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From traps to workable depositsUpdated 02/12/2013, published online 06/04/2010
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.
What are gas and oil traps?
Reservoir rocks, which are both porous and permeable, can hold a given quantity of hydrocarbons. Cap rocks, which seal these reserves, stop the hydrocarbons from migrating upwards towards the surface. But before a deposit can be formed, these hydrocarbons must also be retained in a closed space called a trap.
There are two main types of traps:
• The most common are structural traps, formed by deformations in geological layers. Anticline traps are the result of a rock folding and have a dome-shape. These are the most common type of structural traps.
• Stratigraphic traps are made up of sedimentary layers that have not undergone any 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 traps contain hydrocarbons but also residual water. As they are lighter than this water, the hydrocarbons migrate above the water table. They are then prevented from rising further by impermeable cap rocks.
Hydrocarbon traps can contain:
• only oil but with significant quantities of dissolved gas
• only gas but with light liquid hydrocarbons, known as condensates
• Both oil and gas. In this case, the gas, which is lighter than oil, is collected in the upper part of the trap.
If the reserves are used later, the gas dissolved in the crude oil will be turned into LPG (liquefied petroleum gas, used primarily as fuel). Condensates for their part will be refined to produce naphtha (used in the petrochemicals industry) or kerosene (a fuel used in aviation).
Conserving hydrocarbons to ensure the formation of workable deposits
Once trapped, the hydrocarbons are still at risk for damages that could prevent the formation of a workable deposit.
At depths of less than 1,000 meters, the accumulation of hydrocarbons can be infiltrated by rainwater seeping in (known as meteoric water). This water contains bacteria and oxygen that, when in contact with the gas and oil, trigger chemical reactions that convert the hydrocarbons into water and carbon dioxide.
Bacteria and oxygen start by attacking light and medium hydrocarbon molecules. After a while, the initial oil is significantly degraded, leaving only heavy hydrocarbons that are viscous and solid and that are more difficult to extract than non-degraded oil or gas.
Below 1,000 meters, the temperature is higher than 50°C and in most cases the bacteria that cause this degradation cannot survive. The conditions for preserving oil and gas are therefore optimal beyond this depth.
This can cause fractures and cracks in the rock, breaking the seal of the trap and letting the hydrocarbons leak out. In the case of more violent activity, tectonic tremors can even destroy the trap by substantially reducing or destroying the closure.