Oil and gas origins

Oil and gas are two of the most important fossil fuels. The formation of oil and gas is dependent on the preservation of organic matter and its subsequent chemical transformation into kerogen and other organic molecules deep within the earth at high temperatures over long periods of time. As oil and gas are generated from these organic materials, they migrate upward, where they may accumulate in hydrocarbon traps.

Preservation of Organic Matter

To become a fossil fuel, the organic matter in an organism must be preserved after the organism dies. The preservation of organic matter is a rare event because most of the carbon in organic matter is oxidized and recycled to the atmosphere through the action of aerobic bacteria. (Oxidation is a process through which organic matter combines with oxygen to produce carbon dioxide gas and water.) Less than 1 percent of the organic matter that is produced by photosynthesis escapes from this cycle and is preserved. To be preserved, the organic matter must be protected from oxidation, which can occur in one of two ways: the organic matter in the dead organism is rapidly buried by sediment, shielding it from oxygen in the environment, or the dead organism is transported into an aquatic environment in which there is no oxygen (that is, an anoxic or anaerobic environment). Most aquatic environments have oxygen in the water, because it diffuses into the water from the atmosphere and is produced by photosynthetic organisms, such as plants and algae. Oxygen is removed from the water by the respiration of aerobic organisms and through the oxidation of decaying organic matter. Oxygen consumption is so high in some aquatic environments that anoxic water is present below the near-surface oxygenated zone. Environments that lack oxygen include such places as deep, isolated bodies of stagnant water (such as the bottom waters of some lakes), some swamps, and the oxygen-minimum zone in the ocean (below the maximum depth to which light penetrates, where no photosynthesis can occur). Large quantities of organic matter may be preserved in these environments.

The major types of organic matter preserved in sediments include plant fragments, algae, and microbial tissue formed by bacteria. Animals (and single-celled, animal-like organisms) contribute relatively little organic matter to sediments. The amount of organic matter contained in a sediment or in sedimentary rock is referred to as its total organic content (TOC), and it is typically expressed as a percentage of the weight of the rock. To be able to produce oil, a sediment typically must have a TOC of at least 1 percent by weight. Sediments that are capable of producing oil and gas are referred to as “hydrocarbon source rocks.” In general, fine-grained rocks such as shales (which have clay-sized grains) tend to have higher TOC than do coarser-grained rocks.

Transformation of Organic Matter

The organic matter trapped in sediment must undergo a series of changes to form oil and gas. These changes take place as the sediment is buried to great depths as a result of the deposition of more and more sediment in the environment over long periods of time. Temperature and pressure increase as depth of burial increases, and the organic materials are altered by these high temperatures and pressures.

As sediment is gradually buried to depths reaching hundreds of meters, it undergoes a series of physical and chemical changes, called diagenesis. Diagenesis transforms sediment into sedimentary rock by compaction, cementation, and removal of water. Methane gas commonly forms during the early stages of diagenesis as a result of the activity of methanogenic bacteria. At depths of a few meters to tens of meters, such organic compounds as proteins and carbohydrates are partially or completely broken down, and the individual component parts are converted into carbon dioxide and water, or are used to construct geopolymers—that is, large, complex organic molecules of irregular structure (such as fulvic acid and humic acid, and larger geopolymers called humins). During diagenesis, the geopolymers become larger and more complex, and nitrogen and oxygen content decreases. With increasing depth of burial over long periods of time (burial to tens or hundreds of meters over a million or several million years), continued enlargement of the organic molecules alters the humin into kerogen, an insoluble form of organic matter that yields oil and gas when heated.

