(i) Alkanes (paraffins)

Alkanes are straight-chain normal alkanes and branched iso-alkanes with the general formula C_nH_2n+2. The major paraffinic components of most crude oils are in the range C₁ to C₃₅ (Speight, 1980), although smaller quantities of alkanes up to C₆₀ or higher may be present. Crude oils vary widely in alkane content (Dickey, 1981). The ratio of n-alkanes to isoalkanes is shown in Table 1 for one crude oil sample (Ponca). The ratio ranges from a minimum of 1.7 for heptanes to a maximum of 6.9 for octanes (Speight, 1980). A Pennsylvania crude oil sample had n-alkane:isoalkane ratios of 1.3, 1.7 and 1.5 for pentanes, hexanes and heptanes, respectively (Tiratsoo, 1951). Alkenes are not generally found in crude oils (Speight, 1980).

Table 1

Alkanes isolated from a crude oil sample.

(ii) Cycloalkanes (naphthenes)

Cycloalkanes (or cycloparaffins), also called naphthenes in the petroleum industry, are saturated hydrocarbons containing structures with carbon atoms linked in a ring. The cycloalkane composition in crude oil worldwide typically varies from 30% to 60% (see also Table 3). The predominant monocycloalkanes in crude oil are in the cyclopentane series, having five carbon atoms in the ring, and in the cyclohexanes, having a six-membered ring. The most predominant monocycloalkanes and their composition ranges in crude oil are shown in Table 2 (Bestougeff, 1967). In the higher boiling fractions, such as lubricating oils, cycloalkanes with two or more rings are common, and structures containing up to ten rings have been reported. These polycyclic structures are usually composed of fused five- and six-membered rings (Table 2; Mair, 1964).

Table 3

Composition and physical characteristics of three crude oils.

Table 2

Predominant cycloalkanes isolated from crude oil.

(iii) Aromatic hydrocarbons

The most common aromatic compounds in crude oils are benzene (see IARC, 1982, 1987a), benzene derivatives (e.g., alkylbenzenes) and fused benzene ring compounds. The concentration of benzene in crude oil has been reported to range between 0.01% and 1% (Bestougeff, 1967). Table 3 shows the overall composition of three crude oil samples, including the major classes of aromatic hydrocarbons, and Table 4 gives the levels of seven specific polycyclic aromatics in two of these samples (National Research Council, 1985).

Table 4

Concentrations of individual polynuclear aromatic hydrocarbons in crude oil (10^-6 g/g oil).

(i) Sulfur compounds

Crude oils vary widely in sulfur content, which can range from <0.1% to 10% by weight. The following types of sulfur compounds have been identified in crude oils: thiols (mercaptans), sulfides, disulfides and thiophenes (Costantinides & Arich, 1967).

In the lower distillation range up to about 150°C, the most abundant sulfur compounds are thiols. In the 150–250°C distillation range, the most abundant compounds are thiocyclo-, thiobicyclo- and thiotricycloalkanes and thiophenes. These sulfur compounds are replaced, in turn, by benzothiophenes and more complex ring structures in the higher distillation ranges (Costantinides & Arich, 1967).

(ii) Nitrogen compounds

The nitrogen content of crude oils ranges from trace amounts to 0.9% by weight. The bulk of the nitrogen in fractions that boil below about 200°C is basic nitrogen. The basic nitrogen compounds often found in crude oils include pyridines and quinolines, e.g., 3-methylpyridine and quinoline, while nonbasic nitrogen compounds include pyrroles, indoles and carbazoles, e.g., carbazole, and amides (Costantinides & Arich, 1967).

(iii) Oxygen compounds

The oxygen content of crude oils ranges from 0.06% to 0.4% by weight, the majority of components being alkane and cycloalkane (naphthenic) acids. Other minor components include ketones and phenols (Costantinides & Arich, 1967). The oxygen content of crude oils increases with boiling range, so that more oxygen-containing compounds are found in distillates that boil above 400°C.

(iv) Metal-containing compounds

Traces of many metallic compounds can be found in crude oils. Nickel (see IARC, 1976, 1987b) and vanadium compounds have been identified in crude oils at levels ranging from a few parts per million to 200 ppm (mg/kg) nickel and up to 1200 ppm (mg/kg) vanadium. These metals occur primarily as complexes (porphyrins; Costantinides & Arich, 1967) which are stable and can be distilled at temperatures above 500°C.

