Geochemical characteristics of terrestrial organic matter in the upper paleozoic complex of the vilyui syneclise and some features of its transformation under thermobaric conditions at great depths

Kirill V. Dolzhenko, Alexander N. Fomin, Vasily N. Melenevsky

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Abstract

A combination of geochemical methods were used to study organic matter from Permian rocks in the central part of the Vilyui syneclise (East Siberia) penetrated by the Srednevilyuiskaya-27 ultra-deep well in the depth range of 3370-6458 m. This study discusses variations in the pyrolysis indices (HI, Tmax), hydrocarbon type content (hydrocarbons-resins-asphaltenes), vitrinite reflectance (R0, %), organic vt carbon content (Corg), as well as some trends in the saturated and aromatic hydrocarbon compositions of bitumen extracts from the Upper Paleozoic rocks. Below a depth of about 4.5 km (late mesocatagenesis), the hydrocarbon type composition is characterized by a sharp decrease in the content of asphaltenes from < 30 % (at 4.5-5.0 km) to < 15 % (at 5.0-5.5 km), which are not detected at greater depth. In turn, the resins became the dominant constituent (~ 50-70 %), whereas hydrocarbons account for < 20 % at depths down to 5 km and < 40 % at greater depth. These depths are also characterized by a predominance of saturated hydrocarbons over aromatic compounds with a decrease in the relative contents of high molecular weight compounds in both fractions, as indicated by mass chromatograms. The hydrocarbon index (HI) of organic matter decreases to the first tens from the depth of 4.9 km and to the bottomhole (6519 m); the temperature of the maximum hydrocarbon yield (Tmax) varies between 570-580 °C, showing a slightly increasing trend. Our results show that the generative potential of organic matter from the rocks within the studied depth range (4.9-6.5 km) has been exhausted and that the terrestrial organic matter undergoes significant changes under severe temperature and pressure conditions at great depths.

Original languageEnglish
Pages (from-to)77-84
Number of pages8
JournalGeoresursy
Volume21
Issue number4
DOIs
Publication statusPublished - 14 Nov 2019
Externally publishedYes

Keywords

  • Catagenesis
  • Chromatography-mass spectrometry
  • Hydrocarbon type composition
  • Pyrolysis
  • Terrestrial organic matter
  • Ultra-deep well
  • Vilyui syneclise
  • ultra-deep well
  • hydrocarbon type composition
  • terrestrial organic matter
  • catagenesis
  • pyrolysis
  • chromatography-mass spectrometry

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