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Oil in the Sea IV: Inputs, Fates, and Effects (2022)

Chapter: 2 Petroleum as a Complex Chemical Mixture

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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
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2

Petroleum as a Complex Chemical Mixture

2.1 COMPOSITION OF PETROLEUM AS A COMPLEX MIXTURE OF CHEMICALS AND PROGRESS IN ANALYSES OF THIS COMPLEX MIXTURE

2.1.1 Introduction

This chapter provides a brief overview of the classification of petroleum oils used by industry as it produces, transports, and refines them by means of distillation and petroleum cracking; and the classifications of petroleum oils relevant to oil spill response. Petroleum and many of its distillation products (e.g., fuel oils) and residues such as asphalt are complex mixtures of individual chemical compounds. A few molecular structures of the thousands of chemicals involved are provided as illustrations of this complexity. Analytical chemical methods used in the forensics of connecting environmental occurrence to sources of inputs, and for studies of the fates and effects of inputs are described. Examples of significant advances in analytical chemical methods used in petroleum pollution research are presented and discussed. This informs recommendations for adoption for applications in oil spill response, for assessment of fates and effects, and for further developments of analytical methodologies.

The basic physical chemical properties of gas, oil, and gas and oil mixtures are reviewed to set the scene for Chapter 5, Fates, and Chapter 6, Effects.

“Oil” is a three-letter word used in everyday discussions. This simple term unfortunately masks, for many people, that the chemical composition of petroleum is a complex mixture of thousands of chemicals. Furthermore, each oil has a unique combination and proportion of these chemicals. These chemical composition differences translate to both profound and subtle, but important, differences in each chemical mixture’s (each oil’s) fate and effects in the environment. The chemical composition differences underpin distinguishing one oil from another, that is, the forensics of sources of inputs or spills.

Chemical structures impart properties such as volatility, solubility in water, ease or difficulty of dispersion, environmental persistence, and the ability to react in various ways. These are important to understanding the fates and effects of oil in the environment and in formulating detection, response, mitigation, and cleanup plans.

2.1.2 Definitions and Classifications of Petroleum

The fact that petroleum is a complex mixture of thousands of chemicals has been known for many decades. Many of its components are hydrocarbons that are composed only of hydrogen and carbon, whereas other petroleum chemicals include additional elements such as nitrogen, sulfur, and oxygen (N, S, and O) and thus are not strictly hydrocarbons, even though common use may lump all of these components together as hydrocarbons. A few of the chemicals also contain metals such as nickel and vanadium. This complex chemical composition varies within and between oil reservoirs. This is the result of (1) the sources of the organic matter from which geological processes (designated generally as maturation) generate petroleum in source rock sedimentary formations, (2) migration and accumulation processes in reservoir rock sedimentary formations, (3) in situ biodegradation, and (4) the varied influence of these processes in different geological formations. The details of gas and oil generation and accumulation in reservoirs, as they influence petroleum’s complex composition, are beyond the scope of this report and have been described in several publications including Tissot and Welte (1984), Hunt (1996), Peters and Fowler (2002), Peters et al. (2005), and Philp (2018).

The complexity of petroleum’s chemical composition and the variations between reservoirs necessitated, especially in the early years of the production and use of petroleum, operational definitions for material in reservoirs or obtained from reservoirs. Hunt (1996) has a useful glossary that includes these operational terms and reference to a 137-page detailed illustrated glossary by Miles (1989).

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
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An example is the definition of petroleum: “Petroleum is a form of bitumen composed principally of hydrocarbons and existing in the gaseous or liquid state in its natural reservoir” (Hunt, 1996). In the strictest definition, the term “bitumen” refers to any organic material extracted from ancient sediments by organic solvents, where the exact organic solvent or solvent mixture and conditions of extraction are not specified. “Bitumen petroleum” is a term describing a collective of natural gas, crude oil, and asphalt components at the Earth’s surface temperature and pressure (e.g., Hunt, 1996). However, the term “bitumen” is also used in various ways to describe the heaviest crude oils, some of which are solid or semi-solid rather than liquid in their natural reservoirs. The glossary of a report on spills of diluted bitumen (NASEM, 2016) states “Bitumen—A mixture of hydrocarbons that is too viscous to flow under ambient conditions. Commercial quantities are recovered by thermal processes.” Radović et al. (2018) provide a useful summary explanation: “Technically, under reservoir conditions, we can differentiate ‘heavy’ oils (gravity 100–200 American Petroleum Institute [API], viscosity >100 centipoises, cp) which can flow; and non-flowable (sic) “bitumen” (gravity 100 API; viscosity >10,000 cp). However, since they are part of the same physicochemical continuum, the two terms will be used interchangeably in this chapter.” We use the approach by Radović et al. (2018) in this report with respect to the term “bitumen” rather than that of Hunt (1996).

Crude oil is defined as “petroleum that is removed from the earth in a liquid state or is capable of being so removed” (Hunt, 1996). There are various classifications of crude oils, ranging from those used in the economic arena to gauge prices and investments in stock to those used in the production and processing or refining of crude oils (Hunt, 1996; Wnek et al., 2018; API, 2022, among others). In the latter situation, the crude oils are often classified by their density and chemical composition (see discussion of chemical composition in a following section) as light, medium, heavy crude oils, paraffinic or naphthenic crude oils, and sour or sweet crude oils (Hunt, 1996; Wnek et al., 2018). A sour crude oil or fuel oil is an oil containing quantities of sulfur compounds such as hydrogen sulfide or sulfur-containing compounds such as mercaptans that are noxious to smell. Sweet oils do not contain noxious quantities of those compounds (Hunt, 1996).

There are also designations of live oil and dead oil. Live oil still contains gases, mainly methane, but also ethane, propane, and butane (see Section 5.2.1); dead oil does not contain these gases. Generally, oil well blowout accidents such as the Deepwater Horizon (DWH) involve live oil, and this is important because of the influence of gas bubbles on the initial fate of the discharged petroleum. In comparison, oil spills from oil tankers do not contain methane, ethane, propane, or butane, unless the spill or release is from a liquefied natural gas (LNG) tanker or coastal facility. LNG is mainly methane (85–95%) with 5–15% ethane, propane, butane, and nitrogen (DOE, 2005).

An amalgamation of data from many dead crude oils, showing composition related to percentage of molecular types, general classification of fractions of a crude oil, the molecular type of the groups of chemicals using names common in the oil and gas industry, and boiling point in °C, is presented in Figure 2.1.

Oil classification schemes most relevant to our report are classifications by oil groups (see Box 2.1) and by crude oil density (see Table 2.1). Oil groups are classifications of refined products, which are distinguished by their composition and viscosity. Crude oils are often described by their density through a specialized form of the specific gravity.

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FIGURE 2.1 Components of typical crude oil types.
NOTES: Names of the crude oil types on the x-axis and percentages on the y-axis of the four major commonly used categories of molecules in crude oils: saturates, aromatics, resins, and asphaltenes (identified as SARA within the petroleum industry and the scientific communities involved in responses to oil spills and for oil inputs, fates and effects). See text of this chapter for further explanation.
SOURCE: NASEM, 2016.
Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
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Specific gravity of oils is often reported in units of API gravity, a parameter that evolved and has been standardized over the decades of the petroleum era. API gravity is related to the specific gravity of an oil, by the formula API gravity = (141.5/specific gravity at 15.6°C) – 131.5. An API gravity of 10 is neutrally buoyant in freshwater; lower values would sink, and higher values would float. Table 2.1 provides a few examples of API gravity for some oil types and with some examples connected to geographic locations.

2.1.3 Oils or Fuels of Emerging Importance

There are some categories of petroleum and/or oils that have emerged recently or are of emerging importance for a variety of factors including ease of fulfilling demand for petroleum (diluted bitumen), meeting new environmental regulatory standards (low sulfur fuel oils), improving performance (newer lubricating oils), and reducing net carbon inputs to the environment (biofuels). These are described briefly in the following sections.

TABLE 2.1 Crude Oil Classifications by API Gravity (dead oil composition)

Oil Type API Gravity* Examples (API) References
Condensate >45° Agbami, Nigeria (48°) Speight (2014)
West Texas Intermediate (40°) Speight (2014)
Light oil 35°–45° Macondo (40°) Reddy et al. (2012)
Medium oil 25°–35° Alaska North Slope (32°) Speight (2014)
Heavy oil 15°–25° Venezuela Heavy (17°) Speight (2014)
Tar sands: Orinoco, Venezuela (8°–12°);
Extra heavy oil <15° Oil sands: Athabasca, Canada (6°–10°) Tissot and Welte (1984)

NOTES: For comparison, freshwater has an API gravity of 10°. * API gravity = (141.5/specific gravity at 15.6°C) – 131.5.

SOURCE: Adapted and modified with permission from Rullkötter and Farrington, 2021.

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×

2.1.3.1 Diluted Bitumen

Diluted bitumen, a petroleum product derived from oil sands, is increasing in production. It is transported primarily inland in North America by pipelines and rail tank cars, although increased shipping by tanker to Asia is forecast. Semi-solid bitumen extracted from oil sands is diluted with lighter molecular weight petroleum distillates such as condensates, naphtha, diesel, or synthetic crude oil, creating blends such as “dilbit” and “synbit,” respectively, that enable the blend to be transported as a liquid. Diluted bitumen, being a two-component blend, has bi-phasic characteristics if spilled: the very light diluent volatilizes rapidly (although not completely) leaving a viscous residue.1 A report by the National Academies, Spills of Diluted Bitumen from Pipeline: A Comparative Study of Environmental Fate, Effects, and Response, contains pertinent, important information (NASEM, 2016). See also Chapter 5, Fates, and Section 5.3.2.4.

2.1.3.2 Heavy Fuel Oils and Low Sulfur Fuel Oils

Heavy fuel oil (HFO) is a generic term to describe fuels primarily used to power marine vessels. It is a residual fuel from the distillation of crude oils that is blended with other lighter molecular weight fuels oils to have contents of sulfur in the 3.5% range. They contribute to air pollution in general and especially in port areas. Intermediate fuel oils (IFOs), designated as marine diesel oils by some organizations, are distilled fuels blended with higher proportions of heavy fuel oils. According to a narrower definition, marine diesel oil is a blend of distillate fuels and a very low proportion of heavy fuel oil2.

Recent national and international regulations have phased out HFOs with high sulfur content, that is, high sulfur fuel oils (HSFOs) with 3.5% sulfur, replacing them with low sulfur fuel oils (LSFOs—0.5% sulfur), and ultra-low sulfur fuel oils (ULSFOs—0.1 % sulfur).3 The LSFOs and ULSFOs are blends of various types of both heavy oils and light oils (Sørheim et al., 2020). Burning HSFOs is still allowed if the emissions are scrubbed with an exhaust gas cleaning system, often referred to as a scrubber, to the equivalent sulfur oxide limit as achieved by burning LSFOs (see Section 5.2.3.1).

