Spectral narratives of microstructural restyling and their controls on hydrocarbon generation potential from coal

Abstract

The low to medium-rank Tertiary coals from Meghalaya, India, are explored for the first time for their comprehensive microstructural characterization using the FTIR and Raman spectroscopy. Further, results from these coals are compared with the Permian medium and high-rank coals to understand the microstructural restyling during coalification and its controls on hydrocarbon generation. The coal samples are grouped based on the mean random vitrinite reflectance values to record the transformations in spectral attributes with increasing coal rank. The aliphatic carbon and the apparent aromaticity respond sharply to the first coalification jump ({bar{rm R}}r: 0.50%) during low to medium-rank transition and anchizonal metamorphism of the high-rank coals. Moreover, the Raman band intensity ratio changes during the first coalification jump but remains invariable in the medium-rank coals and turns subtle again during the onset of pregraphitization in high-rank coals, revealing a polynomial trend with the coal metamorphism. The Rock–Eval hydrogen index and genetic potential also decline sharply at the first coalification jump. Besides, an attempt to comprehend the coal microstructural controls on the hydrocarbon potential reveals that the Tertiary coals comprise highly reactive aliphatic functionalities in the type II-S kerogen, along with the low paleotemperature (74.59–112.28 °C) may signify their potential to generate early-mature hydrocarbons. However, the presence of type II-III admixed kerogen, a lesser abundance of reactive moieties, and overall moderate paleotemperature (91.93–142.52 °C) of the Permian medium-rank coals may imply their mixed hydrocarbon potential. Meanwhile, anchizonal metamorphism, polycondensed aromatic microstructure, and high values of paleotemperature (~ 334.25 to ~ 366.79 °C) of the high-rank coals indicate a negligible potential of producing any hydrocarbons.