Crude oil is a complex mixture of hydrocarbons classified as a fossil fuel, but the popular belief that it comes from liquefied dinosaurs is a misconception. Its formation is not primarily derived from large prehistoric animals. Instead, petroleum results from immense volumes of microscopic life transformed over geological timescales by Earth’s internal heat and pressure.
The True Biological Source of Petroleum
The overwhelming majority of crude oil originates from the remains of single-celled marine organisms, specifically ancient algae, bacteria, and plankton, that thrived in vast numbers in oceans and shallow seas millions of years ago. These microscopic organisms possessed a massive collective biomass, far surpassing that of all the dinosaurs that ever lived, providing the necessary source material for the world’s oil reserves.
When these organisms died, their remains drifted down to the seafloor, accumulating in areas with low oxygen levels that prevented complete decay. This organic-rich detritus mixed with fine mud and clay, forming a layer of sediment known as sapropel, the direct precursor to oil. The conditions in these anoxic environments were critical, as they preserved the carbon-rich material.
The contribution of large animals, including dinosaurs, to oil formation is negligible by comparison. When a large creature dies, its remains are quickly exposed to scavengers and decay. Even if some material were to be buried, the total biomass of these larger vertebrates is insignificant next to the continuous “rain” of trillions of planktonic organisms over millions of years.
The Deep Earth Transformation
The organic-rich mud on the seafloor begins its transformation into oil when it is buried by successive layers of sediment. This burial process, which can take millions of years, subjects the source rock to progressively increasing pressure and temperature. As the depth increases, water is squeezed out, and the compacted mudstone or shale prevents oxygen from reaching the organic material.
The preserved organic matter undergoes a process called diagenesis, where it is slowly converted into a solid, waxy material called kerogen. Kerogen represents an intermediate stage in the formation of hydrocarbons. As burial continues, the temperature rises due to the geothermal gradient, the rate at which temperature increases with depth.
The subsequent stage, known as catagenesis, is where the kerogen thermally cracks to form liquid petroleum. This process requires a specific temperature range, often called the “oil window,” which typically spans from about $60^{\circ}\text{C}$ to $150^{\circ}\text{C}$. This temperature range usually corresponds to burial depths between 1 and 4 kilometers. If the kerogen is heated below this window, little oil is produced, and if it is heated above this window, the liquid oil breaks down into natural gas.
How Oil Gets Trapped in Reservoirs
Once the kerogen has been converted into liquid crude oil within the dense source rock, the oil begins to move because it is less dense than the surrounding rock and water. This movement, called primary migration, sees the oil and associated natural gas expelled from the low-permeability source rock into more porous and permeable rock layers nearby. The buoyant oil then continues its secondary migration, moving upward through these porous layers, such as sandstone or fractured limestone, by displacing the water filling the rock’s pore spaces.
The oil will continue to migrate upward until it is halted by a geological structure known as a petroleum trap. These traps prevent the further upward movement of hydrocarbons. They are formed by structural deformations, such as anticlines (arch-like folds in the rock strata) or by faults that juxtapose permeable and impermeable layers.
A trap requires a non-porous and impermeable layer of rock, known as the cap rock or seal, to complete the containment. Common cap rocks include dense shale, salt, or anhydrite, which act as a barrier to seal the reservoir rock. With the cap rock in place, the hydrocarbons accumulate in the reservoir, separated by density with gas at the top, oil in the middle, and water at the bottom.