Physical and Chemical Properties of Crude Oils and Their Geologic

International Journal of Science and Research (IJSR)

ISSN (Online): 2319-7064

Index Copernicus Value (2015): 78.96 | Impact Factor (2015): 6.391

Volume 6 Issue 6, June 2017

www.ijsr.net

Licensed Under Creative Commons Attribution CC BY

Physical and Chemical Properties of Crude Oils and

Their Geologic Significances

Madu Anthony Joseph Chinenyeze

, Ugwu Richard Ekene

1, 2

Department of Geology, College of Physical and Applied Sciences, Michael Okpara University Umudike, Abia State, Nigeria

Abstract: Crude oils are naturally occurring liquid phase of petroleum, composed principally of hydrocarbon compounds and are

extracted from the earth in the liquid state. The hydrocarbon compounds in crude oils include paraffins ranging from pentane to

pentadecane (C

– C

), alkylparaffins, naphthenes, alkylbenzenes and nuclear aromatics. Other associated matters are natural gases,

hydrocarbon waxes, and salt water. Crude oil can be described as paraffin base, naphthene base or mixed base, depending on the most

abundant group of hydrocarbons contained. Crude oils also contain a variety of other chemical constituents comprising of sulphur,

oxygen, carbondixode, nitrogen and trace metals. Crude oils are characterized by some physical and chemical properties, which to a

measurable extent, play important role in the understanding of the oils geologic nature and environment of origin, such a physical

property as optical activity is dependent on hydrocarbon derivatives from organic cholesterine substances, and are destroyed at high

temperatures of about 200

C. They serve as important tools for environmental analysis, reconstruction of temperature history of the oil,

and correlation of crude oils of similar geologic ages. Cloud and pour points of crudes reveal the influence of low temperatures on crude

oils and simultaneously, provide information about the paraffin wax content of the crude. Most of the physical properties of crude oils

such as A.P.I. gravity, viscosity and coefficient of expansion, depend on reservoir pressures and temperatures, chemical composition of

the oil, and sometimes, on the amount of dissolved natural gases. The chemical properties of crude oils vary in relation to changes in

geotemperatures and pressure, coupled with some other elements of katagenesis. The paraffin wax and porphyrins are complex forms of

hydrocarbons which have genetic relationship with living organic matters. Occurrence of isoprenoid hydrocarbons such as pristanes

and phytanes, in crude oils help in the reconstruction of the genetic environments of deposition of the source rocks. Trace metal

substances concentrated in oils are thought to be derivatives from sea water as secretions by marine organisms. The economic value of

any crude is influenced by such properties as A.P.I. gravity, paraffin wax content and sulphur content. The aforementioned properties of

crude oils apply to the Nigerian crudes. Their A.P.I. gravities change abruptly after a certain depth, where there is a marked change in

temperature. Nigerian crude oils are generally light, low sulphur-bearing, and are in great demand by advanced countries.

1. Introduction

Crude oil is a naturally occurring liquid phase of petroleum,

composed principally of hydrocarbons, extracted from the

earth in liquid state, or is capable of being so removed. It is

mostly found in association with natural gases

Other

associated materials include solid hydrocarbon substances

such as waxes and asphalt, and salt water.

2. Chemical Composition of Crude Oils

Crude oils are made up of liquid paraffin hydrocarbon

compounds ranging from pentane to pentadecane (C

– C

These hydrocarbon compounds consist of different groups

such as the normal paraffins, iso-paraffins (branched chain

paraffins), alkyl paraffins, naphthenes (or cycloparaffins),

alkylbenzene and nuclear aromatics. The normal paraffins

are the saturated, low molecular weight hydrocarbons. The

associated gaseous phases are within this group. The

naphthenes (or cycloparaffins) are highly bonded, high

molecular weight hydrocarbons. All crude oils contain some

appreciable amount of the naphthene compounds, (10% by

composition).

Crude oils also contain a great variety of heteroatomic

chemical constituents, comprising of sulphur, oxygen,

carbondioxide, nitrogen and trace metals. Nitrogen varies

from 0.01 to 2% as dissolved gas in the crude oil (Levinson,

1974). Oxygen occurs in different forms in oxygen-bearing

resinous substances.