As sediment is buried to depths of several kilometers, it undergoes a process called catagenesis. At these depths, the temperature may range from 50 to 150 degrees Celsius, and the pressure may range from 300 to 1,500 bars. The organic matter in the sediment, while in a process called maturation, becomes stable under these conditions. During maturation, a number of small organic molecules are broken off the large kerogen molecules, a phenomenon known as thermal cracking. These small molecules are more mobile than are the kerogen molecules. Sometimes called bitumen, they are the direct precursors of oil and gas. As maturation proceeds, and oil and gas generation continues, the kerogen residue remaining in the source rock gradually becomes depleted in hydrogen and oxygen. In a later stage, wet gas (gas with a small amount of liquid) and condensate are formed. (“Condensate” is a term given to hydrocarbons that exist as gas under the high pressures existing deep beneath the surface of the earth but condense to liquid at the earth’s surface.) Oil is typically generated at temperatures between 60 and 120 degrees Celsius, and gas is generated at somewhat higher temperatures, between about 120 and 220 degrees Celsius. Large quantities of methane are formed during catagenesis and during the subsequent phase, which is called metagenesis.

When sediment is buried to depths of tens of kilometers, it undergoes the processes of metagenesis and metamorphism. Temperatures and pressures are extremely high. Under these conditions, all organic matter and oil are destroyed, being transformed into methane and a carbon residue, or graphite. Temperatures and pressures are so intense at these great depths that some of the minerals in the sedimentary rocks are altered and recrystallized, and metamorphic rocks are formed.

Migration and Trapping

Accumulations of oil and gas are typically found in relatively coarse-grained, porous, permeable rocks, such as sandstones and some carbonate rocks. These oil- and gas-bearing rocks are called reservoirs. Reservoir rocks, however, generally lack the kerogen from which the oil and gas are generated. Instead, kerogen is typically found in abundance only in fine-grained sedimentary rocks such as shales. From these observations, it can be concluded that the place where oil and gas originate is not usually the same as the place where oil and gas are found. Oil and gas migrate or move from the source rocks (their place of origin) into the reservoir rocks, where they accumulate.

Oil and gas that form in organic-rich rocks tend to migrate upward from their place of origin, toward the surface of the earth. This upward movement of oil occurs because pore spaces in the rocks are filled with water, and oil floats on water because of its lower density. Gas is even less dense than oil and also migrates upward through pore spaces in the rocks. The first phase of the migration process, called primary migration, involves expulsion of hydrocarbons from fine-grained source rocks into adjacent, more porous and permeable layers of sediment. Secondary migration is the movement of oil and gas within the more permeable rocks. Oil and gas may eventually reach the surface of the earth and be lost to the atmosphere through a seep. Under some circumstances, however, the rising oil and gas may become trapped in the subsurface by an impermeable barrier, called a caprock. These hydrocarbon traps are extremely important because they provide a place for subsurface concentration and accumulation of oil and gas, which can be tapped for energy sources.

There are a variety of settings in which oil and gas may become trapped in the subsurface. Generally, each of these traps involves an upward projection of porous, permeable reservoir rock in combination with an overlying impermeable caprock that encloses the reservoir to form a sort of inverted container. Examples of hydrocarbon traps include anticline traps, salt dome traps, fault traps, and stratigraphic traps. There are many types of stratigraphic traps, including porous reef rocks enclosed by dense limestones and shales, sandstone-filled channels, sand bars, or lenses surrounded by shale, or porous, permeable rocks beneath an unconformity. The goal of the exploration geologist is to locate these subsurface hydrocarbon traps. Enormous amounts of geologic information must be obtained, and often many wells must be drilled before accumulations of oil and gas can be located.

Physical and Chemical Analyses

The origin of oil and gas can be determined using physical and chemical analyses. Petroleum contains compounds that serve as biological markers to demonstrate the origin of petroleum from organic matter. Oil can be analyzed chemically to determine its composition, which can be compared to that of hydrocarbons extracted from source rocks in the lab. Generally, oil is associated with natural gas, most of which probably originated from the alteration of organic material during diagenesis, catagenesis, or metagenesis. In some cases, gas may be of abiogenic (nonorganic) origin. Samples of natural gas can be analyzed using gas chromatography or mass spectrometry and isotope measurements.

