Hydrocarbon Entrapment

Deep within sedimentary rocks, extractable oil and gas deposits take up a lot of space. These hydrocarbon-filled pockets or ‘traps' are often created by deformations in the earth. Their preservation can be compromised if they are attacked by oxygen or bacteria. The traps can also disappear if more rock folding occurs. Therefore, these conditions must be maintained if the oil and gas are to be extracted.

Hydrocarbon entrapment
© Keblow

What Are Gas and Oil Traps?

Reservoir rocks can contain a certain amount of hydrocarbons. Cap rocks sealing these reserves stop the oil and gas from migrating to the earth's surface.









However, this is not enough to form an oil or gas deposit large enough to be worth extracting. Even when located under a cap rock, hydrocarbon molecules continue to slide through the slightest gap in the rock to continue their journey to the surface. This results in sizeable oil or gas leaks, thus reducing the amount of potentially recoverable hydrocarbons.

In order for a deposit to be useable, hydrocarbons must be present in large quantities in a huge closed space.

In order for a deposit to be useable, hydrocarbons must be present in large quantities in a huge closed space - known as a hydrocarbon trap. There are two types of traps, which do not form in the same manner:

   • The most widespread are structural traps, which are created by deformations in the earth.
The rocky layer that covers the earth is in constant motion. It is fragmented into twelve gigantic, constantly moving tectonic plates that sometimes collide. These tectonic movements cause the rocky layers to fold, making cracks appear. This can change the way rocks are arranged. Reservoir rocks are sometimes surrounded on all sides by cap rocks, forming a trap. Anticlinal traps are the result of a rock folding and are dome shaped. These are the most common types 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. Sometimes, salt domes act as cap rocks in this type of trap.

The traps contain hydrocarbons and residual water. Since they are lighter than the water, the hydrocarbons migrate above the water table. They are then prevented from rising further by impermeable cap rocks.

Hydrocarbon traps can also contain:

   • Oil only

   • Gas only

   • Both oil and gas. In this case, the gas, which is lighter than oil, collects at the top of the trap.

Even traps containing only oil have significant quantities of dissolved gas. Similarly, in reservoirs containing only gas, there are always tiny amounts of light liquid hydrocarbons, known as condensate. 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, however, will be refined to produce naphtha (used in the petrochemicals industry) or kerosene (a fuel used in aviation).

Preserving Hydrocarbons - The Final Requirement for Forming Extractable Deposits

Once trapped, the hydrocarbons are not completely safe from degradation.

Thus, if an accumulation of hydrocarbons is located less than 1000 meters underground, rainwater can seep in. This water contains bacteria and oxygen that come into contact with the gas and oil particles, triggering 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, viscous solid hydrocarbons, which are more difficult to extract than liquid oil or gas.

Below 1000 meters, the temperature is higher than 50°C and the bacteria that cause this degradation cannot survive. The conditions for preserving oil and gas are therefore better beyond this depth.

Contrary to hydrocarbon traps located near the surface, accumulations located deep underground are not affected by any upheaval.

False. Although no longer threatened by bacteria or oxygen, just as they can be caused by tectonic activity, traps located deep underground can also be affected by this activity.
It can cause fractures and cracks in the rock, breaking the seal of the trap, thereby allowing hydrocarbon leakage. In the case of more violent activity, tectonic tremors can even destroy the trap by reducing or destroying the closure.