Crude oils accumulate in geologic structures called 'traps'. A

trap can be stratigraphic, paleogeomorphic or a combination

of these. Palecgeomorphic traps includes structural folds and

stratigraphic traps are those caused by lateral changes in

reservoir rock properties within a stratum.

3. Physical Properties of Crude Oils

The physical properties of crude oils are the quantitatively

measurable characteristics of crude oils. They vary

according to the composition of the oil, the relative

abundance of the groups of hydrocarbons, and essentially

depend on reservoir temperatures and pressures.

Specific (or A.P.I) Gravity

This is the weight of a given volume of crude oil. It is

measured in two gravity scales, as stated below:

i. A.P.I. gravity (in degrees) =

ii. Baume gravity (in degrees) =

The A.P.I. (American Petroleum Institute) gravity scale is

more commonly and widely used than the European Baume

gravity scale. But they essentially measure the density of a

crude oil. The A.P

I. gravity of a crude is influenced by the

composition of the oil. Crude oils characterized by high

amount of dissolved gases, are less dense thus, light in

weight and therefore, possess high A.P.I. gravities, while

denser crude oils of low amount of dissolved gases are

characterized by low A.P.I. gravity values.

The group of hydrocarbons predominating in a crude oil also

influences the A.P.I. gravity. For example, paraffin base

Paper ID: ART20174603

DOI: 10.21275/ART20174603

1514

International Journal of Science and Research (IJSR)

ISSN (Online): 2319-7064

Index Copernicus Value (2015): 78.96 | Impact Factor (2015): 6.391

Volume 6 Issue 6, June 2017

www.ijsr.net

Licensed Under Creative Commons Attribution CC BY

crude oils (45-60% paraffin hydrocarbons and less amount

of naphthenes and aromatics) are light, thus, high A.P.I.

gravities. But naphthemic base crudes (consisting

predominantly of naphthene hydrocarbons 60-75%, with

lesser amount of paraffins and aromatics) are heavy and

have low A.P.I gravities?

The A.P.l. gravities of crude oils usually increase with

depth. This is because a combination of source and reservoir

maturation processes, Associated with slow but continuously

increasing geotemperatures, cause the generation of lighter

(or High A.P.I gravity) oils at greater depths of burial.

Viscosity

This is the measure of resistance to flow in crude oils due to

internal friction. It is expressed in 'poise' or ‘centipoise’.

crude oil is said to have a viscosity of 1 poise when a

tangential force of 1 dyne causes a plane surface of 1 square

centimetre area; spaced 1 cm from a stationary plane surface

to move with a constant velocity of 1 cm per second, the

space being filled with the viscous c.

The viscosity of a crude oil is influenced by the amount of

dissolved gases at the prevailing temperature. Crudes

characterized by high amount of dissolved gases have high

A.P.I gravities and low viscosity or moderately high fluidity;

illustrated in figure 2. At high temperatures molecular

agitation (or velocity) of the crude increases, making for a

volumetric expansion and reduction in internal molecular

friction, thus, reducing the oil. The greater the quantity of a

high-molecular weight hydrocarbon group in a crude, the

denser and more viscous.

Refractive Index

The refractive index n, of a crude oil is measured from

where; i = incidence angle

r = angle of refraction

It depends on the density of the oil. Heavy crudes (of low

A.P.I) have high refractive indices. This is because a dense

crude would create a dense medium for a passing ray of

light, which is refracted towards the normal at a low angle

(r). On the other hand, light oils have low refractive indices.

Optical Activity

Optical activity is the power of crude oils to rotate the plane

of polarization of a polarized light. It is commonly expressed

in degrees per millimetre. If any crude oil causes the plane

of polarization to rotate to the right, it is called a

'dextroratory', but if is to the left it is known as a

'levorotary’. This property is destroyed at high temperatures

(250 - 300°C).

Optical activity is also exhibited by some organisms that

contain cholesterine substances (such as cholesterol C

OH). According to Amosov (1951), the amount of optical

rotation shown by a crude oil depends mainly on its sterane-

pentacyclic and triterpane content. And these are

hydrocarbon compounds derived from the microbial

decarboxylation of organic cholesterine substances.