Commonly, rocks are analyzed to determine their potential for producing hydrocarbons. It is important to distinguish between various types of kerogen in the rocks because different types of organic matter have different potentials for producing hydrocarbons. In addition, it is important to determine the thermal maturity or evolutionary state of the kerogen to confirm whether the rock has the capacity to generate hydrocarbons, or whether hydrocarbons have already been generated.

The quantity of organic matter in a rock, referred to as its TOC, can be measured with a combustion apparatus, such as a Leco carbon analyzer. To analyze for TOC, a rock must be crushed and ground to a powder and its carbonate minerals removed by dissolution in acid. During combustion, the organic carbon is converted into carbon dioxide by heating it to high temperatures in the presence of oxygen. The amount of carbon dioxide produced is proportional to the TOC of the rock. The minimum amount of TOC that is adequate for hydrocarbon production is generally considered to be between 0.5 and 1 percent TOC by weight.

Indirect Methods of Analyses

The type of organic matter in a rock can be determined indirectly, through study of the physical and chemical characteristics of the kerogen, or directly, by using pyrolysis (heating) techniques. The indirect methods of analysis include examination of kerogen with a microscope and chemical analysis of kerogen. Microscopic examination can identify different types of kerogen, such as spores, pollen, leaf cuticles, resin globules, and single-celled algae. Kerogen that has been highly altered and is amorphous can be examined using fluorescence techniques to determine whether it is oil-prone (fluorescent) or inert, or gas-prone (nonfluorescent). Chemical analysis of kerogen provides data on the proportions of chemical elements, such as carbon, hydrogen, sulfur, oxygen, and nitrogen. A graph of the ratios of hydrogen/carbon (H/C) versus oxygen/carbon (O/C) is used to classify kerogen by origin and is called a van Krevelen diagram. There are three curves on a van Krevelen diagram, labeled I, II, and III, corresponding to three basic types of kerogen. Type I is rich in hydrogen, with high H/C and low O/C ratios, as in some algal deposits; this type of kerogen generally yields the most oil. Type II has relatively high H/C and low O/C ratios and is usually related to marine sediments containing a mixture of phytoplankton, zooplankton, and bacteria; this type of kerogen yields less oil than Type I, but it is the source material for a great number of commercial oil and gas fields. Type III is rich in oxygen, with low H/C and high O/C ratios (aromatic hydrocarbons), as in terrestrial or land plants; this type of kerogen is comparatively less favorable for oil generation but tends to generate large amounts of gas when buried to great depths. As burial depth and temperature increase, the amount of oxygen and hydrogen in the kerogen decreases, and the kerogen approaches 100 percent carbon. Hence, a van Krevelen diagram can be used to determine both the origin of the organic matter and its relative thermal maturity.

The potential that a rock has for producing hydrocarbons can be evaluated through a pyrolysis, or heating, technique, commonly called Rock-Eval. Rock-Eval yields information on the quantity, type, and thermal maturity of organic matter in the rock. The procedure involves the gradual heating (to about 550 degrees Celsius) of a crushed rock sample in an inert atmosphere (nitrogen, helium) in the absence of oxygen. At temperatures approaching 300 degrees Celsius, heating releases free hydrocarbons already present in the rock; the quantity of free hydrocarbons is referred to as S1. At higher temperatures (300 to 550 degrees Celsius), additional hydrocarbons and related compounds are generated from thermal cracking of kerogen in the rock; the quantity of these hydrocarbons is referred to as S2. The temperature at which the maximum amount of S2 hydrocarbons is generated is called Tmax and can be used to evaluate the thermal maturity of the organic matter in the rock. In addition, carbon dioxide is generated as the kerogen in the rock is heated; the quantity of CO2 generated as the rock is heated to 390 degrees Celsius is referred to as S3. (The temperature is limited to 390 degrees Celsius because at higher temperatures, CO2 is also formed from the breakdown of inorganic materials, such as carbonate minerals.) These data can be used to determine the hydrocarbon-generating potential of the rock, the quantity and type of organic matter, and the thermal maturity. For example, S1 + S2, called the genetic potential, is a measure of the total amount of hydrocarbons that can be generated from the rock, expressed in kilograms per ton. If S1 + S2 is less than 2 kilograms per ton, the rock has little or no potential for oil production, although it has some potential for gas production. If S1 + S2 is between 2 and 6 kilograms per ton, the rock has moderate potential for oil production. If S1 + S2 is greater than 6 kilograms per ton, the rock has good potential for oil production. The ratio S1/(S1 + S2), called the production index, indicates the maturation of the organic matter. Pyrolysis data can also be used to determine the type of organic matter present. The oxygen index is S3/TOC, and the hydrogen index is S2/TOC. These two indices can be plotted against each other on a graph, comparable to a van Krevelen diagram.

