Microorganisms supplied the organic material that over time was converted into hydrocarbons, which are molecules that form crude oil. Ancient microorganisms, such as cianobacteria, were abundant in primitive oceans and lakes. After dying, they mixed with mud and settled to a sea or lake bottom. Layers of sediments deposited on their top increased the pressure; this caused the organic matter to become a waxy material called kerogen, and later, crude oil. Blue-green algae, called estromatolites, dating back to about 3,000 million years ago, were among the microorganisms that became the largest known deposits of oil in the world -- the Green River Formation in Colorado-Wyoming.
The biogenesis of fossil fuel started in warm seas, which were ideal for the incubation of microscopic life. The transformation of the dead microorganisms began when surface waters were so rich in microscopic life that the deposits of rock sediments outpaced the amount of decay on the seabed. The result is an accumulation of thick biologic sludge. The average temperature for oil formation ranges from 120 to 210 degrees Fahrenheit. Another favorable condition for the biogenesis was the presence of big rivers, sending rich flows of nutrients into the seabed, as it happened with the Gulf of Mexico area, which was also cut off from the wider global sea, thus concentrating the nutrients and mud.
In the beginning of the 1900s, scientists believed that crude oil was not the result of microorganism activity but dinosaurs. A breakthrough came in the 1930s when a German chemist analyzed crude oil and discovered fossil remains of chlorophyll, the compound that helps plants, as well as some algae and bacteria, convert sunlight into energy. By the 1960s and 1970s, scientists could find fossils of ancient microbes in crude oil samples.
Some microorganisms that produce carbon dioxide are also useful during the process of oil recovery. The produced carbon dioxide gas helps propel upward the oil from almost exhausted wells. Another example of the role of microorganisms in oil recovery is the bacteria Xanthomonas campestri, which produces xanthan gum, a substance able to loosen oil that clings to underground rock particles.