Irregular BSR: Evidence of an Ongoing Reequilibrium of a Gas Hydrate System

Abstract

Gas hydrate (GH) systems constitute methane sinks sensitive to environmental changes such as pressure, temperature, and salinity. It remains a matter of debate as to whether the large GH system of the Black Sea has reached a steady state since the last glacial maximum (LGM). We report on an irregular free gas distribution in specific sediment layers marking an irregular bottom-simulating reflector (BSR). This anomalous free gas distribution revealed by very high resolution seismic images, acquired by a deep-towed multichannel seismic system, might be evidence of an ongoing migration of the base of the GH stability zone (GHSZ). We show that the reequilibrium is not occurring homogeneously as overpressure from hydrate dissociation slows their decomposition in specific sedimentary layers. The Black Sea example highlights that dissociation and the associated methane release in the water column or even in the atmosphere could be largely delayed by overpressure accumulation. Plain Language Summary Methane hydrate is an ice-like compound composed of a cage of water molecules enclosing a methane molecule. Hydrates can form where water and methane are present under high pressure and low temperatures, for example, in deep-sea sediments. As a result of climate change (e.g., seawater temperature increase), hydrates can melt and release free gas and water. Yet we observe that hydrates are present where they should have melted according to modeling. We explain this irregular melting by differing properties of the host sediments and different quantities of hydrate in the sediments. Methane in the Earth's atmosphere is a strong greenhouse gas. The release of methane from hydrate melting has been proposed as a runaway process where the methane released increases global warming, which further increases hydrate melting and methane release, repeating the cycle. Our results show that the destabilization of a hydrate system is actually a slow process, spanning several millennia. As such, a catastrophic destabilization of a gas hydrate system is unlikely.

Gas origin linked to paleo BSR

“…2 and 4 ), support the fluid circulation. Similar features have been recognised in other areas 66 – 68 . On the basis of the above considerations, we draw the conclusion that the unexpected thick FG zone in this active margin 65 is due to a deep gas supply.…”

Slow Dynamics of Hydrate Systems Revealed by a Double BSR

“…The primary, shallower BSR (BSR1), and the secondary, deeper (BSR2), cross cut stratigraphic layers and mimic the seafloor, while exhibiting a polarity reversal. Their positions can be identified through aligned amplitude terminations (Colin et al., 2020), and by the presence of both attenuation of high frequencies and lower velocity below the distinct BSRs (Figures 3 and 4). BSR2 is patchy, 130 m beneath the BSR1, weaker in amplitude in comparison with BSR1 and attenuation is stronger under BSR2.…”

“…(2019) and Colin et al. (2020) in the study zone, due to the conjunction of low permeability in fine‐grained sediment, and the process of hydrate re‐crystallisation during decomposition, which can slow down the dissipation process below the BGHSZ (Sultan, 2007).…”

A state-of-the-art review and prospect of gas hydrate reservoir drilling techniques

“…To identify gas hydrate, geophysical methods such as multichannel seismic surveys, seabed profiles, and controlled source electromagnetic (CSEM) are frequently used (Minshull et al, 2020;Liang et al, 2020;Crutchley et al, 2010). Bottomsimulating reflectors (BSR), a special physical interface generated by seismic profiles, are the earliest and most widely used, reliable, and intuitive geophysical markers to confirm the occurrence of gas hydrate (Foschi et al, 2019;Colin et al, 2020). Seabed geological sampling, microbial exploration techniques, and seabed visual exploration techniques such as remotely operated vehicles (ROV), ocean floor observation systems (OFOS), TV grabs, and deep-towed systems provide a more robust foundation for determining the presence of gas hydrate (Su et al, 2020).…”