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| Titel |
Natural gas seeps in the French Alps: Sources and pathways |
| VerfasserIn |
Wolfram Kloppmann, Michaela Blessing, Eric Proust, Frédéric Gal, Gaetan Bentivegna, Benoit Henry, Pierrick Defossez, Catherine Lerouge, Pauline Humez, Bernhard Mayer, Romain Millot, Eric Gaucher |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250133831
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| Publikation (Nr.) |
 EGU/EGU2016-14484.pdf |
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| Zusammenfassung |
| Natural gas emanations are part of the geochemical baseline to take into account when
assessing global greenhouse gas emissions and potential impacts of conventional and
unconventional gas exploration and exploitation on groundwater. Examples of such natural
gas macro-seeps are known in several parts of the world (Etiope et al., 2009). Only a limited
number of them have been characterized for their gas and isotopic compositions. Such
analyses can provide essential information for baseline studies, providing insight in the
sources (biogenic vs. thermogenic or modified thermogenic) and pathways of such seeps and
may allow for distinction of natural seeps from stray gas leakage associated with human
activities.
Here, we report gas concentrations and multi-isotope data (δ13C and δ2H of methane and
ethane, δ13C and δ18O of CO2, 3He/4He ratio) of two gas seeps in the French subalpine
chains, both in a similar geological and structural position within Middle Jurassic claystones
along the eastern border of the large synclinal structures of the Vercors and the Chartreuse
massifs (Moss, 1992). The “ardent fountain” (fontaine ardente) of Le Gua, 30 km south of
Grenoble has most likely the longest continuous written record of existence of
any individual natural gas seep, mentioned explicitly as early as the first quarter
of the 5th century (Augustin of Hippo (St. Augustin), approx. 426) This natural
seep was described in the past as a “wet seep” associated with a spring, whereas
the second investigated seep, Rochasson near Meylan north of Grenoble, is a dry
seep.
Both seeps contain methane and ethane with thermogenic C and H isotope signatures,
comparable with a seep in the Northern Swiss Alps at Giswil (Etiope et al., 2010) but with a
higher dryness (C1/(C2+C3)>1000) for the Le Gua seep, possibly due to molecular
fractionation upon advective fluid+gas migration (Etiope et al., 2009). Maturity (R0) of the
reservoir rocks deduced from δ13C(CH4), δ13C(C2H6) is similar to values found by Moss
(1992) for West-alpine Oxfordian shales. CO2 contents of <9% could stem from methane
oxidation. He isotope ratios are radiogenic with only minor mantle contributions, as it is
typical for the Western Alps (Marty et al., 1992). This preliminary investigation
reveals that thermogenic natural gas can migrate naturally to the surface in significant
flux rates over millenaries without anthropogenic causes, in cases that appropriate
leakage pathways exist. This study was co-funded by French ANR and Canadian
NSERC.
ND_ABSTRACT@
Etiope G., Feyzullayev A., Baciu C. L. (2009) Marine and Petroleum Geology, 26,
333-344.
Etiope G., Zwahlen C., Anselmetti F. S., Kipfer R., Schubert C. J. (2010) Geofluids, 10,
476-485.
Marty B., Onions R. K., Oxburgh E. R., Martel D., Lombardi S. (1992) Tectonophysics,
206, 71-78.
Moss S. (1992) Journal of the Geological Society, 149, 503-515.
Augustin of Hippo (St. Augustin) (approx. 426) De Civitate Dei contra paganos, Vol.
Liber XXI, chap. VII |
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