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Evidence of hydrocarbon seepage in the marine environment has been documented back to prehistoric times. Hydrocarbons are held in structural and stratigraphic traps by overlying, impervious layers, and they can escape only if the capping layer is destroyed. Escape channels through the capping layer are formed primarily by faulting, which may be essentially due to tectonic forces, as in the Santa Barbara Channel area, or may be due to diapiric uplift as in the Gulf of Mexico.
Unconsolidated sediments of varying thickness overlie consolidated sediments and bedrock in most marine environments. Because of the incompetency of these sediments, fault traces usually are not transmitted to the surface as open fractures. Hydrocarbons are forced to flow or bubble up through these sediments in a manner that may be analogous to diapirism. The more viscous the hydrocarbon the more probable it is that traces will remain in the sediment. Traces have been found at various locations in the Gulf of Mexico and off the California coast.
Seepage is intermittent and aperiodic with unpredictable rates. Viscosity and the passage up through the sediments may be the controlling factor in the intermittent flow. Tidal effects may be significant.
Hydrocarbons have a specific gravity lower than that of seawater. There is no evidence that floating oil or tar through self distillation will increase enough in specific gravity to sink. It has been suggested that accretion of sediment particles could cause floating tar to sink. This could occur only along the surf zone, or at the mouths of heavily laden streams if at all.
It is, therefore, almost axiomatic that tar or tarry residues found in marine sediments originated from a natural seep. No oil from pumped bilges or ruptured fuel tanks ever sank to the bottom.
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