Gas Hydrates

What are Gas Hydrates?

Gas hydrates are naturally ocurring, crystalline, ice-like substances composed of gas molecules (methane, ethane, propane, etc.) held in a cage-like ice structure. (clathrate).

The formation and stability in the subsurface of these structures are constrained by a relatively narrow range of high pressure and low temperature and depend on the influx of free gas and the amount of gas dissolved in the pore fluid.

Hydrates are a concentrated form of natural gas compared with compressed gas, but less concentrated than liquefied natural gas. It is estimated that a significant part of the Earth's fossil fuel is stored as gas hydrates, but as yet there is no agreement as to how large these reserves are.

Where are they Found?

They are found abundantly worldwide in the top few hundred meters of sediment beneath continental margins at water depths between a few hundred and a few thousand feet. They are present to a lesser extent in permafrost sediments in Arctic areas.

In the marine environment the gas hydrate stability zone is determined by water depth, seafloor temperature, pore pressure, thermal gradient and the gas and fluid composition. The base of the zone in which hydrate can exist is limited by the increase in temperature with depth beneath the seabed.

Bottom-simulating reflectors (BSR's) are a principal indicator of marine methane hydrates

Indicators

The occurrence of hydrates can be estimated in well logs, in particular electrical resistivity and sonic logs.

Gas hydrate bearing sediments show anomalously high electrical resistivity and high acoustic velocities. At the base of the gas-hydrate stability zone, which marks the contact between gas-hydrate and free-gas-bearing sediments, a distinct drop in acoustic velocity often characterizes the acoustic log.

Currently, the principal indicator of marine methane hydrates is the detection of bottom-simulating reflectors (BSR's) on seismic data. Unfortunately, in older data these may have been processed away as they were not recognised for what they are. Reprocessing existing data, concentrating on the shallow section and the BSR, should improve estimates of the extent of this resource.