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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Petrology
Vol. 57 (1987)No. 5. (September), Pages 881-892

Methane-related Carbonate Cements in Pockmarks of the North Sea

Martin Hovland, Michael R. Talbot, Henning Qvale, Snorre Olaussen, Lars Aasberg

ABSTRACT

Some pockmarks on the Norwegian continental shelf contain patches of cemented sediment that can provide hardgrounds attractive to a variety of benthonic organisms. Detailed examination of a sandstone sample from a pockmark in Norwegian Block 25/7 in the North Sea has revealed the presence of Mg calcite and aragonite cements, some of the latter forming botryoids. All cement types are characterized by extremely light carbon isotopic compositions, with a mean ^dgr13C value of -56.1^pmil PDB, which shows that the cements contain carbonate produced by oxidation of biogenic methane. Oxidation occurred in both the oxic and anoxic diagenetic zones. Trace amounts of interstitial methane (mean ^dgr13C = -40.8%^pmil) and higher hydrocarbon gases (up to C5) with a C1/Cn ratio of 0.855-0.874 in the pockmark sediments indicate that some thermogenic methane may be mixed with the biogenic gas.


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