Consider the following statements :
1. There are two distinct types of oceanic deposit with the most common being dominated by methane contained in a structure I clathrate and generally found at depth in the sediment.
2. The methane is isotopically light which indicates that it is derived from the microbial reduction of CO2.
3. The clathrates in these deep deposits are thought to have formed in situ from the microbially produced methane, since the ?13C values of clathrate and surrounding dissolved methane are similar.
4. However, it is also thought that fresh water used in the pressurization of oil and gas wells in permafrost and along the continental shelves world wide, combine with natural methane to form clathrate at depth and pressure, since methane hydrates are more stable in fresh water than in salt water.
5. Local variations may be very common, since the act of forming hydrate, which extracts pure water from saline formation waters, can often lead to local, and potentially significant increases in formation water salinity.
6. Hydrates normally exclude the salt in the pore fluid from which it forms, thus they comprise high electric resistivity just like ice, and sediments containing hydrates have a higher resistivity compared to sediments without gas hydrates .
7. These deposits are located within a mid-depth zone around 300–500 m thick in the sediments (the gas hydrate stability zone, or GHSZ) where they coexist with methane dissolved in the fresh, not salt, pore-waters.
Select the correct answer using the codes given below.
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