Oil and petroleum can come from any kind of organic material if it is well preserved throughout the history of the rock. There are three types of organic material, the one that comes from plants, the one that comes from forams (also called phytoplancton and zooplancton) these are little bugs that float in the sea water, and the one that is a combination of these two, in other words a combination of plants and floating bugs.

A rock that contains organic material is also called "Black Shale". When this rock is buried and undergoes high pressures and temperatures, it expels oil. If the burial continues and the temperature increases, it will expel gas (the one some people use to cook). If the rock gets even hotter, the rock itself may convert into coal.

Think of the oil as the liquid expelled from the black shale, and coal as the black shale that has been heated very much!

Nobody is sure exactly where oil comes from. It is composed of a mixture of the elements carbon and hydrogen, and it is supposed that organic matter (containing both elements) that is heated by the earth's interior creates oil. If this is true, then both plant and animal matter would make oil, because both contain hydrogen and carbon. Other possible sources include comets.

Coal is compressed plant matter and has not been subjected to the same amount of change as oil has, if oil indeed is once-living matter. You can still see roots and the like in blocks of coal, even. Coal is solid because it has not been significantly altered chemically and because roots, twigs, etc. are solid. The phase of petroleum actually depends on the number of carbons per molecule: methane (in a gas stove and has one carbon) is a gas; gasoline (which has six to eight carbons) is a liquid; tar (which has fifteen or more carbons) is a solid.