Concept and chemical composition of petroleum

Petroleum is a combustible organic mineral with liquid hydrocarbon as the main body, which exists in the pores of underground rocks. Oil in underground oil and gas reservoirs is a mixture of gaseous, liquid and solid hydrocarbons and their derivatives. Hydrocarbons are the main components, including various non-hydrocarbons and trace elements. The phase is mainly liquid, with a large amount of hydrocarbon gas and a small amount of non-hydrocarbon gas, as well as different amounts of solid hydrocarbon and non-hydrocarbon substances. The composition and phase ratio of oil in oil and gas reservoirs vary from place to place, so oil has no definite chemical composition and physical constant.

(b) Elemental composition of petroleum

Petroleum has no definite chemical composition, so it has no definite elemental composition. But the chemical elements that make up petroleum are mainly carbon (C) and hydrogen (H), followed by sulfur (S), nitrogen (N) and oxygen (O). There are differences in the composition and content of petroleum elements in different producing areas (table 1- 1).

Generally, the carbon content in petroleum is 80% ~ 88%, and the hydrogen content is 10% ~ 14%. Two elements are absolutely dominant, with a general content of 95% ~ 99%. The total amount of sulfur, nitrogen and oxygen ranges from 0.3% to 7%, generally less than 2% to 3%, and the sulfur content of individual petroleum can be as high as 10%.

Because sulfur is corrosive, the sulfur content is related to the quality of oil. The sulfur content in crude oil varies greatly, from a few ten thousandths (Karamay, 0.05%) to a few percent (Venezuela, 5.48%). According to sulfur content, crude oil can be divided into high sulfur crude oil (sulfur content is greater than 1%) and low sulfur crude oil (sulfur content is less than 1%). Sulfur in crude oil mainly comes from sulfate-containing minerals such as protein in organic matter and gypsum in surrounding rocks, so the sulfur content of oil produced in marine environment is higher than that of oil formed in land environment.

The nitrogen content of crude oil ranges from 0. 1%- 1.7%, with an average value of 0.094%. More than 90% of crude oil contains less than 0.2% nitrogen. The oxygen content of crude oil is between 0. 1%-4.5%, which is mainly related to its oxidative metamorphism.

Table 1- 1 Composition of petroleum elements (mass fraction/%)

(According to Shi Yucheng 1980, there are changes)

In addition to the above five main elements, iron (Fe), calcium (Ca), magnesium (Mg), silicon (Si), aluminum (Al), vanadium (V), nickel (Ni), copper (Cu), antimony (Sb) and manganese (Mn) are also found in crude oil ash (residue after petroleum combustion). Zinc (Zn), molybdenum (Mo), lead (Pb), tin (Sn), sodium (Na), potassium (K), phosphorus (P), lithium (Li), chlorine (Cl), bismuth (Bi), beryllium (Be), germanium (Ge) and silver (Ag). Although there are many kinds of these elements, the total amount only accounts for a few ten thousandths of the oil quality, and they are trace elements in oil, or ash elements.

Among the trace elements in petroleum, vanadium (V) and nickel (Ni) are high in content and widely distributed, and because they are related to the origin of petroleum, they are most valued by petroleum geologists. V/Ni ratio can be used as one of the indicators to distinguish between marine environment and continental environment. Generally speaking, V/Ni > 1 is a marine environment and V/Ni < 1 is a terrestrial environment.

(3) Composition of petroleum compounds

The main elements that make up petroleum are carbon (C), hydrogen (H), sulfur (S), nitrogen (N) and oxygen (O), but the compounds made up of these five elements are huge. Generally speaking, the compounds that make up petroleum are mostly organic compounds; There are not many inorganic compounds mixed as impurities, and the content is very small, which can be ignored. The composition of petroleum compounds can be divided into two categories: hydrocarbons and non-hydrocarbons, among which hydrocarbons are the main ones, which is consistent with the absolute advantages of carbon (C) and hydrogen (H) in the elemental composition.

At present, there are nearly 500 kinds of organic compounds separated from different crude oils analyzed all over the world, not including organometallic compounds. About 200 of them are non-hydrocarbons, and the rest are hydrocarbons. Most crude oil consists of 150 hydrocarbons.

1. Hydrocarbons

Chemically, hydrocarbons can be divided into two categories: saturated hydrocarbons-alkanes, cycloalkanes, unsaturated hydrocarbons-olefins, aromatics and cycloalkanes-aromatics.

(1) saturated hydrocarbon

Saturated hydrocarbons account for the majority in petroleum, generally accounting for 50% ~ 60% of all components of petroleum. It can be subdivided into alkanes and cycloalkanes.

At normal temperature and pressure, alkane C 1-C4 is gaseous, C5-C 15 is liquid, and C 16 is solid (natural paraffin).

Fig. 1- 1 stereochemical structure diagram of isoprenoid alkane homologues.

