I have come across the following paragraph on drilling of oil wells in a book:
Natural gas is found above petroleum, trapped between the rock cap and the petroleum layer. With the help of giant rigs, a hole is drilled in the earth's crust. When the hole reaches the rock cap, the natural gas comes out first with a great pressure. After that, petroleum starts flowing out due to the pressure of natural gas. When the pressure falls further, the petroleum is pumped out by electric motors.
What does the line petroleum starts flowing out due to the pressure of natural gas mean here? After the natural gas comes out with great pressure, petroleum beneath the natural gas flows out but what does due to the pressure of natural gas mean here? Natural gas has already come out (after which petroleum starts flowing out); so how does pressure of natural gas come into the picture after the natural gas has already been released?
Thanks,
Ron