Thermal Maturity and Structural Characterization of Indian Coals: Insights from Petrographic, Pyrolytic and Spectroscopic Methods

ABSTRACT Characterisation of Indian coal samples, collected from various coal-bearing sequences, was performed using spectroscopic techniques (Fourier Transform Infrared and Raman) to understand the structural changes associated with the thermal maturity. The vitrinite reflectance (VRo%) in the studied samples ranges from 0.28 % to 0.92 % and the Tmax ranges between 402 °C and 473 °C. A strong correlation (R2=0.95) between VRo% and Tmax indicates the reliability of these proxies. The elemental data shows carbon (av. 50.51 wt %), hydrogen (av. 2.78 wt %), nitrogen (av. 0.78 wt %) and sulfur (av. 1.34 wt %) contents. Rock-Eval and Fourier Transform Infrared Spectroscopic (FTIR) parameters collectively suggest the kerogen as Type III. The samples exhibit a range of hydroxyl, aliphatic, aromatic and oxygenated functional groups. FTIR semi-quantitative ratios were applied to examine the compositional variations across the rank, revealing an increase in aromaticity and aromatic ring condensation, as evident from the I1, Car/ (Car + C=O, CHar/C=C, and Har/Car ratios. Additionally, an increase in the aromatic hydrogen content is apparent from the fH and Hal/Hratios. The de-functionalisation of oxygenated functional groups is manifested in decrease in the C-factor. Below a VRo of 0.5%, the samples exhibit consistent behaviour; however, beyond this point, the behaviour becomes abrupt, probably due to the physico-chemical factors controlling the transformations beyond the first coalification jump and/or the diverse origin of the samples. The Raman spectra showed variation of the D-band with respect to VRo, and the D1A/GA ratio also supports this trend. The hydrogen index (HI) vs. oxygen index (OI) plot showed distinct clustering of the samples according to maturity, suggesting these variations are well-defined across all samples, irrespective of their location and source characteristics.