Matteo, not sure in what context you ask the question. But on the short term, it could very well be, assuming the bottom shape of the hydrate facilitates a trapping geometry! But the hydrates itself are in-stable in that a sea level change, a temperature change (change of subsea currents), a geotectonic change can affect the hydrates such that the whole system goes back into the gas phase! 

regards

Dear Casper, thanks a lot for your answer. Yes, my question was general. I just wanted a flavor of the scientific community thoughts about this topic, in short and long time scales. I think that in the short term also host lithology, hydrate saturation and mode of occurrence play a key role.

Matteo, you are absolutely right. 

On your other question, re gas leakage. Just looking at the very many gas escape structures on the seabed, I would guess that leakage through capillary invasion is a minimal, almost neglectable process.

In addition the very small differential between the fracture gradient and the reservoir pore pressure at shallow depths will facilitate catastrophic gas escape events, simply because gas charge is dynamic and at some point, when the differential pressure exceeds the fracture initiation gradient, the trap will be breached, creating collapse crater type features on the seabed!

regards

Dear Matteo,

I think that the answer to your first question is yes. The gas hydrate layer forms a self generating near surface seal that can trap free gas below it.  There is a large body of Russian literature on the subject of gas hydrates in Siberia and it appears that the methane is mostly biogenic in origin.

See:-

Skorobogatov, V.A., Yakushev, V.S. and Chuvilin, E.M., 1998. Sources of natural gas within permafrost; North-West Siberia. In Permafrost Proceedings Seventh International Conference, Collection Nordicana (Vol. 57, pp. 1001-1007).

and also:-

Collett, T.S., Lee, M.W., Agena, W.F., Miller, J.J., Lewis, K.A., Zyrianova, M.V., Boswell, R. and Inks, T.L., 2011. Permafrost-associated natural gas hydrate occurrences on the Alaska North Slope. Marine and Petroleum Geology, 28(2), pp.279-294.

With regards to your second question on subsurface identification of free gas, this is the classic shallow gas drilling hazard problem.  Good quality high frequency shallow seismic to identify the low velocity zone associated with the presence of free gas are a requirement.  The key metric of vertical seismic resolution to observe the free gas zone can be an issue. I have observed horizontal near seabed flat spots on deep water marine 3D seismic in the South Atlantic in areas where porous strata are deformed to near vertical by allochthonous salt diapirs.  The vertical geometry of the disturbed beds permits easy identification of the seabed gas hydrate zone and its associated free gas flatspot.