CRUDE OIL PROPERTIES, CLASSIFICATION AND ALTERATIONS

Physical Properties of Crude Oil

Crude oil is a natural mixture of hydrocarbons that is liquid in underground reservoirs and remains liquid at the surface after passing through separating facilities.

In appearance, oil varies from straw yellow, green, and brown to dark brown or black in color. Oils also have a widely varying range of viscosities. On the surface, oils tend to be more viscous than when heated in the subsurface. Viscosity also increases as the density of the crude oil increases.

The density of oil is usually expressed in gravity units, in API degrees as defined by the American Petroleum Institute [oAPI = (141.5/ Relative Density at 60oF) - 131.5]. API degrees are inversely proportional to density. Thus, light oils have API gravities of some 40 degrees, which is equivalent to 0.83 relative density, while heavy oils have low API gravities.

The U.S. Bureau of Mines defines heavy oils as those with API gravities of less than 25°, which is equivalent to 0.9 relative density. When oil reaches an API gravity of 10°, it has a relative density of one, and the same density as fresh water. However, almost all oils are lighter than water.

Crude oil is not the only liquid hydrocarbon which may be produced from an underground reservoir. A light, clear, high API gravity, liquid called condensate, or sometimes distillate, is often obtained in association with natural gas production.

Condensates begin as components of a heavier gaseous phase in the subsurface where they are highly compressed and at elevated temperatures. This gas phase contains some dissolved hydrocarbons which, when brought to lower surface temperatures and pressures, exolve. The subsurface phase then separates into distinct gas and liquid phases, the latter called condensate.

Condensate production from some natural gas wells can be substantial, with amounts reaching several hundred barrels per day, or more.

Chemical Properties of Crude Oil

In terms of its fundamental chemistry, oil consists largely of carbon and hydrogen with traces of oxygen, sulfur, nitrogen, and a rather unusual range of metals which includes vanadium, nickel, and others.

Though the composition of oils is relatively straightforward, the number of hydrocarbon compounds which may be present is immense, and no two oils are exactly identical in their composition.

There are four major groups of compounds which are commonly present in crude oil. These are the paraffins, naphthenes, aromatics, and the resins and asphaltenes. The resins and asphaltenes are not pure hydrocarbons and include elements other than hydrogen and carbon. The paraffins, naphthenes and aromatics are pure hydrocarbons.

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Of these pure hydrocarbons, the paraffins and naphthenes are collectively referred to as saturated hydrocarbons, those in which there is sufficient hydrogen to satisfy the electron requirements of the carbon atoms. The aromatic hydrocarbons are unsaturated with respect to hydrogen.

Figure   1 illustrates the difference between saturated and unsaturated hydrocarbons.

Figure 1

Figure 1

The paraffin molecule ethane has two carbon atoms and six hydrogen atoms. Ethylene, which is unsaturated, has two carbon atoms like ethane, but only four hydrogen atoms.

In addition to these four major groups of compounds, crude oil also contains small amounts of other compounds that contain sulfur, oxygen, and nitrogen, as well as organ metallic compounds.

When they occur in organic molecules, atoms other than hydrogen and carbon are often collectively called heteroatoms. The compounds they form are called heterocompounds.

Paraffins (Alkanes)

Paraffins have a general formula of CnH2n+2.In other words, the next heavier molecule in the series is always obtained by the addition of one carbon atom and two hydrogen atoms.

The simplest and lightest molecule of the paraffin series is the gas methane, with a formula of CH4. Paraffins with less than five carbon atoms are gaseous at normal temperatures and pressures. In addition to methane, these paraffin gases include ethane, propane, and butane. For simplicity, these are sometimes called Cl through C4.

From C5 through C15, the paraffins are liquid at normal temperatures and pressures. For carbon values greater than C15, they are extremely viscous, and may be solid waxes. Individual members of the paraffin series have been recorded up to C40 and beyond.

