Fiery ice from the sea: Marine gas hydrates

Abstract

Marine gas hydrates are cage-like structures of water containing methane or some higher hydrocarbons that are stable under conditions of high pressures and low temperatures. The hydrate structures are formed in sediments of continental margins and are found worldwide. The stability zone extends to about 200 m beneath the sea floor, and hydrates exist in several different forms within the zone, from massive ice-like features at cold seeps on the sea floor to finely distributed deposits in sediment pores over extensive areas. The base of the stability zone is characterized by a strong acoustic impedance change from high velocity hydrated sediments above to low velocity gas below. This acoustic feature generates a strong signal in seismic surveys called the Bottom Simulating Reflector, and it is widely used as an indicator of the presence of hydrates. This paper reviews the current knowledge of hydrate systems from research carried out on the Cascadia Margin off the west coast of Vancouver Island, and in the Gulf of Mexico. The hydrate distributions are different in each of these areas, leading to different effects in acoustic reflectivity.