https://www.planete-energies.com/en/medias/close/how-oil-and-gas-migrate This slow, constant movement away from the source rock is called migration (hydrocarbons) .
Migration is a complicated process. The rate depends on the permeability of the rocks they cross and the size of the molecules: gas molecules rise more quickly than oil molecules, because they are smaller and more mobile. Some hydrocarbon molecules are prevented from moving upward, either because they dissolve in the water contained in the rock they encounter (this affects gas much more often than oil) or because they adhere to the grains that make up the rock. This phenomenon is known as migration loss. These losses can be significant, especially if the oil and gas have a long way to travel. This is why some source rock hydrocarbons will never be suitable for development.
a hydrocarbon deposit will only form if the reservoir rock is capped by a layer of impermeable rock that prevents the oil or gas from rising vertically to the surface and forms a closed space that prevents the oil or gas from rising laterally
Absence of Cap Rock
If the hydrocarbon molecules are not prevented from rising, they will move through the reservoir rock and cannot accumulate.
Oil or gas that reaches the surface at the end of its migration is exposed to bacteria and ambient air. This triggers complex chemical reactions that convert them into water and carbon dioxide. However, when significant quantities of hydrocarbons arrive at the surface more quickly than the final degradation process, the heaviest molecules may remain in the ground in the form of viscous, almost solid bitumen, buried at depths of a few meters. But these bitumen deposits will quickly disappear when the hydrocarbons stop arriving at the surface to replenish them.