The origin of N2-H2-CH4-rich natural gas seepages in ophiolitic context: A major and noble gases study of fluid seepages in New Caledonia
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- 作者：Eric Deville ; eric.deville@ifpen.fr" class="auth_mail" title="E-mail the corresponding author ; Alain Prinzhofer1
- 刊名：Chemical Geology
- 出版年：2016
- 出版时间：15 November 2016
- 年：2016
- 卷：440
- 期：Complete
- 页码：139-147
- 全文大小：1323 K
文摘
The study of natural gas seepages in New Caledonia has shown the occurrence of two gas families, one is N2-H2-CH4-rich, the other is N2-rich. The N2-H2-CH4-rich gases are bubbling in hyperalkaline springs (OH–-rich) known in the peridotite massif of the southern part of the island. This family of gas shows contents of N2 between 50 and 62%, H2 between 26 and 36%, and CH4 between 11 and 16%. δ13C values of methane are ranging from − 39 to − 32‰. We interpret the origin of H2 as a product of fluid-rock interaction between basic-ultrabasic rocks and water with oxidation of Fe2 + and reduction of underground water in the fracture system of the peridotites nappe. Methane is interpreted as the result of a reduction of dissolved inorganic and/or organic carbon in subsurface aquifers. The second family of gas emissions was found in thermal springs in the sedimentary units located structurally below the peridotites nappe. The gas is composed mostly of N2 (between 97 and 98%) associated with relatively high concentration of He. Both gas families show notably high N2 vs fossil noble gas contents (36Ar, 20Ne, 84Kr) suggesting that N2 is not directly issued from gas dissolved in aquifers equilibrated with atmosphere but most likely finds its origin in a deep source, probably within metamorphic sediments which are tectonically buried below the ophiolitic nappe. We interpret the N2-H2-CH4-rich family of gas as a result of a mixing between two end-members, (1) a H2-CH4-rich pole issued from the weathering of peridotite rocks and (2) a N2-rich pole which would be issued from the metamorphosed sediments buried below the ophiolitic units. An unusual inverse correlation between the atmospheric noble gas isotopes 20Ne and 36Ar is interpreted as the result of a degassing of a relatively shallow confined aquifer related to a bubble flow from depth.