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doi:10.2204/iodp.sp.328.2010

Background

Previous drilling at Site 889

Ocean Drilling Program (ODP) Leg 146 was devoted to documenting the evolution of turbidite sediments as they move from the Cascadia Basin into the adjacent subduction zone accretionary prism and experience deformation and consolidation (Westbrook et al., 1994). In this process, pore fluids are expelled and gas, primarily biogenic methane, is transported upward to form hydrates in the upper few hundred meters of the section. An undeformed reference section off Vancouver Island was established seaward of the prism toe at ODP Site 888. ODP Site 889, where Integrated Ocean Drilling Program (IODP) Expedition 328 operations are planned (Fig. F1A), is located landward of the prism toe where the rate of fluid expulsion, estimated on the basis of rates of thickening and consolidation, reaches a cross-prism maximum. To understand the consolidation, fluid expulsion, and hydrate accumulation and dissociation better, a more complete transect across the prism was drilled during IODP Expedition 311 (Riedel, Collett, Malone, et al., 2006). This work included additional coring, wireline logging, and logging-while-drilling operations in the vicinity of Site 889 at IODP Sites U1327 and U1328 (Fig. F1B). As a result, a total of nine boreholes, along with extensive geophysical site surveys, now provide detailed information about the characteristics of this area and an excellent geophysical, geochemical, and lithologic context for the observatory operations of Expedition 328.

Relevant site characteristics

A simplified schematic cross section in Figure F2 illustrates the way gas hydrates are believed to accumulate in accretionary prisms. Pore fluid expulsion, driven by tectonic thickening and consolidation, is rapid near the prism toe (referred to also as the deformation front) and diminishes landward. Vertical migration of water from the prism delivers small amounts of dissolved methane produced in the sediment by biological CO₂ reduction, to the level of gas hydrate stability (a weak function of pressure and strong function of temperature) where hydrates accumulate primarily in permeable fractures and coarse-grained layers. A discrete boundary between sediments containing free gas and gas hydrate in the sediment pore volume eventually develops. This boundary is seen clearly in seismic sections as a bright reflection (the bottom-simulating reflector, or BSR) at a generally uniform depth below the seafloor (a consequence of its depth being primarily temperature controlled), with a polarity opposite to that from the seafloor (Fig. F3).

The seismic reflection profiles crossing Site 889 also provide a clear image of the local sediment structure, which comprises a gently deformed sequence of slope basin deposits draped over highly deformed accretionary prism sediments. These lithologic units have been characterized in detail by coring and logging at the numerous holes drilled in the immediate vicinity of Site 889 (Fig. F4) (Westbrook et al., 1994; Riedel, Collett, Malone, et al., 2006; Riedel et al., in press). The gas/gas hydrate interface was intersected within the deformed prism unit at a depth of 225 meters below seafloor (mbsf). Hydrates above the interface appear to be virtually absent in fine-grained material; most of the hydrate is concentrated in permeable coarse-grained units and massive hydrate lenses that mark present or past pathways of focused fluid flow. Minor quantities of free gas are inferred to occur in an interval a few tens of meters thick below the interface.

Previous attempt to establish a CORK observatory

During Leg 146, two attempts were made to establish Circulation Obviation Retrofit Kit (CORK) hydrologic observatories, one at Site 889 and the other in a similar setting at ODP Site 892 off central Oregon (Westbrook et al., 1994; Davis et al., 1995). These were equipped with sensors to monitor temperatures at multiple formation levels and pressure at the level of perforations in a liner extending below casing. The installation at Hole 892B was successful and operational for roughly 2 y before the instrumentation was removed to facilitate fluid sampling. Owing to unstable formation conditions and deteriorating weather, the installation in Hole 889C did not succeed. Instability of the formation caused sediment to be squeezed into the casing through the perforations, the open end of the liner, and/or the annulus between the liner and the casing (Fig. F5). This prevented the thermistor cable from reaching its intended depth, which in turn precluded a pressure-tight seal of the pressure logging system at the top of the hole. The total hole depth was 385 mbsf; the bottom of the liner was at 323 mbsf; after two aborted attempts to deploy the thermistor cable, a sinker-bar run indicated fill had reached 253 mbsf; the failed third attempt with a cable shortened to 240 m suggested that sediment had intruded the casing up to this depth. The CORK was never refurbished.

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