Petroleum
Petroleum
Petroleum exploration and production
Petroleum is a term describing a variety of liquid hydrocarbons generally found in sedimentary rocks. Some scientists also include natural gas in their definition of petroleum. The most familiar types of petroleum are tar, oil, and natural gas. Petroleum forms through the accumulation, burial, and transformation of organic material, such as the remains of plants and animals, by chemical reactions over long periods of time. After petroleum has been generated, it migrates upward (by virtue of its buoyancy relative to ground-water) until it is either trapped underground by impermeable rocks or seeps out onto Earth’s surface. Petroleum accumulates when it migrates into a porous rock called a reservoir that has an impermeable seal or cap rock that prevents the oil from migrating further.
Petroleum reserves have been discovered in many areas. In the United States, the states of Alaska, California, Louisiana, Michigan, Oklahoma, Texas, New Mexico, and Wyoming are among the most important sources of petroleum. Other countries that produce great amounts of petroleum include Saudi Arabia, Iran, Iraq, Kuwait, Algeria, Libya, Nigeria, Indonesia, the former Soviet Union, Mexico, and Venezuela.
Humans might have used torches made from pieces of wood dipped in oil for lighting as early as 20,000 BC. At around 5000 BC, the Chinese apparently found oil when they were digging underground. Widespread use of petroleum probably began in the Middle East by the Mesopotamians, perhaps by 3000 BC, and probably in other areas where oil seeps were visible at the surface of Earth. Exploration for petroleum in the United States began in 1853, when George Bissell (1821-1884), a lawyer, recognized the potential
use of kerosene, which is derived from petroleum, as lamp fuel. Bissell also recognized that boring or drilling into Earth, as was done to recover salt, might provide access to greater supplies of petroleum than surface seeps. In 1857, Bissell hired Edwin Drake (1819-1880), often referred to as Colonel Drake despite having worked as a railroad conductor, to begin drilling the first successful oil well, in Titusville, Pennsylvania. The well was drilled in 1859. Once the usefulness of oil as a fuel was widely recognized, exploration for oil increased. By 1885, oil was discovered in Sumatra, Indonesia. The famous gusher in the Spindletop field in eastern Texas was drilled in 1901. The discoveries of giant oil fields in the Middle East began in 1908 when the company now known as British Petroleum drilled a well in Persia (now Iran). During World Wars I and II, oil became a critical factor in the ability to successfully wage war.
Petroleum is among the most important natural resources. Modern society uses gasoline, jet fuel, and diesel fuel to run cars, trucks, aircraft, ships, and other vehicles. Home heat sources include oil, natural gas, and electricity, which in many areas is generated by burning natural gas. Petroleum and petroleum-based chemicals are important in manufacturing plastic, wax, fertilizers, lubricants, and many other goods. Thus, petroleum is an important part of many human activities.
Types of petroleum
Petroleum, including liquid oil and natural gas, consists hydrocarbons, which comprise hydrogen and carbon, with small amounts of impurities such as nitrogen, oxygen, and sulfur. The molecules of hydrocarbons can be as simple as that of methane, which consists of a carbon atom surrounded by four hydrogen atoms, abbreviated as CH4. More complex hydrocarbons, such as naphthenes, include rings of hydrogen and carbon atoms linked together. Differences in the number of hydrogen and carbon atoms in molecules as well as their molecular structure (carbon atoms arranged in a ring structure, chain, or tetrahedron, for example) produce numerous types of petroleum.
Different types of petroleum can be used in different ways. Jet fuel differs from the gasoline that automobiles consume, for example. Refineries separate different petroleum products by heating petroleum to the point that heavy hydrocarbon molecules separate from lighter hydrocarbons so that each product can be used for a specific purpose. Refining reduces the waste associated with using limited supplies of more expensive petroleum products in cases in which a cheaper, more plentiful type of petroleum would suffice. Thus, tar or asphalt, the dense, nearly solid hydrocarbons, can be used for road surfaces and roofing materials, waxy substances called paraffin can be used to make candles and other products, and less dense, liquid hydrocarbons can be used for engine fuels and lubricating oils.
Sources of petroleum
Petroleum is typically found beneath the surface of Earth in accumulations known as fields. Fields can contain oil, gas, tar, water, and other substances, but oil, gas, and water are the most common. In order for a field to form, there must exist both a reservoir of porous rock (generally sedimentary rock) and a seal consisting of an impervious geologic structure (such as a fault) or impervious strata to trap the petroleum. To find these features together in an area in which petroleum has been generated by chemical reactions affecting organic remains requires many coincidences of timing of natural processes.
