The newly born hydrocarbons are molecules of small size and they take up more space in the source rock than the original kerogen. They are therefore going to be permanently expelled into the rocks that surround the source rock.
The gas and oil being lighter than water (which impregnates all the rocks in the substratum) then begin a slow rise towards the surface. That is migration. If they can, they slide between the mineral particles of rocks to climb vertically. Their speed of migration depends on the capacity of each rock in their path to allow the circulation of fluids. This capacity is called the permeability. If an impermeable rock stops them, they follow a lateral path along this rock, still in an upward direction, or they pass by paths through cracks and weaknesses in the rock.
The molecules of gas that are smaller and more mobile climb more quickly and slide more easily into rocks that are not very permeable.
A proportion of the hydrocarbons, mainly gas, are dissolved in the water that impregnates the rocks they traverse. Other hydrocarbons remain stuck to the grains of the rocks. These hydrocarbons interrupt the ascent; they represent what are called the migration losses. Such losses can be very significant, especially if the oil and gas take the longest path upwards.
If nothing stops the hydrocarbons reaching the surface, the lightest fractions (gas and volatile liquids) are dispersed into the atmosphere before being destroyed. The heaviest are oxidised or devoured by bacteria. The only ones that continue to exist for some time are the extreme, heaviest fractions, in the form of almost solid tars buried a few meters to tens of meters below the Earth’s surface. |