Statoil, Stavanger, Norway
Abstract: On the Self-Sealing Nature of Marine Seeps
Seeps in ocean and lacustrine environments are often ephemeral and self-sealing. Although the nature of the self-sealing processes vary from one area to the other, field-studies carried out at seeps at the Tommeliten hydrocarbon field in the North Sea, suggest three phases to be common:The methane gas seeps at Tommeliten occur at a water depth of 71 m, centrally in the North Sea above a buried salt diapir.The seeps were surveyed and sampled with the use of an ROV (remotely operated vehicle) in 1983. The gas is mainly methane, but contains small amounts of the heavier hydrocarbons ethane, propane, and butane, and is of thermogenic origin.
These seeps were re-surveyed 15 years later, in 1998, and other features were also mapped out.There are three types of seepage phenomena in the area:
1) Virgin seeps, where the gas comes directly from small vents in the
2) Bacterial mats (probably Beggiatoa sp.) where the gas accumulates in anoxic sediments below a thin layer of bacterial mats before occasionally venting through holes in the mats.
3) Authigenic carbonate cemented "reefs" where no visible gas is evident, but where sampling of carbonate nodules suggests that gas is migrating up to the lower part of the "reefs" within the sediments, where it probably oxidizes and is utilised in various processes, including the production of carbonate cements.
These modern results, therefore, suggest that the formation of the bacterial mats may represent the first phase of natural sealing.The formation of the "reefs", which host numerous sessile and filter-feeding organisms, represents the final phase in the natural seep sealing process. In other regions of the oceans, where other gas compositions, migration rates, and sediment conditions prevail other sealing processes will occur. On the continental shelf off MidNorway for example it is suspected that large bioherms and cold water coral reefs form over ancient seeps.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California