Oil or petroleum is a readily combustable fossil fuel that is composed mainly of carbon and hydrogen , and is thus known as a hydrocarbon.
This is likely because the Mesozoic age was marked by a tropical climate , with large amounts of plankton in the ocean. The formation of oil begins in warm, shallow oceans that were present on the Earth millions of years ago. In these oceans, extremely small dead organic matter - classified as plankton - falls to the floor of the ocean.
This plankton consists of animals, called zooplankton , or plants, called phytoplankton. This material then lands on the ocean floor and mixes with inorganic material that enters the ocean by rivers. It is this sediment on the ocean floor that then forms oil over many years. The energy in oil initially comes from the Sun , and is energy from sunlight that is trapped in chemical form by dead plankton. The process that creates oil is generally the same in most areas, although there may be different types of plant and animal debris that falls to the ocean floor and slightly different conditions.
To form oil, the following steps have to happen: [3] [5]. Dead plankton - both phytoplankton and zooplankton - as well as algae and bacteria sink to the bottom of an ancient ocean and mix with inorganic, clay-like materials that enter these oceans from streams and rivers. This creates an organic-rich mud. This mud can only form in still water environments. This step is shown in Figure 1, panel A. This mud cannot be exposed to too much oxygen , or else the organic matter in the mud would be decomposed by bacteria and disappear quickly.
Therefore environments where oil can form are known as anoxic environments. Before this organic matter is destroyed, it is buried by more sediment and lithifies becomes sedimentary rock , creating organic shale.
The majority of petroleum is thought to come from the fossils of plants and tiny marine organisms. Larger animals might contribute to the mix as well. But another theory holds that more oil was in Earth from the beginning than what's been produced by dead animals, but that we've yet to tap it.
In the leading theory, dead organic material accumulates on the bottom of oceans, riverbeds or swamps, mixing with mud and sand. Over time, more sediment piles on top and the resulting heat and pressure transforms the organic layer into a dark and waxy substance known as kerogen. Left alone, the kerogen molecules eventually crack, breaking up into shorter and lighter molecules composed almost solely of carbon and hydrogen atoms.
Depending on how liquid or gaseous this mixture is, it will turn into either petroleum or natural gas. Scientists aren't really sure, but they figure it's probably on the order of hundreds of thousands of years.
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