What are differences between natural gas, coal, and oil?
All may be considered “fossil fuels”, but they differ in time and conditions of formation and chemical composition.
Coal is a solid, oil is a liquid and natural gas is a vapor (gas). Coal and oil form from deposits of animal and vegetable matter deep in the ground at conditions of high pressure over many years. Natural gas is also formed during those processes, but it can also be generated relatively quickly by organic material decomposition, as in municipal landfills and waste treatment plants. Coal is obtained primarily by mining while oil and natural gas are extracted from wells drilled into the earth from land or sea-based platforms.
Coal is a complex mixture of many different long-chain hydrocarbons. Oil is also a mixture, but of somewhat shorter hydrocarbon molecules than can be separated into products like gasoline, naphthas, kerosene and diesel fuels. Natural gas is primarily methane with several percent of ethane and other traces of light hydrocarbon gases.
In the leading theory, dead organic material accumulates on the bottom of oceans, riverbeds or swamps, mixing with mud and sand. Over time, more sediment piles on top and the resulting heat and pressure transforms the organic layer into a dark and waxy substance known as kerogen. Left alone, the kerogen molecules eventually crack, breaking up into shorter and lighter molecules composed almost solely of carbon and hydrogen atoms. Depending on how liquid or gaseous this mixture is, it will turn into either petroleum or natural gas. http://www.livescience.com/9404-mysterious-origin-supply-oil.html
Major in situ coal fields generally appear to have been formed either in brackish or fresh water, from massive plant life growing in swamps, or in swampland interspersed with shallow lakes. The development of substantial in situ coal measures thus requires extensive accumulations of vegetable matter that is subjected to widespread submersion by sedimentary deposits. http://www.brighthubengineering.com/power-plants/31315-coal-formation-theories/
Other details and options are in that last reference. Basically, coal is purely vegetation in strata. It is always “longer-chain” hydrocarbons. Oil may be formed from animal as well as plant decomposition, and has relatively shorter hydrocarbon chains, making it a liquid instead of a solid.