An anticline commonly forms a geological trap where oil and natural gas accumulate beneath impermeable rock.
An anticline is an arch-shaped fold in layered rock. During deformation, the layers bend upward, and porous reservoir rock may become covered by a nonporous cap rock. Buoyant hydrocarbons migrating through the reservoir can collect near the crest instead of continuing upward.
Successful traps require more than an anticline. They generally need a source rock that generated hydrocarbons, a reservoir rock with connected pores, a seal that prevents escape, and suitable timing between trap formation and hydrocarbon migration. This combination is why petroleum exploration studies both structure and rock properties.
Anticlines are not the only petroleum traps. Fault traps, stratigraphic traps, and combinations of structural and stratigraphic features can also hold hydrocarbons. A syncline is the downward-curving counterpart of an anticline, so the two terms are easy to confuse.