Hokkaido University, Japan

ODP Leg 164 drilled holes at Sites 994, 995, and 997 on the Blake Ridge and penetrated through the predicted depth of the BSR (Bottom Simulating Reflector) into the sediments below. Solid gas hydrate samples were recovered from these sites and were documented to be composed of a structure I methane hydrate with approximately 99% methane and volumetric ratio of gas to water of 130 to 160. High resolution pore water sampling and analysis have revealed that chloride concentrations are highly variable with local anomalously fresh values for the depth interval of 200 to 450 meters below sea-floor. These anomalies in chloride concentration indicate that a minimum of 1 to 2% of the sedimentary section of this variable zone is filled with gas hydrates. Well logs also indicated distinct zones of higher electrical resistivity and acoustic velocity that were coincident with chloride anomalies. The lower limit of hydrate-bearing sediments ( approximately 450 m) coincides with BSR on the seismic profile. A pervasive occurrence of free gas zone below the BSR has been indicated by anomalously low ( approximately 1.4 km/sec.) velocity values. Extrapolation of thermal gradients calculated from in situ temperature measurements predicts that the experimentally determined base of gas hydrate stability (BGHS) occurs 40 to 100 m below the observed BSR depth. This discrepancy may suggest that experimentally determined temperature-pressure conditions for the stability of methane hydrates are not applicable for the prediction of the stability of natural gas hydrate in marine sediments, or alternatively, gas hydrates are actually present in the interval between observed BSR and BGHS but they are so sparse that the drilling and well-loggings could not detect their existence.