Table 5 is a compilation of some other trace elements reported in crude oil and their typical concentrations either in crude oil or in crude oil ash (Magee et al., 1973; Valković, 1978). Most of these elements occur naturally in crude oil as a result of their presence in the rock formation or in salt-water deposits from which the crude oil was drawn, although some

Table 5

Elements in crude oil.

may also be introduced during the process of drilling, pumping, preparing and transporting crude oil to a refinery.

(v) Miscellaneous contaminants

Crude oil, as it emerges from the well-head, is typically a heterogeneous mixture of solids, liquids and gases, including, in addition to the constituents described above, sand and other sediments, water and water vapour, salts and acid gases such as hydrogen sulfide and carbon dioxide. These contaminants are at least partially separated from the crude oil in surface treatment at the well-head (see p. 132) to prepare it for transportation to the refinery (Baker et al., 1986a).

Crude oils are not analysed routinely for their content of various classes of hydrocarbons and nonhydrocarbons; rather, they are usually characterized by their physical properties (specific gravity or density, viscosity) and their sulfur content. Crude oils are also characterized in pilot-scale distillations by the volume or weight percentage in various boiling-point ranges (‘straight-run fractions’).

One of the most important physical properties of crude oil is its specific gravity —the ratio of the density of oil to the density of water, both taken at the same temperature and pressure. From the specific gravity, the ratio of aromatic (high density) to saturated (low density) hydrocarbons in crude oil samples may be estimated. An alternative expression for specific gravity, developed for petroleum applications, is:

The specific gravities of petroleum usually range from about 0.8 (45.3°API) for the lighter crude oils to over 1.0 (10°API) for the heavier asphaltic crude oil (Dickey, 1981).

Crude oil is also characterized by its viscosity. Viscosity is expressed in Saybolt universal seconds (SUS) at 38°C. This value is determined by the time it takes for 60 cm³ of crude oil to flow by gravity through an orifice in a calibrated viscometer (Dickey, 1981). Viscosity may also be expressed in centipoises.

Sulfur content is the third important property of crude oil because of its effect on the refining process (in poisoning catalysts) and the malodorous and toxic properties of hydrogen sulfide and other sulfur compounds. Table 6 gives the API gravity, sulfur content and viscosity of several crude oils.

Table 6

Characteristics of some typical crude oils.

Table 7 summarizes the composition of crude oils throughout the world, based on analysis by the US Department of Energy of 800 crude oil samples from 691 major oilfields in the USA (Coleman et al., 1978) and on analysis by the US Bureau of Mines of the Department of the Interior of 169 samples of crude oil from 122 fields in 27 countries outside the USA (Ferrero & Nichols, 1972).

Table 7

Summary of worldwide crude oil compositions and characteristics.

(i) Preparing the well

The production operation begins after the well has been drilled and has been evaluated as being economically favourable for production. Pipe or casing is inserted into the well bore in a concentric series to prevent contamination by fresh water, loss of circulation, sloughing or charging of shallow sands with abnormal pressures (American Petroleum Institute, 1983). The first such casing placed into a well is the conductor pipe, which may be pile driven or cemented into place and may extend to a depth of 150–1500 m.

The conductor pipe and all other casings are attached at the surface to the casing head (American Petroleum Institute, 1983). Surface casing is inserted through the conductor pipe and deeper into the well to prevent underground formations of fresh water from becoming contaminated with well fluids and to provide a mechanism for controlling the flow of fluid from the well.

(ii) Pumping the crude

Once the well has been completed, oil begins to flow up the well as a result of the inherent reservoir energy, which is manifested by an oil displacement process involving water, gas or a combination of both. Reservoir drive mechanisms — the processes by which the reservoir energy displaces the crude oil — include dissolved-gas drive, gas-cap drive, water drive and combination drive (Baker et al., 1986a; Gray, 1986). Natural drive mechanisms may, at some point in the economic life of the well, lose their inherent energy and the well will require a mechanical force to draw the oil from the reservoir. The common methods of artificial lift are surface pumping, submersible pumping and gas lift (Baker et al., 1986a; Gray, 1986).

