Oil and gas production handbook - Process Control and ...

100 to 200 million years ago, under ,over or with the sand or silt, is transformed by
high temperature and pressure into hydrocarbons.
For an oil reservoir to form, porous rock needs to be covered by a non porous layer
such as salt, shale, chalk or mud rock that can prevent the hydrocarbons from leaking
out of the structure. As rock structures become folded and uplifted as a result of
tectonic movements, the hydrocarbons migrates out of the deposits and upward in
porous rocks and collects in crests under the non permeable rock, with gas at the top,
then oil and fossil water at the bottom. . Ill: UKOOA
This process goes on continuously, even today. However, an oil reservoir matures in
the sense that a too young formation may not yet have allowed the hydrocarbons to
form and collect. A young reservoir (e.g. 60 million years) often has heavy crude,
less than 20 API. In some areas, strong uplift and erosion and cracking of rock above
have allowed the hydrocarbons to leak out, leaving heavy oil reservoirs or tar pools.
Some of the world’s largest oil deposits are tar sands where the volatile compounds
have evaporated from shallow sandy formations leaving huge volumes of bitumen
soaked sands. These are often exposed at the surface, and could be strip mined, but
must be separated from the sand with hot water, steam and diluents and further
processed with cracking and reforming in a refinery) to improve its fuel yield.
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