Existing oil was formed from the remains of microscopic plants and animals that died in the ocean between 10 and 600 million years ago. When these organisms died, they became buried in the mud and sand at the bottom of the sea. Over millions of years, sedimentary layers of decayed organisms were formed.
These layers contained little or no oxygen, so microorganisms broke down the remains into compounds rich in carbon. This led to the formation of organic layers. Combined with sediments, the organic material formed fine-grained shale or source rock. New sedimentary layers exerted extreme heat and pressure on the source rock. This caused the distillation of the organic material into crude oil and natural gas, which flowed and accumulated into thicker sandstone or limestone called reservoir rock. Due to the Earth’s movements, the oil and natural gas in the reservoir rocks became trapped in layers of impermeable rocks such as marble and granite. Movements of the Earth that led to the formation of oil include folding, faulting and pinching out.
In folding, horizontal movements push the layers of rock upward into a fold or anticline. In faulting, one side of the rock layers moves upward or downward. In pinching out, an impermeable rock layer is squeezed toward the reservoir rock.