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Figure F36. A. C1/C2 ratios in mud gas analyzed by on-line mud-gas monitoring and in cuttings and cores by headspace technique, Site C0020. B. Mud-gas C1/C2 ratios versus in situ temperature (based on a geothermal gradient of 24°C/km and a bottom water temperature of 6°C). High C1/C2 ratios are characteristic of biogenic methane formation; low ratios indicate enhanced thermogenic sources. At Site C0020, C1/C2 ratios point to biogenic methane sources. C1/C2 ratios generally decrease with increasing depth as expected, but they show a distinct excursion toward higher values between 1840 and 2054 m MSF (i.e., the depth interval containing 12 coal layers). The excursion of the C1/C2 ratio points to enhanced activity of biological methanogenesis in or between the coal layers. TOC = total organic carbon.

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