Petroleum contains solar energy stored as chemical energy. Many steps are involved in the conversion from the simple radiant energy of the sun to the complex molecules of hydrocarbons. Coastal waters, rich with nutrients brought in by rivers and upwelling deep-sea currents, support an elaborate community of organisms ranging from microscopic, single-celled plants and animals to large predatory fish and mammals. Some of the smallest and simplest of these organisms perform the first capture and conversion of the sun's radiant energy.
The bulk of the living matter in such biotic communities is in the form of microscopic or near-microscopic simple organisms: protozoa (animals) and algae (plants). The algae are photosynthetic: they can synthesize their own food, simple sugars and starches, out of water and car bon dioxide, using the energy of sunlight. Other organisms consume the algae and convert the simple carbohydrates into more complex foods, such as proteins and fats; still larger organisms, in turn, eat these.
Each level of the food chain contributes waste products, and every organism that is not eaten eventually dies. In recycling this organic material, an important role is played by a diversity of bacteria. The two principal types are those that live in aerobic (oxygenated) environments and derive their energy by oxidizing organic matter and those that live in anaerobic (reducing) environments by taking the oxygen from dissolved sulfates and organic fatty acids to produce sulfides (such as hydrogen sulfide) and hydrocarbons. Although aerobic decay liberates certain hydrocarbons that some small organisms accumulate within their bodies, the anaerobics are more important in the formation of oil.
If the process of aerobic decomposition continues indefinitely, all organic matter, including hydrocarbons, is converted into heat, water, and carbon dioxide - the raw materials that photosynthetic plants use to make their carbohydrate food. For an accumulation of petroleum to be formed, the supply of oxygen must be cut off. Most areas along the coast are well aerated by circulation, wind, and wave action. In some areas, however, physical barriers such as reefs or shoals hinder aeration; and in deeper waters far offshore, the water below a certain depth is similarly depleted of oxygen. Here organic waste materials and dead organisms can sink to the bottom and be preserved in an anaerobic environment instead of being decomposed by oxidizing bacteria. The accumulation and compaction of impermeable clay along with the organic matter help seal it off from dissolved oxygen. Thus isolated, it becomes the raw material that is transformed into petroleum by the heat and pressure of deeper burial.
Even in areas with appreciable circulation and oxygenation, organic debris can accumulate so fast that it is quickly buried beyond the reach of aerobic organisms. Locations where this is likely to occur include salt marshes, tidal lagoons, river deltas, and parts of the continental shelf. Epeiric seas such as those that covered much of North America during the Permian and other periods offered broad stretches of warm, shallow water where, unstirred by ocean currents and tides, abundant organic debris could accumulate in an anaerobic environment.
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