2.1.3.3 Lubricating Oils

Petroleum-related lubricating oils have evolved over the decades from initially being mainly composed of higher molecular weight cyclic alkanes from petroleum to the present mixtures of those hydrocarbons with synthetic chemical additives, or composed almost entirely of synthetic chemicals including biosynthetic or biological materials-based lubricants (Bolina et al., 2021; Murru et al., 2021, and references therein). One example, among many, are polyalphaolefinbased products, a synthetic lubricant produced from alpha olefin (see Section 2.1.3.5) and used as a major lubricant in wind-power turbines, including offshore wind turbines.4

2.1.3.4 Biofuels

Since the NRC 2003 report, the production, transportation, and use of biofuels has increased. There are two main biofuels at present: ethanol and biodiesel. Ethanol is produced mainly from corn and similar crops and biodiesel is a liquid fuel consisting of new and/or used vegetable oils and animal fats, which significantly differ chemically from petroleum oils, comprising primarily natural lipids such as esters of fatty acids (e.g., triglycerides—also known as triacylglycerols) rather than hydrocarbons. There is substantial research ongoing in production of biofuels from various sources of biomass, including algal cultures grown for that specific purpose and converted waste biomass. The details of production, release, fates and effects of biofuels are beyond the scope of this report, and furthermore, the chemical compositions and spill behavior of biofuels are substantially different from those of petrochemical fuels. It should be noted that the biological origin of “biofuels” does not mean, a priori, that there will be no adverse effects if such biofuels are spilled or released as chronic inputs to the marine environment and, it is therefore important to keep track of chemical compositions and potential fates and effects of such spills or chronic inputs.

2.1.3.5 Olefins

Olefins are unsaturated hydrocarbons (also named alkenes or cyclic alkenes) with C=C double bonds in various places in the molecule, and with overall structures similar to the n-alkanes and branched alkanes, and cyclic alkanes. They are both biosynthesized naturally occurring compounds and are synthesized for a variety of purposes, including incorporation into drilling muds. The discharge of the untreated synthetic-based drilling muds into the marine environment is not allowed in the United States, Canada, and Mexico. However, past uses of olefins mean that they may be found in sediment environments near previous petroleum drilling operations, (e.g., see Stout and Litman, 2022).

2.1.4 Chemical Compositions

There are various ways that are accepted within the scientific community to depict molecular structures. This brief introduction is provided to capture the range of molecular size,

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1 Further relevant definitions and explanations can be found at https://www.oilsandsmagazine.com/technical/product-streams#streams.

2 See https://www.mabanaft.com/en/news-info/glossary/details/term/marine-diesel-oil-mdo-intermediate-fuel-oil-ifo.html.

3 See https://www.oiltanking.com/en/news-info/glossary/details/term/heavy-fuel-oil-hfo.html.

4 For example, see https://klinegroup.com/wind-turbine-lubricants-an-important-segment-of-the-industry.

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
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FIGURE 2.2 Depictions in three dimensions of example gas and oil molecules in relative size scale.
NOTES: Top row is methane. Middle row is the three ring aromatic compound phenanthrene. Bottom row is an example of a proposed structure of an asphaltene molecule. Left column: molecules shown without illustration of a rough approximation of electron clouds around the atoms. These are also known as ball and stick models. Right column: rough approximation of electron clouds around the individual atoms as they contribute to the overall size of the molecule in three dimensions. Black spheres represent carbon atoms, white spheres represent hydrogen atoms, yellow spheres represent sulfur atoms, and red spheres represent oxygen. The asphaltene molecule is a structure of one asphaltene molecule proposed by Yang et al. (2015).
SOURCE: Heather Dettman, Rafal Gieleciak, Natural Resources Canada.

examples of the classes of chemicals, and the complexity of various classes of chemicals found in oil and the important differences between molecules that contain the same number of elements of mainly carbon and hydrogen arranged in different spatial configurations. The differences in sizes and spatial configurations govern the fates and effects of the molecules in the environment.

Figure 2.2 provides a three-dimensional illustration of the comparison in relative size (to scale) between the simplest of hydrocarbons (methane), the three aromatic ring hydrocarbon phenanthrene, and one of the latest proposed structures for a relatively large asphaltene molecule. The information in this figure should be used as a guide when viewing the more traditional representation of molecular structures used in other figures in this report and in the scientific literature related to inputs, fates and effects of gas and oil chemicals in the marine environment. Also note that the representation of the asphaltene molecule is one of many such molecules in the asphaltenes. These molecules can assume various three dimensional configurations depending on their exact elemental compositions and chemical bonding, and interact with each other to form nanostructures as discussed briefly in a later section of this chapter.

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FIGURE 2.3 Three-dimensional, two-dimensional, and carbon–carbon bond representation of the gases methane and ethane.

A more familiar or traditional, longer used depiction of selected molecules is shown in the other molecular structure figures in this chapter. A few representative structures are depicted here, beginning with two and three dimensional skeletal configurations of methane and ethane. Figure 2.3 depicts the simplest structures, methane and ethane, and illustrates three-dimensional depictions, two-dimensional depictions, and a shorthand notation most often used to depict petroleum hydrocarbons. The shorthand notation for hydrocarbon composition is Cx, where x is the number of carbon atoms and the number of hydrogen atoms may or may not be included. Figures 2.4 and 2.5 present further example structures of increasing complexity.

2.1.4.1 Gases, Volatile Organic Compounds, and BTEX

The simplest manner to begin discussion of the composition of petroleum is with those constituent component chemicals that are volatile (i.e., quickly transform from the liquid to the gaseous phase) at average ranges of atmospheric pressure and temperature conditions at the Earth’s surface. In general, these include chemical compounds of carbon and hydrogen atoms with relatively simple, or branched, short-chain or cyclic molecules of low molecular weight.

Gas in petroleum comprises mostly methane with varying additional quantities of ethane, propane, and butane. These are simple chemical structures (see Figures 2.3 and 2.4). There is overlap in the scientific literature of chemicals designated as gases and those designated as volatile organic compounds (VOCs) from petroleum. Generally, the overlap

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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FIGURE 2.4 Carbon–carbon bond (or skeletal) structures of n-alkanes and iso-alkanes (branched chain alkanes) found in petroleum (gas and oils). As indicated by the boiling points, these are gases or volatile compounds at atmospheric pressure.
SOURCE: Reprinted with permission from Hunt, 1996. Copyright 1997 American Chemical Society.

is in the designation of propane and butane. Examples of chemicals in the petroleum VOCs are the n-alkanes (straight chain hydrocarbons) nC3 to nC10 or propane, butane, pentane, hexane, heptane, octane, nonane, and decane. There are also branched chain alkanes having one or more alkyl substituents in different positions such as 2-methylbutane, 2-methylpentane, and 3-methylpentane (see Figure 2.4 for some examples). Cyclic alkanes include cyclohexane, and methylcyclopentane. A more complete list is presented by Wnek et al. (2018).

Among the petroleum VOCs are the mono-aromatic hydrocarbons benzene, toluene, ethylbenzene, and xylene isomers—commonly referred to as BTEX (see Figure 2.5). These are very important compounds from the perspective of human health concern during oil spills, especially for oil spill responders for at-sea oil spills (see Chapter 4) or the general public for nearshore and shoreline spills (see Chapter 6). They are also of concern regarding potential toxicity to marine mammals that may be in the vicinity of an oil spill (see Chapter 6).

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FIGURE 2.5 Chemical structures of the group of monoaromatic hydrocarbons collectively known as BTEX (benzene, toluene, ethyl benzene, and xylenes). Pi-bond electrons resonate within the ring structure. The structure is often depicted as a circle within the hexagon. If viewed edge-on, this would show clouds of electrons of these C=C bonds above and below the planar hexagon C skeleton.
SOURCE: Montero-Montoya et al., 2018. CC BY 4.0.
Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
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FIGURE 2.6 Examples of a saturated normal-alkane and common iso-alkane and cycloalkane petroleum biomarkers (see Box 2.2) and a nickel porphyrin found in crude oils.
NOTES: The dotted wedge-like bonds indicate C–H bonds into the plane of the page and the solid wedges indicate C–H bonds out of the plane of the page for four of the molecules depicted. (See Figure 2.1 for a simple example for methane and ethane.)
SOURCE: Rullkötter and Farrington, 2021. CC BY 4.0.

It is important to note that the U.S. EPA and similar other national and international agencies use the term “volatile organic compounds” (VOCs) to designate a much wider variety of volatile compounds and differentiate between indoor VOCs and outdoor VOCs.5

2.1.4.2 Saturates, Aromatics, Resins, and Asphaltenes (SARA) and Other Chemicals in Petroleum

One of the most common and useful ways to describe the composition of crude oil and its distillation products is an approach that is coupled to the analytical procedures discussed in the next section of the chapter. Petroleum can be divided into four fractions according to solubility and chemical composition: saturated hydrocarbons (alkanes comprising only carbon and hydrogen, with no unsaturated bonds), aromatic hydrocarbons (comprising unsaturated cyclic structures of carbon and hydrogen, with or without saturated side chains), resins (typically cyclic structures containing one or more heteroatoms in addition to carbon and hydrogen), and asphaltenes (similar in chemical composition to resins but having larger, more complex molecules such as that shown in Figure 2.2). The shorthand used is SARA. Functionally, the first three fractions (i.e., SAR) are soluble in alkane solvents such as n-pentane, n-hexane or n-heptane and are sometimes referred to as maltenes to distinguish them from the asphaltenes that are insoluble in light alkanes but soluble in toluene. Chemically, the saturated and aromatic hydrocarbons comprise only carbon and hydrogen atoms. Resins contain some hydrocabons and chemical compounds with additional elements such as N, S, and O, as is the case for asphaltenes. There is overlap in the solubility definition because some small resins behave like aromatic hydrocarbons.

Commonly, saturates represent the major fraction of oil. Chemical structures of alkanes range from simple methane (CH4), a gas at atmospheric pressure, to volatile, liquid or waxy alkanes and intricate cyclic structures such as steranes and hopanes. Alkanes include three subgroups, with examples shown in Figure 2.6: (1) n-alkanes (or normal alkanes) are straight chains of carbon atoms with hydrogen bonded to the carbon atoms (e.g., n-heptacosane, n-C17). (2) iso-alkanes are branched alkanes where one or more carbon atoms is bonded to a straight carbon chain (e.g. 2-methyl-n-hexacosane, also named 2-methylhexacosane). A special subgroup of branched alkanes is the isoprenoid branched alkanes, so named because of their structural relationship to a repeated unit of the isoprene moiety (e.g., phytane, β-carotane, and lycopane). (3) Cycloalkanes (also called cycloparaffins, naphthenes, alicyclic hydrocarbons) typically range from one ring of five or six carbon atoms to many fused rings, often with alkane substituents on the rings. Some specific examples are depicted in Figure 2.6—a tricyclic diterpane, a sterane, and a pentacyclic triterpane. Although these structures are flat on the page, in reality they are three dimensional “bent” structures as exemplified in the three dimensional example of Figure 2.7 for cholestane,

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5 See https://www.epa.gov.

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
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FIGURE 2.7 Three-dimensional rendering of the petroleum biomarker compound cholestane and comparison to two-dimensional drawing of the same molecule.
NOTE: The positioning of the carbon-hydrogen bond at the three position points above the plane of the molecule is designated 3β and that at the five position points below the plane 5α.
SOURCE: Adapted with permission from Hunt, 1996. Copyright 1997 American Chemical Society.

a sterane. The exact three-dimensional structures of these petroleum compounds are key to the use of steranes, triterpanes, and related compounds as petroleum biomarkers, as will be discussed in more detail in subsequent sections.

Aromatic hydrocarbons range from monocyclic aromatics to fused ring compounds comprising two or more rings (polycyclic aromatic hydrocarbons; PAH or PAHs). Benzene is a single-ring, six-carbon-atom compound as noted previously (see Figure 2.5), whereas naphthalene is a two-ring PAH, phenanthrene has three fused rings, and chrysene has four (see Figure 2.8). The rings are co-planar, and in addition to the carbon–carbon bonds, the ring carbons share a cloud of electrons resonating above and below the ring, a characteristic of aromatic rings, as noted earlier for BTEX compounds. These types of compounds were initially designated aromatic because the first few isolated and characterized had a distinctive aroma; it also is a reminder that smaller aromatic hydrocarbons are volatile.

