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Figure F12. Downhole profile of C1/C2 ratios in mud gas analyzed by online mud-gas monitoring during the drilling of Hole C0020A. High C1/C2 ratios are characteristic for biogenic methane formation and low ratios indicate thermogenic sources because higher hydrocarbon gases are mainly formed by thermal alteration of organic matter. However, minor amounts of C2 can also be generated during early diagenesis of organic matter. At Site C0020, C1/C2 ratios point altogether to biogenic methane sources. C1/C2 ratios generally decrease with depth as expected, but they show a distinct excursion toward higher values between 1840 and 2054 m MSF (i.e., the depth interval in which 12 coal layers were observed). The excursion of the C1/C2 ratio signals the presence of biogenic methanogenesis in coal-bearing horizons.

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