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There are two basic types of paraffin molecules within the series. They have the same chemical compositions, and both increase along the series by the addition of one carbon and two hydrogen atoms. One series consists of normal or straight-chained molecules, the other of branch-chained molecules, called the isoparaffins. Figure 2 illustrates a normal paraffin molecule, n-pentane, and one of its equivalent branched-form isomers, isopentane.

Figure 1

Figure 2

The straight-chained varieties are more common than the branch-chained structures. Although chemically the same, the structural form does affect physical properties. For example, the straight-chained normal paraffins have higher boiling points than the equivalent branch-chained isoparaffins.

The paraffins are a major constituent of the hydrocarbon gases. They are also important quantitatively in the lighter gasoline and kerosene oils where they may represent up to 30 percent of the oils.

Naphthenes (Cyclohexanes)

Naphthenes or cyclohexanes, like paraffins, are saturated hydrocarbons. But the naphthenes occur as closed ring molecular structures with the general formula CnH2n. Naphthenes have been recorded with from three to more than thirty carbon atoms in the rings. However, cyclopentane, C5H10, with a ring of five carbon atoms, and cyclohexane, C6H12, with a ring of six carbon atoms are the dominant naphthenes found in crude oil (Figure 3 ). Complex naphthenes, with paraffin substitutions or condensed-ring structures, form variations within this group.

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Figure 1

Figure 3

Most crude oils contain similar amounts of naphthenes and paraffins. Together these saturated hydrocarbons make up about 60 percent of most crude oil.

Aromatics

Aromatics, unlike the paraffins and naphthenes, are unsaturated with respect to hydrogen. Their structure is based on a ring of six carbon atoms, called the benzene ring, after the simplest member of the family, benzene, C6H6 (Figure 4 ).

Figure 1

F

Figure 4

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One major series of the aromatics is formed by substituting some of the hydrogen atoms with paraffin molecules. This alkylbenzene series (Figure 5) includes toluene C7H8 (C6H5 CH3) and ethylbenzene C8H10C 6H5C2H5).

Figure 2

Figure 5

Another series is formed by straight or branch-chained carbon rings. This includes naphthalene C10H8 and anthracene C14H10 (Figure 6).

Figure 3

Figure 6

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The aromatic hydrocarbons are liquid at normal temperatures and pressures. They are present in relatively minor amounts in light oils but increase in abundance with decreasing API gravity, to more than 30 percent in heavy oils.

Toluene is the most common aromatic component of crude oil, followed by xylene and benzene.

Resins and Asphaltenes

The resins and asphaltenes are complex compounds. They are impure hydrocarbons often referred to as NSO compounds, because they contain nitrogen, sulfur, and oxygen heteroatoms. Some of these heteroatoms substitute for carbon in the aromatic rings (Figure   7 ). The NSO compounds have the highest molecular weights and are the heaviest components in crude oils.

Figure 1

Figure 7

Resins and asphaltenes generally occur in association with heavy aromatic crudes where the combined resin plus asphaltene content ranges from 25 percent up to 60 percent.

Non-hydrocarbon Crude Components

In addition to the four major groups of hydrocarbon compounds, crude oils also contain other forms of sulfur, oxygen, and nitrogen, in addition to their presence in the NSO compounds.

Sulfur is the third most abundant element in crude oils, after carbon and hydrogen, averaging .65 percent by weight. It can occur as free sulfur, hydrogen sulfide, or as various organic sulfur compounds. Crude oils with less than one percent sulfur are referred to as low sulfur oils and those with greater than one percent sulfur, as high sulfur oils.

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The oxygen content of most crude averages .5 percent by weight. It occurs mostly in organic compounds that include acids and alcohols. The acids are especially common in young, immature oil. Some of these compounds are useful indicators that identify the kinds of organisms that gave rise to the crude.

Nearly all crude oils contain small amounts of nitrogen, between .1 and .9 percent. Nitrogen is most abundant in the NSO compounds in the heavy oils, but nitrogen can also occur in lighter compounds.

