Natural gas hydrate was suspected to occur extensively in hemipelagic sediments of the Blake Outer Ridge on Deep Sea Drilling Project (DSDP) Leg 11 (Hollister, Ewing, et al., 1972). Since then marine gas hydrate has been attracting a growing interest in the field of geosciences as well as in resource industries. Enormous amounts of hydrocarbon gases are expected to be trapped in gas hydrates in marine sediments, and dissociation of gas hydrates could possibly cause a sudden and dramatic change in the environment of the earth. Natural gas hydrate samples have been recovered from a number of deep-sea sites by DSDP and Ocean Drilling Program (ODP) drilling (e.g., Kvenvolden and Barnard, 1983; von Huene, Aubouin, et al., 1985; Westbrook, Carson, Musgrave, et al., 1994) and from surface sediments of marginal seas and inland seas by Brooks et al. (1991), Ginsburg et al. (1993), and others. However, our knowledge of fundamental properties of natural gas hydrate, including the composition of gas and water, molecular ratio of gas to water, structure, and crystallography is surprisingly low because of the scarcity of well-preserved gas hydrate samples. During ODP Leg 164, a number of gas hydrate samples were recovered from three locations--Sites 994, 996, and 997 on the Blake Ridge. Upon recovery, the gas hydrate samples were carefully cut and collected from the sediment cores on the catwalk and stored either in pressure vessels or in liquid nitrogen for shore-based experiments and later transported to Tokyo by air. Hydrate samples were often observed as small fragments contained in highly disturbed sediment cores and were collected in bulk phase, but a large, nearly pure, hydrate sample from Site 997 was collected from whole-round cores.
The samples were first measured by X-ray computed tomography (CT) imagery at the Technology Research Center (TRC) of Japan National Oil Corporation (JNOC). Nuclear magnetic resonance (NMR) measurements were carried out on crushed/powdered hydrate samples at Research Center of Japan Petroleum Exploration (JAPEX) in January 1996. Water and gas from selected samples were analyzed for molecular and isotopic composition at Tokyo University and JAPEX. Well-preserved samples were sent to Hokkaido University for the low-temperature, X-ray diffraction analysis and to Hokkaido National Industrial Research Institute (HNIRI) for Raman spectroscopy in August 1996. Also, thermal conductivity was measured at the Fundamental Research Laboratory (FRL) of Osaka Gas and the equilibrium dissociation experiment was conducted at Frontier Technology Research Institute (FTRI) of Tokyo Gas from August through October of 1996.
The results of this series of analyses and measurements are reported in this article. These data improve our ability to estimate the amount of gas hydrate trapped in sediments and to quantify the potential importance of marine gas hydrate as a future natural gas resource.