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FIGURE 2.8 Examples of structures of alkylated aromatic hydrocarbons, aromatic hydrocarbon petroleum biomarkers, and alkylate heterocyclic aromatic hydrocarbons.
SOURCE: Adapted from Rullkotter and Farrington, 2021. CC BY 4.0.
Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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FIGURE 2.9 The 16 parent PAHs currently on the U.S. Environmental Protection Agency (EPA) Priority Pollutant List, plus benzo[f] fluoranthene and benzo[e]pyrene.
SOURCE: Rullkötter and Farrington, 2021. CC BY 4.0.

The aromatic rings can be fused in different specific geometries for PAHs of three or more rings, as depicted in Figure 2.9. For example, compare the three-ring compounds phenanthrene and anthracene, and the four-ring compounds pyrene, benzo[a]pyrene, and chrysene. Some aromatic hydrocarbons have a five-membered carbon ring as well as six-membered rings in their structures (e.g., acenaphthylene and fluoranthene, Figure 2.9). Aromatic hydrocarbons with two or more cyclic rings are designated by the shorthand notation polycyclic aromatic hydrocarbons (PAH or PAHs).

When nitrogen (N), oxygen (O), or sulfur (S) are substituted for a carbon atom in these compounds (see the thiophenes, dibenzofuran, and carbazole in Figure 2.8 as examples) the compounds are no longer strictly hydrocarbons and are known instead as polycyclic aromatic compounds (PACs). Hydrocarbon compounds for which one or more of the carbons has been replaced by the elements O, N, or S are also known as heteroatom compounds. These simple PACs commonly have chemical and physical properties similar to their PAH counterparts.

The various configurations of the fused rings influence physical and chemical properties of the compounds such as volatility, solubility, and also the chemical and biological reactivity of the specific PAH or PAC. A much discussed example is the comparison of benzo[a]pyrene, classified as a strong carcinogen, with its isomer benzo[e]pyrene, which is not classified as carcinogen. Both structures are depicted in Figure 2.9.

PAHs and PACs in petroleum are often “decorated” with substituent alkyl groups of different lengths and in various positions (i.e., alkanes or branched alkanes substituted on the ring structures), forming series of related chemicals having the same aromatic skeleton. Examples depicted in Figure 2.8 include 1- and 2-methylnaphthalene; 1, 3-, and 2, 10-dimethylphenanthrene; and 1, 2- and 1, 3-dimethylchrysene. There are also aromatic hydrocarbons with a combination of saturated cyclic rings coupled with aromatic rings such as acenaphthene (see Figure 2.9) and the monoaromatic and triaromatic steroid hydrocarbons depicted in Figure 2.8.

The chemical structures depicted in Figures 2.2 through 2.9 are only a few examples of the several thousand individual chemical structures present in petroleum.

The resins fraction is not defined by chemical structure but rather by solubility, being insoluble in liquid propane; the structure of individual resins is generally not well known. The resins fraction comprises compounds with heteroatoms where N, S, and/or O replace C in one or more positions (see Figure 2.8). As noted previously, some simple PACs fall into the chemical definition of a resin, but their solubility and reactivity align them functionally with the PAHs.

The asphaltenes fractions, like the resins, are defined by solubility: they are petroleum chemicals that are not soluble in alkane solvents but do dissolve in toluene. They are the compounds that impart a dark color to crude oil. Asphaltenes are high molecular weight compounds typically of undefined structures, although some tentative structures or parts of structures have been proposed. A recent review of asphaltene structure and function by Schuler et al. (2020) suggests that previous reports of average molecular weights in the thousands of Daltons for model asphaltene molecules are overestimates, having been inferred from measuring nanoaggregates of smaller average asphaltene moieties (say, ~600–800 Daltons) (e.g., see Figure 2.2 for a proposed structure for an asphaltene molecule, Yang et al., 2015) that tend to self-assemble into macromolecular clusters.

Naphthenic acids (NAs) are composed of cyclic and non-cyclic carboxylic acids with varied alkyl substituents on these oxygen-containing compounds. They are similar

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×

to the resin fractions in being polar. There is concern that some NAs are toxic to aquatic animals, and some do not biodegrade easily (Lee et al., 2015). While examples of some structures have been noted or proposed, NAs are an example of a class of oil compounds in need of more accurate definition of structure by using advances in analytical chemistry. Other chemicals present in minor quantities in petroleum are metal-containing organic compounds such as the petroporphyrins containing Ni (nickel) and V (vanadium) (Chacón-Patiño et al., 2021, and references therein). See the nickel porphyrin structure in Figure 2.6 as an example. Elemental sulfur and H2S (hydrogen sulfide) are present in some crude oils—the “sour crude oils.”

2.1.4.3 Petroleum Biomarkers

There is a special group of isoprenoid hydrocarbons, cyclic alkanes, and aromatic hydrocarbons commonly referred to in the organic geochemistry and petroleum chemistry literature as “biomarker compounds” or “molecular biomarkers.” It is important to note that these are not the same as “molecular biomarkers” that have come into the lexicon of molecular biology and environmental toxicology. (See Box 2.2 for an explanation of the differences.) To differentiate between toxicological and geochemical/petrochemical usages, the term “petroleum biomarkers” is used to indicate the latter (see Box 2.2). Among the most common of these compounds are the subclasses isoprenoid hydrocarbons, steranes, and hopanes (see examples in Figure 2.6), so named because their molecular structures are derived from biologically produced precursor compounds (e.g., the phytol side chain of chlorophylls, carotenoids, steroids, and hopanoids) that were deposited in sediments with other organic matter of biological origin. Over geologic time through diagenesis and catagenesis, this organic matter was the source of the petroleum that contains these petroleum biomarkers whose backbone chemical structure is the same as or similar to the isoprenoids, carotenoids, steroids, and hopanoids.

These petroleum biomarker compounds are important because their presence and exact proportions to one another in given crude oil or fuels oil can be and have been used to provide a unique, or nearly unique, identification for a given oil. Early in the oil spill literature of the late 1960s and into the 1970s these petroleum biomarkers or molecular signatures were explained in laypersons’ terms as providing the “fingerprint” of an oil. The term “fingerprinting oils” became part of the lexicon of oil spills even though the unique nature of a given oil’s fingerprint was not in the same statistically unique category as a human fingerprint. Since that time, the term “oil spill forensics” has also been used (e.g., Stout and Wang, 2018). Importantly, many of these petroleum biomarker compounds do not readily biodegrade nor are as chemically reactive as many of the other chemicals in petroleum. Thus, they are often used as reference compounds to discern recent alteration of spilled oil (e.g., through biodegradation, volatilization, dissolution, etc.; see Chapter 5). Petroleum biomarkers of oil are noted in various examples and discussion in the Chemical Methods section.

2.1.5 Physical Chemical Properties of Petroleum Hydrocarbons

The molecular weights and various molecular configurations of petroleum hydrocarbons (and any molecule for that matter) control the fundamental physical chemical properties of the individual compounds and of mixtures of compounds. These properties include density, volatility, solubility, and viscosity, among many important properties. As an example, Table 2.2 is a combination of data excerpted from May (1980). Note that molecular weight ranges over about a factor of 3 and the solubilities range over a factor of approximately 106. Table 2.2 also contains the Setschenow constants and the equation using those constants that adjust for solubility reductions as a function of salinity. The reduction of solubility as a function of increasing salt concentrations was noted first in 1889 by Setschenow that can influence hydrocarbon solubility as explained in May (1980). Relatively large changes in salinity are needed to significantly change solubility in contrast to greater sensitivity to small changes in ambient temperature (Whitehouse, 1984). We have noted salinity here as an important factor because large salinity gradients are present in estuarine and many coastal waters.

The presence of dissolved salts is among several parameters such as the presence of other organic chemicals that can influence the solubility of hydrocarbons in the aqueous phase as discussed in May (1980) and Schwarzenbach et al. (2016). These latter authors provide an extensive appendix of solubility data and other physical chemical data for a suite of hydrocarbons and other organic chemicals and reasonably up to date reference citations for these types of data.

There are numerous other properties and parameters of compounds that can be measured or calculated and are useful for understanding the fates and effects of petroleum compounds as discussed in Chapters 5 and 6. One example is the Kiow (also noted in some papers as Kow) or octanol water partition coefficient, a measure of a compound’s partitioning between two liquids of different polarity (i.e., octanol and water). This partitioning coefficient is used to predict partitioning between dissolved chemicals in water and the lipids in marine organisms, and between water and an organic phase (such as an organic coating on a water column or sediment particle or organism exuded polymer). Much has been learned about the fundamentals related to these partitioning processes as noted in the comprehensive text by Schwarzenbach et al. (2016).

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×

TABLE 2.2 Solubility of Some Example Aromatic Hydrocarbons in Pure Water at 25°C and the Corresponding Setschenow Constants Used to Adjust for the Effect of Salinity in Sea Water That Reduces Solubility

Chemical Name Molecular Weight (Daltons) Solubility at 25°C (mg/kg pure water) Setschenow Constant Ks (liters/mole)
Benzene 78.1 1791 +/− 10 0.175 +/− 0.006
Naphthalene 128.2 31.69 +/− 0.23 0.213 +/− 0.001
Fluorene 166.2 1.685 +/− 0.005 0.267 +/− 0.005
Anthracene 178.2 0.0446 +/− 0.0002 0.238 +/− 0.004
Phenanthrene 178.2 1.002 +/− 0.011 0.275 +/− 0.010
2-Methylanthracene 192.3 0.0213 +/− 0.0003 0.336 +/− 0.006
1-Methylphenanthrene 192.3 0.269 +/− 0.003 0.211 +/− 0.018
Fluoranthene 202.3 0.206 +/− 0.002 0.339 +/− 0.010
Pyrene 202.3 0.132 +/− 0.001 0.286 +/− 0.003
Benzanthracene 228.3 0.00094 +/− 0.0001 0.354 +/− 0.002
Chrysene 228.3 0.0018 +/− 0.00002 0.336 +/− 0.010
Triphenylene 228.3 0.0066 +/− 0.0001 0.216 +/− 0.002

NOTES: Solubility data are means +/− one standard deviation. The equation for Setschenow constants is Log S0/Log Ss = KsCs where So is the concentration in freshwater and Ss is the concentration in seawater. Ks is the Setschenow constant and Cs is the molar salt concentration.

SOURCE: Excerpted and adapted from May, 1980.

2.1.6 General Sources of Hydrocarbons in the Marine Environment

The main sources of hydrocarbons in the marine environment need to be described briefly as this is relevant to understanding how marine ecosystems might be predisposed to the presence of certain hydrocarbons and/or mixtures of hydrocarbons such as those found in spilled or chronically released oil. It is also important to set the scene for analytical methods and data interpretation used to distinguish sources of hydrocarbons in forensics and in fates and effects studies.

  1. Biosynthesis and biochemical transformations. Specific n-alkanes and alkenes are synthesized by land plants and marine organisms, as has been documented for decades (NRC, 1975, 1985). Biochemical transformations of biogenic compounds to hydrocarbons have also been well documented. One striking example is the biochemical transformation of the phytol side chain of chlorophyll present in its phytoplankton food to pristane by Calanus copepods (Avigan and Blumer, 1968). Neocalanus copepods in Prince William Sound biosynthesize pristane in a similar manner, and it is transferred through the food web (Short, 2005). Various species of diatoms biosynthesize highly branched C25 alkenes (Belt et al., 2019; Gao et al., 2020; and references therein).

    The significance of C15 and C17n-alkanes by cyanobacteria Prochlorococcus and Synechococcus (Lea-Smith et al., 2015; Love et al., 2021) with an estimated yield of ~308–771 million tons of these hydrocarbon inputs to the global ocean from this source.

