Hydrates are crystalline compounds similar to ice, which form when water comes into contact with small gaseous molecules. The conditions necessary for the formation of natural deposits of methane hydrates are found both in continental sedimentary rocks in the polar regions, with average surface temperatures below 0 °C, and in oceanic sediments at a depth greater than 300 m, where the water temperature is lower at 2 °C.
Although, in theory, thermobaric conditions allow the formation of methane hydrates on 90% of the world's ocean floors, the main deposits have been identified at the margins of continental platforms, where the formation of deposits is favored by the supply of organic material and rapid burial and extraction is favored by easier accessibility.
Currently, the main problems for the exploitation of these deposits are the instability of the submarine slopes, due to the phase transition of the hydrates and the release into the atmosphere of methane gas coming from the dissociation of the hydrates, which could contribute to the greenhouse effect.
Natural gas hydrates were known to chemists as early as the 1900s, although the study of their properties and formation and dissociation phenomena was tackled in the second half of the 20th century, following the blockage of the Siberian gas pipelines.
It was thus ascertained that the Siberian areas possess the ideal environmental conditions for the formation of a solid material similar to ice and containing methane molecules within it. However, only since the end of the 20th century, thanks to international research programmes, has it been established that methane hydrates represent an energy resource open to exploitation, present in the most superficial geosphere on the continental margins within sedimentary sequences and in permafrost of the polar regions.
The release of methane from hydrates is mainly due to two factors. Thermogenic, the gas originates following the thermal alteration of the organic matter contained in the parent rocks during their progressive sinking into the sedimentary basins.
Biogenic, the gas is produced by the decomposition of organic matter due to the activity of methanogenic bacteria which operate roughly from the water-sediment interface up to a few hundred meters of depth.