Oil reservoir

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

An oil reservoir, petroleum system or petroleum reservoir is often thought of as being an underground " lake" of oil, but it is actually composed of hydrocarbons contained in porous rock formations.


The crude oil found in oil reservoirs forms in the Earth's crust from the remains of living things. Crude oil is properly know as petroleum, and is a kind of fossil fuel. Scientific evidence indicates that millions of years of heat and pressure changed the remains of microscopic plant and animal remains into crude oil and natural gas.

Roy Nurmi, an interpretation adviser for Schlumberger described the process as follows: "Something in the order of 500 million years ago there was only simple life in the seas, and these shallow seas would be rich with organic, living organisms. Plankton and algae, proteins and the life that's floating in the sea, as it dies, falls to the bottom, and these organisms are going to be the source of our oil and gas. When they're buried with the accumulating sediment and reach an adequate temperature, something above 50 to 70°C they start to cook. This transformation, this change, changes them into the liquid hydrocarbons that move and migrate, will become our oil and gas reservoir."

In addition to the water environment mentioned, which is usually a sea but might also be a river, lake, coral reef or algal mat, the formation of an oil or gas reservoir also requires a sedimentary basin that passes through four steps: burial under miles of sand and mud, pressure cooking, hydrocarbon migration from the source to porous rock, and trapping by impermeable rock. Timing is also an important consideration; it is suggested that the Ohio River valley could have had as much oil as the Middle East at one time, but that it escaped due to a lack of traps. The North Sea, on the other hand, endured millions of years of sea level changes that successfully resulted in the formation of more than 150 oilfields.

Although the process is generally the same, various environmental factors lead to the creation of a wide variety of reservoirs. Reservoirs exist anywhere from 1,000 to 30,000 ft below the surface and are a variety of shapes, sizes and ages.


The traps required in the last step of the reservoir formation process have been classified by petroleum geologists into two types: structural and stratigraphic. A reservoir can be formed by one kind of trap or a combination of both.

Structural traps

Structural traps are formed by a deformation in the rock layer that contains the hydrocarbons (e.g., fault traps and anticlinal traps).

Stratigraphic traps

Stratigraphic traps are formed when other beds seal a reservoir bed or when the permeability changes ( facies change) within the reservoir bed itself.

An example of this kind of trap starts when salt deposited by shallow seas. Later, a sinking seafloor deposits organic-rich shale over the salt, which is in turn covered with sandstone. As the Earth's pressure pushes the salt up, the shale is "cooked," producing oil that seeps up into the sandstone above. In some places, the salt breaks through the shale and sandstone layers into a salt dome that effectively traps the hydrocarbons beneath it.


To obtain the contents of the oil reservoir, it is usually necessary to drill into the Earth's crust, although surface oil seeps exist in some parts of the world.


Active areas of surface oil reservoirs
  • Texas
Active areas of existing sub-sea oil reservoirs

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