Crude oils also contain minor amounts of organometallic compounds, the most common being nickel and vanadium. Concentrations of these metals range from less than 1 part per million up to 1200 parts per million.

Classification and Occurrences of Crude Oils

Various schemes have been proposed to classify the different types of crude oil.

The triangular diagram (Figure   8 ) is the scheme proposed by Tissot and Welte (1978), based on the ratio of paraffins to naphthenes to aromatics plus NSO compounds.

Figure 1

Figure 8

This classification is preferred because it can be used to demonstrate the alteration paths of oil, either by maturation or degradation (Figure   9 ),

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Figure 2

Figure 9

and to demonstrate the occurrence of sulfur with respect to the classes of crude oil (Figure   10 ).

Figure 3

Figure 10Paraffinic oils are generally light, although they are often waxy and viscous. They usually have low sulfur content and are mature oils.

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Examples of the paraffin type are the Paleozoic crudes of the United States and North Africa, and the Tertiary oils of West Africa, Libya, and Indonesia.

The paraffinic-naphthenic oils occupy the middle path of the graph. These oils are generally of moderate density and viscosity, with low sulfur content.

Examples are Devonian, Cretaceous, and Tertiary oils of West and North Africa.

Aromatic-intermediate oils tend to be relatively heavy, high sulfur content oils. Examples are the Jurassic and Cretaceous reserves of the Arabian Gulf area in the Middle East, Permo-Carboniferous oils from West Texas, and some oils from Venezuela and California. Most oils are paraffinic-naphthenic or aromatic-intermediate. Naphthenic oils are very rare.

With regard to worldwide abundance, the most important classes of oils are the aromatic-asphaltic and aromatic-naphthenic, because these types comprise the huge reserves of heavy oils and tar sands of Western Canada and Venezuela. With respect to conventional crude oil reserves, the aromatic-intermediate class is the most important, being represented by the huge Cretaceous and Jurassic reserves of the Middle East.

Alteration of Crude Oil

Alteration of crude oil by thermal maturation takes place with increasing depth of burial and increasing time. Crude oils become lighter and more paraffinic due to the cracking of their heavier components and increases in their gas content (Figure 11).

Figure 1

Figure 11

The sulfur content decreases as sulfur is driven off in the form of hydrogen sulfide (Figure 12),

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Figure 2

Figure 12

which, in turn, tends to increase the H2S content of natural gas with depth (Figure 13). Eventually the crude may be destroyed and converted into gas and an insoluble residue.

Figure 3

Figure 13

The process of thermal maturation leads to several observations. Young shallow oils tend to be heavy and viscous. They are generally high in sulfur and relatively low in paraffins and rich in aromatics. Young deep oils, on the other hand, are less viscous and of higher API gravity. They are paraffinic and low in sulfur content.

Old shallow oils, because of their maturity, are comparable to young deep crudes both in density, viscosity and paraffin content. Like young shallow oils, however, they may have

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relatively high sulfur, depending on source environment. Deep old oils tend to have the lowest viscosity, the lowest density and the lowest sulfur content.

These are, however, gross generalizations and there are many variations of crude oil type, according to geological parameters. For example, oil may become heavier due to degradation regardless of its age.

Crude oil may also be altered by the process of deasphalting, whereby asphaltenes precipitate by the dissolution of massive amounts of gas and are left behind as a residue. Deasphalting primarily occurs in heavy to medium crude oils. Gas deasphalting, as in thermal maturation, results in oils becoming lighter. Gas deasphalting tends to occur together with thermal maturation and the two processes are often difficult to distinguish from each other. Gas deasphalting with thermal maturation has been demonstrated in Devonian reefs in Western Canada (Evans et al. 1971; Bailey et al. 1974).

It is important to note that oils vary not only with age and depth, but also with the type of source rock.

