Water, which is more dense than oil, could have pushed the oil upward. Another possible cause was the weight of the overlying layers of rock, which would tend to squeeze the oil into holes and cracks in the rock. The most common types of petroleum traps are anticlines, faults, stratigraphic traps, and salt domes. An anticline is an archlike formation of rock under which petroleum may collect. A fault is a fracture in the earth's crust, which can shift an impermeable layer of rock next to a permeable one that contains oil.
Most stratigraphic traps consist of layers of impermeable rock that surround oilbearing rocks.
In a salt dome, a cylinder or cone-shaped formations of salt pushes up through sedimentary rocks, causing the rocks to arch and fracture in its path. Petroleum may accumulate above or along the sides of such a formation. Most crude oil lies in underground formations called traps. In a trap, petroleum collects in the pores of certain kinds of rock. Gas and water are also present in most traps.