  2. Microbial biochemical and geochemical transformations of natural organic matter in soils and surface sediments (often designated as early diagenesis) yield several specific steranes and aromatic hydrocarbons derived from biological precursor molecules, as reaction products of the early diagenesis of sterols in some deposition environments and in sinking particulate matter in the water column (e.g., Wakeham and Canuel, 2016). In particular, the PAHs retene and perylene are often present in analysis of marine sediments and are of known biological/early diagenesis origin (Lima et al., 2005).
  3. Erosion of ancient sediments containing organic matter that has been transformed by diagenesis into material that is oil shale or near to oil shale type material or coal.
  4. Natural seepage at the subsea floor and at the sediment-water interface releases both natural gas and oil to the marine water column (MacDonald et al., 2015; Ruppel and Kessler, 2017). Natural seepage is ubiquitous on the continental slopes and margins in many areas of the world, and natural seep sites are persistent at the annual and decadal time scales (see Section 3.2). Depending on the dynamics of the seep such as the rate of seepage and subsurface interactions with sea water and nutrients, the composition of the petroleum may be altered from that in the reservoir due to the development of microbial communities associated with a seep. (See Chapter 5 for a discussion of microbial degradation of petroleum.)
  5. Human-mobilized coal that may end up in surface sediments as a result of losses due to routine handling or shipwrecks (e.g., Tripp et al., 1981; Hostettler et al., 1999). These authors noted that extraction of surface sediments in the present-day environment that contained coal particles and analysis by gas chromatography methods used to analyze petroleum hydrocarbons (see Analytical Chemistry Methods section) yields results that could be mistaken for the presence of petroleum hydrocarbons.
Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
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  1. Combustion is a source of PAH. Grassland and forest wildfire along with human use of fossil fuels (coal, oil, and gas) and biogenic material (e.g., wood) for heating and power generation are major sources of PAHs in the environment in modern times (e.g., Lima et al., 2005). Grassland and forest fires have been a source for PAHs in the environment before human activities (e.g., Karp et al., 2020). The processes of reactions in combustion that likely yield PAHs and also yield soot and black carbon have been outlined recently by Johansson et al. (2018). Notably, these pyrogenic PAHs differ from oil-derived (petrogenic) PAHs in terms of the relative abundances of parent (i.e., non-alkylated) PAHs and related alkylated PAH homologs and other compositional differences (NRC, 1985; Lima et al., 2005).
  2. There are various sources of groups of hydrocarbons or individual hydrocarbons that are synthesized for specific uses in various products utilized by general public consumers in everyday life. Many of these are disposed via wastewater streams and eventually are discharged in varying amounts to the estuarine and coastal marine environment. A complete review of these is beyond the scope of this report. Two illustrative examples suffice: decalin and linear alkyl benzenes.

    Decalin is a bicyclic saturated hydrocabon (formal name bicyclo[4.4.0]decane) and can be present in two conformations—cis and trans—in petroleum. Also decalin is synthesized as a jet fuel additive because of its energy density and high thermal stability (e.g., Wang et al., 2019), and as solvent used in various manufacturing processes (ScienceDirect, 2004).

    Linear alkylbenzenes (LABs) are involved in the production of linear alkylbenzenesulfonates (LAS) that are used as anionic surfactants in domestic laundry detergents and for washing of dishes. There are 26 congeners (i.e., similar molecular structures) consisting of 10, 11, 12, 13, and 14 carbon atoms arranged in various branched alkane configurations substituted on the benzene molecule. As such they overlap in the jet fuel–fuel oil range of molecular weights for petroleum chemicals. A portion of LABs are not sulfonated in the production and are included in the detergents. The large amount of detergent use results in considerable amounts of LABs in domestic wastewaters, particularly in large urban areas. Despite various wastewater treatment methods, significant amounts of LABs are discharged into the receiving streams or accumulated in sewage sludge (e.g., Eaganhouse et al., 1983; Takada and Ishiwatari, 1990; Sherblom and Eaganhouse, 1991; Takada et al., 1992, 1994; Eaganhouse and Sherblom, 2001; Gustafsson et al., 2001; Macias-Zamora and Ramirez-Alvarez, 2004; Martins et al., 2008, among others). The overlap of molecular structures, physical chemical properties, and biogeochemical fates of LABs with petroleum hydrocarbons leads to LABs being included in hydrocarbons extracted and isolated from marine samples for determination of petroleum hydrocarbons (see the previously cited references). It is incumbent on analysts of samples for assessment of oil spills and chronic inputs of oils, especially in urban harbors receiving wastewater discharges, to be cognizant of the potential presence of LABs in the samples.

  3. Petroleum. Human-mobilized petroleum releases to the environment, either as spills or chronic inputs, will be discussed later in this report. At least two of the sources of chronic releases may contain mixtures of petroleum hydrocarbons and combustion source hydrocarbons. These are (1) motor oils that are dribbled into the environment on roads and driveways by leaking crankcases of cars, trucks, and similar vehicles. The leaking oil contains the original crankcase oil with its mostly higher molecular weight cycloalkanes plus PAHs from combustion of the fuel that have slipped across the rings in the combustion chamber to accumulate in the crankcase oil. (2) A similar process occurs for exhausts of outboard engines and inboard marine engines for boats and ships.

Another category of petroleum that is increasing in production and transport, especially by pipeline and rail tank car, is diluted bitumen. (See previous discussion).

In summary, these key factors are relevant to this report:

  1. Hydrocarbons from biosynthesis or early (modern) diagenesis in the water column, surface sediments, and soils have a composition that is relatively simple compared to the complexity of chemicals in petroleum. Nevertheless, it is reasonable to posit that these hydrocarbons have stimulated the evolution of microbes that can break down and metabolize these types of hydrocarbons in the marine environment (Section 5.2.7).
  2. Hydrocarbons, resins, and asphaltenes present in natural oil seeps have a compositional complexity that is the same as or similar to that of oil inputs from oil spills and several types of chronic releases. Depending on each natural hydrocarbon seep, differences in composition could be a function of physico-chemical dissolution and microbial decomposition processes active during the seepage processes pre-release in the seafloor and at the sea floor.
  3. Combustion sources of PAHs (pyrogenic PAHs) generally have complex compositions similar to PAHs in petroleum. However, the parent PAHs are more abundant than alkylated PAHs in the same grouping of parent and alkylated PAHs (e.g., naphthalenes, phenanthrenes, and chrysenes). In general, the higher the efficiency of the combustion process, the higher the relative abundance of the parent PAHs. In comparison, PAHs in petroleum
Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
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  1. (petrogenic PAHs) have greater relative abundances of the various alkylated PAHs compared to combustion sources. Assessing these relative abundances of parent and alkylated PAHs has been used in numerous instances to ascertain the relative contributions of pyrogenic and petrogenic PAHs to a given sample (e.g., see NRC, 1985; Lima et al., 2005, among others).

2.1.7 Sampling and In Situ Observations and Analyses

The most sophisticated laboratory procedures and analytical chemistry instruments will not provide useful data unless: (1) appropriate precautions are taken to obtain field samples that are not contaminated or compromised by the sampling process, (2) samples are labeled in accord with specifications for the intent of the sampling and analyses (e.g., U.S. Natural Resource Damage Assessment protocols or their equivalent in other countries), and (3) samples are preserved appropriately during transportation and storage prior to analysis. Furthermore, all of the preceding coupled with the appropriate analytical methods will provide the most useful data if there is attention to the details of appropriate sampling plans to answer the questions about responses to oil spills and inputs, fates, and effects of oil spills or chronic inputs (individually or in combination) discussed in the following chapters. This includes familiarity with the physical, chemical, biological, geological, and ecological processes of the ecosystem or ecosystems being sampled.

Several appropriate online documents have been prepared that describe planning and sampling within the context of oil spills (e.g., NOAA, 2014b; IPIECA, 2020, and references therein). These are updated periodically to take into account new knowledge gained from responses and research connected with oil spills. For example, the Royal Society of Canada report (Lee et al., 2015) and this NASEM report (and references herein) provide updated reviews of new knowledge of relevance. In addition, reports such as that by Payne and Driskell (2015) as part of the Natural Resource Damage Assessment (NRDA) studies of the DWH oil spill document sampling equipment and measurement instruments deployed to advantage in the field, including helpful photographs.

There have been advances in refinement and utilization of in situ observations of hydrocarbons within the ocean water column for chronic inputs and episodic spills. These instruments can be mounted on hydrocast-CTD (conduct-temperature-density) rosettes or on underwater vehicles (remotely operated, human-occupied, and autonomous). An important advance is the more extensive use of ultraviolet (UV)-induced fluorescence instruments for detection of aromatic hydrocarbons in seawater in assessment of oil spills. These fluorescence observations are often coupled with other sampling equipment, including water samplers and other analytical platforms. Use of a laser fluorosensor to detect oil on water and substrates such as shoreline, plants, and ice—among other substrates—has been reviewed by Fingas and Brown (2018). A coastal mounted sensor using UV induced fluorescence to detect oil harbors has been reported by Hou et al. (2018). As noted by Part et al. (2021), care must be taken to calibrate the sensors to avoid interference or false positives from the presence of natural colored (or chormophoric) dissolved organic matter (CDOM) and algae-derived chemicals such as chlorophyll A. A brief description of some other advances in UV-fluorescence analytical methods is provided in Section 2.1.8.

Coupling of an underwater mass spectrometer with an autonomous underwater vehicle has expanded capabilities beyond the fluorescence detectors to a broader range of dissolved hydrocarbons. These and other advances with in situ measurement devices such as advanced camera systems for imaging particle sizes and shapes have been summarized in an overview manner in Dannreuther et al. (2021) with relevant references therein. Some of these systems are noted with figures and photos in Payne and Driskell (2015).

A more thorough review of this topic is beyond the scope of this report because of the rapid advances of the past two decades in towed underwater vehicles, remotely operated underwater vehicles, autonomous underwater vehicles, and associated sensor developments.

The preceding applies not only to oil spill response and damage assessment but also to research activities focused on fates and effects.

2.2 PHASES AND STATES OF PETROLEUM FLUIDS IN THE SEA

2.2.1 Gas- and Liquid-Phase Petroleum

As discussed in this chapter, petroleum fluids are a complex mixture of hydrocarbon- and non-hydrocarbon-containing molecules. Depending on the temperature and pressure, they may occur in the gas, liquid, or solid phase of matter. In this report, the focus is on the gas and liquid phases because they are the most often encountered in the marine environment and more problematic. In everyday lives, petroleum fluids are normally used at atmospheric pressure; hence, some hydrocarbon molecules are commonly referred to as gases or liquids. For example, methane, ethane, and propane are gases at standard conditions, and benzene is a liquid. However, propane at 15°C and 1.7 MPa (megapascal, equivalent to about 150 m depth in the ocean) would be in the liquid phase. Because there is concern with petroleum fluids throughout the ocean water column, this report uses the terms gas and liquid when appropriate to refer to the in situ phase of the fluid of interest, rather than describing propane, for example, simply as a gas.

Commonly, the term oil is used to refer to the liquid-phase petroleum since crude oil and refined oil products are normally experienced as liquids at standard conditions. Here, this usage of the term oil may also be followed when appropriate and the phrase liquid petroleum may be used whenever the generic term oil would be ambiguous. However, it is important to remember that crude oil is a complex

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
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mixture that will have a different composition within the gas and liquid phases under different thermodynamic states, and that the term oil is not a precise, scientific description of either the state or the composition of a petroleum fluid.

