Petroleum (oil and natural gas) and coal are fossil fuels. They are called fossil fuels because they are derived from the remains of once living organisms, predominantly plants. Plants store energy by fixing carbon through the process of photosynthesis. When the remains of plants are buried and incorporated into sedimentary rocks they can be converted into fossil fuels.
Fossil fuels occur in sedimentary rocks. An understanding of how sedimentary rocks form is essential if one is to understand how fossil fuels are formed. This image of the Grand Canyon illustrates a characteristic feature of sedimentary rocks , they occur in horizontal layers. All the rocks exposed at the surface in Ohio are sedimentary rocks.
There are three main types of rocks, igneous, sedimentary and metamorphic. Igneous rocks form by the cooling of magma, an example being volcanic rocks which form when lava cools. Sedimentary rocks form by the lithification of unconsolidated sediments. Sandstone, shale, limestone and dolomite are all examples of sedimentary rocks. Metamorphic rocks are rocks that have been changed by an increase in heat and/or pressure. An example of a metamorphic rock is marble which forms by metamorphism of the sedimentary rock limestone.
