Cascadia Margin gas hydrates

Abstract

Expedition 311 Scientists PUBLISHER'S NOTESMaterial in this publication may be copied without restraint for library, abstract service, educational, or personal research purposes; however, this source should be appropriately acknowledged. Katerina Petronotis Yeoperson Martin Rothfuss Pressure Coring Engineer Philip RumfordCuratorial Specialist Derryl Schroeder Drilling Engineer Michael StormsOperations Superintendent Scott TowesMarine Mammal Observer (7-18 October) Floris TuynderPressure Coring Engineer 7 Expedition 311 Preliminary Report ABSTRACTA transect of four sites (U1325, U1326, U1327, and U1329) across the Northern Cascadia margin was cored during Integrated Ocean Drilling Program Expedition 311 to study gas hydrate occurrences and formation models for accretionary complexes. In addition to the transect sites, a fifth site (U1328), representing a cold vent with active fluid and gas flow, was visited.The four transect sites represent different stages in the evolution of gas hydrate across the margin from the earliest occurrence on the westernmost first accreted ridge (Site U1326) to its final stage at the eastward limit of gas hydrate occurrence on the margin in shallower water (Site U1329).Logging while drilling/measurement while drilling carried out at the start of the expedition prior to coring provided a set of measurements that guided subsequent coring and special tool deployments at all five sites. Additional wireline logging at each site and two vertical seismic profiles at Sites U1327 and U1328 were completed. A total of 1217.76 m of sediment core was recovered using the advanced piston corer and extended core barrel systems, interspersed with 24 (16 successful) pressure core sampler runs for onboard degassing experiments and 19 Fugro piston corer/HYACE deployments; 4 of these pressure cores were stored under in situ pressure for subsequent shore-based studies.Indirect evidence of the presence of gas hydrate included increased electrical resistivities and P-wave velocities on downhole logs and low-salinity interstitial water anomalies, numerous infrared cold spots, and decreases in void gas C 1 /C 2 ratios, as well as gas hydrate-related sedimentological moussy/soupy textures in recovered cores. Gas hydrate was also observed directly in the recovered cores, and >30 gas hydrate samples were preserved in liquid nitrogen for shore-based studies. The combined observations show that gas hydrate mainly occurs within coarser-grained turbidite sands and silts.The occurrence of gas hydrate appears to be controlled by several key factors, and the concentration of gas hydrate changes significantly as those factors vary in the sediments along the margin. The key controlling factors are (1) local methane solubility linked with pore water salinity, (2) fluid/gas advection rates, and (3) availability of suitable host material (coarse-grained sediments). In the previous model for gas hydrate formation in an accretionary margin, the highest concentrations of gas hydrate were expected to occur localized near the ba...

A new hybrid pressure-coring system for the drilling vessel <i>Chikyu</i>

“…During the Ocean Drilling Program (ODP), the pressure core sampler (PCS; Pettigrew, 1992;Graber et al, 2002) was developed for the drilling vessel (D/V) JOIDES Resolution. Using the PCS, gas-rich or methane hydratebearing sediments were successfully retrieved from the western Pacific coast during ODP legs 164 (Dickens et al, 1997Paull et al, 2000), 201 (Dickens et al, 2003) and 204 (Shipboard Scientific Party, 2003). During ODP leg 204 at the Cascadia margin, Fugro pressure corer (FPC) and the hydrate autoclave coring equipment (HYACE) rotary corer were also deployed on the D/V JOIDES Resolution, resulting in the successful sample recovery (Shipboard Scientific Party, 2003).…”