δ13C and δD values of n-alkanes from in-reservoir biodegraded oils: Implications for understanding the mechanisms of biodegradation and for petroleum exploration

Nikolai Pedentchouk, Barry Bennett, Steve Larter

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Abstract

This study investigates the magnitude and direction of stable C and H isotope shifts of n-C 15–30 alkanes from biodegraded oils sourced from Type II (Oil suite S) and Type II/III (Oil suite H) kerogens. Compound-specific isotope data show a 2.0‰ 13C-enrichment and no D-enrichment of n-alkanes in the most biodegraded oil from sample suite S. Similarly, there is a 1.5–2.5‰ 13C-enrichment and no D-enrichment in Oil suite H. Overall, there is a <2.5‰ δ 13C and <20‰ δD variability among individual n-alkanes in the whole sequence of biodegradation. N-alkanes from the least biodegraded Oil H samples are 2–4‰ 13C-enriched in comparison with the least biodegraded Oil S. However, there are no differences in the δD values of n-alkanes in these samples. Our indirect isotopic evidence suggests (1) a site-specific biodegradation process, most likely at position C-2 and/or C-3 or another site-specific process, and (2) a significant D/H exchange between organic compounds in the source rock and isotopically similar marine formation waters. We conclude that, unlike δD methodology, investigation of δ 13C composition of n-alkanes has strong potential as a supplementary tool for oil–oil and oil–source-rock correlation even in biodegraded oils when n-alkanes are present.

Original languageEnglish
Article number365
JournalGeosciences
Volume11
Issue number9
Early online date30 Aug 2021
DOIs
Publication statusPublished - Sep 2021

Keywords

  • Carbon
  • Correlation
  • Hydrogen
  • Isotopes
  • N-alkanes
  • Oil biodegradation

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