Rights: © Copyright. University of Waikato. All Rights Reserved. Published 10 June 2008 Download

Adam Vonk explains that a source rock contains the raw materials from which hydrocarbons will eventually form.

These raw materials are rich in organic matter and may contain marine algae or leaf material. This material forms what is called kerogen in the source rock – these are the chemical compounds that make up the organic matter. Carbon is released from kerogen when the source rock is buried.

Burying a rock submits it to both pressure and temperature. As the temperature changes, different compounds are released from the kerogen in the source rock. Initially, it is water and carbon dioxide, then oil and, last, natural gas.

Points of interest: What is the average temperature increase per kilometre burial depth?


Source rocks are rocks that have been accumulated in environments that preferentially preserve carbons. So we may have a series of rocks that contain stuff like marine algae or leaf material or anything that has a lot of, sort of, vegetable or carbon matter in it can actually produce a carbon source for these rocks. There is a concept called kerogen. Now kerogen is the… is the type of material that is within the source rock that has the carbon, which is then driven off by the process of burial and heating. As you bury rocks below the surface of the Earth, you get a temperature increase of… on average, its about 30 degrees per kilometre as you go down, and eventually you end up getting into, what we call, the oil and gas window around 90–150ºC. So as that… that kerogen material in the source rocks gets buried, eventually you get different things driven off from the source rocks. So initially, its organic acids and water, then carbon dioxide and then you get oil, hydrocarbons – this sort of liquid form – and then you go down into… into the gas window, so the last thing to be driven off from those source rocks is the gas.

Richard Sykes – GNS Science