Can hydrocarbons entrapped in seep carbonates serve as gas geochemistry recorder?
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The geochemistry of seep gases is useful for an understanding of the local petroleum system. Here it was tested whether individual light hydrocarbons in seep gases are representatively entrapped in authigenic carbonates that formed near active seep sites. If applicable, it would be possible to extract geochemical information not only on the origin but also on the thermal maturity of the hydrocarbon source rocks from the gases entrapped in carbonates in the past. Respective data could be used for a better understanding of paleoenvironments and might directly serve as calibration point for, amongst others, petroleum system modeling. For this approach, (sub)-recent seep carbonates from the Black Sea (Paleodnjepr region and Batumi seep area), two sites of the Campeche Knoll region in the Gulf of Mexico, and the Venere mud volcano (Mediterranean Sea) were selected. These seep carbonates derive from sites for which geochemical data on the currently seeping gases exist. During treatment with phosphoric acid, methane and higher hydrocarbons were released from all carbonates, but in low concentrations. Compositional studies demonstrate that the ratio of methane to the sum of higher hydrocarbons (C1/(C2+C3)) is (partly strongly) positively biased in the entrapped gas fraction. δ13C values of C1 were determined for all samples and, for the samples from the Gulf of Mexico and the Mediterranean Sea, also of C2 and C3. The present dataset from six seep sites indicates that information on the seeped methane can be—although with a scatter of several permil—recorded in seep carbonate matrices, but other valuable information like the composition and δ13C of ethane and propane appears to be modified or lost during, for example, enclosure or at an early stage of diagenesis.
BGR (PANORAMA project) is thanked for financial support, and the crews and captains of research cruises POS317/2, M72/3, M114 are thanked for excellent cooperation during field work. We also thank two anonymous reviewers and the editors Burg W. Flemming and Monique T. Delafontaine for their helpful comments. Jürgen Poggenburg, Daniela Graskamp, Dietmar Laszinski and Petra Adam are acknowledged for analytical support, and Eckhard Faber, Heiko Sahling, and Georg Scheeder for helpful discussions.
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Conflict of interest
The authors declare that there is no conflict of interest with third parties.
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