Petroleum is a naturally occurring substance consisting of organic compounds in the form of gas, liquid, or semisolid. Organic compounds are carbon molecules that are bound to hydrogen (e.g., hydrocarbons) and to a lesser extent sulfur, oxygen, or nitrogen. The simplest of these compounds is methane with one carbon atom bound to four hydrogen atoms (Figure 1). Asphaltenes are the most complex with more than 136 carbon atoms bound to more than 167 hydrogen atoms, 3 nitrogen atoms, 2 oxygen atoms, and 2 sulfur atoms (Figure 1). Petroleum gas is referred to as natural gas, which should not be confused with the abbreviated term used to describe the refined fuel "gasoline". Natural gas consists predominantly of simple hydrocarbons with only one to five carbon atoms (i.e., methane to pentane, respectively, Figure 1). Liquid petroleum is referred to as crude oil and consists of a wide range of more complex hydrocarbons and minor quantities of asphaltenes (Figure 1). Semisolid petroleum is tar, which is dominated by larger complex hydrocarbons and asphaltenes (Figure 1).
Figure 1. Some examples of organic compounds in petroleum, from the simplest (methane) to the most complex (asphaltene).
Petroleum formation takes place in sedimentary basins, which are areas where the Earth's crust subsides and sediments accumulate within the resulting depression. As the sedimentary basin continues to subside, sediment accumulations continue to fill the depression. This results in a thickening sequence of sediment layers in which the lower sediment layers eventually solidify into sedimentary rocks as they experience greater pressures and temperatures with burial depth. The sediment layers that accumulate vary in character because the sources and depositional settings of the sediments change through geologic time as the sedimentary basin subsides and fills. It is critical to petroleum formation that at some time during the accumulation of sediments at least one of the sediment layers contains the remains of deceased plants or microorganisms. Throughout geologic time, the world oceans have expanded and receded over the Earth’s land surfaces and contributed sediment layers to subsiding sedimentary basins. Development of stagnant water conditions in some of the expanded oceans caused the bottom waters to be depleted in oxygen (anoxic), which allowed portions of decaying plankton (e.g., algae, copepods, bacteria, and archaea) that originally lived in the upper oxygen-bearing (oxic) waters to be preserved as a sediment layer enriched in organic matter (Figure 2). Swamps and marshes may also develop marginal to oceans overlying subsiding basins. In these depositional settings, sediment layers enriched in decaying land plants (e.g., trees, shrubs, and grasses) may occur.
As these organic-rich sediment layers are buried by deposition of overlying sediments in the subsiding basin, the sediments are compressed and eventually lithified into rocks referred to as black shale, bituminous limestone, or coal. Methane producing microorganisms referred to as methanogens may thrive under certain favorable conditions within the organic-rich sediment layer during its early burial. These microorganisms consume portions of the organic matter as a food source and generate methane as a byproduct. This methane, which is typically the main hydrocarbon in natural gas, has a distinct neutron deficiency in its carbon nuclei (i.e., carbon isotopes), which allows microbial natural gas (a.k.a., biogenic gas) to be readily distinguished from methane generated by thermal processes (a.k.a., thermogenic gas) later in a basin's subsidence history. The microbial methane may remain in the organic-rich layer or it may bubble up into the overlying sediment layers and escape into the ocean waters or atmosphere. If impermeable sediment layers, called seals, hinder the upward migration of microbial gas, the gas may collect in underlying porous sediments, called reservoirs (Figure 3).
Figure 2: Formation of organic-rich sediment
Figure 3: Early burial of sediment layers in
Economically significant accumulations of microbial natural gas have been estimated to account for 20 percent of the world’s produced natural gas. Microbial methane may remain trapped in the organic-rich sediment layer through out its lithificaton and contribute to economic accumulations referred to as coal-bed methane and shale gas.
Burial of the organic-rich rock layer may continue in some subsiding basins to depths of 6,000 to 18,000 feet (1830 to 5490 m).At these depths, the organic-rich rock layer is exposed to temperatures of 150 to 350 ºF (66 to 177 ºC) for a few million to tens of millions of years. The organic matter within the organic-rich rock layer begins to cook during this period of heating and portions of it thermally decompose into crude oil and natural gas (i.e., thermogenic gas) (Figure 4).
This overall process of cooking petroleum out of an organic-rich rock layer involves the appropriate combination of temperature and time and is referred to as thermal maturation. If the original source of the organic matter is mostly higher plants (e.g., trees, shrubs, and grasses), natural gas will be the dominant petroleum generated with lesser amounts of crude oil generation. If the original source of the organic matter is plankton (e.g., algae, copepods, and bacteria), crude oil will be the dominant petroleum generated with lesser amounts of natural gas generation. Organic-rich rock layers that have undergone this process of petroleum generation are considered to be thermally mature and referred to as source rocks.
Organic-rich rocks that have not been thermally matured are referred to as being thermally immature. These immature organic-rich rocks may be referred to as oil shale if artificial heating at high temperatures (~1000ºF/~538ºC) in surface or near-surface reactors (a.k.a., retorts) yield economic quantities of oil. Oil shale retorting occurred in Scotland between 1860 and 1960 and is currently active in Estonia and Brazil.
