Kerogen maturation data in the Uinta Basin, Utah, USA, constrain predictions of natural hydrocarbon seepage into the atmosphere

Natural seepage of methane from the lithosphere to the atmosphere occurs in regions with large natural gas deposits. According to some authors, it accounts for roughly 5% of the global methane budget. I explore a new approach to estimate methane fluxes based on the maturation of kerogen, which is th...

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Bibliographic Details
Published in:Journal of geophysical research. Atmospheres 2014-03, Vol.119 (6), p.3460-3475
Main Author: Mansfield, Marc L.
Format: Article
Language:English
Subjects:
Atmosphere
Atmospheres
atmospheric methane budget
Basins
Crude oil
Decomposition
Estimates
Gas seepage
Geophysics
Hydrocarbons
Kerogen
kerogen maturation
Lithosphere
Methane
Natural gas
natural methane seepage
Petroleum hydrocarbons
Polymers
Rocks
Seepage
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Summary:Natural seepage of methane from the lithosphere to the atmosphere occurs in regions with large natural gas deposits. According to some authors, it accounts for roughly 5% of the global methane budget. I explore a new approach to estimate methane fluxes based on the maturation of kerogen, which is the hydrocarbon polymer present in petroleum source rocks and whose decomposition leads to the formation of oil and natural gas. The temporal change in the atomic H/C ratio of kerogen lets us estimate the total carbon mass released by it in the form of oil and natural gas. Then the time interval of active kerogen decomposition lets us estimate the average annual formation rate of oil and natural gas in any given petroleum system, which I demonstrate here using the Uinta Basin of eastern Utah as an example. Obviously, this is an upper bound to the average annual rate at which natural gas seeps into the atmosphere. After adjusting for biooxidation of natural gas, I conclude that the average annual seepage rate in the Uinta Basin is not greater than (3100 ± 900) tonne yr−1. This is (0.5 ± 0.15)% of the total flux of methane into the atmosphere over the Basin, as measured during aircraft flights. I speculate about the difference between the regional 0.5% and the global 5% estimates. Key Points H/C ratios of petroleum source rocks fix the total mass of generated petroleum Duration of active petroleum generation constrains rate of natural gas venting Natural seepage rate for Uinta Basin, Utah, less than 3000 tonne methane/year
ISSN:2169-897X
2169-8996
DOI:10.1002/2013JD020148