The oil reserves that we are currently using are the by-product of the decomposition of the prehistoric organisms that were buried in the crust and were preserved for millions of years. Organic components of the dead bodies are mixed with sedimentary rocks, silts and sand. In time, the dead bodies are covered with increasing amounts of mud, clay, sand and salts that traps the organic component deep down until it is converted into other form. With the extreme temperature and pressure, these materials are turned into crude oil which is one of the main sources of petroleum in the world.
You've just described the formation of fossil fuels. Prehistoric dead organic matter is buried under rock, mud, and sand. As millions of years passed, the organic matter decomposed and decayed. The type of fossil fuel depends on the specific temperature and pressured conditions at the time.
For example, oil and natural gas were created from organisms that lived in the water and were buried under ocean or river sediments. Long after the great prehistoric seas and rivers vanished, heat, pressure and bacteria combined to compress and "cook" the organic material under layers of silt. In most areas, a thick liquid called oil formed first, but in deeper, hot regions underground, the cooking process continued until natural gas was formed. Over time, some of this oil and natural gas began working its way upward through the earth's crust until they ran into rock formations called "caprocks" that are dense enough to prevent them from seeping to the surface. It is from under these caprocks that most oil and natural gas is produced today.
Coal is formed in a similar manner but there are some key differences. Namely, instead of organisms in the ocean; organisms in wetlands caused the formation of coal.