All deposits of coal, petroleum (crude oil), and natural gas come from from the partially decomposed remains of ancient organisms included within sedimentary rocks. This is why they are called "fossil fuels."

Petroleum is therefore produced and accumulated from organic matter, which in turn originates from photosynthesis.

While photosynthesis accounts for the mass production of organic matter from as early as Precambrian time about two billion years ago, it also gets credit (or blame) for creating an oxygen-enriched environment that retards, or slows down, the preservation of the very organic carbons necessary to accumulate as fossil fuel deposits. This is a factor in the extremely long period of time required for generating enough deposits for exploitation, such as in world-class oilfields.

However, in less-disturbed and low-energy environments, oxygen becomes depleted. This deficiency aids in the preservation of organic carbons in sediments. This has held true throughout the Earth's history since the Precambrian, and this same process continues to form oil even now at a rate of hundreds to thousands of barrels per year around the globe.

Beginning in Precambrian time until the Devonian Period, marine phytoplankton was the primary producer of organic carbon. Since then, higher terrestrial plants contributed increasing amounts of primary production of this organic matter. At present day, both marine phytoplankton and terrestrial higher plants produce organic matter in equal amounts. The most important contributors to organic matter in sedimentary deposits are phytoplankton, zoo-plankton, higher plants and bacteria. There is no appreciable contribution from animals or fish, so they are generally ignored.

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