GAS HYDRATE DISTRIBUTION AND DYNAMICS ON HYDRATE RIDGE, CASCADIA ACCRETIONARY COMPLEX

A.M. Trehu and the Leg 204 Science Party
College of Oceanic and Atmospheric Science, Oregon State University

Gas hydrates are widespread in marine sediments on continental margins at water depths greater than 300-500 meters. Accurate quantitative estimates of how much hydrate is present in the sediments, however, are rare and difficult to obtain because much of the gas hydrate has dissociated by the time samples have been retrieved. ODP Leg 204 was only the second ODP leg dedicated to quantifying the abundance of gas hydrates and understanding the dynamics of their formation in marine sediments (the first was Leg 164 to the Blake Ridge offshore North Carolina in 1994) and provided an opportunity to test several new techniques for sampling, preserving and quantifying gas hydrates. Nine sites were drilled and cored on or near southern Hydrate Ridge, a topographic high in the accretionary complex of the Cascadia subduction zone, located approximately 80 km west of Newport, OR. All sites fell within a previously-acquired, high-resolution 3D seismic survey, which imaged the “plumbing” of this gas hydrate system and served as a road map to guide site selection. By integrating results from several techniques with different spatial scales and sensitivity to in situ gas hydrate content, we were able to obtain the first well-constrained quantitative estimates of how gas hydrate and free gas are distributed throughout this system.

Gas hydrate distribution is very heterogeneous. Massive gas hydrate, comprising ~25% of the total volume, is restricted to the upper 20-30 meters beneath the seafloor at the summit, where bubbling methane vents and authigenic carbonates had previously been observed. Coexistence of gas hydrate, free gas and highly saline pore waters in this region requires that gas hydrate formation here be rapid; seismic and geochemical data indicate that an anomalously permeable stratigraphic horizon channels gas-rich fluids from deeper within the accretionary complex to form this focused deposit. This deposit is probably typical of similar shallow, concentrated gas hydrate deposits in which gas is focused from a within a large volume through a stratigraphically or structurally controlled conduit, as is observed in the Gulf of Mexico and at other sites characterized by deep hydrocarbon reservoirs and fluid migration.

Elsewhere, gas hydrate occurs primarily in patchy clusters of veins and lenses up to a few cm thick. Although the regional average gas hydrate content of the sediments is lower than previously thought (~2%), relatively large amounts of gas hydrate (up to 20% of pore space over thicknesses of ~ 10 m) occur near the base of the gas hydrate stability field and within coarse-grained layers. This type of gas hydrate distribution is typical of gas hydrate formed primiarly from locally produced methane.

Bibliography:

Trehu, A.M., G. Bohrmann, F.R. Rack, M.E. Torres, Proc. ODP, Init. Repts. 204, [CD-ROM]. Available from: Ocean Drilling Program, Texas A&M Un., College Station TX 77845-9547, USA, 2003.

Trehu, A.M., G. Bohrmann, F.R. Rack, M.E. Torres, and Leg 204 Science Party, Gas hydrate distribution and dynamics beneath southern Hydrate Rige: ODP Leg 204, JOIDES Journal, v. 29, Fall, 2003.

Trehu, A.M., Bohrman, G., Rack, F.R., Collett, T.S., D.S. Goldberg, P.E. Long, A.V. Milkov, M. Riedel, P. Schultheiss, M.E. Tores, N.L. Bangs, S.R. Barr, W.S. Borowski, G.E. Claypool, M.E. Delwiche, G.R. Dickens, E. Gracia, G. Guerin, M. Holland, J.E. Johnson, Y-J. Lee, C-S. Liu, X. SU, B. Teichert, H. Tomaru, M. Vanneste, M. Watanabe, J.L. Weinberger, 2004. Three-dimensional distribution of gas hydate beneath southern Hydrate Ridge: constraints from ODP Leg 204, Earth and Plan. Sci. Let, in press.

Torres, M.E., K. Wallmann, A.M. Trehu, G. Bohrmann, W.S. Borowski, H. Tomaru, Gas hydrate dynamics at the Hydrate Ridge southern summit based on dissolved chloride data, submitted to Earth and Plan. Sci. Let., Jan. 2004.

Trehu, A.M., P. Flemings, N. Bangs, M. Torres, J. Johnson, M. Riedel, C-S Liu, Lithostatic gas pressures and venting at southern Hydrate Ridge, submitted to Geology, April, 2004.