Oil and natural gas are made from the fossils of ancient marine organisms, primarily phytoplankton and zooplankton. These tiny creatures lived millions of years ago in warm, shallow seas. When they died, they settled to the ocean floor, where they were covered by layers of sediment. Over time, the pressure and heat from these layers transformed the organic matter into oil and natural gas. This process is known as diagenesis.

It's important to note that oil and natural gas are not actually fossils themselves, but rather fossil fuels. They are fuels derived from the decomposition of ancient organisms.