Oil, that vital fossil fuel, owes its existence to ancient life and complex geological processes spanning millions of years. Its story begins in the warm, shallow seas or vegetation-rich lakes of bygone geological eras.
Die to rebirth
When microscopic marine organisms (plankton, algae) and larger life forms died, their remains settled to the bottom. Under oxygen-poor (anaerobic) conditions, they avoided complete decomposition, instead mixing with mineral particles (silt, clay) to form an organically rich sediment. Over time, new sediment layers accumulated above, burying the lower layers ever deeper.
Under the increasing pressure of overlying rocks and gradually rising temperatures (typically 60-120°C at depths of 2-4 km), fundamental transformations occurred. The organic matter within the sediment first converted into a thick, waxy substance – kerogen. This process, known as diagenesis, unfolds over thousands of years.
Catagenesis
The most critical stage is catagenesis. Under continued heat and pressure, chemical bonds within the kerogen broke. Complex organic molecules fragmented into simpler, liquid hydrocarbons – this is crude oil – alongside natural gas (which forms at higher temperatures). This slow “cooking” process takes millions of years within “source rocks.”
See also: Facts About Oil You Didn’t Know
However, oil rarely stays where it forms. Being lighter than surrounding water and rock, it migrates upwards through tiny pores and fractures in permeable rock layers like sandstone or limestone. Movement continues until the oil encounters an impermeable barrier (a layer of shale, a salt dome, a geological fault).
Trap for a purpose
Such a structure, called a trap, holds the oil and gas, forming an accumulation or reservoir. Without a trap, the hydrocarbons would disperse or seep to the surface.
Therefore, oil is the product of the prolonged transformation of organic material deep within the Earth, shaped by heat, pressure, and immense time, ultimately captured by suitable geological structures.