Molecular structure of kerogens from source rocks of the Tarim Basin: A study by Py-GC-MS and methylation-Py-GC-MS |
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Authors: | Email author" target="_blank">Wanglu?JiaEmail author Ping’an?Peng |
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Institution: | State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China |
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Abstract: | Flash pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and in situ methylation-pyrolysis-gas chromatography-mass spectrometry (methylation-Py-GC-MS) have been employed for a study of molecular structure of kerogens from source rocks of the Tarim Basin. The main
products from Py-GC-MS of the Ordovician and Triassic kerogens from the Tarim Basin are n-alkanes and n-alk-1-enes which decrease in relative abundance with increased carbon number. The major products from methylation-Py-GC-MS are normal saturated fatty acid methyl esters which show maxima at C16 and C18. These aliphatic compounds are mainly
from lipids of planktonic algae. Moreover, there are more abundant long-chain fatty acids (C20-C26) and normal n-alkanes and n-alk-1-enes (C15-C25) in pyrolysates of Triassic kerogens than that of Ordovician kerogens, reflecting the discrepancies in
sources of the Triassic kerogens and the Ordovician kerogens. Besides these aliphatic compounds, methoxyl benzoic acid methyl
esters were detected only in products from methylation-Py-GC-MS of late Ordovician kerogen TAC1-1, which possibly was the first molecular evidence for the appearance of moss or
other terrestrial plants in the Tarim Basin. In addition, the relative intensity of 17β(H)-trisnorhopane, 17 β(H), 21 α(H)-30-normoretane
and 17 β(H)-moretane is lower, but 17α(H)-trisnorhopane and 17 α(H)-30-norhopane is higher in the pyrolysates of the Ordovician
kerogens in comparison with the pyrolysates of the Triassic kerogens. Moreover, Pr-1-ene has been detected in pyrolysates
of Triassic kerogens. These differences in pyrolysates consist with maturity of kerogens. In conclusion, combination of methylation-Py-GC-MS and Py-GC-MS are useful tools to investigate the molecular structure of geomacromolecules and can be subject to type
the organic matter, compare the maturity of kerogens and carry out the oil-source correlation. |
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Keywords: | methylation-Py-GC-MS kerogens molecular structure maturity source material Tarim Basin |
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