How does petroleum form from plant and animal debris in sedimentary rock layers?

Petroleum forms through a complex geological process involving the transformation of organic matter over millions of years. It is widely believed that crude oil originates from the remains of microscopic marine organisms, such as phytoplankton and zooplankton, which settle into marine basins and are rapidly buried within layers of mudrock and limestone.1

As these sediments are buried deeper, the organic debris is converted into kerogen, a solid organic material dispersed within sedimentary source rocks. Through a process known as maturation, the increasing temperature and pressure from overlying layers cause the thermal degradation and cracking of kerogen molecules. This occurs within a specific depth and temperature range known as the "oil window," typically between 65 °C and 150 °C.231

Once generated, the liquid hydrocarbons are expelled from the source rock and migrate into more porous and permeable reservoir rocks, where they accumulate. This migration is driven by the lower density of the hydrocarbons and the pressure exerted by compacting sediments.31