 Natural gas deposits are also the main source of helium.Heavy crude oil, which is much more viscous than conventional crude oil, and oil sands, where bitumen is found mixed with sand and clay, began to become more important as sources of fossil fuel in the early 2000s.Oil shale and similar materials are sedimentary rocks containing kerogen, a complex mixture of high-molecular weight organic compounds, which yield synthetic crude oil when heated (pyrolyzed).