From the scene in the movie "Airplane!" in which the character Johnny says, "then the dinosaurs got too fat and died and turned into oil," to the famous Sinclair Oil dinosaur, petroleum and fossil vertebrates have been mistakenly linked together. In reality, dinosaurs and other vertebrate animals did not produce petroleum.
The word "petroleum" comes from the Greek words "petra" meaning "rock" and "oleum" meaning oil, so petroleum is "rock oil"! Petroleum, or crude oil, is a hydrocarbon (a chemical compound that contains hydrogen and carbon), which can be distilled into gasoline, kerosene, oils of different viscosities, and waxes. But where does it come from?
Humans are one of the main forces acting on the Earth's environment today. We generally do not leave large amounts of plant or animal matter lying about in one area long enough for them to accumulate and decompose in large deposits. Millions of years ago, however, plants could die and remain on the ground to decompose over a long period of time, and micro-organisms could accumulate over ocean and lake bottoms.
The best environments for the production of petroleum is restricted basins of water, such as ocean or lake bottoms, where there is little or no water circulation. The poor circulation results in very little oxygen in the water. The micro-organisms decompose partially, using up what little oxygen there is in the water and sediments. When the oxygen is completely used up, the decomposition stops, and what remains of the micro-organisms is preserved. (If the organic matter of the micro-organisms completely decomposes, there will be nothing left to be turned into petroleum.) As more micro-organisms die in the area, an enormous amount of decomposing animal material could accumulate over a period of hundreds or thousands of years.
As more decomposing micro-organisms or soil are deposited on top, the animal material at the bottom of the pile would become compressed by the weight of the material on top. Over millions of years, the animal material at the bottom of the pile would become so compressed that the hydrogen and carbon that composed the micro-organisms when they were alive would become long chains of hydrocarbons. As the sediments become more compacted, the crude oil and natural gas are squeezed out of the sediments, and travel to layers of sand that are more porous and permeable.
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