical and structural characterization of thermally simulated kerogen and its relationship with microporosity

Kerogen was isolated from the Maoming shale at different temperatures to understand changes in chemical, structural and porosity characteristics during artificial maturation. Advanced solid-state 13C nuclear magnetic resonance (NMR) techniques were employed along with Fourier transform infrared (FT-IR), Raman spectroscopy (RS), stable carbon isotopes, and Rock-Eval pyrolysis to characterize the organic matter (OM), whereas the microporosity and surface properties were elucidated using N2 and CO2 adsorption techniques. As the temperature increased, the aliphatic and carbonyl carbons of the kerogen samples showed remarkable decreases in abundance together with an increase of non-protonated and protonated aromatic carbons. Moreover, the kerogen became more enriched in 13C and had a higher degree of crystallization with increasing Ro and Tmax. The aromaticity ranged from 46.9% to 94.7% and the minimum aromatic cluster size varied from 10 carbons at 350 °C to 38 carbons at 500 °C, which was significantly related to the microporosity (Vo, d-co2) of the kerogen samples and their nonlinear sorption of phenanthrene and benzene. The microporosity was extensively affected by the loss of aliphatic carbon and the increase of aromatic fused carbon. When the temperature reached 500 °C, the collapse of aromatic interlayer remarkably reduced the micropore volume in the kerogen, and then resulted in the decreasing adsorption of hydrophobic organic chemicals (HOCs). The above observation confirms the molecular sieve effect in the kerogen samples. In addition, the micro-filling adsorption for HOCs is dominant on the thermally simulated kerogen samples.