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doi:10.2204/iodp.proc.311.109.2006

Drilling objectives

The primary objective of Expedition 311 on the northern Cascadia margin is to constrain geologic models for the formation of gas hydrate in subduction zone accretionary prisms. Natural gas hydrate occurs in marine continental slope and onshore Arctic permafrost environments. The Arctic occurrences can exhibit locally very high gas hydrate concentrations but appear to contain less total gas than marine gas hydrate occurrences. Recent studies have suggested that the largest total amounts of gas hydrate may lie in nearly horizontal layers several hundred meters beneath the seafloor of continental slopes, especially in the large subduction zone accretionary sedimentary prisms. Gas hydrate and underlying free gas produce the ubiquitous bottom-simulating reflectors along numerous continental margins of the world. Gas hydrate does occur on passive margins, but it is less common and appears at usually lower concentrations.

Expedition 311 followed the goals for gas hydrate drilling as proposed by the ODP Gas Hydrates Program Planning Group:

• Study the formation of natural gas hydrate in marine sediments.
• Determine the mechanism of development, nature, magnitude, and global distribution of gas hydrate reservoirs.
• Investigate the gas transport mechanism and migration pathways through sedimentary structures from site of origin to reservoir.
• Examine the effect of gas hydrate on the physical properties of the enclosing sediments, particularly as it relates to the potential relationship between gas hydrate and slope stability.
• Determine no–gas hydrate/no-gas baselines for pore water chlorinity, physical properties (P- and S-wave velocity) and for the empirical Archie parameters to use electrical resistivity.
• Investigate the microbiology and geochemistry associated with hydrate formation and dissociation.

The objectives of this expedition were to test gas hydrate formation models and constrain model parameters, especially models of hydrate concentration through upward fluid and methane transport. The new site survey data and compilation of previous data summarized above are in support of these objectives. The IODP objectives require

• High-quality data on the vertical concentration distributions of gas hydrate and free gas and variation landward in the accretionary prism and
• Estimates of the vertical fluid and methane fluxes through the sediment section as a function of landward distance from the deformation front.

The study will concentrate on the contrast between dispersed pervasive upward flow and focused flow of fluid and methane in fault zones. The pervasive permeability may be on a grain or centimeter scale (the scaly fabric observed in previous ODP clastic accretionary prism cores) or in closely spaced faults.

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