The oil and gas consist of hydrocarbons, molecules
composed of carbon and hydrogen.
We know that these hydrocarbons cannot exist for very long
at the surface of our Earth because they are attacked
by oxygen and devoured by bacteria that live in surroundings where air
is present (aerobic bacteria). Thus, they are quite rapidly transformed
into carbon dioxide (CO2) and water. Incidentally, hydrocarbons do not exist in deep layers of the Earth because, beyond a certain depth (around 10 km), they would be destroyed, since the temperature is too high (the further you plunge underground, the hotter it becomes!).
So, where do these hydrocarbons come from? Their composition shows that they result from a transformation of organic matter consisting of living organisms that died a long time ago. When a plant or an animal dies on the surface of the earth, other living creatures generally recycle its matter. What is not devoured by predators, like vultures or bacteria, is oxidised into carbon dioxide and water, and this carbon dioxide feeds the growth of new plants. Nevertheless, a tiny part, perhaps 0,1%, of this organic mass escapes from the implacable cycle.
In certain cases the remains of dead beings sink to the bottom of the seas. In this environment, very calm and poorly oxygenated, the organic remains are mixed with mineral matter (particles of clay, very fine sand…) to form dark and foul-smelling mud. This bad odour is characteristic of the action of anaerobic bacteria which don’t need air to live (they are a lot less greedy than their cousins on the Earth’s surface).
A part of this organic matter is therefore preserved. The animals that produce it are minuscule or microscopic: principally marine plankton. The plant debris is carried away by the rivers that feed the sea.
This organic matter, mixed with mineral sediments, accumulates little by little. For large quantities of oil or gas to be produced later, the proportion of organic matter must be sufficient, that is to say at least 1 to 2%, to constitute the source rock for our oil. 1 to 2 % does not seem a lot, but exceptional conditions are required to attain this percentage: a lot of plankton or
plant
debris and not too much mineral matter.
A warm climate favourable to plankton; the absence of mountains nearby to limit the volumes of mineral sediment; and a delta or mouth of a major river to carry a lot of plant debris…all of these elements contribute to the formation of the source rock. Nevertheless, whilst this rock remains on the surface of the sea floor, it is unable to produce oil. |