FTIR spectral signatures of Cretaceous-Paleogene sub-bituminous and bituminous coal: Insights into molecular structure evolution during coalification in the Eastern Cordillera Basin, Colombian Andes

The coalification process and maceral composition influence the molecular structure of the organic matter, yet their precise effects remain a significant challenge in coal geology studies. This investigation focused on sub-bituminous and bituminous coal from the Cretaceous-Paleogene Guaduas Formation in the Colombian Andes, concurrently creating an extensive database of FTIR spectral signatures with 118 samples. Analytical techniques employed in this study included Fourier Transform Infrared spectroscopy in the mid-Infrared Region (FTIR-MIR) using the KBr pellet method, micro-FTIR spectroscopy, and X-ray diffraction (XRD). Molecular structural parameters, such as Aromaticity (AR1 and AR2), Degree of Condensation (DOC1 and DOC2), methylene-to-methyl ratio (CH₂/CH₃), and Factor A, were determined. The random vitrinite reflectance (VRr%) of the studied samples ranged from 0.46 % to 1.65 %, indicating the occurrence of sub-bituminous and bituminous coals. The results revealed AR1 ranging from 0.026 to 0.182, AR2 from 0.208 to 1.282, DOC1 from 0.048 to 0.244, DOC2 from 0.373 to 1.797, CH₂/CH₃ from 2.444 to 6.430, and Factor A from 0.559 to 0.701. The prominent inorganic peaks in the FTIR spectra corroborated with the coal mineralogy, kaolinite and quartz, which are dominant, with minor occurrences of siderite, pyrite, and iron oxides. Micro-FTIR data indicated higher aromaticity and condensation in macerals of the inertinite group compared to vitrinite and liptinite groups. The correlation of structural parameters showed an increase in condensation and aromaticity with higher VRr% (or increased coal rank). The aromaticity and condensation increased with depth, suggesting that burial affected the rank of the Guaduas Formation coal. Coal with a higher inertinite content showed higher aromaticity/condensation and shorter aliphatic chain length (CH₂/CH₃). The FTIR spectra of the analyzed dataset reveal that coalification was the dominant factor that affected the organic molecular structure of coal in the Eastern Cordillera Basin. It was found that maceral composition is also crucial in explaining the variations in molecular structural parameters observed in isorank coals. In this sense, liptinite-rich coal was enriched in aliphatic components, whereas vitrinite and inertinite-rich coals were enriched in aromatic compounds, exhibiting gas-prone characteristics.