Cascadia Margin

ODP drilling on the Cascadia margin during Leg 146 was designed to study the occurrence of gas hydrate in an accretionary prism associated with an active subduction zone and to examine the role of upward fluid and methane expulsion in hydrate-bearing sediments [Westbrook et al., 1994], Strong BSRs are widespread over much of the continental slope and provide an opportunity to examine whether they are due to high-velocity hydrate or low-velocity free gas [Hyndman and Spence, 1992], During Leg 146, only minor amounts of hydrate were recovered in the core. At Site 889, drilled off the west coast of Vancouver Island, hydrate is inferred to be present from approximately 127 to 228 mbsf based on geochemical core analysis and wireline logs [Westbrook, et al, 1994], Only after integration of downhole data and regional seismic velocities was it possible to estimate the concentration of hydrate and free gas above and below the BSR [Yuan et al, 1996]. This velocity contrast was interpreted to be about 2/3 due to high-velocity hydrate and 1/3 due to underlying low-velocity gas.

Figure 4 shows the logs recorded from seafloor to about 250 m bsf at Site 889. From the velocity log, hydrate is interpreted to increase in concentration downward from the seafloor, with an increase at about 130 m associated with the contact between slope basin and underlying accreted sediments. From 130 m to the BSR at about 230 m, the velocity is about 1.8 km/s compared to a reference of about 1.6 km/s. Likewise, the electrical resistivity at Site 889 increases downward from the seafloor with an abrupt increase at about 130 m. From this depth to the BSR, resistivity averages about 2.1 ohm-m compared to the reference of about 1.0 ohm-m [Hyndman et al, 1999]. In addition to the general increasing trend in the logs at Site 889, there are several layers of high velocity and electrical resistivity near 190 and 210 m. There is no corresponding lithologic cause evident in the natural gamma-ray, neutron porosity, and bulk density logs so these thin zones may represent especially high hydrate concentration. The free gas zone underlying the BSR is not expressed in the sonic and resistivity logs, but only as low velocities in the VSP data [MacKay et al., 1994], Hyndman, et al [1999] interpret the relatively high resistivity log over this interval to indicate that cold drilling fluid has induced hydrate to form from free gas present near the borehole. Alternatively, drilling circulation may have rapidly washed the free gas away from the vicinity of the borehole and beyond the depth of penetration of the logs, but not sufficiently deep to affect the VSP measurements.

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