There are different definitions of standard thermodynamic conditions in different fields of chemistry and engineering. In this chapter, the definition of standard conditions is used from the Society of Petroleum Engineers (SPE), which defines standard temperature to be 15°C (288.15 K or 59°F) and standard pressure to be 100 kPa (kilopascal, also equal to 1 bar). We use this definition since quantitative metrics describing oil spills, especially as related to offshore oil and gas exploration or hydrocarbon transport, are likely to be expressed using this standard.

Within a petroleum reservoir, all of the crude oil components are present as a mixture in equilibrium with the ambient pressure and temperature (McCain, 1990). Depending on the composition and ambient conditions, this mixture may or may not contain a gas phase. When no gas phase is present, all compounds are present in the liquid-phase petroleum, and some of the lightest components, such as methane, may be thought of as fully dissolved in the liquid phase. When a gas phase is present, all of the components of the whole petroleum will equilibrate between the gas and liquid phases. This means that a large fraction of the methane may be found dissolved in the liquid phase and that some of the higher molecular weight hydrocarbons, such as pyrene, with molecular weight of 202.25 g/mol, will partially partition into the gas phase. As an example, over half of the methane released from the DWH wellhead during the spill was released in the liquid-phase petroleum (Gros et al., 2016). The phase partitioning will be at equilibrium in the petroleum reservoir and may be in disequilibrium for some time as crude oil is conducted from the reservoir to surface storage.

The gas-to-oil ratio (GOR) is a measure of the fraction of compounds that would be gases at standard conditions within a crude oil. In petroleum engineering, GOR is commonly expressed as the number of cubic feet of gas that can be extracted at standard conditions to the number of stock barrels of liquid crude oil that would remain. This form of the GOR of a reservoir is very important in petroleum engineering as it explains what types of petroleum products can be extracted through a well and the volume flow rate of produced gas and liquid petroleum. The GOR is evaluated as an equilibrium condition for the petroleum mixture in a closed system (i.e., not in equilibrium with the atmosphere). Hence, the light, highly volatile and volatile components of the original petroleum fluid would still be present in the petroleum mixture for the purposes of evaluating the reservoir GOR. The GOR may also be expressed using in situ conditions or different units. For example, the GOR of the DWH crude oil at the sea floor conditions of the release was 29–44% gas and 56–71% liquid (Gros et al., 2016). When expressed this way, using the same volume units for oil and gas, the reported GOR may be unitless. In whatever way the GOR is specified, it is always critical to know the thermodynamic conditions and units used to make the evaluation.

For a surface spill of a crude oil or refined product, as from a tanker or ship accident, the original gaseous components of the oil would no longer be expected to be present, and the GOR would not be important. For submarine oil spills, especially for oil well blowouts, the GOR is a critically important parameter and must be considered in the context of the thermodynamic state for which it is evaluated. As oil and gas traverse the ocean water column, the thermodynamic state (notably, temperature and pressure) is continually changing, and the composition of the oil and gas may be changing, both directly through phase changes as well as via fate processes such as dissolution, biodegradation, and other interactions with seawater.

Because liquid petroleum is immiscible in water, it may also be referred to as a non-aqueous phase liquid (NAPL). NAPLs include hydrocarbons that exist as a separate, immiscible phase when in contact with water and/or air; they may either be crude oils or refined products. NAPLs are typically classified as either light nonaqueous phase liquids (LNAPLs), which have densities less than that of water, or dense nonaqueous phase liquids (DNAPLs), which have densities greater than that of water (Newell et al., 2015). Normally, liquids with an API gravity less than 10°API would be classified as DNAPLs. (See Table 2.1 for comparison to other API gravity of some other oil types.)

Coal tar is an example of a multicomponent DNAPL, which consists of polycyclic aromatic hydrocarbons; phenols; benzene, toluene, ethylbenzene, and xylene; and other compounds. Produced for medical and industrial uses, coal tar is a by-product of the production of coke and coal gas from coal. Coal tar, creosote, and No. 6 fuel oil DNAPL mixtures generally have very low bulk solubility and may result in small to negligible dissolved plumes when spilled. However, these DNAPL mixtures often contain soluble constituents, such as naphthalene, that dissolve in water creating plumes after a spill.

2.2.1.1 Live Oil and Gas

When a petroleum mixture is in equilibrium with standard conditions, most of the gases originally dissolved in the liquid-phase petroleum will have escaped, and the remaining compounds would remain in the liquid phase. This state of liquid petroleum is referred to as dead oil. Petroleum mixtures in most other states of equilibrium would be referred to as live oil. The term live is used to indicate that some gaseous compounds, such as methane, are present in the liquid-phase petroleum at concentrations such that some gas would evolve out of the liquid phase if brought to standard conditions.

Both the gas and liquid phases of a petroleum fluid can be termed as live gas or oil whenever they are out of equilibrium with standard conditions. Fluids released from a subsea oil well blowout, for example, would be live gas and

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×

oil as the composition of the gas and liquid phase petroleum fluids would be that at the high pressure of the release point, generally including much more of the light hydrocarbons, such as methane, within the liquid phase than will remain at surface conditions. The disequilibrium of the released fluids relative to standard conditions is important both for predicting bulk properties, such as density, and for evaluating the composition of each phase of petroleum. These parameters are critical for evaluating fate, effects, and safety issues, including explosivity.

2.2.1.2 Weathered Oil

Historically, the general term weathering has been used to describe any process that changes the composition of an oil from its release conditions. This term came into use when many early studies applicable to surface floating oil considered processes linked to the weather conditions. Such processes may include evaporation, natural dispersion, or emulsion formation. Today, oil spill scientists may use weathering to describe a much wider array of fate processes occurring throughout the ocean water column, including dissolution, photo-oxidation, biodegradation, sorption, sedimentation, and so on. Correspondingly, weathered oil is any oil whose composition has changed by any fate process since its release to the environment. Weathering or fate processes are described in detail in Chapter 5.

2.2.1.3 Gas Hydrates

Gas hydrates consist of gas molecules surrounded by cages of water molecules that form a crystalline lattice (cage) via hydrogen bonds under certain pressure and temperature conditions (Sloan and Koh, 2008; Hassanpouryouzband et al., 2020). Gas molecules small enough to fit inside the lattice structure are encaged, or enclathrated, within the water lattice. Natural gases that may form hydrates in seawater include methane, ethane, propane, isobutane, n-butane, nitrogen, carbon dioxide, and hydrogen sulfide, among others (Sloan and Koh, 2008). Natural gas hydrates can concentrate and store large quantities of natural gas within their water cage cavities. When heated or depressurized, they become unstable and dissociate into water and natural gas.

Gas hydrates are important in petroleum engineering because they can cause blockages of wells and pipelines. Methods to understand and prevent such hydrate formation are the topic of flow assurance (Sloan and Koh, 2008; Koh et al., 2011). In marine oil spills, hydrates may play a role in both response and petroleum fluid fate when spills occur within the hydrate stability zone, the region of the oceans for which the temperature is low enough and pressure high enough for hydrate stability. Hydrates cause issues for response because they may clog collection systems, freeze equipment, or otherwise interfere in sensors or operations. Gas hydrates may alter the fate of spilled oil and gas by creating a barrier between the free oil and gas and water. Because hydrate forms as a crystalline matrix of gas and water, it will grow from the gas–water interface. Natural gas molecules may be supplied from either the gas or liquid phase of the spilled fluids. Warzinski et al. (2014a) show the nature and dynamics of hydrate armoring on methane and natural gas bubbles in a high pressure water tunnel. Though hydrate itself is soluble in seawater, it is less soluble than the free gas; hence, hydrate armoring may interfere with dissolution of natural gas bubbles in seawater. The implications of hydrates on marine oil spills are discussed in more detail in Section 5.3.3, on deep-water processes for acute marine oil spills, and Section 5.4.1, on natural seeps.

Naturally formed gas hydrates occur beneath the seafloor, in permafrost areas, and beneath some ice sheets where temperature and pressure conditions for formation of methane hydrates are favorable (see Figure 2.10). Except on upper continental slopes (depths less than 300–700 m water depth), the seafloor of most of the oceans is within the hydrate stability zone. About 99% of the world’s gas hydrates are in the uppermost hundreds of meters of marine sediments at water depths greater than ~500 m and close to continental margins (Ruppel and Kessler, 2017).

2.2.2 Advances in Analytical Chemistry Methods

There have been significant advances in chemical methods applied to analyzing gas and oil chemicals released to the environment since the NRC (2003) report. That report did not address analytical chemistry methods in detail because of the large amount of other information presented. Thus, advances since the NRC 1985 report are reviewed briefly, but the focus of this section is on the significant advances since the NRC (2003) report. More details of the analytical chemistry procedures are provided in Stout and Wang, 2018, and Wise et al., 2022, and specific references cited in this section.

2.2.2.1 Ultraviolet Fluorescence Analyses as a Sample Screening Method and Related Advances

The initial analyses of samples and extracts often involve ultraviolet fluorescence spectrometry to assess the presence of aromatic hydrocarbons and other aromatic compounds in petroleum—mostly the low-to-medium molecular weight compounds that have greater solubility in seawater than the higher molecular weight PAHs and PACs. This method has been used for decades as a quick scanning method to assess the presence or absence of these petroleum compounds (NRC, 1985).

Over the past several decades, the instrumentation and methodology has advanced to the point where there is now a robust excitation–emission matrix spectroscopy (EEMS) methodology that provides a reasonably rapid analyses of water samples and some extracts (Bugden et al., 2008). This

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
Image
FIGURE 2.10 Schematic of the occurrence and dynamics of methane hydrate in terrestrial and marine systems.
SOURCE: Adapted from Ruppel and Kessler, 2017.

has been enhanced by coupling EEMS with parallel factor analysis (PARAFAC) to enhance the specificity of the methodology (e.g., Miranaghia et al., 2018; Araújo et al., 2021; Matheus et al., 2021; Oliveira et al., 2021). This methodology has also been used to provide insight into the fate of petroleum aromatic compounds (PACs) in sea water 2 years after the DWH oil spill (Bianchi et al., 2014).

Depending on the circumstances and the objectives of the analyses, these UV-fluorescence methods can have an important role in providing a first screening and identification of samples to be subjected to a progressive cascade of more detailed analyses as noted in the following section.

The basics of the flow schemes of detailed analyses are summarized in outline form in Figure 2.11. Oil samples or samples from the environment (e.g., air, water [dissolved and particulate fractions], sediments, organisms) are analyzed by (1) progressive physical and/or chemical separation of oil constituents from other organic materials in the samples; (2) solution-based separation of the resulting isolated oil into groups of chemicals, such as the SARA fractions previously described, by using chromatographic means such as column or thin-layer chromatography and high performance liquid chromatography; (3) further physico-chemical separation and quantification by gas chromatography (GC) based on properties of volatility and polarity of the individual constituent chemicals corresponding to their molecular weight and structure; (4) mass spectrometric (MS) analysis of compounds separated by GC or high-performance liquid chromatography (HPLC) (often via coupled GC-MS or HPLC-MS); or (5) direct analysis by MS of extracts, isolated fractions, or oil or weathered oil samples (see weathering in Chapter 5).

This analytical methodology of glass capillary gas GC and glass capillary gas chromatography–mass spectrometry (GC-MS) coupled to computer systems for data processing had become more routine just at the time of the 1985 NRC report. Since that time, there has been extensive application and demonstration of its practicality and utility. An informative, brief history of GC-MS computer systems is provided by one of the pioneers of these methods (Hites, 2016).

