Context: Scientists have said that the massive methane hydrate deposits of biogenic origin in the Krishna-Godavari (KG) basin and near the coast of Andaman and Mahanadi make it necessary to study the associated methanogenic community.
- Even the lowest estimate of methane present in the methane hydrates in KG Basin is twice that of all fossil fuel reserves available worldwide.
- Methane hydrate deposits are believed to be a larger hydrocarbon resource than all of the world’s oil, natural gas and coal resources combined.
- The current challenge is to inventory this resource and find safe, economical ways to develop it.
What is Methane Hydrate?
- Methane hydrate is a crystalline solid that consists of a methane molecule surrounded by a cage of interlocking water molecules.
- Methane hydrate is an “ice” that only occurs naturally in subsurface deposits where temperature and pressure conditions are favorable for its formation.
- Most methane hydrate deposits also contain small amounts of other hydrocarbon hydrates.
- These include propane hydrate and ethane hydrate.
Where are the Methane Hydrate Deposits?
- Four Earth environments have the temperature and pressure conditions suitable for the formation and stability of methane hydrate.
- Sediment and sedimentary rock units below Arctic permafrost;
- Sedimentary deposits along continental margins;
- Deep-water sediments of inland lakes and seas; and,
- Under Antarctic ice.
- With the exception of the Antarctic deposits, methane hydrate accumulations are not very deep below Earth’s surface.
- In most situations the methane hydrate is within a few hundred meters of the sediment surface.
How are Methane Hydrates produced?
- Methane gas is primarily formed by microorganisms (methanogens) that live in the deep sediment layers and slowly convert organic substances to methane.
- These organic materials are the remains of plankton that lived in the ocean long ago, sank to the ocean floor, and were finally incorporated into the sediments.
- Methane hydrate is formed when hydrogen-bonded water and methane gas come into contact at high pressures and low temperatures in oceans.
- But with increasing depth into the thick sediment layers on the sea floor, the temperatures begin to rise again because of the proximity to the Earth’s interior. In sediment depths greater than about 1 kilometre the temperatures rise to over 30 degrees Celsius, so that no methane hydrates can be deposited.
- This, however, is where the methane formation is especially vigorous.
- First, small methane gas bubbles are produced deep within the sediment.
- These then rise and are transformed to methane hydrates in the cooler pore waters near the sea floor.
- So, the methane is formed in the deep warm sediment horizons and is converted and consolidated as methane hydrate in the cold upper sediment layers.
- No methane hydrates are found in marginal seas and shelf areas because the pressure at the sea floor is not sufficient to stabilize the hydrates.
- At the bottom of the expansive ocean basins, on the other hand, where the pressure is great enough, scarcely any hydrates are found because there is insufficient organic matter embedded in the deep-sea sediments.
- The reason for this is that in the open sea the water is comparatively nutrient poor, so that little biomass is produced to sink to the sea floor.
- Methane hydrates therefore occur mainly near the continental margins at water depths between 350 and 5000 metres.
- For one reason, enough organic material is deposited in the sediments there, and for another, the temperature and pressure conditions are favourable for methane to be converted to methane hydrates.
Greenhouse Gas Formation
- At low temperatures the methane hydrates on the sea floor are stable, but if the water and the sea floor become warmer, then the hydrates can break down.
- Because microorganisms then oxidize the resulting methane gas to form the greenhouse gas carbon dioxide (CO2), methane hydrates have recently become a topic of intense discussion within the context of climate change.
- Methane, which itself acts as a strong greenhouse gas, does not escape directly out of the sea as methane because it is transformed into CO2.
- But the formation and release of carbon dioxide are considerable.
- An additional problem is that the oxygen in seawater is consumed through the formation of carbon dioxide.
- Many bacteria use methane to provide energy for their metabolism. Some bacteria break the methane down with the help of oxygen. This is called aerobic oxidation. Other bacteria do not need oxygen. This kind of oxidation is called anaerobic.
- Scientists therefore fear that large quantities of methane hydrate will melt there in the future, releasing increased amounts of CO2 into the ocean and the atmosphere. The oxygen content of the seawater will decrease accordingly.
- Furthermore, the CO2 released not only contributes to further global warming, it also leads to acidification of the oceans.
- Ocean acidification is the name given to the ongoing decrease in the pH and increase in acidity of the Earth’s oceans, caused by the uptake of anthropogenic carbon dioxide (CO2) from the atmosphere.