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Last edited at 3:38PM on 11/27/2012
Coal is carbon mixed with minerals. Being a solid, it retains physical features that divulge its origin. The existence of fossil fern leaves in coal tells us that coal came from prehistoric plant life. If that weren't enough of a clue, the world has many peat beds where decaying vegetable matter, in the absence of oxygen is found in various stages that will ultimately lead to coal.
Oil, however, is not so easily understood. How, one may ask, can coal arise in one place and petroleum in another? It is all too easy to say, without providing specifics, that sometimes geologic heat and pressure lead to coal, and sometimes they lead to oil. There are no fossil remnants in oil. If oil is actually the remnants of living matter of bygone geologic eras, the ferns and dinosaurs could hardly leave a trace in liquid oil. .
Oil is said to come from layers of dead organisms lying on the sea floor. They include the microscopic sea plants and sea creatures which are later incorporated into the sediments. With time, these sediments are squashed and heated thus are changed to oil. Later, this oil flows through the rocks towards the surface until it becomes trapped against a layer of rocks through which it cannot flow. Visit http://www.oilprimer.com/where-does-oil-come-from.html for details.