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. The most common types of traps are anticlines, faults, stratigraphic traps, and salt domes.