When companies drill for oil, they are looking for oil traps. Oil seeps up through rocks and collects inside the ground, forming an oil trap. This seepage is called migration. Oil originates in a source rock and migrates through the layers. Shale is the most typical type of impermeable rock that traps oil. It has a lot of kerogen, which is basically solid organic matter. Kerogen is altered by heat and pressure from the earth. The deeper a shale rock or rock layer is buried, the more pressure and heat it withstands. Oil and gas are squeezed out and begin to migrate. They are able to migrate through permeable rock and cracks. Permeable rocks like sandstone and limestone have loosely packed grains that oil can travel through. Oil doesn’t mix with water, so oil is pushed up by water in the ground. It eventually gets trapped in a pool underground when it encounters an impermeable rock layer, also known as trap rock or a cap rock.



There are four major kinds of oil traps that occur inside the earth. The first is an anticline trap. Companies looking for oil deposits can start from the sky. Anticline traps are usually along long oval domes of land that can be seen from above or on geological maps. An anticline is a place where layers, or strata, of rock have been pushed up into an arch. The movement of the earth’s crust pushes up on the surface and creates high pressure at the arches. If there is an impermeable rock layer, then oil and natural gas can be trapped there. Anticline traps hold most of the world’s oil.

The next type of trap is called a fault trap. A fault is a place in the earth’s crust where layers of rock slide up against each other. These layers can contain permeable and impermeable rock. If oil is migrating through the impermeable rock and the layers begin sliding up or down against each other, then the migrating oil can be cut off by an impermeable layer and trapped against the fault. This is what we refer to as a fault trap.

The anticline trap and the fault trap are generally formed by the earth’s layers moving. The next type is called a salt trap and is created in a little different kind of way. Masses of salt form underground and are pushed up by pressure inside the earth to form a salt dome. The salt dome breaks through and pushes layers of rock aside as it rises to the surface. When it crosses layers of permeable rock, it blocks the path of the migrating oil, much like a fault would, and causes the oil to accumulate up against the pillar of salt.

Pinch-out traps are formed by variations in the rock layers themselves. Types of stratigraphic traps, pinch-outs are almost always formed in stream beds. Basically, it’s where sand, which is permeable, gets trapped in layers of impermeable rock, such as shale and siltstone.

Rock layer knowledge is very important to oil prospectors today. Supporting our modern world, which is dependant on oil, requires an extensive knowledge of geology and an on-going study of our earth and its layers.