Recommended GC-MS methods for analyzing oil or oil-contaminated samples in environmental settings have been set forth by national agencies, for example, Lauenstein and Cantillo (1998), Olson et al. (2004), and NIST (2021).6

There are hundreds if not thousands of scientific papers and reports on uses of GC-MS relevant to inputs, fates, and effects of oil in the marine environment since the NRC 2003 report. An instructive collection of recent examples are provided by Stout and Wang (2016, 2018) and the recent review by Wise et al. (2022).

Advances in mass spectrometry leading to different types of commercially available and easier to use mass spectrometers coupled to GC or HPLC have enabled widespread adoption and significant technical advances, such as GC-orbitrap

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6 See https://www.canada.ca/en/environment-climate-change/services/canadian-environmental-protection-act-registry/agreements/related-federal-provincial-territorial/standards.html.

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
Image
FIGURE 2.11 Simplified flow diagram of extraction and chemical analysis of bulk oil, oil slick, emulsions, and environmental samples emphasizing long established methods of instrumental analysis and newer instrumental analysis methods.
NOTES: Not all newer and potentially useful instrumental analysis methods are shown. Black text and arrows describe various environmental samples and their handling, blue text and arrows show the pathways of different extraction processes, green text and arrows show different types of separations or isolation, brown text and arrows describe the main types of chemical compounds in a petroleum sample (saturates, aromatics, resins, and asphaltenes), and purple text describes the chemical analysis systems.

MS (Heska et al., 2021), multiple mass spectrometers connected in sequence such as GC-triple quadrupole MS (e.g., Szulejko and Solouki, 2002; Adikari et al., 2017). Advances in this field of analytical chemistry are documented by numerous papers in the scientific literature (e.g., in the journal Mass Spectrometry Reviews, Wiley online). A recent review by Wise at al. (2022), although focused on advances since the DWH oil spill, provides references from numerous studies other than those focused on the DWH.

Several of the advances in methodology have focused on a progressively more detailed analysis of the GC detected portion of petroleum, that is, what is termed the GC-amenable fraction (see Box 2.3). It is important to emphasize that most of the compounds in resins and all of the asphaltenes are not GC-amenable. This is emphasized because the GC-amenable fraction of an oil might represent only a portion—and sometimes a minor portion—of some spilled and chronic oil inputs. In general, the proportion of the GC-amenable fraction decreases as the asphaltene fraction increases over the continuum of light—medium—heavy oils.

Given the complexity of the mixture of GC-amenable compounds in fuel oils and crude oils, analysis by capillary GC will not completely resolve all the compounds present. Some of the compounds present in greater relative abundance will be recorded as peaks and can be quantified and identified by GC-flame ionization detector (FID) signal and/or mass spectrometry in a GC-MS system. As time increases, many

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×

other compounds of increasing molecular weight and boiling point elute. Because of the similarity of their structures, the signals as recorded by the detector, overlap with one another. This gave rise to the term “unresolved complex mixture” (UCM) for this signal as explained by Farrington and Quinn (2015). Even routine analyses with GC columns could not resolve that signal into individual compounds or peaks in many of the gas chromatograms. The UCM is especially prominent as a feature in some weathered and biodegraded fuel oils and crude oils.

An example is depicted in Figure 2.12 that shows glass capillary GC of Macondo crude oil from the DWH accident, a surface slick from June 2010, a beach sand pattie (see Chapter 5) from April 2011, and a rock scraping from June 2011 on the shoreline (Aal from Wise et al., 2022). This time sequence from fresh to weathered and biodegraded oil is typical of a spilled oil when lower molecular weight compounds are lost to evaporation, and the resolved peaks representing compounds such as n-alkanes and branched alkanes are biodegraded, leaving behind the UCM compounds of cyclic and branched cyclic alkanes. It is important to note that if the detector signal for the Macondo Well oil was expanded on the y-axis so that the resolved peaks were off the top of the scale, then the UCM would be apparent in that chromatogram. That is, those UCM compounds are present in the original oil but only become more apparent in the signal as the other compounds are removed by weathering and biodegradation processes.

GC-MS and GC-MS-MS, GC/FT-ICR-MS methods allow for identifying and quantifying some of the compounds that make up or co-elute with the UCM. Comprehensive two-dimensional gas chromatography (GC×GC) has resolved

Image
FIGURE 2.12 Capillary gas chromatograms of oil samples after the DWH oil spill: (a) original Macondo Well oil, (b) surface slick June 2010, (c) sand patty on beach April 2011, (d) rock scraping July 2011.
NOTES: The x-axis is increasing molecular weight as indicated by n-alkane carbon number. The y-axis is GC FID (flame ionization detector) signal intensity.
SOURCES: Courtesy of C. R. Reddy and R. K. Nelson, Woods Hole Oceanographic Institution (Wise et al., 2022).
Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×

the components as will be discussed below in more detail as an example of the advances in analytical methods relevant to the task and scope of this report.

The higher molecular weight and more polar fractions of crude oils and fuel oils are not amenable to conventional gas chromatography, that is, many of the resins and the asphaltenes had not been analyzed to an appreciable extent until developments in the past two decades involving Fourier transform-ion cyclotron resonance- mass spectrometry (FT-ICR-MS).

Examples of important advances in analytical chemical methodology relevant to the charge to this committee are summarized in Box 2.4 and explained briefly in the following paragraphs. A more detailed description of the methods is presented in Wise et al. (2022) and papers in Stout and Wang (2018).

2.2.2.2 Two-Dimensional Gas Chromatography (GC×GC) and GC×GC Mass Spectrometry (GC×GC-MS)

The analytical methodology of two-dimensional gas chromatography was introduced by Liu and Phillips (1991). Within a few years, Frysinger et al. (1999) had utilized the method to analyze benzene, toluene, ethylbenzene and xylenes, and total aromatic compounds in gasoline, Frysinger and Gaines (1999) had reported GC×GC use in analyzing petroleum, Gaines et al. (1999) had utilized the method for oil spill identification, and Frysinger and Gaines (2001) reported on the separation and identification of petroleum biomarkers.

The use of GC×GC methodology provides a significantly more comprehensive separation analysis of samples for petroleum chemicals compared to GC with glass capillary columns alone or interfaced with mass spectrometry. When combined with mass spectrometry, GC×GC-MS provides greater certainty in some cases of forensic analyses of sources of spilled oil (e.g., Lemkau et al., 2010; Nelson et al., 2016). In fates and effects assessments and research, the method provides significantly better chemical composition data to unravel or support important aspects of these assessments and research.

The basic principle of GC×GC is to utilize the resolving power of two sequential GC columns, the first of which has a nonpolar coating, to separate the compounds progressively mainly by boiling point/molecular weight. During that process, and with rapid periodicity, the effluent of that column is trapped by a freezing process that is then followed rapidly by a heating process. The heated compounds are “switched’ into a second GC column having a more polar interior coating that then progressively separates the groups of trapped compounds from the first column by their polarity properties, derived from their specific molecular structure. The effluents of the second column pass to a GC detector or, most often, to an interfaced mass spectrometer with data collected by an interfaced computer. This is explained in more detail by Nelson et al. (2016).

Examples of GC×GC separations of the samples shown in Figure 2.12 are depicted in Figure 2.13, in which all or the vast majority of the individual compounds have been resolved. Admittedly, these depictions are currently less familiar to many oil spill responders and non-chemistry researchers. The power of these sets of data is that various expansions of the two-dimensional data can be faithfully generated from the data sets. For example, in Figure 2.14, the GC×GC-FID (flame ionization detector) analyses data for the DWH or Macondo Crude oil are shown in both three-dimensional plots Figure 2.14A and in a two-dimensional plot Figure 2.14B, with color codes indicating the intensity

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
Image
FIGURE 2.13 Comprehensive GC×GC chromatograms of samples shown in Figure 2.12.
NOTES: The x-axis shows separation in the first dimension: molecular weight, depicted by n-alkane number. The y-axis is separation in the second dimension: polarity (a function of separation by a slightly less non-polar phase coating the second GC column), given as retention time in seconds. Evaporation front refers to loss of petroleum compounds of greater volatility to the left of the line by initial and ongoing evaporation.
SOURCES: Courtesy of C. R. Reddy and R. K. Nelson, Woods Hole Oceanographic Institution (Wise et al., 2022).

of the signal. Sections of these types of GC×GC data can be expanded to a zoomed-in view to provide more detail as is the case in Figure 2.15A for hopane petroleum biomarkers (A) and sterane petroleum biomarkers (B). Further explanations of shorthand notations in petroleum biomarker nomenclature are given by Philp (2018) and Peters et al. (2005).

Despite the high resolution and identification power of GC×GC and GC×GC-MS there have been only a few notable applications of the methodology to routine identification of spilled oil and to fate and effects assessments and research (e.g., the M/V Cosco Busan oil spill [Lemkau et al., 2010], the Hebei Spirit oil spill [Yim et al., 2012], and the DWH oil spill as noted in the Figures 2.13, 2.14, and 2.15 above). The reasons are explained in more detail by Górecki (2021) who addressed the lack of wider adoption of GC×GC in general in analyses for a multitude of chemicals in a variety of samples—not specifically the subject of this report. Górecki notes that a major impediment to wider use in general is the need to incorporate GC×GC and GC×GC-MS advances in standardized protocols. This applies in the case of responses to spilled oil and assessments of the accompanying fates and effects. This convinces and enables a wider range of laboratories in all sectors—government, industry, commercial analytical laboratories, spill response, environmental companies and academia—to invest in, and utilize, GC×GC and GC×GC-MS.

2.2.2.3 Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS)

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) is a powerful analytical instrument method for a variety of organic chemicals (e.g., Marshall et al., 1998). There has been substantial progress in the past two decades in the evolution of instrumentation and its applications. This has involved various configurations of varying powers of magnets used to generate the magnetic fields (e.g., see the review by Cho et al., 2014). These instruments have been interfaced in various configurations with GCs (GC-FT-ICR-MS) or

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
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Image
FIGURE 2.14 Comparison of three-dimensional surface rendering (mountain plot) (A) and a color contour plot plan view (B) for GC×GC-FID (flame ionization detector) chromatograms from the analysis of Macondo crude oil.
SOURCES: Courtesy of C. R. Reddy and R. K. Nelson, Woods Hole Oceanographic Institution (Wise et al., 2022).
Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
Image
FIGURE 2.15 GC×GC FID chromatograms of petroleum biomarkers from Macondo crude oil. A zoomed in view of a GC×GC FID Mountain plot (see Figure 2.14) of the region where the hopanoid petroleum biomarkers are shown in panel (A). A zoomed-in view of the diasterane/steranes petroleum biomarker region is shown in panel (B).
SOURCES: Courtesy of C. R. Reddy and R. K. Nelson, Woods Hole Oceanographic Institution (Wise et al., 2022).
Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×

high performance liquid chromatography (HPLC-FT-ICR-MS). Also, several types of ionization techniques have been developed. For example, Benigni et al. (2016) report the application of atmospheric pressure laser ionization (APLI) for the screening of PAHs in various samples. Kujawinski et al. (2011) utilized a combination of liquid chromatography—mass spectrometry and FT-ICR-MS to identify and quantify the dioctyl sodium sulfosuccinate (DOSS) introduced into the Gulf of Mexico water column plume by subsurface use of COREXIT 9500A during the DWH oil spill.

More detailed discussion of the many advances in these analytical chemistry methods is beyond the scope of this report. References cited in Cho et al. (2014) and Benigni et al. (2016) provide a reasonable introduction and overview for the advances in these analytical methodologies. The next section discusses briefly the importance of the use of one of the higher magnetic field FT-ICR-MS instruments to advance our knowledge related to inputs, fates, and effects of oil in the marine environment.