The natural and artificial lift mechanisms provide a means of raising reservoir fluids capable of flowing into the well bore. However, fractures, channels and perforations through which the fluids flow often become blocked and diminish the production capacity of a well. These passages may be cleared and new ones created by using reservoir stimulation techniques such as acidizing and hydraulic fracturing (Baker et al., 1986a).

Acidizing is the process of treating the formation — limestone or dolomite — with hydrochloric, acetic or hydrofluoric acid. Additives such as corrosion inhibitors, surface active agents, sequestering agents and antisludge agents are mixed with the acids to prevent acid attack on tubing and casing, to help disperse the acid in the formation, to prevent precipitation of ferric iron during acidizing and to prevent formation of insoluble sludge (Giuliano, 1981; Baker et al., 1986a).

Hydraulic fracturing is used extensively and successfully on formations composed of sandstone. A fluid, such as water charged with nitrogen, is pumped under high pressure at high rates into the well to create deep penetrating fractures in the reservoir. Charging the water with nitrogen facilitates the flow of water back out of the well (Giuliano, 1981; Baker et al., 1986a).

(iii) Surface treatment

When the crude oil has been brought to the surface, the final production step is to reduce it to the form in which it will be sent to the refinery for processing. Contaminants such as sediment and water are removed, and volatile components are separated and treated by the use of separators (Giuliano, 1981; Baker et al., 1986a; Gray, 1986).

Natural gases must be treated to remove water vapour and acid gases such as carbon dioxide and hydrogen sulfide. Water vapour may be removed by bubbling the gas through a solid or liquid desiccant; the acid gases may also be removed from a natural gas stream by adsorption or absorption with an appropriate liquid or solid desiccant. This process of removing acid gases from a natural gas stream is commonly referred to as sweetening (Baker et al., 1986a).

(iv) Transportation and storage

The primary means of transporting crude oil are tankers and pipelines; trucks and railways fulfil much smaller yet significant roles. Barges are used to transport oil on inland waterways and to off-load large tankers.

Modern tankers carry over two-thirds of all crude oil produced to modern industrial societies (Baker et al., 1986b). Oil can be loaded onto tankers either from onshore facilities after transport from inland fields or from offshore platforms. The single-point (or single-buoy) mooring system is a common method for loading tankers. Oil is pumped from an offshore or onshore facility through a pipeline on the ocean bed to a marine riser which is suspended at the surface by a large mooring buoy. The oil is passed from the pipeline into a flexible hose connected to the riser, through the riser to a floating base and from there to the ship (Giuliano, 1981).

An individual oil field may contain several hundred wells. Flow lines connect individual wells in an oil field to field storage tanks and transport oil to a central location for treatment, testing and measurement. Following treatment, oil is transported from a central tank battery by intermediate ‘gathering’ lines which, like the flow lines, generally range from 5 to 30 cm in diameter (Giuliano, 1981; Baker et al., 1986b).

Pumps at a pump station move the oil into and through a pipeline. A gathering station in or close to an oil field receives oil from producers' tanks via a pipeline gathering system and moves it on to a trunk-line station located on the main ‘trunk’ line. Trunk lines are large-diameter (up to 120 cm) pipelines that carry oil over long distances to refineries, central storage or ports. Booster pump stations are placed along the trunk line as necessary to compensate for loss of pressure as the oil is moved through the line (Giuliano, 1981; Baker et al., 1986b).

Tank farms may be located along pipelines, where oil can be temporarily side-tracked from transit for holding, sorting, measuring or rerouting. A tank farm may function as a receiving station for oil that is to be moved into the pipeline transportation system. Pipelines from a tank farm converge at a station manifold which can split, merge or reroute the flow of oil as needed (Baker et al., 1986b). Highly viscous crude oil can be heated and transported via an insulated pipeline, along which reheating stations may be employed (Watkins, 1977).

Deposits accumulate on the inside wall of a pipe during the course of operations. Some crude oils deposit substantial coatings of wax on cooling; salts and other foreign materials may also build up. To clean the pipeline and remove deposits, ‘pigs’ equipped with scrapers and brushes are run through it periodically, entering and leaving via locks or pig traps, so that the line can continue to operate under pressure (Anderson, 1984).

