Petroleum, or crude oil, is a naturally occurring oily liquid composed ofvarious organic chemicals. It is found in large quantities below the surfaceof the earth and is used as a fuel and as a raw material in the chemicalindustry. Modern industrial societies use it primarily to achieve a degreeof mobility on land, at sea, and in the air that was barely imaginableless than a hundred years ago. In addition, petroleum and its derivativesare used in the manufacture of medicines and fertilizers, foodstuffs,plastic ware, building materials, paints, and cloth and to generateelectricity.In fact, modern industrial civilization depends on petroleum and itsproducts; the physical structure and way of life of the suburbancommunities that surround the great cities are the result of an ample andinexpensive supply of petroleum. In addition, the goals of developingcountries to exploit their natural resources and to supply foodstuffs for theunderdeveloped populations, are based on the assumption of petroleumavailability. In recent years, however, the worldwide availability ofpetroleum has steadily declined and its relative cost has increased.Petroleum will probably no longer be a common commercial material bythe mid-21st century. Characteristics The chemical composition of all petroleum is principally hydrocarbons.Petroleum contains gaseous, liquid, and solid elements. The consistency ofpetroleum varies from liquid as thin as gasoline to liquid so thick that it willbarely pour. Formation Petroleum is formed under the earth s surface by the decomposition ofmarine organisms. The remains of tiny organisms that live in the sea and,to a lesser extent, those of land organisms that are carried down to thesea in rivers and of plants that grow on the ocean bottoms, are mixed withthe fine sands and silts that settle to the bottom in quiet sea basins. Suchdeposits, which are rich in organic materials, become the source rocks forthe making of crude oil. The process began many millions of years agowith the development of abundant life, and it continues to this day. Thesediments grow thicker and sink into the seafloor under their own weight.As additional deposits pile up, the pressure on the ones below increasesseveral thousand times, and the temperature rises by several hundreddegrees. The mud and sand harden into shale and sandstone; carbonateprecipitates and skeletal shells harden into limestone; and the remains ofthe dead organisms are transformed into crude oil and natural gas.Once the petroleum forms, it flows upward in the earth s crust because ithas a low density. The crude oil and natural gas rise into the microscopicpores of the coarser sediments lying above. Frequently, the rising materialencounters a dense layer of rock that prevents further movement. In otherwords, the oil has become trapped, and a reservoir of petroleum is formed.A significant amount of the upward-migrating oil, however, does notencounter impermeable rock but instead flows out at the surface of theearth or onto the ocean floor. Historical Development These surface deposits of crude oil have been known to humans forthousands of years. In the areas where they occurred, they were longused for such limited purposes as caulking boats, waterproofing cloth andfueling torches. By the time of the Renaissance, some surface depositswere being distilled to obtain lubricants and medicinal products, but thereal exploitation of crude oil did not begin until the 19th century. TheIndustrial Revolution had by then brought about a search for new fuels,and the social changes it effected had produced a need for good, cheapoil for lamps. People wished to be able to work and read after dark. The search for a better lamp fuel led to a great demand for rock oil ,thatis, crude oil, and various scientists in the mid-19th century weredeveloping processes to make commercial use of it. Thus the quest for greater supplies of crude oil began. For several yearspeople had known that wells drilled for water and salt occasionallycontained petroleum, so the concept of drilling for crude oil itself soonfollowed. The first such wells were dug in Germany in 1857-59, but theevent that gained world fame was the drilling of an oil well near Oil Creek,Pennsylvania, by Colonel Edwin L. Drake in 1859. Exploration In order to find crude oil underground, geologists must search for asedimentary basin in which shales rich in organic material have beenburied for a sufficiently long time for petroleum to have formed. Thepetroleum must also have had an opportunity to migrate into porous trapsthat are capable of holding large amounts of fluid. The occurrence ofcrude oil in the earth s crust is limited both by these conditions, whichmust be met simultaneously, and by the time span of tens of millions to ahundred million years required for the oil s formation. Petroleumgeologists and geophysicists, however, have many tools at their disposalto assist in identifying potential areas for drilling. Thus, surface mappingof sedimentary beds makes possible the interpretation of subsurfacefeatures, which can then be supplemented with information obtained bydrilling into the crust and retrieving samples of the rock layersencountered. In addition, increasingly sophisticated scientific techniques,having to do with the reflection and refraction of sound waves propagatedthrough the earth, reveal details of the structure and interrelationship ofvarious layers in the subsurface. Ultimately, however, the only way toprove that oil is present in the subsurface is to drill a well. An oil field, once found, may comprise more than one reservoir, that is,more than one single, continuous, bounded accumulation of oil. Indeed,several reservoirs may be stacked one above the other, isolated byintervening shales and impervious rock . Such reservoirs may vary in sizefrom a few tens of hectares to tens of square kilometers, and from a fewmeters in thickness to several hundred