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fault because it moves as the sediments are being deposited. This is in contrast to other faults
such as normal, reverse, and strike-slip faults that occur in sedimentary rocks millions of years
old. It is also called a down-to-the basin fault, because the basin side is moving down.
Drag folds. Drag folds are formed by friction generated along a fault plane when a fault
moves. Friction causes the beds on either side of the fault to be dragged up on one side and down
on the other side of the fault. Most mountain ranges on land were formed by compressional
forces, and the rocks display compressional features such as folds.
Tilted fault blocks. Buried, tilted fault blocks can form large petroleum traps. During the
geological past, horizontal sedimentary rocks were broken by normal faults into large, tilted fault
blocks. Some of the blocks can contain reservoir rocks. Later, the seas covered the tilted fault
blocks and deposited caprocks of shale or salt on them. The oil and gas then formed and
migrated up along the reservoir rock to below the sealing fault or caprock.

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Stratigraphic Traps
Secondary stratigraphic traps—angular unconformities. An angular unconformity
can form a giant gas and oil trap when a reservoir rock is terminated under an angular
unconformity that is overlain by a seal.
Primary stratigraphic traps. Reefs, beaches, river channels and updip pinchouts of
sandstones form primary stratigraphic traps.
Reefs. Reefs are prolific gas and oil traps in North America. Permian age reefs in west
Texas and New Mexico, Devonian age reefs of Alberta, and Cretaceous age reefs of Mexico
form giant oil fields. Petroleum production can come not only from the reef but also from a
compaction anticline overlying the reef. The compaction anticline forms in porous sediments,
such as sands and shales, deposited on a hard rock mound or ridge, such as a limestone reef or
bedrock hill. The sediments are deposited thicker to the sides of the reef than directly over the
top. When the sediments are buried deeper, the weight of the overlying sediments compacts the
loose sediments. The reef, composed of resistant limestone, compacts less. Because more
compaction occurs in the thicker sediments along the flanks of the reef, a broad anticline forms
in the sediments over the reef. Any reservoir rocks in the sedimentary rocks overlying the reef
can trap petroleum.
Beach sands. Beach sands, called buttress sands, can be deposited on an angular
unconformity during rising seas and form giant oil and gas field reservoirs.

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Combination Traps
Combination traps have both structural and stratigraphic trapping elements.
Bald-headed structures. When an anticline or dome is formed, the crest of the structure
is exposed to erosion. Most or all potential reservoir rocks can be removed from the top of the
structure. Seas later cover the area, and sediments are deposited, burying the eroded structure in
the subsurface. When the petroleum migrates up the reservoir rocks, it is trapped below the
angular unconformity. Because the crest of the structure is barren, but the flanks are productive,
it is called a baldheaded structure or anticline.
Salt domes. A salt dome is a large mass of salt, often miles across, rising from a
subsurface salt layer through overlying sedimentary rocks to form a plugshaped structure. Salt,
composed primarily of halite, is a solid that can flow slowly as a very viscous liquid under
pressure. A salt layer is formed by the evaporation of water. When sands and shales are later
deposited on the salt layer, the weight of the overlying sediments presses down on the salt layer.
The salt starts to flow and lifts up a weak area in the overlying sedimentary rocks. As the salt
rises, it uplifts and pierces overlying sedimentary rocks to form a piercement dome. Because the
salt is lighter in density than the surrounding sediments, buoyancy also helps the salt rise.