Petroleum geochemistry

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Petroleum geochemistry is a branch of geochemistry (the application of chemical concepts to understand geological systems) which deals specifically with petroleum and its origin, generation, and accumulation, as well as its extraction, refinement, and use. [1] [2] Petroleum, also known as crude oil, is a solid, liquid, and/or gaesous mix of hydrocarbons. [3] These hydrocarbons are from the burial and metamorphosis of organic matter from millions of years ago; [4] the organic matter is from marine animals, plants, and algae. [5] Petroleum is extracted from the Earth (above or below its surface, depending on the geology of the formation), refined, and used as an energy source. [3]

Contents

Crude oil is most commonly organised into four types - light, heavy, sweet, and sour. [6] Petroleum is a non-renewable energy source (also known as a "fossil fuel"), so the efficacy of extraction and refining is important for its continued use; multiple techniques are used to detect and to extract crude oil, based on the source rock it is found in and the type of oil itself. [1]

Types of Petroleum

Petroleum is differentiated into types based on its American Petroleum Institute (API) gravity and by how much sulphur it contains. [7]

API Gravity

The API gravity of a crude oil is a measurement of purity - i.e., amount of impurities, such as sulphur, nitrogen, or oxygen. [8] Impurities increase the density of the crude. [9] [6]

Light Crude Oil

Light crude oils have higher API gravity figures, due to having fewer impurities. [9] It is more commonly used to produce diesel and gasoline than heavier oils are. [6] Due to its lower viscosity, it is easier to extract and to transport. [9]

Heavy Crude Oil

Heavy crude oils have lower API gravity figures, and a larger percentage of impurities. [9] It is used in the making of heavier outputs - e.g., asphalt [6] - and has a higher viscosity, making it more difficult to transport and extract. [9]

Sulphur, a prevalent constituent of crude oil, and quantity found is used in considering crude oil 'sweet' or 'sour'. Sulfur - El Desierto mine, San Pablo de Napa, Daniel Campos Province, Potosi, Bolivia.jpg
Sulphur, a prevalent constituent of crude oil, and quantity found is used in considering crude oil 'sweet' or 'sour'.

Sulphur Content

How 'sweet' or 'sour' a crude oil is is based on the amount of sulphur it contains. [6]

Sweet Crude Oil

'Sweet' crude oil has lower sulphur content [7] - lower than 0.5%. [6] It can be refined into kerosene, high-quality diesel, and gasoline. [6]

Sour Crude Oil

'Sour' crude oil has high natural sulphur content (at least 0.5%). [7] Extra treatment is required in the refining process [6] ; impurities are removed to refine the crude into gasoline. [9] Due to the greater cost associated, it is more commonly refined into fuel oil and diesel - less valuable outputs than products of sweet crude oil. [9]

Hydrocarbon Compounds

The three main hydrocarbon compounds in petroleum are paraffins, naphthenes, and aromatics.

Paraffins

Paraffinic hydrocarbons are part of the alkane series [10] , and are the most common hydrocarbon found in crude oil. [11] Paraffins are often a part of gasoline, making them comparatively more valuable. [11]

Paraffinic hydrocarbons are also known as alkanes, and are represented by the formula CnH2n+2, where n is a positive integer. [12]

Naphthenes

Naphthenic hydrocarbons are saturated cyclic hydrocarbons [10] , and are very important in the refining of liquid crude oil. [11]

Also known as cyclic alkanes, they are represented by the formula CnH2n, where n is a positive integer. [13]

Aromatics

Aromatic hydrocarbons are cyclic [10] , and are much less abundant than the other two main hydrocarbon compounds. [11] They are represented by the formula CnHn, where n is a positive integer. [14]

Petroleum Geochemical Techniques

Techniques are used for finding the source rock (the solid material in which the petroleum is found), as well as the type and amount of the petroleum within. [1] They are also used to note migration timing and pathways, which are then used to predict when and where petroleum can be found; [1] petroleum sources can be predicted if material associated with source rock is found. [1]