Crude oil is also transported by truck, especially from new fields where pipeline gathering lines have not been built. However, motor carrier transport represents only a small fraction of US domestic transportation of crude oil, accounting for less than 0.3% of that total in 1982. An even smaller percentage (0.05% in 1982) of domestic crude oil transportation is by rail. Rail tank cars are used to move crude oil from ocean tankers or waterways to small inland refineries (Baker et al., 1986b).

(v) Production volumes

World crude oil production from 1947 to 1986 is shown in Table 8 by geographical region. Over the past 40 years, production has increased more than seven fold, from 3000 million barrels to 22 000 million barrels per year. Table 9 gives production data for the 20 countries that produced the most crude oil in 1976 and 1986.

Table 8

World crude oil production, 1947–86 (millions of barrels per year)^a.

Table 9

World crude oil production (thousands of barrels per year): 20 leading regions.

In 1986, proven worldwide reserves of crude oil were estimated to be 700 000 million barrels (Table 10).

Table 10

Proven reserves at end 1987.

Toxic effects

Oral administration of Prudhoe Bay crude oil (5.0 ml/kg bw daily for two days) to male Charles River CD-1 mice resulted in increases in liver weight, hepatic proteins, RNA, glycogen, total lipids, cholesterol, triglycerides and phospholipids (Khan et al., 1987a).

Epidermal ornithine decarboxylase was induced following application of Prudhoe Bay crude oil to the backs of female Charles River CD-1 mice; a maximal induction of over 60 fold was seen 6 h after application of 50 µl. Concurrently, epidermal putrescine levels were elevated 4.7 fold over those in controls. Intraperitoneal administration of the crude oil led to an increase (15–20 fold, maximal activity 12 h following administration of 4 ml/kg bw) in hepatic ornithine decarboxylase activity but to a 45% decrease in the renal enzyme activity. Hepatic putrescine levels were elevated 34 fold over those in controls (Rahimtula et al., 1987).

Application of Kuwaiti crude oil (0–200 µg) to the skin of male Sprague-Dawley rats increased dermal benzo[a]pyrene 3-hydroxylase by 15 fold and diphenyloxazole hydroxylase by six fold (Rahimtula et al., 1984).

Platelets isolated from male Sprague-Dawley rats 24 h after oral treatment with a Prudhoe Bay crude oil showed a substantial inhibition of aggregation induced by adenosine diphosphate, arachidonic acid or epinephrine (Chaudhury et al., 1987a). Inhibition of aggregation was effected with as little as 0.1 ml crude oil/kg bw (Chaudhury et al., 1987b). Aggregation was also inhibited by aliphatic, heterocyclic and aromatic fractions of the crude oil (Chaudhury et al., 1987a).

Alteration in cellular calcium sequestration has been postulated to be a primary mechanism in initiating irreversible cell damage. Administration of 5 ml/kg bw Prudhoe Bay crude oil intraperitoneally or orally daily for two days to male Sprague-Dawley rats that were sacrificed 24 h later resulted in an abrupt drop in liver mitochondrial and microsomal adenosine triphosphate-dependent calcium uptake. In-vitro incubation of either mitochondria or microsomes with dimethyl sulfoxide (DMSO) extracts of the crude oil resulted in a concentration-dependent inhibition of calcium influx. The release of calcium from calcium-loaded mitochondria and microsomes was also observed in the presence of the crude oil extract. At concentrations which affect calcium sequestration, the crude oil extract produced swelling of mitochondria. Microsomal adenosine triphosphatase activity in the presence or absence of calcium was unaffected by the crude oil. The results indicate that increased permeability of the mitochondrial and microsomal membranes to calcium is a contributing factor in the inhibition of calcium uptake by Prudhoe Bay crude oil (Khan et al., 1986).

Administration of a single oral dose (5-10 ml/kg bw) of Prudhoe Bay crude oil to pregnant Sprague-Dawley rats resulted in induction in maternal hepatic microsomal cytochrome P450 levels and various monooxygenases in a dose-dependent manner after 24 h. Maximal induction of glutathione S-transferase, uridine 5'-diphospho (UDP) glucuronyl-transferase and DT-diaphorase (NADH, NADPH quinone oxido reductase) activities were observed 72 h after administration of the crude oil (Khan et al., 1987b).

