Better explicating the strengths and shortcomings of these models will help refine projections and improve transparency in the years ahead.
Image credit: Witsawat.S.
Edited by Andrea Rinaldo, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland, and approved September 11, 2020 (received for review January 31, 2020)
Significance
A major uncertainty in the sources of atmospheric methane is the role of geologic seepage from petroleum-bearing sedimentary basins. Hydrocarbon seeps located onshore, shallow offshore, and coastal areas can play a major role. Methane released by deep ocean seeps typically does not reach the atmosphere. Here, we provide evidence for a single, large, and sudden expulsion of methane from a deep‐water reservoir during the Pliocene. We use geophysical evidence and fluid-flow modeling to estimate that this single event would have amounted to ∼10% of present‐day annual natural methane emissions. Although no ultraseepage events, such as this one, have been documented in modern times, the relatively common geologic circumstances of this type of event suggest that they are not exceptional.
Abstract
Geologic hydrocarbon seepage is considered to be the dominant natural source of atmospheric methane in terrestrial and shallow‐water areas; in deep‐water areas, in contrast, hydrocarbon seepage is expected to have no atmospheric impact because the gas is typically consumed throughout the water column. Here, we present evidence for a sudden expulsion of a reservoir‐size quantity of methane from a deep‐water seep during the Pliocene, resulting from natural reservoir overpressure. Combining three-dimensional seismic data, borehole data and fluid‐flow modeling, we estimate that 18–27 of the 23–31 Tg of methane released at the seafloor could have reached the atmosphere over 39–241 days. This emission is ∼10% and ∼28% of present‐day, annual natural and petroleum‐industry methane emissions, respectively. While no such ultraseepage events have been documented in modern times and their frequency is unknown, seismic data suggest they were not rare in the past and may potentially occur at present in critically pressurized reservoirs. This neglected phenomenon can influence decadal changes in atmospheric methane.
Footnotes
- ↵1To whom correspondence may be addressed. Email: martino.foschi{at}earth.ox.ac.uk.
Author contributions: M.F., J.A.C., and C.W.M. designed research; M.F. and C.W.M. performed research; M.F., J.A.C., C.W.M., and G.E. analyzed data; and M.F. and C.W.M. wrote the paper with contributions from J.A.C. and G.E.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2001904117/-/DCSupplemental.
Data Availability.
The codes and scripts used here are available by request to M.F. (martino.foschi{at}earth.ox.ac.uk) or C.W.M. (christopher.macminn{at}eng.ox.ac.uk). The multichannel reflection seismic data and the well 214/4-1 were made available by Petroleum Geo-Service and Total SA, respectively. These data are proprietary, and may be available upon request via the UK National Data Repository (UK Oil and Gas Authority; https://ndr.ogauthority.co.uk/).
- Copyright © 2020 the Author(s). Published by PNAS.
This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).
Evidence for massive emission of methane from a deep‐water gas field during the Pliocene
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- Earth, Atmospheric, and Planetary Sciences
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