Characterization of kerogen using combined pyrolysis- GC-MS and FT-IR in weathered and unweathered coals and coaly shales from the Central Myanmar Basin, Myanmar

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Kerogens from weathered and unweathered coals and coaly shales from Myanmar have been characterized based on Rock-Eval pyrolysis, pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS), CNS elemental composition, and Fourier transform infrared (FT-IR) spectroscopy. Three moderately weathered coaly shales and five unweathered coals/coaly shales were collected from relatively thin carbonaceous layers (15-150 cm) in the Upper Eocene sediments in the western margin of the Central Myanmar Basin. Although organic maturities are similar for all samples (Tmax417-426℃; immature), kerogens from the weathered samples exhibit low Hydrogen Index (HI, 29-42 mgHC/gTOC) and high Oxygen Index (0I, 96-168 mgCO2/gTOC), whereas those from unweathered samples have higher HI (86-250 mgHC/gTOC) and lower OI (25-115 mgCO2/gTOC), respectively. Kerogens with high S3 (pyrolysed at 300-400℃) by Rock-Eval also had high S3' (pyrolysed at 400-650℃), and show strong correlation. The ratio of S3' to S3 is higher in the weathered samples, suggesting an increase in S3' or a decrease in S3 values. Py-GC-MS pyrolysates from the weathered kerogens showed that lighter n-alkanes/alkenes less than n-C10 and methyl/dimethyl phenols are decreased. FT-IR analysis of the weathered kerogen showed lower peaks of aliphatic bonding at 2925-2850 cm-1, and relatively constant peaks of C=0 and C-0 bonds at 1716 cm-1 and 1000-1300 cm-1, respectively. These trends in the FT-IR were more apparent in the bitumen samples. These results suggested that rapid decomposition of alkyl chain moieties in kerogen by weathering faster than that of organic oxygen group had caused an increase in OI.

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