or more. Most of the oil that hasbeen discovered and exploited in the world has been found in a relativelyfew large reservoirs. In the U.S., for example, 60 of approximately 10,000oil fields have accounted for half of the productive capacity and reserves. Primary Production Most oil wells in the U.S. are drilled by the rotary method that was firstdescribed in a British patent in 1844 assigned to R. Beart. In rotarydrilling, the drill string, a series of connected pipes, is supported by aderrick. The string is rotated by being coupled to the rotating table on thederrick floor. The drill bit at the end of the string is generally designedwith three cone-shaped wheels tipped with hardened teeth. Drill cuttingsare lifted continually to the surface by a circulating-fluid system driven bya pump.Trapped crude oil is under pressure. Thus, when a well bore is drilled intothis pressured accumulation of oil, the oil expands into the low-pressuresink created by the well bore in communication with the earth s surface.As the well fills up with fluid, however, a back pressure is exerted on thereservoir, and the flow of additional fluid into the well bore would soonstop, were no other conditions involved. Most crude oils, however, containa significant amount of natural gas in solution, and this gas is kept insolution by the high pressure in the reservoir. The gas comes out ofsolution when the low pressure in the well bore is encountered and thegas, once liberated, immediately begins to expand. This expansion,together with the dilution of the column of oil by the less dense gas,results in the transporting of oil up to the earth s surface.The fluid may not reach the surface, so that a pump (artificial lift) must beinstalled in the well bore to continue producing the crude oil.Eventually, the flow rate of the crude oil becomes so small, and the cost oflifting the oil to the surface becomes so great, that the well costs more tooperate than the revenues that can be gained from selling the crude oil.The well s economic limit has then been reached and it is abandoned. Oil Drill Rig and ReservoirThe rotary drilling rig uses a series of rotating pipes, called the drill string, to tap into oil reservoirs.The drill string is supported by a derrick, and turned by the rotary table on its floor. Circulating,mudlike fluid driven by a pump removes cuttings as the teeth of the drill bit dig into the rock aroundthe reservoir. Reservoirs occur in many places. They form as a result of intense pressure on top oflayers of dead marine and land organisms mixed with sand or silt. This reservoir abuts a salt dome,which has trapped a layer of oil and natural gas between itself and nonporous rock. Because theyhave no place to expand, the gas and crude oil are under high pressure and will tend to rush
explosively out the channel opened by the drill rig. Enhanced Oil Recovery The oil industry has developed schemes for supplementing the productionof crude oil that can be obtained mostly by taking advantage of thenatural reservoir energy. Such supplementary schemes, collectivelyknown as enhanced oil recovery technology, can increase the recovery ofcrude oil, but only at the additional cost of supplying extraneous energy tothe reservoir. In this way, the recovery of crude oil has been increased toan overall average of 33 percent of the original oil. Two successfulsupplementary schemes are in use at this time: water injection and steaminjection.Water Injection In a completely developed oil field, the wells may be drilled anywherefrom 60 to 600 m (200 to 2000 ft) from one another, depending on thenature of the reservoir. If water is pumped into alternate wells in such afield, the pressure in the reservoir as a whole can be maintained or evenincreased. In this way the rate of production of the crude oil also can beincreased; in addition, the water physically displaces the oil, thusincreasing the recovery efficiency. In some reservoirs with a high degreeof uniformity and little clay content, water flooding may increase therecovery efficiency to as much as 60 percent or more of the original oil inplace. Water flooding was first introduced in the Pennsylvania oil fields,more or less accidentally, in the late 19th century, and it has since spreadthroughout the world.Steam Injection Steam injection is used in reservoirs that contain very viscous oils, thosethat are thick and flow slowly. The steam not only provides a source ofenergy to displace the oil, it also causes a marked reduction in viscosity(by raising the temperature of the reservoir), so that the crude oil flowsfaster under any given pressure. This scheme has been used extensively inthe states of California, in the United States, and of Zulia, in Venezuela,where large reservoirs exist that contain viscous oil. Offshore Drilling Another method to increase oil-field production and one of the mostexciting engineering achievements in recent decades has been theconstruction and operation of offshore drilling rigs. The drilling rigs areinstalled, operated, and serviced on an offshore platform in water up to adepth of several hundred meters; the platform may either float or sit onlegs planted on the ocean floor, where it is capable of resisting waves,wind, and in Arctic regions ice floes. Refining Once oil has been produced from an oil field, it is treated with chemicalsand heat to remove water and solids, and the natural gas is separated.The oil is then stored in a tank, or battery of tanks, and later transportedto a refinery by truck, railroad tank car or pipeline. Large oil fields all havedirect outlets to major, common-carrier pipelines. Basic Distillation The basic refining tool is the distillation unit. In the U.S., after the CivilWar, more than 100 still refineries were already in operation. Crude oilbegins to vaporize at a temperature somewhat less than that required toboil water. Hydrocarbons with the lowest molecular weight vaporize at