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anticlinal trap A fold structure with an arch of non-porous rock overlying porous strata (reservoir rock), providing a trap in which oil, gas, or water may accumulate. In Middle East oilfields, large, upright folds occur in thick, competent limestones which extend for many kilometres; fracturing along the crests of the anticlines increases permeability in the reservoir rocks. See PETROLEUM; and NATURAL GAS. Compare FAULT TRAP; REEF TRAP; STRATIGRAPHIC TRAP; STRUCTURAL TRAP; and UNCONFORMITY TRAP.