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Geology; October 2003; v. 31; no. 10; p. 833-836; DOI: 10.1130/G19689.1
© 2003 Geological Society of America
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In situ methane concentrations at Hydrate Ridge, offshore Oregon: New constraints on the global gas hydrate inventory from an active margin

Alexei V. Milkov*,1, George E. Claypool*,2, Young-Joo Lee*,3, Wenyue Xu*,4, Gerald R. Dickens*,5, Walter S. Borowski*,6 and ODP Leg 204 Scientific Party

1 Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
2 8910 West 2nd Avenue, Lakewood, Colorado 80226, USA
3 Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea
4 School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
5 Department of Earth Science, Rice University, Houston, Texas 77005, USA
6 Earth Sciences Department, Eastern Kentucky University, Richmond, Kentucky 40475, USA

The widespread presence of bottom-simulating reflectors (BSRs) on continental margins has bolstered suggestions that gas hydrates and free gas constitute a large dynamic reservoir of CH4 carbon and a vast potential source of energy. However, only a few hydrate-bearing areas have been drilled, and of these, the amount of CH4 has only been directly quantified in 18 discrete samples from 3 holes on Blake Ridge, east of Georgia. Here we report and discuss 30 direct measurements of CH4 concentration in sediments above and below the BSR at Hydrate Ridge on a tectonically active margin offshore Oregon. High CH4 concentrations (71–3127 mM) support abundant gas hydrate (occupying an average of ~11% of porosity) and free gas (occupying ~4% of porosity in 1 sample) in a restricted area where hydrocarbon gases migrate from the deep accretionary complex to the seafloor. In a larger area lacking this hydrocarbon supply, lower CH4 concentrations (10–893 mM) indicate less gas hydrate (average ~1% of porosity) and little or no free gas. Overall, the amount of CH4 at Hydrate Ridge is significantly less than that at Blake Ridge. These results challenge certain interpretations, including the global volume of hydrate-bound CH4, which though large, may be four to seven times less than widely cited estimates. Speculations on the distribution and role of gas hydrate and free gas need revision.

Key Words: methane • gas hydrate • Hydrate Ridge • ODP Leg 204




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