Fossil Fuels: How many T. Rex does it take to power a car?

It was once a common misconception that fossil fuels (also known as hydrocarbons or simply oil, gas, and coal) are the remains of dinosaurs, such as Tyrannosaurus Rex.  In fact, this belief was so prominent in the early oil and gas industry, that Sinclair Oil featured a green Brontosaurus as its mascot.  However, we now know that nearly all fossil fuels are remnants of bacteria and algae.  In order to produce the amounts of oil and gas required by our daily lives, these small organisms must have been present in significant quantities and were much more prominent than dinosaurs.   

The Brontosaurus used as Sinclair Oil's mascot during the 1960s. Image from American Oil and Gas Historical Society.

So where did these organisms live and how did they become extractable fossil fuels? Well, let's focus on the fossil fuels we use daily, oil and natural gas. Most of the organic material (organisms containing carbon) that became oil and gas were once bacteria and algae in the oceans, lakes, rives, and lagoons of primeval Earth.  These organisms were deposited on the bottoms of bodies of water when they died.  For the organic matter to become oil, they must first be preserved. Preservation occurs where there is a high productivity rate (i.e., organic matter being deposited quickly) and anoxic conditions (i.e., minimal oxygen).  If one of these conditions isn’t right, there either won’t be enough organic matter to produce oil or other living organisms will decompose the organic matter.  If both of these conditions are met and the organic matter is preserved, it can then be buried by the sediments that are deposited over them. As the organic matter is buried, the temperature of the surrounding rock increases, due to heating from Earth's interior, baking the organic matter at 80-150°C for millions of years.  The baking of these sediments is known as generation, during which the combination of temperature and time converts the organic matter into oil and natural gas.

Cross section of a petroleum system modified from Magoon and Dow (1994).  Red arrows indicate heating from below.  Blue arrows show migration of hydrocarbons through the reservoir. Dashed lined mark the extent of the oil trap. 

After the organic rich sediments have been converted into oil, they are now a potential oil deposit.  In order to become extractable, the oil must first migrate. Migration is the process where hydrocarbons, such as oil, move from the source rock, into a permeable and porous reservoir (e.g., sandstone).  After migrating the hydrocarbons may be entrapped, which is the process where hydrocarbons are trapped in a specific area due to that areas geometry.  Common hydrocarbon traps include folds, uplifts, and faults. In order for the hydrocarbons to stay in the trap an impermeable layer, or seal, is required to prevent fluid flow and loss of hydrocarbons.  At this point the organism has become oil and is in a prime position be extracted and ultimately power your car.  So how many T. Rex does it take to power your car? Well, a live one might be able to pull it along, but their remains won’t be able to keep it running.