Large craters and associated melt-water channels on the seabed may indicate the leakage of hydrocarbons from deeper reservoirs or source rocks.
An investigation of Quaternary glacial sediments using three-dimensional seismic and high-resolution aeromagnetic data in and around quadrant 25 in the northern North Sea has revealed numerous circular depressions.
The largest occurred at or near to the points where sub-glacial melt-water channels had been initiated.
With diameters ranging from 500 to 3,000 metres, these craters are encountered at depths of 20 to 300 metres below the seabed.
They are thought to have formed in a similar fashion to mud volcanoes through such processes as the expulsion of gas from melting gas hydrates (ice-like substances) and the expulsion of glacial melt water which subsequently carved out the channels.
The driving forces were temperature and pressure changes induced by alternating advances and retreats of glacial ice during the Quaternary period.
An increase in the number of craters above a major fault and their general correlation with Tertiary gas fields suggest that some of the gas may have originated from greater depths.
Areas containing clusters of large craters may be indirect indicators of hydrocarbon leakage from underlying reservoirs or source rocks, thus providing additional evidence of regional prospectivity.
Phenomena such as these can also provide timely warning of drilling hazards, which can result in delays or – at worst – catastrophic blowouts.
