The notion that petroleum or crude oil comes from dinosaurs is fiction. Surprised? Oil formed from the remains of marine plants and animals that lived millions of years ago, even before the dinosaurs. The tiny organisms fell to the bottom of the sea. Bacterial decomposition of the plants and animals removed most of the oxygen, nitrogen, phosphorus and sulfur from the matter, leaving behind a sludge made up mainly of carbon and hydrogen. As the oxygen was removed from the detritus, decomposition slowed. Over time the remains became covered by layers upon layers of sand and silt. As the depth of the sediment reached or exceeded 10,000 feet, pressure and heat changed the remaining compounds into the hydrocarbons and other organic compounds that form crude oil and natural gas.
The type of petroleum formed by the plankton layer depended largely on how much pressure and heat were applied. Low temperatures (caused by lower pressure) resulted in a thick material, such as asphalt. Higher temperatures produced a lighter petroleum. Ongoing heat could produce gas, though if the temperature exceeded 500°F, the organic matter was destroyed and neither oil nor gas was produced.