thelowest temperatures, whereas successively higher temperatures arerequired to distill larger molecules. The first material to be distilled fromcrude oil is the gasoline fraction, followed in turn by naphtha and then bykerosine. The residue in the kettle, in the old still refineries, was thentreated with caustic and sulfuric acid, and finally steam distilled thereafter.Lubricants and distillate fuel oils were obtained from the upper regionsand waxes and asphalt from the lower regions of the distillationapparatus. In the later 19th century the gasoline and naphtha fractionswere actually considered a nuisance because little need for them existed,and the demand for kerosine also began to decline because of thegrowing production of electricity and the use of electric lights. With theintroduction of the automobile, however, the demand for gasolinesuddenly burgeoned, and the need for greater supplies of crude oilincreased accordingly. Oil Refining and Fractional DistillationCrude oil is refined into products such as gasoline, asphalt, and waxes by a process calledfractional distillation. During the process, the parts, or fractions, of crude oil are divided outsuccessively by their increasing molecular weight. For instance, gasoline has a lowmolecular weight and vaporizes at a fairly low temperature. This means that at theappropriate temperature, while all of the rest of the oil is still in liquid form, gasoline may beseparated out. The remaining oil goes through the same process at a slightly highertemperature, and jet fuel is divided out. Repeating the distillation process several times willseparate out several constituents of crude oil, which are then processed and put to a widerange of uses. Product Percentages In 1920 a U.S. barrel of crude oil, containing 42 gallons, yielded 11gallons of gasoline, 5.3 gallons of kerosine, 20.4 gallons of gas oil anddistillates, and 5.3 gallons of heavier distillates. In recent years, bycontrast, the yield of crude oil has increased to almost 21 gallons ofgasoline, 3 gallons of jet fuel, 9 gallons of gas oil and distillates, andsomewhat less than 4 gallons of lubricants and 3 gallons of heavierresidues. Petroleum Engineering The studies and tasks carried out by petroleum engineers are drawn fromvirtually every field of science and engineering. Thus the explorationstaffs include geologists who specialize in surface mapping in order to tryto reconstruct the subsurface configuration of the various sedimentarystrata that will provide clues to the presence of petroleum traps.Subsurface specialists then study drill cuttings and interpret data on thesubsurface formations that is relayed to surface recorders from electrical,sonic, and nuclear logging devices lowered into the bore hole on a wireline. Seismologists interpret sophisticated signals returning to the surfacefrom sound waves that are propagated through the earth s crust.Geochemists study the transformation of organic matter and the meansfor detecting and predicting the occurrence of such matter in subsurfacestrata. In addition, physicists, chemists, biologists, and mathematiciansall support the basic research and development of sophisticatedexploration techniques.Petroleum engineers are responsible for the development of discoveredoil accumulations. They usually specialize in one of the importantcategories of production operation: drilling and surface facilities,petrophysical and geological analysis of the reservoir, reserve estimationand specification of optimal development practices, or production controland surveillance. Although many of these specialists have formal trainingas petroleum engineers, many others are drawn from the ranks ofchemical, mechanical, electrical, and civil engineers; physicists,chemists, and mathematicians; and geologists. Production Volumes and Reserves Crude oil is perhaps the most useful and versatile raw material that hasbecome available for exploitation. By 1995, the United States was using 7billion barrels of petroleum per year, and worldwide consumption ofpetroleum was 25 billion barrels per year. Reserves The world s technically recoverable reserves of crude oil the amount ofoil that experts are certain of being able to extract without regard to costfrom the earth add up to about 2300 billion barrels, of which some 110billion barrels are in the United States. However, only a small fraction ofthis can be extracted at current prices. Of the known oil reserves that canbe profitably extracted at current prices, more than half are in the MiddleEast; only 2 percent are in the United States.Projections It is likely that some additional discoveries will be made of new reservesin coming years, and new technologies will be developed that permit therecovery efficiency from already known resources to be increased. Thesupply of crude oil will at any rate extend into the early decades of the21st century. Virtually no expectation exists among experts, however, thatdiscoveries and inventions will extend the availability of cheap crude oilmuch beyond that period. For example, the Prudhoe Bay field on theNorth Slope of Alaska is the largest field ever discovered in the westernhemisphere. The ultimate recovery of crude oil from this field isanticipated to be about 10 billion barrels, which is sufficient to supply thecurrent needs of the U.S. for less than two years, but only one such fieldwas discovered in the West in more than a century of exploration.Furthermore, drilling activity has not halted the steady decline of U.S.crude oil reserves that began during the 1970s.Alternatives The only alternative fuel that is capable of supplying the huge energyneeds of today s world is coal, the availability of which in the U.S. andelsewhere throughout the world is well established. Associated with itsprojected increased utilization would be an increase in the use ofcoal-based electrical power to do more and more of the chores ofindustrialized nations.