Natural sources of hydrocarbons
Oil spills are always thought
of as affecting an otherwise hydrocarbon-free
marine environment so the percentage of discharge from “natural sources”
may come as a surprise to some readers.
Various coastal areas located around eroded sedimentary basins or faults between plates of the Earth’s crust harbour natural seepage of fossil fuels. This is the case in particular on the coasts of Alaska, California, the Gulf of Mexico, the Red Sea, the Caspian Sea and Borneo Island, where sheen and oil slicks are regularly seen independently of any pollution caused by man. There also exist, inland and on certain shores, outcrops of asphaltic sands, from which hydrocarbons are carried into the water system or directly into the sea. These sites are historically the source of the first human exploitations of oil.
The reference works consulted agree on an estimated total of around 250,000
tonnes per year of seepage from natural sources for the period 1990-2000,
which represents 10% of all oil released into the marine environment. However,
no exhaustive inventory of natural seepage locations exists and this figure
could therefore be out by a factor of ten. We should be cautious of routinely
accusing man when confronted with a localised oil spill, evidence of tar
balls or sheen in inshore pools. Nature itself may have a role to play in
these unpleasant phenomena.
Did you know?
Natural seep at Coal Oil Point
The most studied natural seep field is in California, near Santa Barbara, off the coast from Coal Oil Point Natural Reserve. The seepage is estimated at 150,000 m3 of gas and 20 tonnes of oil per day. This seep is the subject of many publications of the University of California.
It is important to know the locations of natural oil seeps. These sites are important sources of information on the influence of oil on the natural environment. They also rightly reduce the notion of the purity of natural sites, which is asserted whenever an oil spill is caused by human activity.
We can neither stop natural seepage today, nor predict seeps which may appear tomorrow. We can only respond to their effects where necessary, with the same techniques and means as for pollution provoked by man.
The submarine the Nautile discovered an oasis of life 2,000 m deep, a haven for flourishing fauna thriving on methane
Seeps of mineral hydrocarbons are not the only natural source of hydrocarbons in the marine environment. Life is also a source of hydrocarbons. Many hydrocarbons are in fact natural minor components of living matter. A number of oils and aromatic essences are present in many vegetal and animal species as biogenic hydrocarbons. When organisms which produce these substances die, hydrocarbons are released on the seabed and into sediments. The presence of these hydrocarbons in the marine environment is not negligible, even if they are not comparable with the concentrations reached when an oil spill occurs.
A site’s return to its original state following an oil spill does not therefore necessarily imply a total absence of hydrocarbons in the water and sediment. The mineral hydrocarbons from the spill disappear in time, either by mechanical removal or by degradation into products which can be assimilated by the natural environment.