ABSTRACT

Multiple deployments of a pressurized core barrel (PCB) during drilling of DSDP-IPOD Site 533A on the Blake-Bahama Outer Ridge has provided conclusive evidence for the presence of gas hydrate in sediments exhibiting a well-defined bottom simulating reflector. Pressure decline curves monitored during degassing and sampling of two pressurized sediments clearly indicate a pressure plateau at about 1, 500 kPa resulting from gas hydrate decomposition. The pressure decline curve also includes a region attributable to hydrostatic pressure and a sediment degassing region. The molecular compositions of volatile hydrocarbons in PCB gas samples were characterized by (a) less than 2% variation in the C1/C2 ratio during pressure release, (b) a fourfold increase in C3-C6 hydrocarbons over the duration of the experiment, and (c) a tenfold increase in CO2 content of the expelled gases. Gas hydrate solids were recovered from Core 13-1 (~ 238 m depth) in sufficient quantity to permit several pressure/volume measurements. Gas expansion of the solids was measured at 13:1. Mole ratio of water to gas was found to be 50:1 after correcting for pore fluid content. Sediments remaining after gas hydrate decomposition were 93% H2O by weight. Most of the water contained in the sediments appeared to be associated with the gas hydrate. The hydrates recovered were extremely fine grained and decomposed rapidly upon removal from in-situ pressures and temperatures.

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