Geophysical signatures for low porosity can mimic natural gas hydrate: An example from Alaminos Canyon, Gulf of Mexico

摘要

Natural gas hydrate in sand sediments can increase both the measured compressional velocity and resistivity. The same geophysical signatures occur, however, in low-porosity sand. We investigate the possible occurrence of natural gas hydrate in a sand interval in Alaminos Canyon Block 21 (AC 21) in the Gulf of Mexico, drilled by the U.S. Gas Hydrate Joint Industry Project. The sand interval has an increase in resistivity (~2.2Ωm) and a strong peak and trough at the top and bottom of the sand on exploration seismic, which has been interpreted as natural gas hydrate. We reexamine the logging data and construct a new reservoir model that matches the measured resistivity, the high-density sublayers in the sand, and the surface seismic trace. Our model shows that the sand interval in AC 21 is most likely water saturated; and the slight increase in resistivity, higher-measured density, and the seismic amplitudes are caused by a reduction in porosity to ~30% in the sand interval relative to a porosity of ~42% in the surrounding marine muds. More broadly, we show that the average depth where the porosity of marine muds becomes lower than sand sediment is 900 mbsf, though it could be as shallow as 600 mbsf for high-porosity sands. In any case, the similar geophysical signatures for water-saturated sand and low saturations of natural gas hydrate in sand probably occur throughout the gas hydrate stability zone at most sites worldwide.