Surface Prospecting

Petroleum, or evidence of its immediate occurrence, can be found on the surface of the Earth. Oil seeps can be found near a fault zone, where the movement of Earth's crust can expose petroleum source rock, and thus the crude oil itself. [15] They can also be found on the ocean floor, and can be found using satellite imaging. [16]

Distillation

While not used as commonly as other techniques today, distillation is used in the process of refining petroleum. It involves the dividation of the crude oil into hydrocarbon categories, and products are recovered from the heated material. [17] A distillation tower is used in separation of the oil, with anywhere between 2 and 300 theoretical plates. [16]

Gas Chromatography

Similar to the process of distillation, gas-liquid chromatography (typically referred to as gas chromatography, or, more simply, GC) utilises a distillation tower to separate the petroleum. However, compared to distillation's 2 to 300 theoretical plates, gas chromatography includes more than 25,000. This provides a greater degree of separation. [16]

In order to achieve more complete analyses, gas chromatography is used along with mass spectrometry (to make gas chromatography/mass spectrometry, or GCMS), with infrared spectrometry (to make gas chromatography/infrared spectrometry, or GCIR), and with isotope ratio mass spectrometry (to make gas chromatography/isotope ratio mass spectrometry, or GSIRMS). [16]

Pyrolysis

While the crude oil from a petroleum source rock is easily separated using gas chromatography and gas chromatography/mass spectrometry, the organic matter found is not soluble in the solvents used in these techniques, and thus cannot be properly analysed. Pyrolysis is used to characterise kerogens (insoluble hydrocarbons) [18] and asphaltenes (limited solubility in common solvents). [19] There are multiple methods of pyrolysis; fingerprinting methods - which use flash pyrolysis or rapid temperature-programmed pyrolysis - involve rapid transfer of the product to the gas chromatography tower. [16] Rock-Eval is a commonly used process to determine the content of the source rock. [20] Hydrous pyrolysis is performed within water and in high pressures; this method can simulate different depths of burial, demonstrating the possibilities of the fate of the source rock and the associated patroleum. [16]

Measurement of Stable Isotopes

The bulk isotope ratio value of stable isotopes for petroleum depict the average isotopic compositions of the oil's components. Carbon stable isotopes are often used in this method. Whether a sample of petroleum originated in a marine environment or a non-marine environment can be seen using this ratio value, as can method distance and age of the oil. [16] [21]

Biological Markers ("Biomarkers")

With credit to the previously listed techniques, biomarkers were found in petroleum and source rock extract. These are fossils from organisms, but are closer in size to molecules than to visible hand samples. They display the same structure as their parent biomolecules and are used in the identification of the organic matter from which the petroleum is derived. Biomarkers are also used in correlating oils and source rocks, finding the oil's maturity, regional differences found between multiple samples, and the history of the basin in which the source rock was located. [16]

Correlation

Before the use of gas chromatography-mass spectrometry and biomarkers, correlation of locations' geology was used to find how different formations relate to each other and to their environment. Oil-oil correlations (comparing petroleum to other oil found locally or in other areas) and oil-source correlations (comparing petroleum and its source) were performed; infrared spectrometry, refractive indices, solvent extractable organic matter, compound class distribution, and elemental analysis are all methods of doing oil-source correlations.

Related Research Articles

<span class="mw-page-title-main">Hydrocarbon</span> Organic compound consisting entirely of hydrogen and carbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may be similar to that of gasoline or lighter fluid. They occur in a diverse range of molecular structures and phases: they can be gases, liquids, low melting solids or polymers.

<span class="mw-page-title-main">Petroleum</span> Naturally occurring flammable liquid

Petroleum, also known as crude oil or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name petroleum covers both naturally occurring unprocessed crude oil and petroleum products that consist of refined crude oil.

<span class="mw-page-title-main">Kerogen</span> Solid organic matter in sedimentary rocks

Kerogen is solid, insoluble organic matter in sedimentary rocks. It consists of a variety of organic materials, including dead plants, algae, and other microorganisms, that have been compressed and heated by geological processes. All the kerogen on earth is estimated to contain 1016 tons of carbon. This makes it the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold.