Isoalkanes with branched chains (side chains) in petroleum are mainly ≤C 10, which are common in C6-c8; C 1 1-C25 is less, and isoprene alkane is the most important. Isoprene alkanes in petroleum (figure 1- 1) are generally thought to have evolved from phytol, a side chain of chlorophyll, so they are the marker compounds of petroleum biogenesis. At present, many isoprene compounds have been separated from petroleum, accounting for 0.5% of petroleum. Among them, 2,6, 10, 14- tetramethylpentadecane (palmitane) and 2,6, 10, 14- tetramethylhexadecane (phytane) are widely studied and applied. Studies have shown that various isoprene compounds from the same petroleum source are very similar. Therefore, it is often used as a sign of oil source correlation.

The proportion of naphthenes in petroleum is 20% ~ 40%, with an average of about 30%. Low molecular weight cycloalkanes (< < C 10), especially cyclopentane (C5- five-membered ring) and cyclohexane (C6- six-membered ring) and their derivatives are important components of petroleum, and cyclohexane is generally more than cyclopentane. Medium to high molecular weight cycloalkanes (C 10-35) can be monocyclic to hexacyclic. Cycloalkanes in petroleum are mainly monocyclic and bicyclic, accounting for about 50% ~ 55%, tricyclic accounts for about 20%, and tetracyclic accounts for about 25%. The content of polycyclic naphthenes in petroleum decreases with the increase of maturity, so the naphthenes with 1-2 rings in highly mature crude oil increase obviously.

At normal temperature and pressure, cyclopropane (C3H6) and methyl cyclopropane (C4H8) are gaseous; In addition, other monocyclic cycloalkanes are all liquids, and cycloalkanes with more than two rings (> C 1 1) are solids.

(2) Unsaturated hydrocarbons

Unsaturated hydrocarbons in petroleum are mainly aromatic hydrocarbons and naphthenic aromatic hydrocarbons, accounting for 20% ~ 45% of crude oil on average. In addition, linear olefins are occasionally found in crude oil. Olefins and unsaturated cyclic hydrocarbons are rare because they are extremely unstable.

Aromatic hydrocarbons identified in petroleum can be divided into three categories according to their structures: monocyclic, polycyclic and condensed rings, and the main molecules of each category are often alkyl derivatives rather than substrates.

Monocyclic aromatic hydrocarbons include benzene, toluene, xylene, etc. Polycyclic aromatic hydrocarbons include biphenyl, triphenylmethane, etc. Polycyclic aromatic hydrocarbons include naphthalene (bicyclic condensation), anthracene and phenanthrene (tricyclic condensation), benzoanthracene and osmium (tetracyclic condensation).

Benzene, naphthalene and phenanthrene are the most abundant aromatic hydrocarbons in petroleum, and their main molecules often appear in the form of alkyl derivatives. For example, toluene, rather than benzene, usually occurs in the former.

Cycloalkanes contain one or several fused aromatic rings, and are fused with saturated rings and alkyl groups. The most abundant naphthenic aromatic hydrocarbons in petroleum are indane, naphthalene and their methyl derivatives, which are composed of two rings (an aromatic ring and a saturated ring). Tetracyclic and pentacyclic alkylaromatic hydrocarbons are the most important, and their content and distribution characteristics are often used in the study of petroleum genesis and oil source correlation. Because most of them are related to steroids and terpenoids (aromatization), and steroids and terpenoids are typical biological genetic marker compounds.

2. Nonhydrocarbon compounds

Non-hydrocarbon compounds in petroleum refer to a large class of compounds containing sulfur, nitrogen or oxygen or metal atoms (mainly vanadium and nickel) in addition to carbon and hydrogen. The content of these elements in oil is not much, but there are many compounds containing these elements, sometimes reaching 30% of the oil quality. There are mainly compounds containing sulfur, nitrogen and oxygen.

(1) sulfur-containing compound

Sulfur is the third important element in petroleum after carbon and hydrogen, and sulfur-containing compounds are also the most common. At present, there are nearly 100 kinds of sulfur-containing compounds identified in petroleum, most of which are mercaptan, sulfide, sulfide (H2S) and thiophene (in the form of sulfur-containing heterocyclic compounds, which are abundant in heavy petroleum).

(2) Nitrogen compounds

There are few nitrogen compounds in petroleum, and the average content is less than 0. 1%. At present, there are more than 30 nitrogen-containing compounds isolated from petroleum, mainly in the form of nitrogen-containing heterocyclic compounds. Can be divided into two groups. One is alkaline compounds, including pyridine, quinoline, isoquinoline, acridine, porphyrin, indole, carbazole and their homologues. Among them, metalloporphyrin compounds containing vanadium and nickel are the most important.

