Oil gas is store in the natural wells within the crust. Oil gas have protection layer in underground we call it cover layer. If the oil gas move up unrestricted to the earth surface, they will loss. In order to prevent the oil gas loss from the earth surface, a dense and impervious rock covered in the upper of reservoir stratum is needed. This kind rock we call it cover layer. The rock suitable for cover layer include shale, mudstone etc. Sometimes, dense marlite and limestone also can used as cover layer. Cover layer should have certain of thickness. If it is too thin, it can’t bear the pressure the oil gas put on it. So it can’t play the role of cover to prevent the oil gas exit. Cover layer should be homogenized which namely the variation in cover thickness should not be great. But the condition that some places have cover layer and some not have is must never permitted. If not, there will have leak in up of the reservoir stratum and the oil gas will loss.
The underground oil warehouse- trap. In the underground, the geological structure that could stop the oil gas flow and be dispersed to droplets , bubble is called geological trap, hereinafter referred to as the trap. It like a underground oil storage warehouse and can store oil and natural gas. So trap is the terminal of oil gas migration and is the seat of reservoirs. And also it is the goal oil workers looking for. The trap is the unity of reservoir rock and cover layer. Reservoir rock is covered by cover layer and oil gas is stored in the porosity of reservoir rock. There are three levels in the trap because of the different density among oil, gas and water when the oil gas go to the trap The lightest natural gas is in the up ,the heavier oil in the middle and the heaviest water is in the bellow. So far the oil completes the whole process from generation, migration to go to trap.
The following is a dome shape the oil gas well scheme. (The solid line above represents our external environment)