Recognition of areas effected by petroleum seepage: Northern Gulf of Mexico continental slope

The presence of large amounts of gas and/or liquid hydrocarbon seepage in near surface sediments can produce distinct features including an irregular topography (on several scales, ranging from meters to kilometers); seismically transparent/chaotic sediments; oil staining; gas plumes; sediments containing elevated concentrations of extractable organic matter, organic carbon, and calcium carbonate; associated brine seepage and anoxic conditions; extensive bacterial mats; hydrate formation and decomposition; and dense chemoautotrophic communities. Although no single characteristic is always uniquely associated with seepage, the co-occurrence of several of these features is strongly suggestive of an area being exposed to non-indigenous upward migrating hydrocarbons.