Oil and gas

Oil and Gas Deposits

Updated 11/12/2013, published online 06/04/2010

Brazil's pre-salt oil reserves. © AFP

Both crude oil and methane gas (the main component of natural gas) are hydrocarbons that are formed deep underground and over geological time migrate from reservoirs to form deposits. Oil or gas deposits can be found throughout the world, both underground and beneath the ocean depths. However, a number of conditions have to be met before these reserves can form and accumulate. Oil formation is a long, slow process, with a number of fundamental stages spanning several millions of years at least.

When oil engineers study an area, they try to see if the different stages, formation and accumulation, could have taken place. This helps them to determine the presence and potential value of workable deposits.

The formation of hydrocarbons

Deep under the earth, oil and methane gas are formed out of organic matter from dead plants and animals. This can take tens of millions of years and requires a specific set of physical, chemical and geographical conditions. [Read more].

The migration of hydrocarbons

Starting out from the source rock where they are formed, hydrocarbon molecules, which are lighter than the water impregnating the upper part of the earth's crust, set off on an upward journey through the rocky layers towards the earth's surface. When they reach porous and permeable rocks, which characterize oil or gas reserves, they can accumulate in great quantities. [Read more].


From traps to workable deposits

Workable oil and gas deposits occupy closed spaces created by deformations in geological layers. These spaces, known as “traps”, must be large enough to make the deposit economically viable. [Read more].

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