Trapping

		Impermeable stratum Gas Oil Water Sand Mother or source rock

But if, in the course of their migration, the hydrocarbons encounter an impermeable later, they become entrapped in the microscopic clefts and fissures of the rock, which is then known as reservoir rock. Inside this reservoir rock, the gaseous hydrocarbons slowly rise above the oil. What are the chances of a geologist ever suspecting the existence of this reservoir, a few million years later?