Unlike conventional natural gas, the methane in gas hydrates is largely the result of anaerobic bacteria acting on organic matter in the sediments below the sea floor. In areas of low sedimentation rates, low organic content and high oxygen content, aerobic bacteria acting on the organic matter generate carbon dioxide. However, where the sedimentation rate and organic content are high, the environment becomes anoxic (deficient in oxygen) at shallow sediment depths and anaerobic bacteria acting on organic matter generate methane. In certain of these environments, low temperature and high pressure act in concert to create the frozen hydrates.
Gas hydrates occur naturally where combinations of temperature and pressure favour the stability of gas hydrate over a gas-water mixture. The stability of gas hydrates increases with increasing temperature and pressure, but not as quickly as the rate at which temperature increases. Therefore, gas hydrates are stable in two settings; one, in very cold regions, such as in the Canadian north, where temperatures at the surface of the earth are low enough that gas hydrates are stable to a depth of about one kilometre, and two, at the bottom of the sea, where the water temperature is above freezing but the pressure of the overlying water column creates conditions where gas hydrates are stable.
Structure of a gas hydrate (methane clathrate) block embedded in the sediment of hydrate ridge, off Oregon, USA. The gashydrates have been found during a reaserch cruise with the German reseach ship FS SONNE in the subduction zone off Oregon in a depth of about 1200 meter in the upper meter of the sediment. The shown gas hydrate (white) has been deposited in thin layers into the sediment (Photo credit: Wikipedia)
As a result there are three major modes of occurrence for gas hydrates:
- Terrestrial accumulations that occur in regions of very cold surface temperature such as in permafrost regions and shallow arctic seas. During the last glacial period, very large amounts of seawater were locked up in the continental ice sheets and sea level was lower. As the ice sheets melted, sea level rose very quickly but the increase in temperature that is associated with this flooding has not yet been “felt” by the gas hydrates at greater depth in the earth. So, in regions like the Beaufort Sea, gas hydrates are relic hydrates that formed during the last glaciation.
- Sub-sea marine accumulations found tens to hundreds of metres below the seafloor.
- Sea floor accumulations. There are some relatively recently discovered pure gas hydrate outcrops at the sea floor in the deep waters of the Gulf of Mexico.
Natural gas hydrates occupy the pore space of conventional reservoirs in a manner similar to the occurrence of natural gas in a conventional reservoir. Gas hydrates can also act as seals for underlying natural gas reservoirs. Some of the gas in the gas hydrate stability zone is conventionally derived natural gas that is seeping toward the surface from lower reservoirs or source rocks.
English: Structure sI of gas hydrates Polski: Struktura sI gazowych hydratów (Photo credit: Wikipedia)
Seismic surveys are the most common tool used to locate marine gas hydrates. The contrast between the high-velocity hydrate-bearing strata and the low-velocity gas-bearing strata results in the bottom-simulating reflector (BSR), a seismic reflector that parallels the sea floor, since the sea floor is approximately a surface of equal temperature. Current seismic techniques cannot commonly resolve either the top of gas hydrate occurrence or the extent of gas possibly occurring below the gas hydrate occurrence.
However many gas hydrates, particularly in permafrost settings, have been found by drilling through them without prior knowledge of their existence. The same is true in many marine settings, including Canada’s East Coast, where few seismic indications for gas hydrates occur relative to the number of wells that have encountered them.
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