Formation

For the formation of oil and natural gas deposits, several processes have to take place:

1. Source Rock Formation

Source rock is predominantly formed in oceans and is derived from planktonic life forms and plant material transported from land into the sea. Sedimentation of this organic matter must take place under anaerobic conditions to prevent the organic material being broken down by bacteria. Such conditions are found in the oxygen-poor bottom layers of deep oceans; alternatively, high rates of sedimentation can also preserve the remains of animals and plants and prevent them from decomposing.

2. Maturation

The ongoing process of sedimentation results in the source rock being to increasingly greater depths, concurrently with increasingly higher temperatures. Over time, the increase in compaction and temperature of the source rock causes the organic matter to be converted into crude oil and natural gas (hydrocarbons). Crude oil is formed at slightly lower temperatures (at approx. 60 – 150 degrees Celsius) from marine plankton, while natural gas is formed at higher temperatures (at approx. 140 – 180 degrees Celsius), mainly from the remains of land vegetation.

3. Mobilisation and Migration (of crude oil and natural gas)

Crude oil and natural gas have a lower specific gravity than the water filling all the pores of the sedimentary rock. This causes the hydrocarbons located in the source rock to migrate upward. This type of migration occurs in porous and permeable rock formations and through cracks and fissures (faults).

4. Formation of an Accumulation

Once the ascending oil or natural gas reaches a location where porous rock meets with impermeable rock (usually some form of clay) and forms a 'trap'. The ‘trapped' hydrocarbons are prevented from escaping and a hydrocarbon accumulation is formed. In cases where ascending hydrocarbons are not trapped, they emerge on the earth’s surface or on the seabed. Crude oil emerging on the earth’s surface or in very shallow deposits tends to be degraded by bacteria in combination with the action of surface water.