2.2.2.4 High Magnetic Field Fourier Transform-Ion Cyclotron Resonance–Mass Spectrometry (High Mag FT-ICR-MS) or Ultra High Resolution Mass Spectrometry

Since publication of Oil in the Sea III (2003), the development and application of an instrument involving high magnetic fields coupled with different ion sources (e.g. positive or negative ions) in high magnetic Fourier transform-ion cyclotron resonance-mass spectrometry (High Mag FT-ICR-MS) has expanded the analytical chemistry window to encompass the higher molecular weight compounds of oil composition beyond the GC-amenable fraction (described previously). High mag FT-ICR-MS provides high resolution insights into elemental composition from C10 to C100+ and heteroatom content of sulfur, oxygen and nitrogen (0–20+ atoms). This has enabled a major advance in our understanding of the elemental composition of higher molecular weight components of oils especially resins, asphaltenes, and also asphalt. Of equal, or perhaps greater importance, this has also enabled a major advance in our understanding of the processes important in the fates and effects of inputs. Specific advances are described in the fates and effects section of this report.

These advances have also set forth some challenges. While high resolution elemental compositions of thousands of compounds have been presented, the exact molecular configurations linked to exact chemical compositions are largely unknown.

There are only a few high-mag FT-ICR-MS instruments available worldwide. The National High Magnet Laboratory Facility7 at Florida State University–Tallahassee is funded as a user facility by the National Science Foundation, currently advancing research on fates and effects of spilled oil, among other research foci. There may be special cases of very high molecular weight oils and dilbit spills, used motor oil inputs, natural seeps, and chronic inputs of road runoff with asphalt chemicals and asphalt reaction products for which the method is well suited.

2.2.2.5 Pyrolysis GC-MS

Several advances in the utilization of pyrolysis GC-MS offer a complementary quantitative or semi-quantitative approach to analysis of some of the same higher molecular weight material that is analyzed by ultra high resolution or high mag FT-ICR-MS. Thermoslicing pyrolysis GC-MS is a specialized pyrolysis GC-MS method that provides interesting insights into those compounds that may be trapped or absorbed into the resin/asphaltene fraction during weathering. It also provides insights into molecular components of asphaltene and asphalt polymeric-type substances (e.g., Kruge et al., 2018; Seeley et al., 2018).

2.2.2.6 Carbon Isotope (13C, 14C) Measurements of Environmental Samples

The capability exists now to routinely measure isotopic ratios of the stable carbon isotopes 13C/12C and radioactive 14C in individual compounds or groups of compounds. Biogenic hydrocarbon sources have δ13C signatures that are different for marine organisms and for land plant sources. There are some distinctive signatures for δ13C among and between various hydrocarbons in petroleum that assist with identification of the presence of petroleum hydrocarbons and, on occasion, with forensics analyses of spilled oils.

Analyses of δ14C provides an assessment of whether a specific chemical (or group of chemicals) has a source in a fossil fuel (i.e., is devoid of a radioactive carbon signal due to decay over geological time) or is modern in origin (e.g., from recent biosynthesis). This also allows for assessing whether carbon from a fossil fuel source has become incorporated into the food web by assaying either δ14C of bulk organic material or δ14C specific classes of biological compounds such as carbohydrates, proteins, and lipids.

Similar approaches are now available for the utilization of stable isotopes of nitrogen, oxygen, and sulfur to elucidate reactions involving the fates of petroleum compounds in marine ecosystems. This adds to the methods appropriate for tracing and understanding biodegradation and photochemical reaction pathways, discussed in later sections of this report (Chapter 5).

2.2.2.7 Expanding the Utilization of the Above Advancements to Assessment of Fate, Effects, and Forensics of Inputs

There is understandable concern about rapidly introducing newer analytical methods into a portfolio of standard methods which have a proven track record in a regulatory and legal framework. There have been examples with

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7 See https://nationalmaglab.org.

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
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GC×GC-MS where the use of this newer methodology has enabled a more complete forensic type analysis of spilled oil (e.g., Nelson et al., 2016) or sources of a collection of pelagic tar balls (Green et al., 2018).

It is recognized that there are trade-offs in the use of any given analytical methodology, such as instrument cost and availability, time of analysis, ease of data interpretation including requisite software for the interfaced computers, and other factors. The issue of analysis time and ease of data interpretation are often important factors for forensic analysis after sampling of oil spill samples and potential sources. For example, a recent exercise using a combination of GC-MS and ion mobility spectrometry-mass spectrometry (IMS-MS) in a table-top exercise demonstrated the feasibility of utilizing IMS-MS (minutes for analysis) compared to GC-MS (1 to 2 hours) to obtain comparable results (Roman-Hubers et al., 2021).

The advances in analytical chemistry methods described previously require judicious choices, depending on the questions being asked related to the samples to be analyzed, such as how rapidly answers are needed, whether the question is related to forensics, other regulatory activities, or research related to inputs, fates, and effects. For the arenas of forensics and regulatory activities, appropriate regulatory authorities should conduct periodic reviews every few years to ascertain which of the newer methods should be introduced into the portfolio of recommended methods for oil spill forensics and other regulatory activities. This should be accompanied by increased availability of appropriate standard reference materials and quality assurance and quality control interlaboratory and intra-laboratory exercises.

2.2.2.8 The Challenges with Reporting of Petroleum and Other Hydrocarbon Concentrations in Environmental Samples

The reporting of hydrocarbon concentrations or sums of hydrocarbons in oils and environmental samples is common in oil pollution studies. The difficulties arise when terms such as total petroleum hydrocarbons (TPH), total alkanes, or total polycyclic aromatic hydrocarbons (PAHs) are used without a clear specification of the identities of the compounds included and their individual concentrations. This becomes a very important problem when attempting to compare studies of fates and effects of the same oil and the same oil at various stages of weathering and biodegradation (see Chapters 5 and 6) in which concentrations of TPH, total alkanes, and total PAHs are reported that contain summations of different mixtures of analytes. The problem becomes even more complicated when comparing fates and effects for different oils, or for differentiating the presence of given concentrations of PAHs from petrogenic or pyrogenic sources, or mixtures.

One approach that some studies have taken is to specify “target analytes,” for example a given set of alkanes, cyclic alkanes, and/or PAHs that the analysts suspect could be present in the samples and can be separated and quantitatively assessed by the analytical methods employed. There is a need for general agreement among relevant agencies, organizations, and groups of scientists with respect to the chemicals to be measured and what group of compounds to include in summations (i.e., total of some specific group of chemicals). Such agreements should maximize the contributions to understanding and assessing fates and effects of spills and inputs and enable advantageous use of analytical chemistry methods for identification and tracking of spills and inputs within a forensic setting. The forensics needs may emphasize different sets of chemicals and therefore different types of measurements than assessments of fates and effects. All of the preceding most likely will vary depending on the type of oil spilled or variations in other types of input.

Irrespective of what methods are used and which compounds or summations of compounds are reported, there is a need for adequate quality assurances and quality control (QA/QC) for sampling and analytical methods within and between laboratories.

Quality Control and Quality Assurance

The need for QA/QC in applications of the analytical chemistry methods have been recognized for decades and implemented in recommended standard methods. Recent reviews have set forth the state of knowledge (e.g., Murray et al., 2015; Litman et al., 2018; Wise et al., 2022, and references therein).

The preparation and curation of standard reference materials, their utilization in QA/QC interlaboratory comparison exercises for the analyses of selected petroleum oils and petroleum and fossil fuel combustion PAHs in bivalve molluscs and surface sediments have been ongoing since the late 1970s (Kimbrough et al., 2008, and references therein; Wise et al., 2022, and references therein). Similar types of samples and/or interlaboratory comparison exercises have been ongoing within the international community during that same period of time. An example is an International Council for the Exploration of the Seas/International Oceanographic Commission (ICES/IOC) intercomparison exercise (Farrington et al., 1988, and references therein).

Interlaboratory comparisons and instrument calibration can be done using standard reference materials. These are mixtures of compounds with accepted composition and properties. Repositories of standard reference materials are maintained by various entities. See NIST (2021), among others, for details on the importance and usage of standard reference materials.

This has become standard protocol for official assessments such as those for forensics in oil spill identification and for activities such as NRDA by groups reporting to official trustees for a given spill in the United States. This is also the case for similar activities in Canada. It is becoming more common among academic and other independent researchers as is appropriate.

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
Authentic Samples of Sources of Spills and Inputs

An essential aspect of forensics and regulatory activities, and research studies of the fates and effects of any oil input to the environment, is to collect samples of original spilled oil or other sources of oil to the extent practicable. These samples are useful in forensic analyses to trace the geographic and temporal extent of a given input using petroleum biomarker compounds. Collections of potential sources of a mystery spill can also be used in forensic analyses to tie the mystery spill to a specific source or, at the very least, eliminate some potential sources. Stout and Wang (2016) provide an interesting, relevant collection of examples.

“Big Data” Issue

Sampling and analytical methods for forensic analyses and for assessing the fates and effects of oil spills and other inputs are now generating large sets of data because of the increased ability to analyze for more of the myriad compounds of interest. As with other sciences, oil pollution studies have entered the realm of “big data.” It is not only in the analytical chemistry data, but data related to “omics” (see Chapter 5) and Effects (see Chapter 6). A related issue is bringing the most relevant “big data” together for the same samples. The inclusion of metadata such as sampling location, time, sample type, and other related environmental information into these data bases is essential.

While expense is often cited for the reasons for not bringing the most appropriate chemistry and biological analysis methods together for the same samples, the inefficiency connected with the “cost” of lost knowledge should enter the considerations in a more appropriate manner.

The need for appropriate data archives, readily accessible for all interested users is clear (e.g., McNutt et al., 2016). NOAA’s DIVER database (NOAA, 2021e) and the recent Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) database (Gibeaut, 2016; GoMRI, 2021) are examples of a response to this need. Science and technology are poised for important expansion of content and interactive data archives for chemical analyses interfaced with archives of inputs, fates and effects data as noted in Chapters 5 and 6.

2.3 THERMODYNAMICS OF MIXTURES OF OILS

As described previously, oils are complex mixtures of molecules, each molecule having its own unique properties. At the same time, these molecules mix together into fairly homogeneous fluids that have their own properties, including mixture density, viscosity, interfacial tension, and others. These properties can be measured when samples are available. However, there are many instances where these properties need to be estimated, as for example in predicting the behavior of a spilled oil during its transport and fate in the marine water column.

The purpose of a model for the thermodynamic behavior of an oil is to predict its bulk properties and its interactions with seawater. Important bulk properties include density and viscosity; some models may also predict the gas–liquid phase partitioning. When released in seawater, it is also critical to predict interfacial tension, potential for hydrate formation, and the solubility and diffusivity of individual components of the oil. Each of these properties is thermodynamic, meaning that they depend on the composition of the oil–seawater system and its thermodynamic state, typically defined by the temperature and pressure. Some of these properties are important themselves for determining oil behavior; others are inputs to additional process models for predicting fate effects, such as breakup of oil and gas into droplets and bubbles, mass transfer from oil or gas to a dissolved state in seawater, and many other processes (see Chapter 5).

Irrespective of the thermodynamic model used, oil composition is normally described by quantifying the abundance of both individual molecules and groups of molecules having similar behavior. When oil composition is simplified by grouping molecules together, these groups are called pseudo-components. To make predictions, oil property models require additional data for several physical properties of each compound or pseudo-component. These properties may include molecular weight, boiling point, or critical point properties, among others, depending on the needs of the model. Using the analytical methods described previously, the molecular composition of oils can be known in great detail. Because of the added burden of also quantifying the physical properties of each of these compounds, models of oils may be based on individual molecules up to about n-C8 at most (Gros et al., 2016). For longer-chain hydrocarbons, pseudo-component groups become more efficient than itemizing all possible chemical structures individually.

