Once cosily installed in their trap, the hydrocarbons are not completely protected from change. We know that they don’t like oxygen and bacteria. However when an accumulation of oil is too close to the surface, rainwater always ends up by entering into contact with it. This water carries oxygen and voracious bacteria which begin to attack the oil, provoking a significant diminution in the proportion of light and average liquid hydrocarbons, as well as a liberation of gas.
After a while, all that remains are the heavy and viscous hydrocarbons that are difficult to exploit. If it has not escaped the gas
is less attractive to us than the initial oil. This latter will have undergone serious deterioration: what a waste! The bacteria responsible for the alterations cannot survive at temperatures in excess of 50/55°C.
The oil therefore remains protected as long as the temperature remains above this level. Broadly speaking, we can say that it is necessary to start worrying when the accumulations of hydrocarbons are found at a depth of less than 1000 meters.
The accumulations found deeper are nevertheless not completely protected from upheaval. This time the threat comes from rock movements. These tectonic movements, if they happen, can destroy the trap, by significantly reducing its closure, even destroying it, or more often,
breaking the seal by fractures or by cracks which the trapped hydrocarbons are going to rush into and escape. The trap is thus finished; empty!
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