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How are oil sands and heavy oil formed?

Like other forms of petroleum, the formation of heavy oil and bitumen began with plants using solar energy to convert carbon dioxide and water into oxygen and carbohydrates through a process known as photosynthesis. When the plants, primarily algae, and small organisms (plankton) that fed on them died, the sediments containing the remains became buried at the bottom of a vast inland sea. As the depth of burial increased, heat and pressure transformed the carbohydrates into hydrocarbons. Coal is a solid hydrocarbon derived from land plants. Oil is a liquid hydrocarbon derived primarily from simple marine plants and animals, and natural gas is a gaseous hydrocarbon derived from either terrestrial or marine materials at a higher temperature and pressure than coal or oil.

Oil formation takes place in source rocks, usually very fine-grained rocks known as black shales. Once the oil is formed, continued pressure from overlying rock strata forces the oil to migrate through permeable rock layers until it is trapped in reservoirs of porous sedimentary rock such as sandstone or limestone, or until it escapes at the surface.

The liquid hydrocarbons that comprise the oil include a range of light to heavy compounds. The molecules of light compounds contain a few atoms of carbon surrounded by hydrogen atoms. The molecules of heavy compounds consist of many more carbon atoms and relatively fewer hydrogen atoms. Wax, grease, tar and asphalt are examples of heavy compounds.

As the eons passed, the oil-bearing sediments were covered by more than a kilometre of sedimentary rock. Then, about 50 million years ago, vast amounts of the liquid hydrocarbons migrated more than 100 kilometres eastward and upward until they reached and saturated large areas of sandstone at, and just below, the surface of what is now northern Alberta.

Micro-organisms present in the sandstone slowly consumed the hydrocarbons, beginning with the lightest. The heavy oil and bitumen now being produced from the area are the remnants of that migration. They are still the world’s largest known hydrocarbon resource deposit, but scientists believe the amount of crude oil digested by the micro-organisms was two or three times what remains.

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