Where does oil come from anyway?

Oil forms due to burial and heating of organic sediment, eg coal. The organic sediment breaks down, initially to oil. The 'oil window' refers to the temperature and pressure conditions at which oil forms. While geothermal gradients vary, the typical depth of the oil window is 4-6 kms. If heated to temperatures above the oil window the chemical reaction continues, forming natural gas and so on until there is nothing useful left.

Oil and natural gas, once formed, are more buoyant than the encompassing host rock and will rise due to buoyancy. Therefore, to 'trap' oil there must be a suitable structure above the buried sediment such as an anticline with a suitable reservoir rock such as a sandstone, and suitable capping rock such as a shale. Sandstone is generally porous and therefore oil can accumulate within this rock, while shale is considered an aquiclude; a rock type which prevents fluid from passing through it.

Therefore, from the discussion above, it can be seen that the process of oil formation requires the accumulation of large amounts of organic sediment, there burial to depths greater than 4 km BUT not greater than 6, and further requires suitable structures to trap the oil. If these structures are not in place the oil will simply escape to the surface and evaporate over geologically short timescales.

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To allow significant accumulations of organic material subsidence is generally required, this continuously lowers base level allowing sediment to continue to accumulate. Furthermore a low energy environment is needed as foreign sediments, eg silicates, will contaminate the organic sediment. Subsequent burial to the depths occurs over the timescale of 10's of millions of years, and deformation of overlying rocks to form suitable trapping structures generally requires orogenic (mountain building) events, which also occur over a similar time scale.

In conclusion, both the timescale, and the specific events required, make oil formation, and equally important; oil accumulation, very rare events. Currently there are locations in the world which are accumulating significant organic sediments, eg Bangladesh, however it will only be a matter of luck if they are exposed to the conditions suitable for oil formation in the future. And of course this won't be for millions of years.