Cloud and Pour points

The pour point is the temperature at which a crude oil will

no longer flow, when a tube containing it is first heated in a

bath, in order to dissolve all its wax content and then

gradually cooled. At this temperature, the crude oil is in

semi-solid to solid form, and thus loses its fluidity. Slightly

above this temperature of no-flow (the pour point), there

might had been an appearance of cloudy substances in the

crude

which is due to the settling out of the solid paraffin

waxes contained in the crude oil. That temperature is called

the cloud point.

This property determines the influence of low temperatures

on crude oils. Simultaneously, it provides information about

the amount of solid paraffin waxes contained in the oil. This

property is common in paraffin base crudes but wax-free

naphthenic oils do not show cloud point. If the pour point of

a crude is above the surface temperature, it will precipitate

its paraffin waxes on approaching the surface of the ground

will only flow on heating. The range of pour points of crude

oil varies greatly from -70

F to +90

F or higher.

Volume

The volume of a crude oil in its reservoir rock differs from

the volume it occupies in the surface. This is due to

formation gas-oil ratio and reservoir pressures. The

formation gas-oil ratio expresses the volume of gas

contained in one barrel of a crude oil as it comes from the

reservoir rock. Under high reservoir pressure, the volume of

oil in the reservoir increases because of the influence of

dissolved gases. But on release of the reservoir pressures,

the dissolved gases escape, leading to the shrinkage of the

volume of the crude oil at the surface.

Flourescence

It may be yellow, green or blue. For example, when a

paraffin base crude oil (gasoline-rich) is exposed to

ultraviolet fluorescence light, it emits yellow colour, while

naphthenic oils emit brownish colour This property is

important in testing for cutting, core and drilling mud

samples and in well-logging interpretation, for location of

different oil horizons.

Colour

This is the light transmitted through crude oils. It is

yellowish to red for light oils and dark or even opaque for

heavy (or low A.P.I gravity) oils.

Some other Physical Properties

Odour

This varies greatly in crude oils. High content of light

hydrocarbons (paraffins and naphthenes) in a crude gives

rise to a gasoline-like odour. A pleasant odour is produced if

the crude has abundant aromatic hydrocarbons. But with

high amount of unsaturated hydrocarbon compounds,

sulphur and nitrogen compounds in the oil, it produces a

repugnant odour.

Coefficient of Expansion

This is the measure of volumetric increase of a crude under

thermal influence. It increases with increase in A.P.I

gravity. Oils containing high amount of (dissolved) gases

Paper ID: ART20174603

DOI: 10.21275/ART20174603

1515

International Journal of Science and Research (IJSR)

ISSN (Online): 2319-7064

Index Copernicus Value (2015): 78.96 | Impact Factor (2015): 6.391

Volume 6 Issue 6, June 2017

www.ijsr.net

Licensed Under Creative Commons Attribution CC BY

and possibly of high A.P.I gravities possess high values of

coefficient of expansion. Heavy crude oils (low A.P.I

gravity) have lower coefficients of expansion.

Aqueous Solubility

The aqueous solubility of crude oil and its fractions

increases linearly with temperature. The rate of solubility

becomes significant at temperatures of about 100°C. At

temperatures above 180°C, crudes occur as molecular

solutions in mixed phase with water. According to Cartmill

and Dickey (1970), at such high temperatures, the nature of

the phase enhances primary migration of oil by molecular

solution mechanism. And salinity of about 150,000ppm of

sodium chloride results in the seperation of liquid

hydrocarbons from the aqueous phase.

Surface tension Effect

Crude oils possess some intermolecular forces of cohesion,

expressed as force per unit peripheral outline. Because of

this force, oil in dispersed state cannot move through water-

wet sand, much less, fine-grained shales. The small forces

created by natural hydrodynamic gradients do not overcome

those created by surface tension. Consequent the oil is

dispersed in the form of globules.

Flash Point

This is the temperature at which the volatiles rising off the

surface of heated oil will ignite with a flash, on passing a

flame over the surface. This provides some clew about the

gaseous content of the crude oil.

