Geophysical Signatures for Low Porosity Sand Can Mimic Natural Gas Hydrate

Abstract:

Natural gas hydrate is identified in sand reservoirs by an increase both the measured compressional velocity and resistivity. The same geophysical signatures can 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 in 2009 by the US Gas Hydrate Joint Industry Project. The sand interval in AC21 has an increase in measured resistivity (~2.2 Ω-m) on geophysical well logs and a strong peak and trough at the top and bottom of the sand on exploration seismic, which has been interpreted as a natural gas hydrate reservoir with saturations up to 20%. We reexamine the geophysical data and construct a new reservoir model that matches the measured resistivity, the high-density sub layers in the sand, and the surface seismic trace. Our modeling shows 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 relatively to a porosity of ~42% in the surrounding marine muds. More broadly, we show that the mean depth where the porosity of marine muds becomes lower than sand sediment is ~900 mbsf, meaning that the similar geophysical signatures for water-saturated sand and low saturations of natural gas hydrate probably occur at most gas hydrate sites worldwide.