Methane in serpentinized ultramafic rocks in mainland Portugal

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• 作者：Giuseppe Etiope ; Steve Vance ; Lance E. Christensen ; Jos¨¦ M. Marques ; Isabel Ribeiro da Costa
• 关键词：Methane ; Serpentinization ; Peridotites ; Hyperalkaline waters ; Igneous reservoir rocks
• 刊名：Marine and Petroleum Geology
• 出版年：2013
• 出版时间：August, 2013
• 年：2013
• 卷：45
• 期：Complete
• 页码：12-16
• 全文大小：804 K

文摘

We report a new case of methane (CH₄) of apparent abiotic origin in continental serpentinized ultramafic rocks. Multiple analytical techniques, on-site and in the laboratory, revealed methane and ethane degassing from hyperalkaline (pH?>?11) Ca²⁺-OH^? mineral waters in boreholes drilled in the Alter-do-Ch?o igneous intrusion, at Cabe?o de Vide, in mainland Portugal. The C and H isotopic composition of CH₄ (¦Ä¹³C?¡«??20¡ë; ¦Ä²H:??283¡ë) suggests a dominant abiotic origin, although minor thermogenic contributions cannot be excluded. Similarly, low methane-to-ethane ratios suggest a predominantly non-microbial source, consistent with previous microbiological data showing the lack of methanogenic archaea in these waters. Heavier hydrocarbons, CO₂ and H₂ are below detection limits. This case study confirms that CH₄ from serpentinized ultramafic rocks can be transported by hyperalkaline fluids linked to deep circulation of meteoric waters. Maximum depth of Cabe?o de Vide serpentinized rocks is less than 1?km, and present temperatures are likely lower than 50?¡ãC. Serpentinization and related gas formation may have occurred at any time during thermal evolution of the igneous intrusion, so gas formation temperature cannot be easily determined. This case is an opportunity to test thermometry provided by CH₄ isotopologue analyses. The existence of methane in continental serpentinized igneous rocks is more widespread than previously thought and petroleum systems with similar serpentinized ultramafics in reservoir rocks may have traces of the observed ¹³C-enriched CH₄.