OCEANIC METHANE HYDRATES - A FRONTIER GAS RESOURCE

Methane hydrates are ice-like compounds consisting of natural gas (mai nly methane) and water, whose crystal structure effectively compresses the methane: each cubic metre of hydrate can yield over 150 cu.m of m ethane. Hydrates ''cement'' sediments and impart considerable mechanic al strength; they fill porosity and restrict permeability. Both biogen ic and thermogenic methane have been recovered from hydrates. Hydrates occur in permafrost regions (including continental shelves), and are stable in ocean-floor sediments below water depths of about 400 m in t he ''Hydrate Stability Zone '' (HSZ). This is a surface-parallel zone of thermodynamic equilibrium that extends down from the sediment surfa ce to a depth determined by temperature, pressure and local heat flow. Methane and water are stable below the HSZ. Although the economic rec overy of hydrates has taken place in Arctic regions,oceanic hydrates o ffer far greater potential as an energy resource. A variety of traps f or methane gas can be formed by oceanic hydrates. In addition to the g as within the hydrates themselves, simple gas traps in closures beneat h the HSZ in the vicinity of bathymetric highs, and complex traps invo lving both hydrate and structural/stratigraphic components, have been observed. It has been estimated that at least twice as much combustibl e carbon occurs associated with methane hydrates as in all other fossi l fuels on Earth. The evaluation of methane in, and associated with, o ceanic hydrates therefore constitutes a major energy exploration front ier.