Pseudo-components are most often composed of molecules having a similar fate. Solubility, octanol and Kow, biodegradation rate, and boiling point are common properties used to distinguish them (McKay, 2003). For example, these groups may be distinguished by boiling point and SARA identification. More recently, Gros et al. (2016) identified pseudo-components by common regions in two-dimensional gas chromatography (GC×GC) spectrograms. They also present methods to compute several important properties for each pseudo-component using property correlations and group contribution methods. However, it is not always necessary to group molecules having similar fates in the aquatic environment. For example, the equation of state for DWH crude oil developed by Zick (2013) and used in the court case of the United States against BP (British Petroleum) grouped benzene and n-hexane together into a single pseudo-component despite these compounds having solubilities that differ by a factor of 151. Rather than predicting the oil fate, the purpose of the Zick (2013) model was to predict fluid phase density and gas–liquid phase equilibrium, for which this pseudo-component selection was appropriate. Irrespective of how pseudo-components are defined or their properties estimated, an oil mixture is defined by stating the compound and pseudo-component groups and quantifying the mass fraction of the whole petroleum fluid within each group.

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×

Property models for oil normally fall into two broad categories: thermodynamic equations of state (EOS) or correlation methods. Thermodynamic EOSs have developed significantly since the 1970s within the petroleum chemistry and engineering fields, where they are mostly applied within systems having limited exposure to seawater (McCain, 1990). Common practical models are variations of the cubic equations of state (see the review by Valderrama, 2003). Recently, these models have also been adapted to predict the behavior of natural gas (McGinnis et al., 2006) and oil in the oceans (Gros et al., 2016, 2017; Dissanayake et al., 2018). Though powerful, cubic equations of state require significant input data for each component and are limited to predicting some of the fundamental properties of a fluid mixture, including density and component fugacity, hence, solubility. The alternative correlation methods include group-contribution methods and property correlations. Group-contribution methods predict oil properties based on discrete contributions of individual molecular components within a compound. As these methods provide component properties, these may be used to estimate the input data required by the cubic equations of state (e.g., Gros et al., 2016). The Poling et al. (2001) method for predicting the critical point pressure of a compound is a good example. Property correlation methods similarly develop empirical relationships between the properties of a given molecule or pseudo-component, for example, the molecular weight and boiling point, and another property, such as the vapor pressure. These methods are especially important for estimating oil properties not predicted by the cubic equations of state, such as viscosity, interfacial tension, and diffusivity in seawater.

Current oil spill models differ mostly in their approaches to predicting density, phase equilibrium, and component solubility of an oil mixture: some use thermodynamic EOS and others use property correlations for density and solubility with database lookup methods for phase equilibrium. An example using the thermodynamic EOS approach is presented in Gros et al. (2016). They applied the Peng–Robinson (PR) cubic EOS (Peng and Robinson, 1976) with volume translation (Péneloux et al., 1982) to various compositions of the DWH oil. The PR EOS predicts the density of the gas and liquid phases of the mixture and the fugacities of each component in the mixture given the component properties of the molecular weight, acentric factor, critical point temperature, pressure and volume, and the binary interaction coefficients. The gas–liquid phase equilibrium is determined by adjusting the estimated partitioning of components into the gas and liquid phases at a given thermodynamic state and iterating until the fugacities of each component in each phase converge (Michelsen and Mollerup, 2007).

Using the alternative property correlation method, the density of the liquid-phase petroleum is reconstructed from estimates of the component densities and their mass fractions in the whole oil. The gas-liquid phase equilibrium would normally be read from a thermodynamic table of properties, developed by way of another EOS, such as the Gros et al. (2016) or the Zick (2013) model of a crude oil (e.g., French-McCay et al., 2015) or based on laboratory measurements.

Using the EOS approach, the solubility of each component is derived by applying the modified Henry’s law (King, 1969; Dhima et al., 1999) in which the partial pressure is replaced by the fugacity coefficients. The Henry’s constant for each component at standard conditions is adjusted to the in situ thermodynamic state using standard corrections for temperature, pressure, and salinity that depend on the component properties of the partial molar volume at infinite dilution, the enthalpy of transfer from the gas phase to the liquid phase, and the Setschenow coefficient (Gros et al., 2016). Gros et al. (2016) reported each of these required model inputs for 131 individual molecules identified in the DWH oil and for 148 additional pseudo-components defined from the GC×GC-FID chromatogram and simulated distillation data. To estimate the properties for many of these components, they present several group-contribution and property correlations. Similarly, Gros et al. (2018) present a method to estimate the required parameters from distillation data. Through rigorous comparison to available laboratory data, they conclude that this model and its parameter estimation techniques is valid for pressures corresponding to ocean depths ≤2,500 m, temperatures between −2 and 30°C, and salinities up to 35%.

An alternative and more direct approach to estimate solubility using property correlations was introduced by Mackay and Leinonen (1977). There, the solubility of each component of an oil is given by the product of the solubility of the pure component in seawater, the mole fraction of that compound in the whole oil, and the solubility enhancement factor that accounts for non-ideal behavior of the petroleum liquid. Mackay and Leinonen (1977) report four values of the solubility enhancement factor, corresponding to components containing alkanes, cyclic alkanes, aromatics, and olefins, respectively. In situ effects of temperature, pressure, and salinity are captured through the pure-component solubility estimates; non-ideal, real-fluid effects are contained in the solubility enhancement factors. Many other similar methods exist. For example, Lehr et al. (2002) predict pure-component solubility for >C6 aromatics based on the molecular weight and component density. They then estimate the solubility of each component from the whole petroleum liquid using the oil–water partition coefficient, which they compute from a correlation with the pure-component solubility. An implicit assumption of these methods is knowledge of the oil-phase composition, which may have to be linked to an EOS model when the release involves oil in equilibrium with gas.

Because pure compounds, such as benzene, have a constant chemical activity regardless of thermodynamic state, their solubilities in seawater can be reported in thermodynamic tables, for instance as a function of temperature and pressure. When these compounds are present in a petroleum mixture, however, their activity and hence, solubility, are altered as described previously, and solubility data become composition-dependent. The situation is made even more complicated when considering reservoir fluid versus dead

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×

oil (see Section 2.2). The composition of fluids within a petroleum reservoir are in equilibrium with the local temperature and pressure, both of which are typically very high. In this state, a significant light fraction of the mixture, for example, methane, ethane, propane, and so on, may be dissolved in the liquid petroleum phase. Such a liquid is commonly termed live. Dead oil, by contrast, is the liquid petroleum composition after coming to equilibrium with standard conditions in a closed system; in this state, little of the gas fractions remains dissolved in the oil. Whether the oil enters the ocean as dead or live oil, hence, can significantly affect the solubility of all fractions of the oil. Moreover, the composition of the liquid phase depends on the chain of gas–liquid separations occurring after release. For example, once liquid oil droplets separate from gas bubbles, further evolution of the liquid phase composition will depend on this discrete composition and not on the composition of the whole oil, upstream of the initial separation into droplets and bubbles. Phase separation is discussed in more detail in Section 2.2 and the diversity of fate processes in Chapter 5. The effect of these processes on the solubility of benzene from various oil compositions is illustrated in Box 2.5.

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×

Because of the recent advances in predicting oil properties and their importance to fate and effects (e.g., evolving oil compositions, solubilities of complex mixtures, phase equilibria, and so on), databases of more complete oil composition data are needed. These could include elemental composition to n-C8 or lighter compounds and complete SARA analysis. Results of GC or GC×GC output in standardized formats could also be housed within the repository. The committee recognizes that these data may not be available for spills from exploration wells and that some of these data are proprietary for producing wells. However, there are existing databases of oil properties for well-known oils, and where these more comprehensive analyses can be made public, they should be stored in standardized data formats in dedicated public repositories.

2.4 CONCLUSIONS

Conclusion—Analytical Chemistry Methodology: There have been significant advances in analytical chemistry methodology utilized in forensics pertaining to oil spills and other inputs of petroleum to the marine environment and assessing and understanding the fates and effects of such inputs. Previous advances of the 1980s–1990s such as glass capillary–mass spectrometry (GC-MS) have been utilized routinely. Recent advances in analytical chemistry methodology such as glass capillary two-dimensional gas chromatography–mass spectrometry (GC×GC-MS), GC Orbitrap MS, GC-MS-MS-MS, FT-ICR-MS, and high magnetic field FT-ICR-MS have been tested in several studies and are ready for more routine use to provide much needed more detailed chemical compositional data pertinent to the questions being asked in forensic analyses for spilled oil and chronic inputs, and understanding fates and effects of various inputs of oil to the environment.

Conclusion—Elemental Composition: Remarkable insights have been gained into the elemental compositions and classes of compounds in resins, asphaltenes, and asphalt, as well as reaction products of processes governing the fate of oil spills and other petroleum inputs by utilization of FT-ICR-MS and in particular by ultra high resolution high magnetic FT-ICR-MS. Despite these advances, the exact molecular structures of these thousands to tens of thousands of chemicals have yet to be elucidated and are one of the most important challenges of analytical chemistry relevant to the subject of this report.

Conclusion—Big Data: Utilization of the advances in analytical chemistry methods yield large databases of concentrations and relative abundances of petroleum oil chemicals and reaction products resulting from physical, chemical, and biological processes acting on such chemicals in the environment. There are existing databases such as those maintained by NOAA and Environment and Climate Change Canada (as noted in this chapter) that might be expanded to meet the need for archiving these datasets in an accessible and useful manner. It is importance to incorporate appropriate metadata for the samples in these larger databases.

Conclusion—Reporting of Chemical Composition: The reporting of concentrations of individual chemicals, and summations of chemicals of different compositions, varies among and between scientific papers and reports. This can lead to inappropriate comparisons of different compositions of chemicals when elucidating fates and effects results between and among different studies and assessments.

Conclusion—Modeling: Models to predict the properties of petroleum fluids, including mixtures with gas, have improved recently, buoyed partly by the better elucidation of the composition of petroleum mixtures using advanced analytical methods.

Conclusion—New Fuels: Continued attention to the chemical compositions of biofuels and other evolving fuels as their production and use expands will inform response actions and the potential fates and effects of such spills or chronic inputs.

Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×

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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
Page 48
Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
Page 49
Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Suggested Citation:"2 Petroleum as a Complex Chemical Mixture." National Academies of Sciences, Engineering, and Medicine. 2022. Oil in the Sea IV: Inputs, Fates, and Effects. Washington, DC: The National Academies Press. doi: 10.17226/26410.
×
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Oil and natural gas represent more than 50 percent of the worldwide energy supply, with high energy demand driven by population growth and improving standards of living. Despite significant progress in reducing the amount of oil in the sea from consumption, exploration, transportation, and production, risks remain. This report, the fourth in a series, documents the current state-of-knowledge on inputs, fates and effects of oil in the sea, reflecting almost 20 additional years of research, including long-term effects from spills such as the Exxon Valdez and a decade-long boom in oil spill science research following the Deepwater Horizon oil spill.

The report finds that land-based sources of oil are the biggest input of oil to the sea, far outweighing other sources, and it also notes that the effects of chronic inputs on the marine environment, such as land-based runoff, are very different than that from an acute input, such as a spill. Steps to prevent chronic land-based oil inputs include reducing gasoline vehicle usage, improving fuel efficiency, increasing usage of electric vehicles, replacing older vehicles. The report identifies research gaps and provides specific recommendations aimed at preventing future accidental spills and ensuring oil spill responders are equipped with the best response tools and information to limit oil’s impact on the marine environment.

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