Petroleum has a lower density than the water that occupies pores, voids, and cracks in the source rock and the overlying rock and sediment layers. This density difference forces the generated petroleum to migrate upwards by buoyancy until sealed reservoirs in the proper configurations serve as traps that concentrate and collect the petroleum. Some of the generated natural gas may not migrate out and away from its source rock, but instead remains within microscopic pores and dissolved in the organic matter of its source rock. This retained natural gas has proven to be an economically significant resource that is referred to as shale gas. The Barnett Shale in the Fort Worth basin of Texas is a good example of this type of accumulation.
In some basins, petroleum may not encounter a trap and continue migrating upward into the overlying water or atmosphere as petroleum seeps. Crude oil that migrates to or near the surface of a basin will lose a considerable amount of its hydrocarbons to evaporation, water washing, and microbial degradation leaving a residual tar enriched in large complex hydrocarbons and asphaltenes (Figure 5). Tar deposits range in size from small local seeps like the La Brea tar pits of California to regionally extensive occurrences as observed in the Athabasca tar sands of Alberta.
Figure 4: Continued burial of sediment and
rock layers in subsiding basin.
Figure 5: Deeper burial of rock layers in
Burial of the source rock may continue to depths greater than 20,000 ft. (6100 m) in some sedimentary basins. At these depths, temperatures in greater than 350ºF (177ºC) and pressures greater than 15,000 psi (103 MPa) transform the remaining organic matter into more natural gas and a residual carbon referred to as char. Oil trapped in reservoirs that are sometimes buried to these depths also decomposes to natural gas and char. The char, which is also called pyrobitumen, remains in the original reservoir while the generated natural gas may migrate upward to shallower traps within the overlying rock layers of the basin. The Gulf Coast basin that extends into the offshore of Louisiana and the Anadarko basin of the US mid-continent are good examples of these deep basins.
Further burial to temperatures and pressures in excess of 600ºF (316ºC) and 60,000 psi (414 MPa), respectively, represent metamorphic conditions in which the residual char converts to graphite with the emission of molecular hydrogen gas. The resulting metamorphic rocks are graphitic slate, schist or marble. Thermodynamic considerations indicate that water remaining in these rocks should react with the graphite to form either methane or carbon dioxide depending on the amount of molecular hydrogen present. Currently, the deepest wells in sedimentary basins do not exceed 32,000 ft (9760 m). Therefore, the significance of natural gas generation under these extreme conditions remains uncertain.
Sedimentary basins vary considerably in size, shape, and depth all over the Earth’s crust (Figure 6).
Figure 6: General outline of major sedimentary basins.
A large number of variables and different combinations of these variables determine whether a sedimentary basin contains microbial methane, natural gas, crude oil, tars, or no petroleum. Not all basins have organic-rich sediment layers deposited during their subsidence history. As a result, these basins will contain no appreciable quantities of petroleum regardless of how deep the basin subsides. Other basins that do have an organic-rich rock layer may not have been buried to sufficient depths to generate natural gas or crude oil through thermal maturation, but may contain microbial methane accumulations. An organic-rich rock layer in some basins may thermally mature to generate mostly natural gas because of the dominance of higher plant debris contributing to its organic matter. Conversely, an organic-rich rock layer in other basins may thermally mature to generate mostly crude oil because of the dominance of lower plant debris contributing to its organic matter. More than one organic-rich rock layer may be deposited in the burial history of some basins with all, one, or none subsiding deep enough to thermally mature to generate petroleum. In other basins that have an organic-rich rock layer and sufficient burial to generate petroleum, the lack or scarcity of seals and reservoirs to collect generated petroleum may result in natural gas losses to the atmosphere or large degraded oil and tar deposits at or near the basin surface.
Research on these variables is critical to understanding the occurrences of known petroleum accumulations from which predictions can be made as to where undiscovered petroleum still resides within the Earth's crust. Research depends heavily on data collected from rock outcrops around and subsurface drilling in sedimentary basins. This geological data is essential to understanding of the development of sediment and rock layers (i.e., stratigraphy) within a basin and the history of their subsidence and trap development (i.e., tectonics). However, the vastness of sedimentary basins, limited well data, and migration of petroleum away from its source also requires research to 1) establish fingerprinting methods to determine genetic correlations among different petroleum types and their source and 2) conduct laboratory experiments to simulate petroleum generation and alteration to predict types, amounts, and extent of petroleum generated under varying subsurface conditions. Collectively, this understanding of genetically related petroleum, source rock identification, levels of thermal maturation, migration distances, and degrees of near-surface degradation allows construction of computer models of petroleum generation, migration, and accumulation through time within an evolving sedimentary basin. The USGS Energy Resources Team addresses these research issues under the Petroleum Processes Research Project.
*Figures are modified after those in Public Issues in Earth Sciences, USGS Circular 1115 entitled “The Future of Energy Gases” By P.J. McCabe, D.L. Gautier, M.D. Lewan, and C. Turner (1993).
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