Many studies on the toxic effects of crude oil in non-laboratory mammals, birds, and aquatic organisms have been reported and reviewed (Rice et al., 1977; Engelhardt, 1984; Holmes, 1984; Engelhardt, 1985; Leighton et al., 1985; Payne et al., 1987).

Effects on reproduction and prenatal toxicity

The effects of petroleum and petroleum products on reproduction have been reviewed (Schreiner, 1984).

Prudhoe Bay crude oil was administered orally to pregnant Sprague-Dawley rats as a single dose (5 ml/kg bw) on various gestation days (3, 6, 11, 15 or 17), as a single variable dose (2–10 ml/kg bw) on gestation day 6, or as daily doses (1 or 2 ml/kg bw) on days 6–17 of pregnancy. Administration during the earlier stages of pregnancy (day 3, 6 or 11) significantly increased the number of resorptions and decreased fetal weight and length. No adverse effect was observed following administration on gestation day 15 or 17. Multiple exposure to crude oil also caused a significant reduction in maternal body weight (Khan et al., 1987c). [The Working Group noted that no information on gross external abnormalities was reported and that the embryotoxic effects might have been a consequence of maternal toxicity.]

Both placental and fetal hepatic enzyme systems were induced on gestation day 18 following treatment of pregnant Sprague-Dawley rats with a single 5 ml/kg bw dose of Prudhoe Bay crude oil on gestation days 11, 15 or 17. Liver microsomal P450 levels, benzo[a]pyrene hydroxylase and ethoxyresorufin O-deethylase activities were increased two, two to three and 10–12 fold, respectively in 18-day-old fetuses. Similar trends were noticed in the placenta. Activities of phase II enzymes such as glutathione S-transferase, UDP glucuronyltransferase and DT-diaphorase were also significantly elevated (Khan et al., 1987b).

Several studies have demonstrated pronounced effects of crude oil on the reproductive capacity of birds (decreased hatchability, deformed bills, incomplete ossification, incomplete feather formation, gross structural abnormalities, dead embryos) after application on the shell surface or after oral administration (Grau et al., 1977; Albers, 1978; Holmes et al., 1978; Hoffman, 1978, 1979a,b; Lee et al., 1986; Walters et al., 1987). [The Working Group noted that the avian system is a sensitive model for embryotoxic effects; results should be interpreted with caution with respect to possible effects in mammalian systems.]

Genetic and related effects

A large number of studies have been reported on the mutagenicity of crude oil and its fractions to Salmonella typhimurium (Table 15). Crude oil did not induce mutagenicity in any of the studies reported, either in the presence or absence of an exogenous metabolic system. Some neutral/aromatic (including polycyclic aromatic) fractions of crude oil were mutagenic in the presence of an exogenous metabolic system.

Table 15

Mutagenicity of crude oils and their fractions in Salmonella typhimurium.

Aromatic fractions (one to three rings and four rings and more) of Prudhoe Bay crude oil caused a significant increase in the frequency of sister chromatid exchange in cultured Chinese hamster ovary cells only in the presence of an exogenous metabolic system; no increase in the frequency of chromosomal aberrations was observed (Ellenton & Hallett, 1981). Wilmington crude oil did not increase the number of sister chromatid exchanges in human lymphocytes in vitro in the presence of an exogenous metabolic system (Lockard et al., 1982).

Intraperitoneal administration of Wilmington crude oil (five doses of 1 or 2.1 g/kg bw) did not induce sperm abnormalities in B6C3F₁/Hap mice, and micronuclei were not induced in bone marrow of outbred Swiss male mice given 6.1 g/ kg bw intraperitoneally; an increase in the number of sister chromatid exchanges in bone-marrow cells of male outbred Swiss mice was observed at 7.2 g/kg bw intraperitoneally, but not at 1.8 or 3.6 g/kg (Lockard et al., 1982).

Absorption, distribution, excretion and metabolism

No data were available to the Working Group.

Toxic effects

A labourer who had aspirated crude oil developed aspiration pneumonia and hepatic and renal toxicity, from which he recovered completely (Wojdat & Winnicki, 1964).