4. Chemical Properties

The chemical properties of crude oil deal with the chemical

nature and the changes in composition in relation to

temperature and pressure variations occurring at all times

within the oil pool. Some of the chemical properties are

related to the origin, migration, and accumulation of the

crude oil.

Chemical Nature

Crude oils are composed of liquid hydrocarbon compounds

of: paraffin series ranging from pentane to pentadecane (C

), varying amounts of naphthenes, alkyl benzenes,

amomatics, trace amount of semi-solid to solid phase

hydrocarbons such as asphaltic matters and waxes. Other

secondary constituents are sulphur, nitrogen, oxygen,

carbondioxidc, trace metals which include silicon, iron,

aluminum, calcium, magnesium, copper, lead, tin, zinc,

nickel, molybdenum and vanadium. Various hydrocarbon

compounds in crudes have been isolated and analysed at

different temperatures ranging from 40 – 180

C. These

hydrocarbons consist essentially of hydrogen and carbon.

Some of the compounds are saturated with hydrogen while

others are not.

A crude oil is categorized on the basis of the most abundant

group of hydrocarbons it contains. A crude oil can be

described as paraffin base,

naphthene base or mixed base (i.e

naphthenic-paraffin base) depending on the percentage

composition of its hydrocarbon fractions, as demonstrated in

Table 1.

The paraffin base crude oils have low density (high A.P.I

gravities), and in chemical refineries yield high amount of

gasoline and kerosene. The naphthene base crudes produce

high amount of lubricating oils, less gasoline and kerosene

as the refinery products.

Hydrogenation of crude oils

During the early stages of crude oil formation, there is a

remarkable thermal cracking of the organic materials into

decomposed complexes, and the coupling activity of

anaerobic bacteria processes, and the catalytic influence of

such available trace metals as vanadium and nickel, lead, to

the transformation of the complex organic matter into alkene

rich paraffinic oil. And according to Zobel (1947), reservoir

catalytic chemical reactions lead to the dissociation of avail

sulphides into free sulphur and hydrogen. The elemental

hydrogen would convert the alkene rich paraffinic crude oil

into an accumulation of gaseous paraffinic oils (of high

A.P.I gravity), in relatively close association with the

kerogen (or organic source rock).

Paraffin wax content

Paraffin waxes in crude oils are semi-solid to solid forms of

hydrocarbons, consisting mainly of normal paraffins. These

n-paraffins range from about C

– C

. Admixtures of

branched-chain paraffins are also contained. Hedberg

(1968), described waxes as complex petroleum substances

whose complexity is caused by molecular mixture of

branched chain and n-paraffin hydrocarbons, with molecular

weights, high enough to be solids at ordinary temperatures.

Paraffin waxes in crude oils have melting points above 30-

C. The amount of wax in crude oils varies very greatly.

According to Levinson (1974), paraffin base and mixed base

crude oils have high amount of paraffin wax up to about 1-

6%. Hedberg (1968) stated that paraffinic-naphthenic (or

mixed) base crude oils usually possess over 10% of paraffin

wax, Naphthenic oils contain trace amounts of waxes

(usually <1%)

Waxy oils may also contain soft waxes and nonwaxes

composed of complex naphthene chains, naphthenic rings

and polycyclic aromatic. They are separated by solvent

crystallization into viscous oils, soft wax-like materials, and

hard, dry microcrystalline solids.

High content of paraffin waxes in oils may lead to the

clogging of pores of the reservoir rocks. Such oils congeal

at atmospheric temperatures and exhibit high pour points.

Paraffin waxes are believed to be derived from land plant

materials and other terrigenous organic matter of non marine

environments.

Odd carbon Chain Lengths

Chemical analysis shows that some crude oils exhibit a

detectable predominance of n-paraffins of odd number

carbon chains over those of even Members. The odd carbon

chains range from C

to C

. The ratio of the sum of the

mole percentages of odd carbon n-paraffins to the sum of

even Carbon n-paraffins in a specific molecular weight

range, serves as an index of odd carbon preference. Welte

(1965) believed that crude oils from different environments

Paper ID: ART20174603

DOI: 10.21275/ART20174603

1516

International Journal of Science and Research (IJSR)

ISSN (Online): 2319-7064

Index Copernicus Value (2015): 78.96 | Impact Factor (2015): 6.391

Volume 6 Issue 6, June 2017

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possess correspondingly different odd carbon preference

indices.