Finite Fossil Fuels

Oil and gas are derived from the alteration of kerogen, an insoluble organic material, under conditions of high temperatures (50 to 150 degrees Celsius) and pressures (300 to 1,500 bars). After oil and gas are generated, they migrate upward out of organic-rich source rocks and come to be trapped and accumulate in specific types of geologic settings. The search for oil and gas deposits trapped in the subsurface can be expensive and time-consuming, requiring trained exploration geologists. Once a promising geologic setting has been located, the only way to determine whether oil and gas deposits are actually present in the subsurface is to drill a well.

Oil and gas are two of the earth’s most important fossil fuels. It is important to understand that a finite amount of these hydrocarbons is present within the earth. They cannot be manufactured when known reserves are depleted.

Principal Terms

fossil fuel: a general term used to refer to petroleum, natural gas, and coal

geopolymer: a large molecule created by linking together many smaller molecules by geologic processes

hydrocarbons: solid, liquid, or gaseous chemical compounds containing only carbon and hydrogen; oil and natural gas are complex mixtures of hydrocarbons

kerogen: fossilized organic material in sedimentary rocks that is insoluble and generates oil and gas when heated; as a form of organic carbon, it is one thousand times more abundant than coal and petroleum in reservoirs, combined

methane: a colorless, odorless gaseous hydrocarbon with the formula CH4; also called marsh gas

natural gas: a mixture of several gases used for fuel purposes and consisting primarily of methane, with additional light hydrocarbon gases such as butane, propane, and ethane, with associated carbon dioxide, hydrogen sulfide, and nitrogen

petroleum: crude oil; a naturally occurring complex liquid hydrocarbon, which after distillation yields a range of combustible fuels, petrochemicals, and lubricants

reservoir: a porous and permeable unit of rock below the surface of the earth that contains oil and gas; common reservoir rocks are sandstones and some carbonate rocks

Bibliography

Durand, Bernard, ed. Kerogen: Insoluble Organic Matter from Sedimentary Rocks. Paris: Éditions Technip, 1980. Discusses various aspects of kerogen, ranging from its origin and appearance under the microscope to its chemical composition and structure. Written by specialists, mostly in English, but with a few articles in French. Contains a number of beautiful color plates illustrating the appearance of kerogen-rich rocks and organic microfossils (pollen, spores, acritarchs, dinoflagellates) as seen through the microscope.

Gluyas, Jon, and Richard Swarbrick. Petroleum Geoscience. Malden, Mass.: Blackwell Science, 2004. Describes the tools and methods used in petroleum exploration. Includes limited images, drawings, and tables. Provides extensive references, further reading lists, and an index. Appropriate for graduate students, academics, and professionals.

