What are oil and natural gas? The common term used for oil and natural gas is petroleum. Petroleum is the product of the decomposition of organic matter trapped in sediments. Once formed, the petroleum migrates from its source, usually a carbonaceous shale (see Chattanooga Shale), through permeable rocks until it becomes trapped in a reservoir. The trapping is generally caused by impermeable layers that have been deformed by tectonic processes. Briny sedimentary water aids in the upward migration after the petroleum is squeezed from the shale-oil, since oil floats on water, and natural gas is lighter than oil

When was it deposited? Oil and gas are formed by heat and pressure to organic material which is buried in the process of form sedimentary rocks. A good description of how oil and gas forms is in Jean-Marc Jancovici's energy website. Shell oil also has a introduction to oil and gas.

How is it used? Petroleum is a fossil fuel. It can be refined to form a variety of products other than gasoline or kerosene. It is used in plastics, herbicides, insecticides, rubber, etc. And, of course, both oil and natural gas are used for sources of industrial and domestic heat.