The porphyrin compounds in crude oil were first discovered by Trebbs (C.Treibs, 1934). Including protoporphyrin and deoxy-rose protoporphyrin, and put forward that the porphyrin in petroleum is transformed from plant chlorophyll and animal hemin. This discovery provides strong evidence for the theory of organic origin of petroleum, which has aroused widespread concern and attention. At present, the research on porphyrin has been gradually deepened and many types have been found. Porphyrin is a nitrogen-containing compound with four pyrrole nuclei as the basic structure and connected by methyl Sichuan bridge. In petroleum, porphyrin often forms complexes with metal elements such as vanadium and nickel, so it is also called organic metallization (complex), and its basic structure is very similar to chlorophyll (Figure 1-2).

Fig. 1-2 comparison of chloroPhyll (a) with porphyrin (b), phytoalkane (ph) and sulfolane (Pr) in crude oil (according to G.D.Hobsohetal, 198 1).

However, not all crude oils contain porphyrin, and quite a few crude oils contain no or only trace amounts. Generally, the crude oil formed in Mesozoic and Cenozoic strata contains more porphyrin, while the crude oil in Paleozoic strata contains little or no porphyrin. This may be related to the poor stability of porphyrin. Porphyrin will be destroyed by ring-opening cracking at high temperature (> 250℃) or under oxidation conditions.

In addition, the types of porphyrins in crude oil are also closely related to the sedimentary environment. Offshore oil is rich in vanadium porphyrin, and onshore oil is rich in nickel porphyrin.

(3) Oxygenated compounds

More than 50 oxygenates have been identified in petroleum. Include organic acids, phenols and ketones. Among them, there are mainly organic acids related to acid functional group-COOH, including fatty acids of C 1-24, naphthenic acids of C5- 10, and isoprenoid acids of C10/5. Organic acids and phenols (acidity) in petroleum are collectively called petroleum acids, among which naphthenic acids account for the most, accounting for 95% of petroleum acids, mainly pentatomic acids and hexatomic acids. Almost all petroleum contains naphthenic acid, but the content varies greatly, ranging from 0.03% to 1.9%. Naphthenic acid is easy to combine with alkali metals to form naphthenate, which is particularly soluble in water. Therefore, the existence of naphthenate in groundwater is one of the signs of petroleum exploration.

(4) Fraction composition of petroleum

Petroleum is a mixture of hundreds of hydrocarbon and non-hydrocarbon organic compounds, and each compound has its own boiling point and freezing point. Petroleum fraction is the distillation of crude oil by heating, and the petroleum is fractionated into several fractions with different boiling points, each of which is a fraction. The temperature range (distillation range) used for fractionation is different, and the distillate (fraction) is also different (Table 1-2).

Table 1-2 approximate distillation range of petroleum products

Usually, the refining process of petroleum can be regarded as petroleum fractionation, and the control of distillation range is determined according to the quality of crude oil and the specific requirements for oil quality. In order to improve the output and product quality of light fractions in petroleum, modern oil refining industry has adopted a series of technical measures besides direct distillation, such as catalytic thermal cracking, hydrocracking, thermal cracking and platinum reforming of petroleum. For example, gasoline fractionated at atmospheric pressure only accounts for 15% ~ 20% of crude oil. After catalytic pyrolysis, the gasoline output can be increased by 50% ~ 80%, which meets the needs of gasoline as energy fuel.

(5) Petroleum composition analysis

The composition analysis of petroleum is to use organic solvents and adsorbents to selectively dissolve and adsorb the compounds that make up petroleum. Choosing different organic solvents and adsorbents, crude oil is divided into several parts, each part is a component.

Generally speaking, before component analysis, crude oil is fractionated to remove the light fraction below 2 10℃, and the fraction above 2 10℃ is cut for component analysis. The components that can be dissolved in chloroform and carbon tetrachloride are called oil, which is the weakest polar part of petroleum, and its components are mainly saturated hydrocarbons and some low molecular weight aromatic hydrocarbons. The component dissolved in benzene is called benzene gum, and its components are mainly aromatic hydrocarbons and some compounds containing impurity elements of aromatic ring structure (mainly polycyclic aromatic hydrocarbons containing sulfur, nitrogen and oxygen). Alcohol-benzene gel (or other corresponding components) can be obtained by using mixed solution of alcohol and benzene (or other more polar solvents such as methanol and acetone). The composition of this gum is mainly non-hydrocarbon compounds containing impurities. Separated by petroleum ether, the part dissolved in petroleum ether is oil and colloid. Among them, the part that can be adsorbed by silica gel is colloid, the part that cannot be adsorbed by silica gel is oily, and the rest is asphaltene that is insoluble in petroleum ether (but soluble in neutral organic solvents such as benzene, carbon disulfide and chloroform, and can be adsorbed by silica gel in colloidal solution). The latter is the main component of residual oil, and its main component is macromolecular non-hydrocarbon compounds with complex structure.