Feature Report: About Oil and Gas Deposits

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From Traps to Commercial Deposits

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Commercial oil and gas deposits occupy closed spaces created by deformations in geological layers. These spaces, known as traps, must be large enough to make developing the deposit economically viable.

Analysis of aerial images to understand the geological features around Kampala, Uganda. © L. ZYLBERMAN / TOTAL

Reservoir rock, which is both porous and permeable, can hold a given quantity of hydrocarbonsThe final phase in petroleum system formation, after a deposit has accumulated.... Cap rockAn impermeable barrier that blocks the migration of oil and gas to the surface..., which seals these reservoirs, stops the hydrocarbons from migrating upwards to the surface (See Close-Up: "How Oil and Gas Migrate")

But before a depositAn accumulation of natural resources, such as oil, natural gas, coal, uranium, metal ore or another commodity... can be formed, these hydrocarbons must also be sealed in a closed space called a trapA volume of “sealed” rock whose configuration or type prevents the oil and gas trapped in the reservoir....

About Oil and Gas Traps

There are two main types of trap:

  • Structural traps, which are formed by changes in geological layers caused by the movement of tectonic plates. Reservoir rock is sometimes deformed until it forms a completely sealed space. These anticlinal traps are dome-shaped and the most common type of structural trapTrap that forms as a result of changes in the structure of subsurface....
  • Stratigraphic traps are made up of sedimentary layers that have not undergone tectonic deformation. In this case, a cap rock completely seals off the reservoir rock. For example, salt domes can act as cap rocks in this type of trap.

The trap contains hydrocarbons, but also residual water. Because they are lighter than the water, the hydrocarbons migrate above the water table.

HydrocarbonOrganic compound consisting of carbon and hydrogen. Hydrocarbons are the principal constituents of crude oil, natural gas and petroleum products. traps can contain:

If the reserves are developed, the gas dissolved in the crude oilOil that has not been refined. will be turned into liquefied petroleum gas (LPG)Combination of light hydrocarbons produced partly from the refining of crude oil (about 40%) and partly from the processing of natural gas.., used primarily as fuelFuel is any solid, liquid or gaseous substance or material that can be combined with an oxidant.... Condensate will be refined to produce naphtha, used as a feedstock in the petrochemical industry, or kerosene, a fuel used in aviation. 

Conserving Hydrocarbons

Once trapped, the hydrocarbons are still at risk of deterioration that could prevent the formation of a commercial deposit.

At depths of less than 1,000 meters, the accumulation can be infiltrated by meteoric water (precipitation). This water contains bacteriaTiny living organisms that are made up of a single cell that does not have a distinct nucleus, such as a prokaryotic cell... and oxygen that come into contact with the gas and oil, triggering chemical reactions that separate them into water and carbon dioxide.

Bacteria and oxygen start by attacking light and medium hydrocarbon molecules. After a period of time, the initial oil is significantly degraded, leaving only viscous, solid hydrocarbons that are more difficult to extract than non-degraded oil or gas.

Hydrocarbons must be contained in natural traps to be developed.

Below 1,000 meters, the temperature is in most cases higher than 50°C and the bacteria that cause the degradation cannot survive. Although no longer threatened by bacteria or oxygen, traps located deep underground can be affected by tectonic activity — some traps have even been created this way. This can cause fractures and faults in the rock, breaking the seal and letting the hydrocarbons leak out of the trap. More violent tremors can even destroy the trap by substantially reducing or destroying the seal.

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