Colorado School of Mines, United States

Leg 164 of the Ocean Drilling Program (ODP) was designed to investigate the occurrence of gas hydrate in the sediments beneath the Blake Ridge on the southeastern continental margin of North America. Resistivity and acoustic well logs from Sites 994, 995, and 997 indicate the presence of gas hydrate in the depth interval between 185 and 450 mbsf on the Blake Ridge. Resistivity log calculations suggest that the gas-hydrate-bearing sedimentary section on the Blake Ridge may contain between 2 and 11 percent bulk volume gas hydrate. However, because the resistivity calculations are based on empirical relations developed for conventional hydrocarbon accumulations, their usefulness in a gas hydrate system is unknown. A logging tool that shows promise of yielding gas hydrate reservoir data is the Schlumberger Gamma Ray Spectrometry Tool (GST). By combining elemental yields from neutron spectroscopy logs, reservoir parameters including porosities, lithologies, formation fluid salinities, and hydrocarbon saturations (including gas hydrate) can be calculated. Carbon/Oxygen (C/O) elemental data from the GST has been used to determine gas hydrate saturations at all three sites drilled on the Blake Ridge. Detailed analyses of the carbon-oxygen content of the sediment matrix, borehole and formation fluids have been used to construct C/O fan charts for the geological conditions on the Blake Ridge, which were used to calculate gas hydrate saturations. The C/O calculated gas hydrate saturations (ranging from 3 to 14 percent bulk volume gas hydrate) from the Blake Ridge compare favorably to, although are higher than, the resistivity-derived gas hydrate saturations.