Gas hydrates in coarse-grained reservoirs interpreted from velocity pull up: Mississippi Fan, Gulf of Mexico: COMMENT

Madof (2018) invokes the presence of high-velocity natural gas hydrates to describes a velocity pull up (VPU) associated with a nearseafloor turbidite channel complex on the Mississippi Fan. If the channel complex is indeed a high-saturation hydrate reservoir, it is by far the largest hydrate reservoir in the world. Consequently, seismic VPUs may become a powerful tool in remote detection of hydrate systems. The VPU below the turbidite complex indicates that the velocity of the system within the turbidite complex is larger than surrounding sediments. Madof (2018) suggests that ‘dense water-filled sands encased in uncompacted (porous) muds’ may be a possible cause of the VPU but then dismisses this possibility suggesting that the observed VPU is larger than any sediment-related velocity variation. We argue, however, that the variation in sediment velocity for water-saturated sands within the channel and water-saturated marine muds outside the channel complex probably explains part, and may potentially explain all of the VPU. Clay-rich marine muds settle at very high porosities of 60–90% at the seafloor, but undergo significant compaction (reducing 20–30 porosity units) within the first several hundred meters of sediment (Kominz et al., 2011). In contrast, sand sediments deposit at the seafloor at much lower porosities (28%–42%) and undergo very minor compaction and porosity reduction in the same interval (Beard and Weyl, 1973; Kominz et al., 2011). This difference in depositional porosity and compaction rates causes a significant difference in the physical properties in the near seafloor environment, where sands have significantly higher density and higher velocity in the first several hundred meters (Fig. 1). On average, in fact, sand sediments have a higher velocity than marine muds for the first kilometer from the seafloor, though the velocity contrast reduces with depth in the sediment column (Cook and Sawyer, 2015). These traveltime variations between coarser-grained sediment in the turbidite complex and surrounding marine muds can accumulate and could result in a VPU.