<span class="mw-page-title-main">Oil refinery</span> Facility that processes crude oil

An oil refinery or petroleum refinery is an industrial process plant where petroleum is transformed and refined into products such as gasoline (petrol), diesel fuel, asphalt base, fuel oils, heating oil, kerosene, liquefied petroleum gas and petroleum naphtha. Petrochemical feedstock like ethylene and propylene can also be produced directly by cracking crude oil without the need of using refined products of crude oil such as naphtha. The crude oil feedstock has typically been processed by an oil production plant. There is usually an oil depot at or near an oil refinery for the storage of incoming crude oil feedstock as well as bulk liquid products. In 2020, the total capacity of global refineries for crude oil was about 101.2 million barrels per day.

Organic geochemistry is the study of the impacts and processes that organisms have had on the Earth. It is mainly concerned with the composition and mode of origin of organic matter in rocks and in bodies of water. The study of organic geochemistry is traced to the work of Alfred E. Treibs, "the father of organic geochemistry." Treibs first isolated metalloporphyrins from petroleum. This discovery established the biological origin of petroleum, which was previously poorly understood. Metalloporphyrins in general are highly stable organic compounds, and the detailed structures of the extracted derivatives made clear that they originated from chlorophyll.

<span class="mw-page-title-main">Vacuum distillation</span> Low-pressure and low-temperature distillation method

Vacuum distillation or Distillation under reduced pressure is a type of distillation performed under reduced pressure, which allows the purification of compounds not readily distilled at ambient pressures or simply to save time or energy. This technique separates compounds based on differences in their boiling points. This technique is used when the boiling point of the desired compound is difficult to achieve or will cause the compound to decompose. Reduced pressures decrease the boiling point of compounds. The reduction in boiling point can be calculated using a temperature-pressure nomograph using the Clausius–Clapeyron relation.

Shale oil is an unconventional oil produced from oil shale rock fragments by pyrolysis, hydrogenation, or thermal dissolution. These processes convert the organic matter within the rock (kerogen) into synthetic oil and gas. The resulting oil can be used immediately as a fuel or upgraded to meet refinery feedstock specifications by adding hydrogen and removing impurities such as sulfur and nitrogen. The refined products can be used for the same purposes as those derived from crude oil.

<span class="mw-page-title-main">Catalytic reforming</span> Chemical process used in oil refining

Catalytic reforming is a chemical process used to convert petroleum refinery naphthas distilled from crude oil into high-octane liquid products called reformates, which are premium blending stocks for high-octane gasoline. The process converts low-octane linear hydrocarbons (paraffins) into branched alkanes (isoparaffins) and cyclic naphthenes, which are then partially dehydrogenated to produce high-octane aromatic hydrocarbons. The dehydrogenation also produces significant amounts of byproduct hydrogen gas, which is fed into other refinery processes such as hydrocracking. A side reaction is hydrogenolysis, which produces light hydrocarbons of lower value, such as methane, ethane, propane and butanes.

Heavy crude oil is highly viscous oil that cannot easily flow from production wells under normal reservoir conditions.

<span class="mw-page-title-main">Asphaltene</span> Heavy organic molecular substances that are found in crude oil

Asphaltenes are molecular substances that are found in crude oil, along with resins, aromatic hydrocarbons, and saturates. The word "asphaltene" was coined by Boussingault in 1837 when he noticed that the distillation residue of some bitumens had asphalt-like properties. Asphaltenes in the form of asphalt or bitumen products from oil refineries are used as paving materials on roads, shingles for roofs, and waterproof coatings on building foundations.

Pyrolysis oil, sometimes also known as bio-crude or bio-oil, is a synthetic fuel with limited industrial application and under investigation as substitute for petroleum. It is obtained by heating dried biomass without oxygen in a reactor at a temperature of about 500 °C (900 °F) with subsequent cooling, separation from the aqueous phase and other processes. Pyrolysis oil is a kind of tar and normally contains levels of oxygen too high to be considered a pure hydrocarbon. This high oxygen content results in non-volatility, corrosiveness, partial miscibility with fossil fuels, thermal instability, and a tendency to polymerize when exposed to air. As such, it is distinctly different from petroleum products. Removing oxygen from bio-oil or nitrogen from algal bio-oil is known as upgrading.