Adverse skin effects including dryness, pigmentation, hyperkeratosis, pigmented plane warts and eczematous reactions have been observed among petroleum field workers in contact with crude oil (Mierzecki, 1965; Dzhafarov, 1970; Gusein-Zade, 1982). In one study in the USSR, a higher prevalence of skin effects was noted among transport workers in crude oil production than among petroleum field workers (Gusein-Zade, 1982). Skin diseases (hyperkeratosis and follicular lesions) were 1.5–2.5 times more frequent in petroleum field workers than in control groups (Chernov et al., 1970).

Effects on reproduction and prenatal toxicity

No data were available to the Working Group.

Genetic and related effects

No data were available to the Working Group.

(i) Lung cancer

In an attempt to explain an excess of lung cancer cases observed in a cluster of parishes in Louisiana, USA, Gottlieb et al. (1979) conducted a case-control study, the design of which is described in the monograph on occupational exposures in petroleum refining (p. 102). An elevated risk for lung cancer was observed among black men aged over 53 years who had been employed in petroleum exploration and production (odds ratio, 1.6; 95% CI, 1.0–2.6). By logistic analysis, the ratio associated with crude oil exploration and drilling was three fold among persons over the age of 62 in parishes with petroleum or paper industries. [The Working Group noted that, since information used in this study was extracted directly from death certificates and since no account was taken of cigarette smoking, caution should be applied in interpreting the results.]

Gottlieb (1980) reanalysed the risk of lung cancer in relation to work in the petroleum mining and refining industry in the men included in the previous study. A group of 200 men with lung cancer and 170 control men who had worked in petroleum mining (125 cases, 112 controls) and refining (75 cases, 58 controls) were identified. The odds ratio for lung cancer associated with employment in the petroleum industry (mining and refining combined) was estimated at 1.2 (95% CI, 1.1–1.4). For welders, operators, boiler makers and painters, and oil-field workers taken as a group (mining and refining combined), the odds ratio was 2.3 (95% CI, 1.4–3.9). [The Working Group noted that information on exposure was extracted directly from death certificates; that no information on cigarette smoking was available; that cases were older than controls, which, in itself, may explain the difference observed; and that mining and refining occupations were combined.]

(ii) Testicular cancer

Mills et al. (1984) studied 347 hospital patients with histologically confirmed germ-cell tumour of the testis in the USA and matched them by age, sex, race and residence with 347 hospital controls, most of whom had tumours other than cancer of the testis. The ascertainment period was from 1 January 1977 to 31 August 1980. Occupational histories were extracted from medical records; when the type of industry was not apparent in the record, this was ascertained from the employer. An excess risk for testicular cancer was observed among petroleum and natural gas extraction workers (odds ratio, 2.3; 95% CI, 1.0–5.1). [The Working Group noted that information was obtained only on current occupation.]

Sewell et al. (1986) conducted a population-based study in New Mexico, USA, in which cases were identified at the New Mexico Tumor Registry. In order to be included in the study, the cases had to have had histologically confirmed testicular cancer registered in 1966–84, to have been 15 years old or more at the time of diagnosis and to have died of the disease. Controls consisted of persons who had died from other cancers, matched by age, year of diagnosis, race and sex. A total of 81 cases and 311 controls was identified. The source of occupational data was either death certificates (99%) or information on file at the tumour registry (1%). No excess risk for testicular cancer was observed among petroleum and gas workers (odds ratio, 0.57; 95% CI, 0.16–2.0). The authors noted the limited power of the study, that an association might have been obscured by the restriction to fatal cases and that information on exposure was limited.

(iii) Multiple sites

In a large case-control study of cancer at many sites conducted in Montreal, Canada, which is described in detail in the monograph on gasoline, p. 185, an association was seen between exposure to crude oil and rectal cancer (five cases; adjusted odds ratio 3.7; 90% CI, 1.3–10.6) and squamous-cell lung cancer (seven cases; adjusted odds ratio, 3.5; 90% CI, 1.5–8.2) (Siemiatycki et al., 1987). It was indicated, however, that these associations might only be apparent since they are based on very small numbers. The authors suggested that one of the main groups exposed to crude oil, namely seamen, would probably have had life styles very different from those of the rest of the study population.