Abstract: Hydrates of Natural Gas--Important Agent in Geologic Processes

By

Richard D. McIver

Solid, ice-like mixtures of natural gas and water, called "gas hydrates" or "clathrates," have been found under the permafrost in Arctic basins; and they are almost certain to occur in the first few hundred meters of some sediments under deep (>300 m) water in the oceans.

Gas-water clathrates may be important in explaining some geologic phenomena because they are ice-like; thus hydrated formations retain their water, and normal consolidation cannot take place. Hydrates can also act as barriers and prevent movement of other fluids through them; they may form reservoir seals and traps where there is neither structure nor facies change. Moreover, hydrates decompose when increased temperatures or decreased pressures render them unstable, and the large supply of gas released may help maintain formation pressure when contiguous reservoired hydrocarbons are produced.

When the pressure-temperature regime of a hydrated formation is disturbed by deeper burial, change of sea level, etc., and the hydrate decomposes, the once-solid sediment contains anomalous volumes of water and large volumes of gas, and the internal pressure is increased. If the volume of decomposed hydrate is large, enormous stresses may be imposed on surrounding sediments and may trigger movement of overlying sediments as mudslides or turbidites down slopes, or of the unconsolidated sediment as diapirs, mud volcanoes, etc., at points of weakness.

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