It All Started With Small Aquatic Organisms
The term "fossil fuels" has led to a great deal of confusion and misunderstanding regarding the ancient organisms that eventually formed the fuel we use today. In actuality, it wasn't the remains of dinosaurs, reptiles, mammals, birds, or any other large creatures that eventually turned into fossil fuels, but rather incredibly small organisms that lived in the sea. Specifically, the origin of fossil fuels is phytoplankton and zooplankton, as well as plant matter such as algae and microorganisms that lived in aquatic environments.
Unlike the fossils many people imagine, the remnants that eventually became petroleum were the tiny, floating organisms that largely served as a food source for fish and other marine life, and now, millions of years later, serve as a primary fuel source across the planet.
The Majority Of Oil Deposits Were Formed During The Mesozoic Era
It's important to understand exactly how long the entire process takes, and the conditions required to initiate it. One fact that starkly outlines both is that the majority of oil deposits in existence today were formed in the Mesozoic Era, approximately 66-252 million years ago.
An estimated 70% of deposits were formed in this window; scientists believe the highly tropical climate of that era resulted in huge amounts of plankton in the oceans. This is also one reason so many people believed dinosaurs played a role in petroleum formation, as dinosaurs only existed during the Mesozoic era. However, it's further estimated that 20% of oil deposits formed in the Cenozoic Era - as in the last 65 million years - and only 10% formed in the Paleozoic Era, which includes the hundreds of millions of years before the Mesozoic.
When Plankton Perished, They Sank To The Bottom Of The Sea
After the life cycle of the phytoplankton and zooplankton came to an end, the tiny marine organisms would sink and eventually float to the bottom of the oceans, many of which were much more shallow than the oceans of our current geological era. The incredibly small bits of plant and algae material and skeletal remains of the living organisms would mix with the clay and silt on the seafloor, forming what is known as pelagic sediment.
At these lower levels, known as anoxic waters, very little dissolved oxygen is present, meaning the decomposition of the organisms' remains happened much more slowly.
Marine Organisms Were Buried Under Layers Of Sediment And Debris
Once the layer of sediment containing the biomass settled, new deposits and layers of sediment would form on top of them. Layers and layers of mud and silt would bury the biomass deeper and deeper, exposing it to high levels of pressure. Increased pressure, as well as subduction of lower levels of the seafloor, would mean the layer of biomass was also exposed to increasing levels of heat.
Through the crucible of pressure and heat, the sediment led to diagenesis - or the process in which sediment is turned into sedimentary rock. However, this process is only one aspect of the eventual formation of petroleum. The other aspect, which it works in conjunction with, is anaerobic decomposition.