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Origin of Oil and Gas
Nowadays there are two main theories explaining the origin of petroleum or oil and natural gas - organic and inorganic ones. However, it has not been possible to determine the exact origin because it has not been possible to identify the exact place or materials from which any particular oil accumulation originated. The precise details regarding the problems of origin, migration and accumulation of petroleum have yet to be fully answered. Recent advances in analytical chemistry and geochemistry have advanced the knowledge and understanding, but issues remain to be resolved. The oil pool (field) is an end product to a 5-stage sequence of events: raw materials, accumulation, transformation, migration and geologic time. But the complication is that petroleums are complex mixtures of many hydrocarbons occurring in series with no two petroleums exactly alike in composition. This is probably due to variations in primary source materials and subsequent processes during formation such as pressure and temperature changes. Although the components of petroleum unite to form complex mixtures, the typical elemental chemical analysis indicates 10- 15% hydrogen and 82-87% carbon weight.
· heavy crude
· light crude
· methane gas
· propane gas
· butane gas
· cyclo-hexane gas.
The organic theory presumes that hydrogen and carbon that make up petroleum came from plants and animals living on land and in sea. This explanation is most generally accepted by scientists. Heat and pressure transformed the organic materials into solid, liquid or gaseous hydrocarbons known as fossil fuels- coal, crude oil or natural gas. Oil is typically derived from marine plants and animals. Natural gas can be formed from almost any marine or terrestrial organic materials, under a wide variety of temperatures and pressures.
The inorganic theory holds that hydrocarbons were trapped inside the Earth during the planet’s formation and are slowly moving upwards. According to this theory, the hydrogen and carbon were brought together under great pressure and temperature deep in the Earth to form oil and gas, which then found its way through porous rocks to collect in natural traps in the underground formations of the earth.
Due to the force of gravity and the pressure created by the overlaying rock layers, oil and natural gas seldom stay in the source rock in which they are formed. Instead, they move through the underground layers of sedimentary rocks until they either escape at the surface or are trapped by a barrier of less permeable rock. Most of the world’s petroleum had been found trapped in porous rocks under relatively impermeable formations. These reservoirs are often long distances away from the original source. A seep occurs when hydrocarbons migrate to the Earth’s surface. Over time, huge amount of these hydrocarbons have escaped into atmosphere. Flowing water can also wash away hydrocarbons. Sometimes only lighter, more volatile compounds are removed, leaving behind reservoirs of heavier types of crude oil.
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