Petroleum production: (1) schematic section through a drilling rig: (A) upper block; (B) drill pipes; (C) platform; (D) lifting cable; (E) lower block with hook; (F) mud pipe; (G) rotation head; (H) drill string; (J) rotating table; (K) working stage; (L) blow-out preventer; (M) mud filter; (N) mud pump; (O) drawoff pipe; (P) mud reservoir; (Q) rotating-table motor; (R) lifting-gear motor. (2) The borehole (a) and the same in more detail (b): (A) drill string; (B) tricone drill bit; (C) heavy drill collar. Each time the drill bit needs to be replaced, the whole drill string has to be removed from the bore hole.
Petroleum is a naturally-occurring mixture of hydrocarbons, usually liquid 'crude oil,' but sometimes taken to include natural gas. It is believed to be formed from organic debris, chiefly of plankton and simple plants, which has been rapidly buried under marine conditions unfavorable to oxidation. After some biodegradation, increasing temperature and pressure cause cracking, and oil is produced. As the source rock is compacted, oil and water are forced out, and slowly migrate, laterally and vertically, to porous reservoir rocks, chiefly sandstone and limestone. Finally, secondary migration occurs within the reservoir as the oil coagulates to form a pool, generally capped by impervious strata, and often associated with natural gas.
Some oil seeps to the Earth's surface: this was used by the early Mesopotamian civilizations. But most petroleum is extracted via oil wells from reservoirs in the Earth's crust sealed by upfolds of impermeable rock or by salt domes which form traps. The first oil well was drilled in western Pennsylvania in 1859. The industry thus begun has grown so fast that it now supplies about half the world's energy, as well as the raw materials for petrochemicals.
Modern technology has made possible oil-well drilling to a depth of 5 kilometers, and deep-sea wells in 150 meters of water. Rotary drilling is used, with pressurized mud to carry the rock to the surface and to prevent escape of oil. When the well is completed, the oil rises to the surface, usually under its own pressure, though pumping may be required. After removing salt and water, the petroleum is refined by fractional distillation, producing the fractions gasoline, kerosene, diesel oil, fuel oil, lubricating oil, and asphalt (see fractionation). 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. The chemical composition of crude petroleum is chiefly alkane's, saturated alicyclic compounds, and aromatic compounds, with some sulfur compounds, oxygen compounds (carboxylic acids and phenols), nitrogen, and salt.