Petroleum generation occurs over millions of years. In order for petroleum generation to form, organic matter such as dead plants or animals must accumulate in large quantities. The organic matter can be deposited along with sediments and then buried as more sediments accumulate. The organic-rich sediments are compressed to form a source rock via a process known as diagenesis. After burial, chemical activity in the absence of oxygen allows the organic material in the source rock to change into petroleum. A good petroleum source rock is a sedimentary rock such as shale or limestone that contains between 1% and 5% organic carbon. Rich source rocks occur in many environments, including lakes, deep areas of the seas and oceans, and swamps. The source rocks must be buried deep enough below the surface of Earth to heat up the organic material, but not so deep that the rocks metamorphose or that the organic material changes to graphite or materials other than hydro-carbons. Temperatures less than 302°F (150°C) are typical for petroleum generation, and the range of temperatures in which petroleum can be generated is known as the petroleum window.
Once a source rock generates and expels petroleum, the petroleum migrates from to a porous reservoir rock that can store the petroleum. Sandstone, limestone, and highly fractured rocks can all serve as petroleum reservoirs. A good reservoir rock might have pore space that exceeds 30% of the rock volume. Poor quality reservoir rocks have less than 10% void space capable of storing petroleum. Rocks that lack pore space also tend to lack permeability, the property of rock that allows fluid to pass through the pore spaces of the rock. With very few pores, it is not likely that the pores are connected and less likely that fluid will flow through the rock than in a rock with larger or more abundant pore spaces. Highly porous rocks tend to have better permeability because the greater number of pores and larger pore sizes tend to allow fluids to move through the reservoir more easily. The property of permeability is critical to producing petroleum: If fluids can not migrate through a reservoir rock to a petroleum production well, the well will not produce much petroleum and the money spent to drill the well has been wasted. In some cases, fluids are pumped under great pressure into poor quality reservoir rocks in order to fracture the rock and increase permeability. This procedure is known as hydrofracturing.
Petroleum reservoirs are generally sealed by a less porous and permeable rock known as a seal or cap rock, which prevents the petroleum from migrating further. Rocks like shale and salt provide excellent seals for reservoir rocks because they do not allow fluids to pass easily. Seal-forming rocks tend to be made of small particles of sediment that fit closely together so that pore spaces are small and poorly connected. The permeability of a seal must be virtually zero in order to retain petroleum in a reservoir rock for millions to hundreds of millions of years, the time span between formation of petroleum to the discovery and production of many petroleum fields. Likewise, the seal must not be subject to forces within Earth that might cause fractures or other breaks in the seal to form.
Reservoir rocks and seals work together to form a trap for petroleum. Typical traps for petroleum include buried hills shaped like upside-down bowls below the surface of Earth, known as anticlines, or traps formed by faults along which rock has been pulverized to form a seal. Abrupt changes in rock type can form good traps, such as sandstone deposits next to shale deposits, especially if a sand deposit is encased in a rock that is sufficiently rich in organic matter to act as a petroleum source and endowed with the properties of a good seal.
An important aspect of the formation of petroleum accumulations is timing. The reservoir must have been deposited prior to petroleum migrating from the source rock to the reservoir rock. The seal and trap must have been developed prior to petroleum accumulating in the reservoir, or else the petroleum would have migrated farther. The source rock must have been exposed to the appropriate temperature and pressure conditions over long periods of time to change the organic matter to petroleum. The necessary coincidence of several conditions is difficult to achieve in nature.
Petroleum exploration and production
Petroleum exploration and production activities are performed primarily by geologists, geophysicists, and petroleum engineers. Geologists look for areas likely to have the necessary combination of source rocks, reservoir rocks, and geologic history to accumulate large amounts of petroleum. They examine the rocks at the surface of Earth and information from wells drilled in the area. Geologists also examine satellite images of large or remote areas to evaluate the rocks more quickly.
Geophysicists examine seismic data, which are derived from recording waves of energy introduced into the rock layers of Earth through explosions or other means, to determine the shape of the rock layers beneath the surface and whether or not traps such as faults or anticlines exist.
Once the geologist or geophysicist has gathered evidence of potential for a petroleum accumulation, called a prospect, an engineer assists in determining how to drill a well or wells to assess the prospect. Drilling a well to explore for petroleum can cost as little as $100,000 and as much as $30,000,000 or more, depending on how deep the well must be drilled, what types of rocks are present, and how remote the well location is. Thus, the scientists must evaluate how much the well might cost, how big the prospect might be, and how likely the scientific predictions are to be correct. In general, approximately 15% of exploration wells are successful.
Once a successful exploration well has been drilled, the oil or gas, or both, are pumped to the surface through the well. At the surface, the petroleum either moves through a pipeline or is stored in a tank or on a ship until it can be sold.