Hunt, John Meacham. Petroleum Geochemistry and Geology. 2d ed. New York: W. H. Freeman, 1996. Covers petroleum composition, origin, migration, accumulation, and analysis, and the application of petroleum geochemistry in petroleum exploration, seep and subsurface prospects, crude oil correlation, and prospect evaluation. Requires a background in chemistry and algebra.

Hyne, Norman J. Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production. 2d ed. Tulsa, Okla.: PennWell, Corporation, 2001. Provides a well-rounded overview of the processes and principles of gas and oil drilling. Covers foundational material. Appropriate for professionals in the oil-drilling field, geologists, and students of similar fields.

North, F. K. Petroleum Geology. Boston: Allen & Unwin, 1985. Covers a wide variety of topics related to petroleum geology, including the nature and origin of petroleum; where and how oil and gas accumulate; exploration, exploitation, and forecasting; and the distribution of oil and gas. Designed to introduce students to many topics in exploration, drilling, and the basics of the origin of oil and gas with practical application. Well illustrated with maps and geologic cross-sections representing oil-producing areas around the world. Suitable for geologists and college students.

Peters, K. E. “Guidelines for Evaluating Petroleum Source Rock Using Programmed Pyrolysis.” The American Association of Petroleum Geologists Bulletin 70 (March, 1986): 318-329. Provides information on Rock-Eval pyrolysis, one of the major analytical techniques for analyzing rocks to determine their hydrocarbon potential. Provides a brief summary of the technique and goes into detail using numerous examples, discussing some of the problems encountered in interpreting samples. Suitable for geologists and advanced college students.

Raymond, Martin S., and William L Leffler. Oil and Gas Production in Nontechnical Language. Tulsa, Okla.: PennWell Corporation, 2005. Provides a good overview of the industry. Includes pictures, charts, graphs, and drawings. Describes oil exploration, and how oil and gas are found and extracted from the earth. Appropriate for general readers.

Selley, Richard C. Elements of Petroleum Geology. 2d ed. San Diego: Academic Press, 1998. Covers the specifics of oil and gas and their relationship to geology. Designed for students near the end of their coursework in geology or for geologists beginning careers in the petroleum industry. Requires basic understanding of geological concepts. Contains subject and proper name indexes, useful illustrations, and appendices that include a well classification table, a glossary of oil terms and abbreviations, and a table of conversion factors.

Tissot, Bernard P., Bernard Durand, J. Espitalié, and A. Combaz. “Influence of Nature and Diagenesis of Organic Matter in Formation of Petroleum.” American Association of Petroleum Geologists Bulletin 58 (March, 1974): 499-506. Discusses the generation of hydrocarbons and changes in kerogen that occur during burial. Provides a concise summary of the types of kerogen and depths at which oil and gas are generated. Well illustrated with graphs.

Tissot, Bernard P., and D. H. Welte. Petroleum Formation and Occurrence. 2d ed. Berlin: Springer-Verlag, 1984. One of the most comprehensive guides to the origin of petroleum and natural gas. Organized according to the production and accumulation of geologic matter (a geological perspective); the fate of organic matter in sedimentary basins (generation of oil and gas); the migration and accumulation of oil and gas; the composition and classification of crude oils and the influence of geological factors; and oil and gas exploration (application of the principles of petroleum generation and migration). Well illustrated with line drawings and graphs. Appropriate for college-level students and indispensable for geologists.

Waples, Douglas W. Geochemistry in Petroleum Exploration. Boston: International Human Resources Development Corporation, 1985. A leading reference in the field. Provides an overview of the origin of oil and gas. Concise and well illustrated with line drawings and graphs. Appropriate for college-level students.

Welte, Dietrich H., Brian Horsfield, and Donald R. Baker, eds. Petroleum and Basin Evolution: Insights from Petroleum Geochemistry, Geology, and Basin Modeling. New York: Springer, 1997. Explores the origins of oil and gas from the perspective of mathematical modeling of sedimentary basins. Somewhat technical, but illustrations and maps help clarify many of the difficult concepts. Bibliography.