Petroleum, naturally occurring mixture of hydrocarbons, usually liquid “crude oil,” but sometimes taken to include natural gas. Petroleum is believed to be formed from organic debris, chiefly of plankton and simple plants, which was rapidly buried in fine-grained sediment under marine conditions unfavorable to oxidation. After some biodegradation, increasing temperature and pressure caused cracking, and oil was produced. As the source rock was compacted, oil and water were forced out and slowly migrated to porous reservoir rocks, chiefly sandstone or limestone. Finally, secondary migration occurred within the reservoir as the oil coagulated to form a pool, generally capped by impervious strata and often associated with natural gas. Some oil seeped to the earth's surface; this was used by the early Mesopotamian civilizations. The first oil well was drilled in western Pennsylvania in 1859. The industry now supplies about half the world's energy, as well as the raw materials for petrochemicals. The chief world oil-producing regions are the Persian Gulf, the US, the former USSR, northern and western Africa, and Venezuela. After the removal of salt and water, the petroleum is refined by fractional distillation, producing the fractions natural gas, gasoline, naphtha, kerosene, diesel oil, fuel oil, lubricating oil, and asphalt. Undesirable compounds may be removed by solvent extraction, treatment with sulfuric acid, etc., and less valuable components converted into more valuable ones by cracking, reforming, alkylation, and polymerization.