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doi:10.2204/iodp.proc.311.101.2006

Preliminary scientific assessment

The revised IODP U.S. Implementing Organization (USIO) riserless vessel Phase 1 schedule included an additional 15 operational days for Expedition 311. The revised schedule included a port call in Astoria, Oregon, at the start and in Victoria, British Columbia, at the end of Expedition 311, with 37 total operational days on site. The original operations plan for Expedition 311 was based on 22 days at five sites.

The main change to accommodate the additional 15 days was in expanding the operations plan to a three-hole approach at each site. Hole A was dedicated to LWD and MWD operations. Hole B was a continuous coring hole for APC/XCB coring, with additional temperature measurements using the APCT/APC3/DVTP/DVTPP tools as well as monitoring of pressure/temperature conditions by deploying the advanced piston corer methane (APCM) tool. We also planned to deploy the PCS at three intervals in Hole B. These three PCS cores were to be used for degassing experiments only. Because core recovery in gas hydrate–bearing sediments is challenging, we took advantage of the third hole to spot-core in missed intervals from Hole B. Hole C was mainly dedicated to pressure coring, with a total of six deployments alternating the PCS, HPC, and FPC systems. Hole C was wireline logged after coring operations were completed. The additional time and associated flexibility in the program allowed us to accommodate weather problems and operational complexities.

For this expedition, we carried out LWD/MWD operations prior to coring each site. The LWD/MWD logging program surpassed all expectations. The newly developed LWD/MWD safety protocol provided an effective means to deal with concerns associated with shallow gas hazards. The logging data guided special tool deployments (PCS, FPC, and HRC) in addition to providing high-quality downhole measurements, which were used to identify and characterize gas hydrate occurrences.

Uncertainty in the detailed velocity profile at each site complicates our effort to resolve critical issues related to the depth of the BSR. Additional VSPs and walkaway VSPs will be needed to address these issues.

Weather conditions impacted operations throughout the expedition. A combination of adverse weather and severe sea state resulted in >4 days of schedule delays. In addition, the ship heave conditions reduced core recovery and quality. Temperature tool and pressure core deployments were also affected by adverse weather conditions.

Many of the gas hydrate accumulations encountered during the expedition occurred in sand-rich sediment sections. This condition negatively affected recovery with standard XCB and specialized pressure core systems.

IODP and Transocean staff members are complimented for their ability to execute complex operations and demonstrated outstanding flexibility in adapting to changing requirements.

The original proposal 553-Full2 Cascadia margin gas hydrate included deployment of long-term monitoring devices such as ACORK instruments and fiber-optic distributed temperature sensing (DTS) cables as well as coring and logging of a reference site (proposed Site CAS-04B) in the deep Cascadia Basin west of the deformation front to characterize the incoming, undeformed, gas hydrate–free sediment column. The long-term monitoring aspect of the proposal is closely linked to the NEPTUNE cable observatory and is planned to connect the ACORK and DTS instruments by a node near Site 889. The NEPTUNE program is laying a cable during the next 2 y that follows the transect established during this expedition.

Although Expedition 311 addressed many key issues of gas hydrate occurrence on this margin, several important questions remain unanswered and need to be addressed during future drilling. One of the most important aspects in gas hydrate research is establishing the extent of the GHSZ. Capturing the temperature field with coarsely spaced single-point measurements and regression analyses to establish linear temperature gradients was shown to be challenging because of weather conditions and inappropriate in a region dominated by advective fluid/gas flow. It is, therefore, crucial to deploy a system like DTS that allows measurements on a higher vertical resolution to map the GHSZ in more detail, especially around the base of the stability zone. This system will provide the answers to critical questions (e.g., of the potential occurrence of Structure II gas hydrate and temporal changes in the system).

Expedition 311 provided evidence for strong lateral heterogeneity between adjacent bore holes, especially within sites where the gas hydrate occurs mainly within the accreted sediment complex (e.g., Site U1327). The occurrence of gas hydrate appeared to be driven by local variation of methane solubility as well as proximity of suitable host sediments (coarser-grained turbidite sands). A key question is how sediment permeability controls fluid and gas migration. Permeability may be on a sediment grain scale, a centimeter scale (the scaly fabric observed in previous ODP clastic accretionary prism cores), or on a fault/fracture level scale. A key experiment to measure permeability on these scales is given by the proposed dual ACORK deployment. Two boreholes spaced several tens of meters apart equipped with the ACORK instruments are required to carry out a cross-hole hydrogeologic fluid-flow experiment. The two boreholes additionally offer the opportunity to deploy sensors for geophysical measurements (e.g., multicomponent geophones for active and passive seismic experiments, resistivity receivers for CSEM experiments, and strainmeters to measure long-term deformation [to be linked with the newly installed strainmeters at the Pacific Geoscience Centre in Sidney and other sites along the margin and as part of the proposed NEPTUNE cable observatory]).

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