Sea change. Natural gas was formed from plant and animal residues, millions of years ago.
Organic material, heat and pressure – these were the ingredients needed for crude oil and natural gas to form. Today’s energy source was cooked up millions of years ago, according to a complex recipe.
First precondition - lack of oxygen
Natural gas formed in ocean basins where there was no water cirulation. The absence of currents was crucial because the dead aquatic organisms and land plants in the water used up the oxygen dissolved in it when they decayed. Decomposition ceased when the oxygen ran out, and the partly decayed substances collected on the sea bed. Together with fine clay particles, they formed what is known as “sapropel”.
Kerogen - the mother of all hydrocarbons
The sapropel was pressed further into the depths by new layers of sand and clay, raising the pressure and temperature. At about 50°C kerogen, the precursor of crude oil and natural gas, was created. This turned into the fine-grained “bedrock” in which crude oil and natural gas would later form.
Still deeper and hotter
The pressure and temperature continued to rise, due to the formation of mountains, for example. At 2,000 m below the earth’s surface and temperatures of 70°C or more, it was mainly oil that came into being. Further down, at temperatures of about 200°C, only natural gas occurred.
More in store
Due to their higher volume and still more pressure, oil and gas were forced from the bedrock and migrated upwards, before being trapped again, in porous sandstone. They rose further until they reached an impermeable layer such as clay, collected there and formed reservoirs. It is reassuring to know that each year more natural gas is discovered than is produced.