The Gulf of Mexico Gas Hydrate Joint Industry Project (JIP) identified the Diana Basin, located in the Alaminos Canyon and East Breaks Blocks of the Gulf of Mexico, as a potential gas hydrate bearing sand reservoir. In 2009, JIP Leg II drilled two logging-while-drilling (LWD) holes in Alaminos Canyon to evaluate the sand reservoir. Initial results from the logging-while-drilling (LWD) geophysical logs were inconclusive, showing a moderate increase in measured resistivity and compressional velocity. A significant borehole washout also occurred in the sand reservoir. A recently published modeling study concluded that a relatively low saturation of hydrate (less than 30%) occupies the pore space in the sand reservoir. However, our modeling results contradict this finding. We used the propagation resistivity logs, which were not affected by the borehole washout, to calculate porosity. Porosity values derived from the resistivity logs range from approximately 20% to 24% in the sand. Velocities derived from the calculated resistivity/porosity curve were significantly higher than the LWD velocity, ranging from 1700 to 2450 meters per second (m/s). The derived velocities were used to generate synthetic seismic traces. The synthetic seismic traces show excellent agreement with the original surface seismic trace. Our results suggest that no gas hydrate exists within the sand reservoir in the Diana Basin. The moderate increase in resistivity and compressional velocity is due to the decrease in porosity to 20–24% in the sand, in comparison to a porosity of about 45% in the surrounding unconsolidated clay layers.
Natural gas hydrate is a solid, ice-like substance that forms when a gas molecule is enclosed by a host of water molecules at high pressures and low temperatures (Sloan, 1991; Sloan, 1998). Because of the low temperatures required for gas hydrate formation and stability, the majority of global gas hydrate are found in permafrost and marine sediments on continental margins. In the Gulf of Mexico, the required pressure-temperature regime typically occurs in continental margin sediments in water column depths greater than 400 m (Milkov and Sassen, 2001). The base of the gas hydrate stability zone (GHSZ) is controlled by the heat flow in the sedimentary column. In general, the deeper the water column is, the thicker the GHSZ is in the sediments.
The recent successful production of offshore gas hydrate in Japan suggests that gas hydrates may be a energy resource exploited in the future. Exploration for gas hydrate in shallow marine sediments will likely increase. Entities seeking to develop gas hydrate deposits for energy will need a reliable method for interpreting exploration measurements in order to accurately locate and quantify gas hydrate deposits. Seismic prospecting is one exploration method which has been demonstrated to successfully locate gas hydrate deposits. The Gulf of Mexico Gas Hydrate Joint Industry Project (JIP) used seismic prospecting to locate gas hydrate at Walker Ridge (McConnell, et al., 2009a) and Green Canyon (McConnell, et al., 2009b) in the Gulf of Mexico. However, gas hydrate's effects on sediment elastic properties are not completely understood, making interpretation of seismic exploration measurements inconclusive.