Oil and Gas Formation
In terms of ingredients, oil and gas are derived mostly from the remains of dead plants, small marine organisms and bacteria. Any of these ingredients separately, or combined together, are referred to as "organic matter". In places such as swamps, forests, lakes and certain undersea environments, large amounts of organic matter can accumulate and become buried and preserved by other sediments. After burial, these deposits of organic matter are chemically altered into a substance called "kerogen". As time passes, more sediments continue to accumulate above and the kerogen is buried further and further beneath the surface. As the kerogen gets moved deeper within the earth, the temperatures around it begin to increase according to what is called the "geothermal gradient". (Deeper within the earth it gets hotter and hotter.) The geothermal gradient is, on average, about 30°C/km of burial.
Slowly over time, and if given enough heat (generally at least 100°C), the kerogen begins to change into oil. If the temperature continues to increase, the oil will then make the change into gas. However, if temperatures get too hot (exceeding approximately 200°C), any useful petroleum products will be combusted.
Once oil and gas have been generated they tends to flow or migrate away from the source rock (still containing the kerogen) through microscopic pores and cracks to other rock layers nearby where the pressure is lower. Rocks that have pores or cracks large enough to allow oil and gas to flow are called "permeable". If they don't contain pores or cracks, they they are called "impermeable". Oil and gas fields most often occur as petroleum migrates towards the surface and accumulates in rocks that have high porosity and permeability, and that are surrounded above and to the sides by impermeable layers. These permeable rocks are also referred to as "reservoirs". The impermeable layer(s) are then referred to as "seals". Collectively, these types of geologic structures are called "traps". The are two main types of traps: 1) Structural traps, whereby rock layers are folded (bent) by tectonic forces into suitable shapes, or rock layers are rearranged by large faults; 2) Stratigraphic traps, whereby the correct combination of source, reservoir, and seal rocks are deposited in a specific geologic setting. In the Queen Charlotte Basin and a majority of the other basins world-wide, most traps are a combination of the two.
For an excellent review of oil and gas formation, please visit the UK offshore operators association web site.
Some credit from above text to UKOOA: The UK's Oil and Gas Site, 2004. |
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A good
analogy to understanding oil and gas (petroleum) formation is the making
of a pizza. One must have all the proper ingredients, along with a correct
combination of temperature and time to cook them. In the case of oil and
gas, the time frame involved is millions of years, and the temperatures
involved are anywhere from 50-200°C.
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