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How do anticlines form structural traps for petroleum?

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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.

Anticlines are the most common type of structural trap, accounting for approximately 80 percent of the world’s discovered petroleum. They form as upfolds of strata, typically resulting from lateral tectonic pressure or the draping and compaction of sediments over topographic highs.1

These structures function as traps because they create a concave roof—viewed from below—that prevents the upward migration of petroleum. Within the porous reservoir rock, natural gas, oil, and water segregate by density, with the gas occupying the highest point of the fold. An overlying layer of impermeable cap rock acts as a seal, preventing the fluids from escaping. The capacity of the trap is defined by its closure, which is the vertical distance between the highest point of the anticline and the spill plane, the level at which petroleum would leak if the reservoir were filled beyond capacity.21