How Hydrocarbon Migrates to the Earth's Surface

06/04/2010

Hydrocarbon molecules begin from the source rock where they are formed and set off on an upward journey through rocky layers. When they encounter porous rocks in which liquids can circulate, the oil and gas particles tend to accumulate in large pockets. Sometimes, these huge reservoirs are capped by a layer of impermeable rock that prevents the hydrocarbons from moving upwards towards the surface. These are the conditions under which an oil or gas field is formed.

Oil and Gas, a Slow Upwards Rise

Within the source rock where they are formed, oil and methane gas are made up of tiny hydrocarbon molecules. These molecules are formed from kerogen, which is a mixture of water, CO₂, carbon and hydrogen formed from decomposing organic matter.






However, hydrocarbons take up more room than kerogen in the rock. They are therefore expelled into the rocky layers around the source rock that are saturated with water. Since they are lighter than the water in these rocks, gas and oil tend to rise towards the earth's surface by sliding between the rocks' mineral particles. This slow, non-stop movement is known as migration.

The speed at which hydrocarbons migrate depends on the permeability of each rock they cross, i.e. its capacity to allow liquids to circulate. Moreover, gas molecules rise more quickly than oil molecules because they are smaller and more mobile.

Sometimes, hydrocarbon molecules are impeded in their upward journey by less permeable rocks. In this case, they can continue their migration by:

   • passing through cracks in the rock

 or

   • passing through a neighboring, more permeable rock.

However, sometimes a proportion of the hydrocarbons can't move upwards because:

   •  they are dissolved in the water contained in the rocks they pass through - this is mostly the case with gas

 or 

   •  they remain stuck to the rock grain.

This phenomenon is known as migration losses. These losses can be significant, especially if the oil and gas have a long way to travel. This is why some source rock hydrocarbons will never be suitable for drilling.

How Hydrocarbon Pockets Are Formed

Oil and gas molecules that survive migration and are not dissolved in the atmosphere can accumulate in huge hydrocarbon reservoirs. However, for this to happen the rocks that they encounter on their journey must have certain characteristics.

Sedimentary basins are made up of sedimentary rocks that allow oil and gas migration. They are formed from grains that are deposited in seas, oceans, lakes or lagoons.
The rocks differ according to the size of the grains from which they are formed:

   • Very large grains form gravel rocks

   • Small grains bond together to form sand

   • The smallest grains of all form mud

Within the rocky layers, these grains come into contact with each other, but there are also empty spaces between them. These spaces define the porosity of each rock- the higher the percentage of these spaces within the rock, the more porous the rock.

The empty spaces in porous rocks may contain large quantities of hydrocarbons.
These rocks are also often have good permeability, although it does not always go hand in hand with porosity. So for example, although clay is porous because it is made up of particles with large empty spaces between them, it is impermeable because these particles are in the form of flat sheets piled tightly on top of each other, which impedes fluid flows.

Oilmen are particularly interested in rocks that are both porous and permeable.
These rocks, known as reservoir rocks, are potentially filled with gas or oil which can move rapidly when pumped. The best rock reservoirs are:

   • sandstone

   • carbonates (limestone and dolomite)

Rocks that are fractured or cracked make good reservoirs because the hydrocarbons can move around more freely.

However, hydrocarbon migration through a reservoir rock is not enough to form an oil or gas pocket. Unless the hydrocarbon molecules are prevented from rising, they will only pass through the reservoir rock instead of accumulating within it. To form a hydrocarbon reserve, an impermeable rock - called a seal or cap rock - is needed to form a barrier above the reservoir rock. The best cap rocks are the most impermeable, which are:

   • clays

   • layers of crystallized salt

However, any rock that is sufficiently impermeable - such as highly compact carbonates - can serve as a cap rock.