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Geology; May, 2007; v. 35; no. 5; p. 455-458; DOI: 10.1130/G23557A.1
© 2007 Geological Society of America
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Geochemical evidence of secondary microbial methane from very slight biodegradation of undersaturated oils in a deep hot reservoir

Alexei V. Milkov*,1 and Leon Dzou1

1 BP America, 501 Westlake Park Boulevard, Houston, Texas 77079, USA

A rare finding of early mature undersaturated oils with low gas/oil ratios enables us to document secondary microbial methane generation during very slight biodegradation in a deep hot reservoir in the ultradeep-water of the Gulf of Mexico. In three studied gas samples, methane is enriched in 13C ({delta}13C is from -63{per thousand} to -64{per thousand}) relative to pure thermogenic methane (estimated {delta}13C is from -71{per thousand} to -67{per thousand}) and pure primary microbial methane ({delta}13C is -68{per thousand}). Carbon dioxide in gases has {delta}13C values that negatively correlate with {delta}13C values of pure thermogenic methane. Methane is unusually enriched in heavy isotope 2H relative to associated ethane. Some extracted oils are depleted in long-chain alkyl aromatics. These lines of geochemical evidence suggest anaerobic microbial degradation of oil and subsequent reduction of resulting carbon dioxide to methane. Although specific geobiological details of secondary microbial methane generation are unclear, this process may be partially responsible for charging some of the largest gas and gas hydrate fields in the world.

Key Words: microbial methane • oil • biodegradation • Gulf of Mexico • hydrocarbons • gas




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