Joint Oceanographic Institutions, United States

Most published gas hydrate volumetric estimates have of necessity been made by broad extrapolation based on only general knowledge of local geologic conditions. The amount of gas that might be stored in a gas hydrate accumulation is dependent on a number of reservoir parameters, including the areal extent and thickness of the gas hydrate occurrence, sediment porosities, and the degree of gas hydrate saturation. Downhole logs often serve as a source of critical gas hydrate reservoir data. Before Ocean Drilling Program (ODP) Leg 204, however, only about seven gas hydrate occurrences had been sampled and surveyed with open hole well logging devices. From 16 July through 25 July 2002, a complex logging-while-drilling (LWD) and measurement-while-drilling program was conducted on Hydrate Ridge during ODP Leg 204. During the cruise, and for the first time during ODP, the LWD tool string included Schlumberger's Nuclear Magnetic Resonance While Drilling tool (proVision). Postcruise research with the proVision data has consisted of estimating bound fluid volumes and total fluid porosities, along with comparing nuclear magnetic resonance (NMR)-derived porosities with those derived from downhole measured neutron and density logs and core measurements. These efforts have focused on calculating gas hydrate concentrations by indirect methods using NMR data. It has been shown that it is possible to obtain gas hydrate saturations (percent of pore space occupied by gas hydrate) from downhole logging-measured NMR porosities. In this study, porosities derived from the NMR proVision tool were used along with porosity data from the Azimuthal Vision Density Neutron tool to calculate gas hydrate saturations at all nine LWD logging sites on Hydrate Ridge. The NMR-density-porosity relation yielded gas hydrate saturations ranging from high values near the crest of the ridge of approximately 50% and higher in Hole 1249A to much lower values along the flanks of the ridge.