Phytane is the isoprenoid alkane formed when phytol, a chemical substituent of chlorophyll, loses its hydroxyl group. When phytol loses one carbon atom, it yields pristane. Other sources of phytane and pristane have also been proposed than phytol.

<span class="mw-page-title-main">History of the petroleum industry</span>

While the local use of oil goes back many centuries, the modern petroleum industry along with its outputs and modern applications are of a recent origin. Petroleum's status as a key component of politics, society, and technology has its roots in the coal and kerosene industry of the late 19th century. One of the earliest instances of this is the refining of paraffin from crude oil. Abraham Gesner developed a process to refine a liquid fuel from coal, bitumen and oil shale; it burned more cleanly and was cheaper than whale oil. James Young in 1847 noticed a natural petroleum seepage when he distilled a light thin oil suitable for use as lamp oil, at the same time obtaining a thicker oil suitable for lubricating machinery. The world's first refineries and modern oil wells were established in the mid-19th century. While petroleum industries developed in several countries during the nineteenth century, the two giants were the United States and the Russian Empire, specifically that part of it that today forms the territory of independent Azerbaijan. Together, these two countries produced 97% of the world's oil over the course of the nineteenth century.

<span class="mw-page-title-main">Unresolved complex mixture</span>

Unresolved complex mixture (UCM), or hump, is a feature frequently observed in gas chromatographic (GC) data of crude oils and extracts from organisms exposed to oil.

<span class="mw-page-title-main">Kent Refinery</span>

The BPRefinery (Kent) was an oil refinery on the Isle of Grain in Kent. It was commissioned in 1953 and had a maximum processing capacity of 11 million tonnes of crude oil per year. It was decommissioned in August 1982.

<span class="mw-page-title-main">Petroleum refining processes</span> Methods of transforming crude oil

Petroleum refining processes are the chemical engineering processes and other facilities used in petroleum refineries to transform crude oil into useful products such as liquefied petroleum gas (LPG), gasoline or petrol, kerosene, jet fuel, diesel oil and fuel oils.

Petroleum naphtha is an intermediate hydrocarbon liquid stream derived from the refining of crude oil with CAS-no 64742-48-9. It is most usually desulfurized and then catalytically reformed, which rearranges or restructures the hydrocarbon molecules in the naphtha as well as breaking some of the molecules into smaller molecules to produce a high-octane component of gasoline.

A separation process is a method that converts a mixture or a solution of chemical substances into two or more distinct product mixtures, a scientific process of separating two or more substances in order to obtain purity. At least one product mixture from the separation is enriched in one or more of the source mixture's constituents. In some cases, a separation may fully divide the mixture into pure constituents. Separations exploit differences in chemical properties or physical properties between the constituents of a mixture.

<span class="mw-page-title-main">BTX (chemistry)</span> Mixtures of benzene, toluene, and the three xylene isomers

In the petroleum refining and petrochemical industries, the initialism BTX refers to mixtures of benzene, toluene, and the three xylene isomers, all of which are aromatic hydrocarbons. The xylene isomers are distinguished by the designations ortho –, meta –, and para – as indicated in the adjacent diagram. If ethylbenzene is included, the mixture is sometimes referred to as BTEX.

<span class="mw-page-title-main">Pyrobitumen</span> Type of solid, amorphous organic matter

Pyrobitumen is a type of solid, amorphous organic matter. Pyrobitumen is mostly insoluble in carbon disulfide and other organic solvents as a result of molecular cross-linking, which renders previously soluble organic matter insoluble. Not all solid bitumens are pyrobitumens, in that some solid bitumens are soluble in common organic solvents, including CS
2
, dichloromethane, and benzene-methanol mixtures.

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