Hydrocarbon trap

The reservoir has the capacity to accumulate very large quantities of hydrocarbons. The seal prevents them from rising towards the surface. But all that is insufficient for the accumulation of the hydrocarbons and for the formation of an oil or gas field. Indeed, once they arrive under the seal, these hydrocarbons slide into the empty spaces where they can continue to rise, via all the escape points. It is therefore necessary to have a large, closed volume so that the hydrocarbons accumulate in sufficient quantity to be profitably exploitable. This closed volume is called a trap. It is created by deformations in the rock layers. The lower its escape point compared to its summit, the bigger the trap.

A trap filled with hydrocarbons can – depending on conditions – contain just oil or gas, or both. If there is oil and gas, the latter, being lighter, will collect at the top of the trap with the oil lying beneath. It is important to remember that even when it appears there is only oil, significant quantities of gas is present. The gas is dissolved in the liquid. Similarly when it appears there is an accumulation of gas only, there is invariably present a fraction of light liquids called condensate. Furthermore, a little water always remains stuck to the grains of the reservoir rock. This is called the residual water.