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As the petroleum is formed at the bottom of the sea, it sits on an impermeable layer of silt and sand. Since it is lighter than water, it will float to the top. This migration of petroleum allows it to contact porous rock. The liquid petroleum can now flow through the porous rock and accumulate in a structural trap (Instructional).

http://www-class.unl.edu/geol101i/09_structural.htm

Petroleum and natural gas climb up the rock and become trapped at the top, at the fold axis. Gas is lighter than petroleum and accumulates at the very top (Instructional).

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Structural traps are formed by the folding and faulting of rocks. Tension from the earth's crust and uplifts caused by salt or shale form the foldings and faultings in the earth. A "U" shape in the rocks is formed-impermeable to petroleum, creating a trap (The Origin). There are three basic types of structural traps (The Paleontological). They are:


1). ANTICLINE TRAP


2). FAULT TRAP


3). SALT DOME TRAP

http://www.cartage.org.lb/en/themes/Sciences/Earthscience/Geology/OilandGas/PetroleumGeology/Traps/t

http://www.abix.com/Oil%20and%20Gas%20Traps.htm