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Where do We Find Oil and Gas?

Unfortunately oil and gas do not exist is liquid pools underneath the earth's surface. In fact, you could hold a rock containing oil in your hand and would not be able to see the oil.

Important Properties of Rocks

 For rocks to contain oil and gas they must have two very important properties. The rock must have a place for the oil and gas to be stored. These storage areas are called pores. These pores are not visible to the naked eye and must be viewed using a microscope. The number of pores that a rock contains indicates how much oil or gas can be stored. The more pores the more oil or gas that can be contained in the rock. 



Scientists measure the amount of pore space by determining the rock's porosity. Porosity is expressed as the volume percent of the rock that contains open space and can range from 5 to 30%. 

The greater the porosity, the more oil and gas can be stored in the rock. Porosity does not mean that there is oil or gas in the the rocks. In fact the pores could be filled with water.

Not only must the rocks have pores, the pores must be connected so that the oil and gas can move through the rock. Scientists measure the ability of oil and gas to move through rock by determining its permeablity. Oil and gas in low permeability rock have difficulty moving out of the rock and into a well. Permeability is measured in thousandths of a darcy or millidarcys. Sandstone and carbonate rocks are generally the most porous and permeable rocks and are therefore where scientists usually find oil and gas.  

How Does Oil Collect Under the Earth's Surface

The oil and gas formed millions of years ago did not stay in the same place. The permeability of the rocks allowed oil and gas to move through the rocks until they reached a layer of rock that had low permeability. This is like having a sponge filled with water. Water can be added to the sponge and will drip out of it. 

However, if we put the sponge in a plastic container, the water will stop dripping and will be trapped. Oil and gas will move toward the surface until they reached a barrier.

Scientists call these barriers that hold oil and gas traps. A trap prevents oil and gas from rising to the earth's surface. A trap consists of a layer of rock that oil and gas cannot penetrate (impermeable) above a porous, permeable layer that holds the oil and gas. These traps can take many forms but a common type would be in the shape of an inverted bowl formed from impermeable rock. Traps are formed by changes in the earth's crust such as folding or faulting. Scientists categorize traps by how they were formed or how they look. The major types of traps are anticlinal, fault, plug, stratigraphic, and lenticular.

In an anticlinal trap, the layers of rock are altered by geological forces to create a concave shape or dome. Hydrocarbons move into the porous layers and are trapped by an impermeable rock layer or caprock. 

A fault trap is formed by a shift or movement of rock layers that caused an impermeable layer to create a barrier next to a layer of porous rocks containing hydrocarbons. Plug traps are traps where a core of impermeable rock has moved through layers of rock to form a trap. 

These plugs are often composed of salt and salt domes are an important formation that scientists look for when trying to find oil and gas. A stratigraphic trap is created by either an impermeable rock layer over the top of a porous rock layer or by a change of porosity and permeability within the reservoir itself. Lenticular traps are formed by a change of permeability within the formation. These changes in the amount of connected pore space can create hydrocarbon pockets within a formation. The different trapping mechanisms can also occur in combination.








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