Because of the implications of shallow gas accumulations to drilling and the integrity of platform foundations, it is important to determine whether gas accumulations can be considered as effectively static, unchanging for the duration of offshore operations, or dynamic Shallow gas is frequently seen In association with seabed features, such as mud lumps, gas domes and pockmarks These are considered to have been caused by the escape of gas through the seabed, consequently it can be implied that gas has migrated to, and through, the seabed. This indicates that shallow gas is not static, but it does not suggest a time-scale over which migration occurs. Is shallow gas mobile on a geological or a human time-scale' During 1977 and 1978 the British Geological Survey (then the Institute of Geological Sciences) undertook repeat analogue geophysical surveys of their South Fladen Pockmark Study Area as part of a programme of investigations into pockmarks, the mechanism by which they were formed and their hazard potential (McQuillinn et al, 1979). During these surveys three anomalously large pockmarks were discovered One of them, christened the ‘Witch's Hole’ during a previous survey, is close to an area of acoustic turbidity within the topmost sediments, the Witch Ground Formation.
This paper describes an analysis of data from these and other surveys collected from around the Witch's Hole, and considers whether or not there is any evidence of shallow gas migration, and whether any deductions can be made about the rate of migration.
Evidence of gas migration is mainly confined to seabed or near-seabed features and processes.
Gas rising from the seabed has been observed as plumes on echo sounder records, although there is a danger of confusion with the similar signature given by shoals of fish. In most instances these occurrences are observed only once, however the example shown In Figure 1 has been observed five times over a SIX year (1983 to 1989) time period
(Fig. 1 is available in full paper)
Geochemical surveys used in petroleum prospecting take the form of on-line analyses of the hydrocarbon content of near-seabed waters (‘Sniffer’ surveys) and analyses of the hydrocarbons in the pore spaces and adsorbed on clay particles of seabed sediments collected by coring (Sweeney, 1988). These surveys indicate a positive flux of hydrocarbons, principally methane, and may provide evidence of the source. Faber and Stahl (1984) related the hydrocarbon content of seabed sediments to source rock potential.
Fluids rising to and through the seabed are responsible for the formation of several seabed features including mud volcanoes, mud mounds and pockmarks Of these the most widespread are pockmarks which occur over an area of approximately 100,000 km2. In the North Sea even though they are restricted to areas in which the seabed sediment is suitable for their formation Although this implies a vast number of gas seepage sites it is probable that only a small proportion of the pockmarks are currently ‘active’.