The geochemical signatures of variable gas venting at gas hydrate sites

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Diverse evidence suggests that gas-venting rates at sites of hydrate crystallization are variable in space and time, but the magnitude of these variations has been difficult to quantify. The hydrate crystallization model of Chen and Cathles [J. Geophys. Res. (Solid Earth) 108 (2003)] is used here to analyze 10 years of vent gas chemistry measurements at the Bush Hill hydrate mound and gas-venting site, Green Canyon 185, offshore Louisiana, Gulf of Mexico. The analysis suggests that, at any instant of time, gas vents at variable rates in different gas channels at the same site, and that the compositional differences in these vent gases are nearly as large as can be produced by hydrate crystallization. Almost two orders of magnitude differences in venting rate between individual gas channelways are suggested. Changes in the average vent gas composition over the last 10 years suggest the average venting rate varied by a factor of ∼2 or more over a few years. The average C + C composition of Bush Hill hydrates are leaner than could be crystallized from vent gases sampled over the last decade, indicating that the venting gas flux was slower in the past by a factor of ∼2. This is compatible with geologic generalizations that venting evolves from fast (mud volcano), to intermediate (hydrate crystallization), to slow (carbonate precipitation) if venting organized into more discrete vents with time. © 2004 Elsevier Ltd. All rights reserved. 3 4

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Marine and Petroleum Geology

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