Friday 16 March 2012

Gas hydrate pingoes in the Kwanza Basin off the coast of Angola.

Gas (or clathrate) hydrates are ice-like compounds that form from a mixture of water and gases under pressure at low temperatures. Pingoes are dome shaped hills formed by ice in waterlogged soils in arctic conditions. In recent years geologists have become aware of structures referred to as 'gas hydrate pingoes', which are analogous to the arctic pingoes, but formed on continental shelves in sediments soaked by pressure liquified gasses.

In a paper in the March 2012 edition of the journal Geology, Christophe Serié and Mads Huuse of the School of Earth, Atmospheric and Environmental Sciences at the University of Manchester and Niels Schødt of Maersk Oil describe the discovery of a series of exceptionally large gas hydrate pingoes in the Kwanza Basin off the coast of Angola during seismic surveys carried out by the oil industry, and the conclusions drawn from this.

Map showing the area in which the structures where found. Serié et al. (2012).

The pingoes were located at depths of 800-1000 m on the flanks of a large salt diapir. A salt diapir is a hill formed by the evaporation of salt-saturated subsurface water, caused by the salt crystals taking up more space than the saturated fluid it replaced.

The diapir and accompanying hydrate pingoes. M3, M4, M5, M7 & M8 are interpreted as collapsed pingoes, M1, M2 & M6 as still standing pingoes. Serié et al. (2012).

Serié et al. interpret the presence of such structures on the flanks of gas hydrate pingoes as indicative of the gases being of hydrothermal origin; the same source dehydrating the lower (diapir) sediment layers (with a constant new supply of seawater brining more salt) and releasing gas, which rises through the diapir then forms hydrates when it meets seawater in the upper sediment layers, close to the surface.