PRELIMINARY SCIENTIFIC ASSESSMENT
The revised IODP U.S. Implementing Organization (USIO) riserless vessel schedule included an additional 15 operational days for the 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 on site and included a visit of five sites in total.
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/DVTPP tools as well as monitoring of pressure/temperature conditions of each core 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 hydratebearing sediments is challenging, we took advantage of the third hole to spot-core in missed intervals from Hole B. Hole C was mainly dedicated toward pressure coring, with a total of six deployments alternating the PCS, HPC, and FPC systems. Hole C was then wireline logged after coring operations were completed. The additional time and associated flexibility in the program allowed us to work through 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 remains in the detailed velocity profile at each site along the transect 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 certainly impacted operations throughout the duration of the expedition. A combination of adverse weather and severe sea state resulted in schedule delays adding up to >4 days. In addition, the deteriorating conditions resulted in reduced core recovery and quality. Both temperature tool and pressure core deployments were also adversely affected by general deterioration of weather conditions throughout the expedition.
An unforeseen problem arose with the recognition of the fact that many of the gas hydrate accumulations encountered during the expedition occurred in sand-rich sediment sections. Recovery with standard XCB and specialized pressure core systems were both negatively affected by the presence of sands.
The IODP and Transocean staff members need to be complimented in their ability to execute the complex operations and to demonstrate outstanding flexibility to adapt to changing conditions in requirements.
The original proposal 553-Full2 Cascadia margin gas hydrates included deployment of long-term monitoring devices such as ACORK instruments and fiber-optic DTS cables as well as coring and logging of a reference site in the deep Cascadia Basin west of the deformation front to characterize the incoming, undeformed, and gas hydratefree 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 out a cable route within the next 2 years that exactly follows the transect established during this expedition.
Although Expedition 311 addressed many key issues of gas hydrate occurrence on this margin, several important questions are still unanswered and need to be addressed during Phase II drilling. One of the most important aspects in gas hydrate research is establishing the extent of the gas hydrate stability zone. Capturing the temperature field with coarsely spaced single-point measurements and regression analyses to establish linear temperature gradients was shown to be not only challenging due to the weather conditions but also 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 gas hydrate stability zone in much 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 further showed 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 here is permeability in the sediments and how fluid and gas can migrate through the system. The permeability may be on a sediment grain scale, on a centimeter scale (the scaly fabric observed in previous ODP clastic accretionary prism cores), or in closely spaced faults. A key experiment to measure such permeability on these various 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 Sydney and other sites along the margin and as part of the proposed NEPTUNE cable observatory]).
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