Oil and natural gas are hydrocarbons, molecules of carbon and hydrogen atoms. Gas molecules are much smaller than oil molecules. Some oil molecules are chains of over 60 carbon atoms, surrounded by hydrogen, others have a more complex structure such as a ring of carbon atoms.
The steps involved in formation of a, petroleum reserve include accumulation followed by burial of organic matter. As the organic matter is buried it is heated and converted into petroleum by the process of maturation. Finally, the petroleum needs to migrate into a reservoir where it can be produced. There are 3 requirements for an economical petroleum occurrence; favorable source rocks, reservoir rocks, and a trap. In addition the timing needs to be just right, in that the reservoir rock and trap need to be in place at the time migration occurs.
Organic matter (remains of plants and animals) in sediments has two main sources marine plankton and continental plant material. Plankton, one celled plants and animals, that live in the ocean, may settle to the bottom where they accumulate. Continental organic matter, which is mostly plant material, may be washed into oceans along with sediments. Clay rich sediments are highest in organic matter, these will eventually become lithified to form shale.
There is an increase in temperature with depth in the earths crust. As organic matter is buried it is heated and transformed into kerogen, oil and gas. The most oil is produced between the temperatures of 60 and 120 degrees C, at a depth range known as the oil window.
A source rock is a rock that is capable of producing hydrocarbons. There are two key requirements for source rocks; 1) they need to have a high enough concentration of organic matter and 2) they should have been heated to a high enough temperature to reach thermal maturation.
Oil & Gas 2
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