Carbon Isotope Ratio

Most crude oils have been proved to contain varying

amounts of carbon 13 and carbon 12 isotopes. And the

concentration of these isotopes varion crudes of different

source environments. According to Silvermann and Epstein

(1959), the ratio of C

isotopes in crude oils is found

to be nearly analogous to those of living organic matter.

This property thus, establishes a relationship between crude

oils and organic sediment. Welte (1965) found that the

quantity of isotope carbon-13 in crude oil is significantly

different for oils from reservoirs of varying geologic ages.

Sulphur and Its Isotope Ratio

In crude oils, sulphur occurs as free sulphur, hydrogen

sulphide and in organic sulphur compounds such as thiols

and disulphides. Sulphur

occurs in trace quantities of about

0.1 to 5.5% by weight in oils.

Lijubach (1975) traced sulphur occurrence in oil to the

genesis of the oil. He believes that bacterial reduction

processes convert sulphates into hydrogen sulphides and free

sulphur. And if the environment is rich in trace metals,

available meteoric water would enhance the oxidation of the

sulphides into metallic sulphides and hydrogen. These metal

sulphides do not influence the composition of the crude. But

if the environment is deficient of trace metals there would be

direct bacterial oxidation of hydrogen sulphide into

elemental sulphur thus producing a sulphur rich oil. This is

illustrated' in Fig. 4. High amount of sulphur is found in

association with high molecular weight hydrocarbons,

(asphaltenes inclusive) Fig. 5. High content of sulphur and

its compounds corrodes the refinery installations when such

crudes are refined.

At temperatures higher than 150

C, sulphur reacts with

some hydrocarbon compounds in the crude to produce thiols

and mercaptans. Therefore the presence of elemental sulphur

in crude oils suggests a relatively lev: temperature history

(Lijmbach, 1975).

Porphyrins in Crude Oil

Porphyrins are complex hydrocarbon compounds that

originate from living organic matters such as chlorophyll

and hermins. The basic structure of porphyrins consists of

four interconnected rings, each ring containing four carbon

atoms and four nitrogen atoms.

According to Hodgson et al (1967), porphyrins have high

affinity for trace metals of vanadium, nickel, and iron. This

property enhances the hybridization of unstable porphyrin

substances with nickel or vanadium to produce metal-

porphyrin complexes. The latter is produced by the

simplification of the ring structures of the chlorophyll matter

coupled with the replacement of the magnesium metal atom

in the centre, by such trace metals as V, Ni or Fe, thus

producing a metallic porphyrin complex. The latter exists

more stably in the crude oil, as it offers more resistance to

degradation and alteration.

Chemical investigations have shown that paphyrins are one

of the important constituents of crude oils. They are

commonly associated with isoprenoid hydrocarbon forms;

pristane and phytane. According to Hunt (1968), these

hydrocarbons are derivatives from marine phytoplankton.

The long-side chain characteristics of the chlorophyll

molecule readily break off to form phytol. And the phytol

can either be oxidized to form pristance or be reduced to

form phytane, depending on the redox potential of the

environment. Other hydrocarbon groups associated to

porphyrins include carotene and terpenoid structures.

Porphyrin substances are of moderately low temperature

origin, usually destroyed at temperatures of about 200°C.

Thus, their presence in crude oils is an indication that the

crude was formed at temperatures below 200

Trace Metals in Crude Oils:

Crude oils contain varying amounts of trace elements some

of which include, iron, aluminium calcium, magnesium,

copper, lead, tin, astemum, antimony, zinc, silver, nickel,

chromium, molybdoniura and vanadium. But the most

important of these trace elements are vanadium, nickel and

iron. The concentration of any of these trace metals is so

small that the value is expressed in parts per million. The

concentration of trace metals in any crude oil is found to be

inversely proportional to the A.P.l gravity of the oil.

