Ocean Drilling Program, United States

Ocean Drilling Program (ODP) Leg 164 recovered a number of large solid gas hydrate from Sites 994, 996, and 997 on the Blake Ridge. Sites 994 and 997 samples, either nodular or thick massive pieces, were subjected to laboratory analysis and measurements to determine the structure, molecular and isotopic composition, thermal conductivity, and equilibrium dissociation conditions. X-ray computed tomography (CT) imagery, X-ray diffraction, nuclear magnetic resonance (NMR), and Raman spectroscopy have revealed that the gas hydrates recovered from the Blake Ridge are nearly 100% methane gas hydrate of Structure I, cubic with a lattice constant of a = 11.95+ or -0.05 Aa, and a molar ratio of water to gas (hydration number) of 6.2. The delta (super 18) O of water is 2.67 per mil to 3.51 per mil SMOW, which is 3.5 per mil-4.0 per mil heavier than the ambient interstitial waters. The delta (super 13) C and delta D of methane are -66 per mil to -70 per mil and -201 per mil to -206 per mil, respectively, suggesting that the methane was generated through bacterial CO (sub 2) reduction. Thermal conductivity values of the Blake Ridge hydrates range from 0.3 to 0.5 W/(m.K). Equilibrium dissociation experiments indicate that the three-phase equilibrium for the specimen is 3.27 MPa at 274.7 K. This is almost identical to that of synthetic pure methane hydrate in freshwater.