Petroleum reserves
Estimates of the amount of recoverable oil and natural gas in the United States are 113 billion barrels of oil and 1,074 trillion cubic feet of natural gas. Worldwide estimates of recoverable oil and natural gas are 1 trillion barrels of oil and 5 quadrillion cubic feet of natural gas. These worldwide reserves are expected to supply 45 years of fuel at current production rates with expected increases in demand. However, such estimates do not take into account reserves added through new discoveries or through the development of new technology that would allow more oil and natural gas to be recovered from existing oil and natural gas fields.
Daily consumption of oil in the United States exceeds 17 million barrels of oil per day, of which approximately 7 million barrels are in the form of gasoline for vehicles. Over half the petroleum consumed in the United States is imported from other countries. While the United States has large reserves of petroleum, the undiscovered fields that remain tend to be smaller than the fields currently producing petroleum outside of the United States. Thus, less expensive foreign reserves are imported to the United States. When foreign petroleum increases in price, more exploration occurs in the U.S. as it becomes more profitable to drill wells in order to exploit smaller reservoirs.
Current research
Current research in petroleum includes many different activities. Within companies that explore for and produce petroleum, scientists and engineers try to determine where they should explore for petroleum, how they might recover more petroleum from a given field, and what types of tools can be lowered into wells in order to enhance our understanding of whether or not that individual well might have penetrated an oil or gas field. They also study fundamental processes such as the deposition and diagenesis of sedimentary rocks form, the response of rocks to tectonic stress, and the evolution of life on Earth. The United States Geological Survey (USGS) evaluates petroleum reserves and new technology to produce oil and gas. The federal government operates several facilities called Strategic Petroleum Reserves that store large
KEY TERMS
Barrel— A unit of volume typically used for oil. A barrel contains 42 gal (160 L).
Field— An accumulation of oil or natural gas (or both) that can be produced, usually for a profit.
Hydrocarbon— Compound made from atoms of hydrogen and carbon. Methane (CH4) and propane (C3H8) are simple, gaseous hydrocarbons. Oil can vary from tar to very light liquid hydrocarbon to natural gas.
Natural gas— Gaseous hydrocarbon.
Oil— Liquid hydrocarbon.
Petroleum— Substances made of hydrogen and carbon compounds (hydrocarbons), typically also containing impurities such as nitrogen, sulfur, and oxygen.
Reservoir rock— A rock that has sufficient pore space and connection between pores to allow oil or gas to be stored in the rock and to flow out of the rock. Sandstones and limestones can be excellent reservoir rocks.
Seal— Rock made of fine particles and having little pore space or connection between pores that prevents fluids from leaking out of a reservoir rock. Shale and salt provide some of the best seals for petroleum reservoirs.
Sedimentary rock— Rock formed by deposition, compaction, and cementation of weathered rock or organic material, or by chemical precipitation. Salt and gypsum form from evaporation and precipitation processes.
Source rock— Sedimentary rock containing sufficient organic matter (0.5-5% organic carbon from organic matter in a source rock is typical) to generate petroleum.
Trap— A structure in which petroleum can accumulate and be stored. Anticlines (dome shaped structures below the surface of Earth) can form good traps. Traps can also form along faults and in areas where rock types change rapidly.
quantities of petroleum for use in times of supply crisis.
Petroleum exploration specialists are using a type of geophysical data known as three-dimensional seismic data to study the structures and rock types below the surface of Earth in order to determine where exploration wells might successfully produce petroleum. Geochemists are assessing the results of studies of the chemistry of the surface of Earth and whether or not these results can improve the predictions of scientists prior to drilling expensive exploratory wells.
Significant recent discoveries of petroleum have been made in many areas of the world: Algeria, Brazil, China, Egypt, Indonesia, the Ivory Coast, Malaysia, Papua New Guinea, Thailand, the United Kingdom, and Vietnam, among others. In the United States, the Gulf of Mexico, Gulf Coast states, California, and Alaska continue to attract the interest of explorationists.
See also Air pollution; Fossil and fossilization; Internal combustion engine; Oil spills; Plastics.
Resources
BOOKS
Blatt, H., R. Tracy, and B. Owens. Petrology: Igneous, Sedimentary, and Metamorphic. New York: Freeman, 2005.
Gow, S. Roughnecks, Rock Bits and Rigs: The Evolution of Oil Well Drilling Technology in Alberta, 1883-1970. Calgary, Alberta: University of Calgary Press, 2006.
Lyons, W.C. and G.J. Plisga. Standard Handbook of Petroleum & Natural Gas Engineering. Woburn, Mass.: Butterworth-Heinemann, 2004.
Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, New Jersey: Prentice Hall, 2004.
Gretchen M. Gillis