|MadSci Network: Earth Sciences|
Well actually, Stephen, it doesn't.... Or at least, most of it doesn't. Hydrocarbons, (Oil and Gas) come from buried organic matter which is either derived from plants, bacteria, algae, simple organisms, or rarely from more complex wriggly things that you would actually call 'animals'. This organic material (called Kerogen) gets buried in sediment (sometimes it IS the sediment) and preserved from oxidation by the nature of the environment of deposition. Normally, wood and leaves rot down over a few years to re-enter the carbon cycle, but if the groundwater/ lake water/ or seawater is stagnant and anoxic, then oxidation cannot take place. Anaerobic bacteria can live in these environments and do often modify the kerogens, but don't destroy them. Note that only the lower layers of the water need to be stagnant, the surface layers may be clean and well-oxygenated, overlying this stagnant smelly stuff. With time and burial the rocks are compressed and heated (because the inside of the Earth is hot and heat slowly leaks out, so deeper buried rocks are hotter than shallow ones). The kerogens are gently cooked and oil sweats out of them. Later, if burial continues, the rocks begin to bake and gas is released. Finally the rocks can get so hot that the organic material would "burn" if there was any oxygen. What happens insted is that the material is transformed into graphite (pure carbon) films and amorphous coke. How do we know all this? There is a clear progression from recently buried organic material through the whole process of kerogen transformation to oil and gas. Millions of samples have been recoved from rock outcrops and from drilling. In addition, special chemicals called BIOMARKERS are clearly derived directly from parent organic molecules in the original living plants, algae, bacteria, and animals. We can even measure these biomarkers and match particular oils to a source rock of known age, environment, depositional facies, flora/fauna and degree of burial. For example, Indonesian Oils generally come from Tertiary age Pro-delta shales with a high land-plant content. North Sea Oils come from Jurassic Marine shales from a strongly anoxic environment. One aspect of my job is investigating these links between source rocks and oils, but we have specialists called GEOCHEMISTS who do it full-time. Nick Hoffman Petroleum Geophysicist
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