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Petroleum, which includes both oil and natural gas, can be a gaseous, liquid, or semisolid substance. A. Petroleum comes from organic matter that has undergone a complex series of chemical changes under the seafloor. B. Although most of the petroleum formed leaks away into the ocean, some migrates from shale to sandstone or limestone, and is caught in pools. C. Porous rocks made of quartz or carbonate minerals are particularly likely to house oil pools because of their strong molecular attraction with oil. D. Petroleum forms best when organic matter is evenly distributed over a large area and does not exceed 8 percent of the material in the clay. E. More than 60 percent of the petroleum discovered so far has been found in rocks that are less than two-and-a-half million years old. F. It is difficult to estimate the total amount of petroleum in the world, but experts believe that 1,500-3,000 billion barrels will eventually be discovered.