Britannica Sources used in this answer

Why does petroleum become trapped beneath impermeable cap rock?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Multiple large offshore petroleum oil rigs with cranes and equipment stand in the ocean, connected by walkways above the water. — petroleum Liquid petroleum (crude oil)—which is refined into gasoline, diesel, heating fuels, asphalt, plastics, and other useful chemical compounds—is often extracted from the ground using a series of drills and pumps.

Petroleum becomes trapped beneath impermeable cap rock because of the physical properties of the fluids and the surrounding geologic environment. Because petroleum and natural gas are less dense than the water typically found in subsurface sedimentary rock formations, they naturally migrate upward through porous and permeable reservoir rocks over geologic time.12

A trap functions as a closed, inverted container. When this migrating petroleum encounters a layer of denser, relatively impermeable rock—known as cap rock, such as shale or evaporites—its upward and lateral movement is blocked. This barrier prevents the hydrocarbons from continuing to the surface, where they would otherwise dissipate or be broken down by environmental factors. The petroleum then accumulates in the highest portion of the trap, typically forming a layer of gas at the top, followed by oil, with water at the bottom.132