Natural Gas Hydrate Concentration in Sandy Sediments Closely Relevant to Pore Water And Methane Accumulation

Takashi Uchida¹, Amane Waseda¹, and Takatoshi Namikawa^2
1 JAPEX Research Center, Chiba, Japan
² JOGMEC Technology Research Center, Chiba, Japan

The Mallik and Nankai Trough wells have retrieved abundant gas hydrate-bearing cores successfully from sandy sediments in the Arctic Canada and Japan. Gas hydrate-dominant layers were identified in sandy strata typically ranging from 10 cm to a few meters in thickness. Pore-space hydrates are very small in size and fill intergranular porosity of sandy sediments, whose saturations in most gas hydrate layers were high up to 80% in pore volume. Muddy sediments usually contain low concentrations.

According to grain size distributions gas hydrates are dominant in medium- to very fine-grained sands, and concentrations of gas hydrate may need gas accumulation and original pore space large enough to occur within host sediments. Subsequent analyses in sedimentology and geochemistry performed on gas hydrate-bearing sandy cores also revealed important hydrological controls on formation and preservation of natural gas hydrate. It is necessary for evaluating subsurface fluid flow behaviors to know both of porosity and water permeability of gas hydrate-bearing sediments. Porosities and pore-size distributions were obtained by the mercury porosimetry, which revealed that porosities of gas hydrate-bearing sands range from 25 to 40 %. Water permeabilities were measured for gas hydrate-bearing sand cores indicating that highly gas hydrate-saturated sands may have permeability of a few millidarcies. The distribution of a porous and coarser-grained host rock should be one of the important factors to control the occurrence and concentration of gas hydrate, as well as physicochemical conditions. This appears to be a similar mode for conventional oil and gas accumulations.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005