Oils generated from sediments deposited in a marine, reducing environment tend to be aromatic-intermediate type oils with high sulfur content. Examples of these marine-type oils are the Jurassic and Cretaceous of the Middle East; the Devonian of Alberta, Canada; the Cretaceous of West Africa; and the Mesozoic from the Paris and Aquitaine Basins (Tissot and Welte, 1978). Oils from coastal or deltaic sediments tend to be paraffinic to paraffinic-naphthenic and contain less than one percent sulfur. Examples of these types include most oils from the lower Cretaceous of West Africa, Brazil, South Argentina and Chili; the lower Tertiary of the Uinta basin in Utah; Tertiary oils from Nigeria, the U.S. Gulf Coast and from some Tertiary basins in Indonesia (Tissot and Welte, 1978). High wax crudes, which are included in this type, have been extensively reviewed by Hedberg (1968).

Figure 4

Figure 12

Figure 14 illustrates the occurrence of oils of marine and non-marine origin with respect to crude oil composition.

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In addition to thermal maturation, and deasphalting, oils may be altered by degradation. Degradation causes oils to become heavier and more viscous and leads to the formation of heavy oil and tar sand deposits.

Groundwater flushing causes various degrees of degradation of oils, since flushing removes the lighter and more mobile components of the oils. In addition, oil at the surface and at very shallow depths may be degraded due to the action of aerobic bacteria, a process termed "biodegradation". Although groundwater flushing and biodegradation may act independently, they apparently act together in producing degradation. Figure 15, by means of gas chromatographs, illustrates how the lighter hydrocarbon compounds in crude oil are broken down by bacterial oxidation over a 21-day period.

Figure 5

Figure 15

The gas chromatographs show disappearance of n-paraffin packs first in the C16 - C25 range, and, later, in the entire range, during incubation of Saskatchewan crude oil with a mixed microbe population at 30 °C. Figure 16 summarizes the four major processes by which crude oil alteration occurs:

1. Thermal maturation;

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Figure 6

Figure 162. Deasphalting;

3. Degradation by water washing, and;

4. Degradation by bacterial action (biodegradation).

Most heavy-degraded oil and tar sand deposits fall into one of two categories. They may be high sulfur, aromatic-asphaltic class oils derived from the degradation of aromatic-intermediate class oils, or they may be low sulfur, aromatic-naphthenic class oils, derived from the degradation of paraffinic or paraffinic-naphthenic crudes. One important example of the aromatic-asphaltic type can be found in the high sulfur, Cretaceous-age tar sands of Western Canada. Figure 17 is a map that shows the extent of these tar sand deposits.

Figure 7

Figure 17

These are estimated to contain some three trillion barrels of oil in place.

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The oil occurs in sands derived, for the most part, from areas uplifted to the West, laid down in deltaic and fluvial environments, and subsequently tilted gently westwards.

Normal, unaltered, light oils occur in the western, deeper part of the Alberta basin. However, oil becomes progressively heavier in more shallow positions going in an upward direction to the east. Finally, severely altered, degraded, heavy oils are found close to the surface (Deroo et al., 1974). The tar sands are encountered in stratigraphic pinchout traps and in combination (fold and pinchout) traps along the eastern margin of the basin.

The origin of the Athabasca tar sands is still controversial, even though they are some of the best known deposits in the world. The debate revolves around the question of whether the oil is immature or mature oil which has been subsequently degraded. Supporters of the immature origin believe that the chemical composition of the oil shows that it has not undergone significant thermal maturation.

On the other hand, advocates of the degradation theory point to the gradual lightening of the oil basinward to the west, accompanied by a gradual increase in formation water salinity.

It is also difficult to understand how viscous, heavy oil could have migrated into the reservoir. It is much easier to explain migration of less viscous, light oil up the basin margin with subsequent degradation. The sands, however, are highly permeable, so perhaps, given sufficient time, even heavy oil might have been able to move up through them.

Examples of the low-sulfur, aromatic-naphthenic type of degraded oils are found in the lower Cretaceous of West Africa and in the lower Jurassic of Malagasy (Tissot and Welte, 1978).

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