Statistical determination of geochemical parameters of coal and kerogen macerals from transmission micro-infrared spectroscopy data

A set of samples including type I and type II kerogens and coals (type III), as well as two series of artificially oxidized and matured coals were analyzed by Fourier transform infrared spectroscopy (FTIR). Major infrared bands (aliphatic and aromatic CH, C=O and C=C) were integrated in order to draw correlations with geochemical parameters obtained on bulk powdered samples: Rock-Eval HI, H/C, O/H and O/C atomic ratios. Coal macerals (bituminite, resinite, vitrinite), megaspores and torbanite components (algal bodies and matrix) were characterized by in situ micro-infrared spectroscopy. The correlations derived from FTIR analysis of bulk samples were used to deduce the geochemical parameters of coal and kerogen macerals from their micro-infrared characteristics. Results indicate that H/C atomic ratios and Rock-Eval Hydrogen Indices exhibit good correlations (0.9 < r < 0.95) with FA′ = 2860 cm−1/(2860 cm−1 + 1600 cm−1) and CHali infrared parameters that allow the HI and H/C of single macerals to be calculated. However a significant scattering of the data points around the regression line is observed when using large sets of samples. Correlations were recalculated on a limited series of oxidized coals in order to improve their quality. The derived equations facilitate the determination of the evolution of HI as well as H/C and O/C atomic ratios of different coal macerals during artificial oxidation. Results are compared to the recomputed data on coal, deduced from both the micro-infrared analysis of macerals and the maceral composition of coal.