The Slow Formation of Oil and Natural Gas Deposits
5 min read
It takes millions of years for decaying organic matter to be transformed into hydrocarbons within source rock. As sedimentation occurs, chemical reactions create the carbon and molecules that make up liquid or gaseous hydrocarbons. The hydrocarbons then slowly travel upwards until they are trapped by impermeable rock and form a in a reservoir.
The Slow Formation of Oil and Natural Gas Deposits
Organic wastes are composed of carbon, hydrogen, nitrogen and oxygen. The majority, resulting from the death of living creatures, animal or vegetable, are generally destroyed and digested by .
But certain are deposited on the beds of enclosed seas, lagoons, lakes, deltas or other aquatic environments poor in oxygen. They are therefore protected from bacterial action
There, the organic matter mixes with (sand, clay, salt…) and accumulates in successive layers over millions of years. The most ancient layers are buried under the more recent.
These layers are pushed down by their own mass. The phenomenon of Plate Tectonics, disturbing the earth's mantle, breaks them up and takes them deeper into the Earth's crust.
The further the sedimentary layers subside, the more the temperature and pressure increase. Chemical reactions eliminate the nitrogen atoms and the remains of oxygen.
They leave only molecules formed from carbon and hydrogen, which constitute the liquid and gaseous hydrocarbons. These hydrocarbons are to be found at the heart of a rock, called the source rock.
The hydrocarbons move around below ground because, being lighter than water, they tend to make their way towards the surface of the Earth. If nothing stops them, they will escape and ooze out onto the surface or solidify into bitumen, losing their volatile constituents.
But if, during the course of their migration, the hydrocarbons meet an impermeable layer, which is called a seal or , they are trapped below in the microscopic interstices and cracks of a rock, called the reservoir rock.
In this reservoir rock, the gaseous part of the hydrocarbons rises slowly above the oil, pushing the water below.
Several tens of millions of years later, will a geologist suspect the existence of this trap and consequently discover an oil or gas field?
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