Some Other Chemical Properties

Effect of Carbondioxide and Saline Water

High amount of anaerobic microbacteria in some reservoir

rocks generate carbondioxide, which in association with

hydrocarbon gases, constitute dissolved gases that mobilize

the liquid hydrocarbon. The carbondioxide facilitates a

decrease in viscosity of the oil and generates internal gas

pressure to drive the crude oil from dead-end pockets and

through interstitial spaces.

When these gases are dissolved in saline water, sufficient

reservoir drive to flow light paraffin base oils is provided.

The saline nature of the water reduces its surface tension,

thus, creating molecular contact between water and the

crudes, subsequently, leading to the effective mobilization of

the crude oil.

5. Geologic and Economic Significance of Some

Properties of Crude Oil

Criteria for Environmental Interpretation

Some properties of crude oils have contributed immensely

towards the analysis of the environments of occurrence of

the crudes and the understanding of their mode of origin.

High wax content in crude oils is a property commonly

associated to the genetic environment or the kind of organic

matters from which the oil was derived. Paraffin waxes are

however conceived to be derived from terrigenous materials

(land plants) in the source rock. This is characteristic of

paralic to paralic marine environments. Its lithologic

sequences are characterized by sandstone and shale

intercalations, often waxy oils are associated with inter-

Paper ID: ART20174603

DOI: 10.21275/ART20174603

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International Journal of Science and Research (IJSR)

ISSN (Online): 2319-7064

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Volume 6 Issue 6, June 2017

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fingering of continental and marine sediments, and may

contain some lenses of coal and oil shales. Hedberg (1968)

thought that waxy oils are of fresh to brackish water origin,

occurring in continental, paralic or nearshore environments,

examples of waxy crude oils of such environments are those

of Nigeria and Eastern Venezuela.

The environment of deposition of source rocks of crude oils

can be discerned from the hydrocarbon-sulphur-nitrogen

compositional association. This is shown on Table V.

Optical activity is a physical property of oils which

establishes some relationship between crude oils and marine

or near marine environments. This is because of the

occurrence of cholesterine substances (source of optical

activity) in both exude oils and some marine organisms.

Trace metal complexes occurring in crude oils are

considered as important indicators for environmental

interpretation. Such trace metals as vanadium, nickel, iron,

silicon are believed to have formed from marine organic

secretions or from planktonic organisms. And the presence

of these elements in a crude suggests that the oil must have

formed in a marine environment. Hodgson and Baker

(1957) held the view that the presence of iron-porphyrins in

crude oils is an indication of source of marine origin. They

argued that the characteristic high pH values of about 8.5 for

marine environments favour high concentration of iron

Carbon isotope ratios (C

) in oils are suggestive of the

source rock environments. The carbon isotope ratios of

crude oils vary with their environments of formation.

Silvermann (1960) found that the average C

ratio for

oils of marine environment is greater than 1% whereas the

value decreases in oils of nonmarine environment.

Hydrocarbon compounds such as porphyrins and isoprenoid

compounds which occur in crude oils contribute immensely

to the reconstruction of temperature and paleoenvironraental

history of the oils. The ratio of some isoprenoid compounds

(pristane-to-phytane), contained in any crude oil is a

function of the redox potential of the oil's source rock

environment of deposition. According to Lijmbach (1975),

certain relationship has been observed between isoprenoid

composition of crude oils and their source rock environment

of deposition.

1) Peat swamp conditions were characterized by low pH, of

5, low oxygen content and presence of toxic compounds,

thus enhancing anaerobic bacterial activity and

decarboxylation of phytanic acid. This leads to

generation of pristine rich oils characterized by the

Pristane-to-heptadecane ratio greater than unity. But the

redox potential of the environment do not favour

generation of phytane-rich crude oils.

2) Source rocks deposited at open water environment

generated oils of low pristane/C

(heptadecane) ratios.

3) Source rocks of alternating swampy and open water

conditions produce oils of intermediate-Pristane/ n-

heptadecane ratio.

Criteria for Geologic Interpretations

Optical activity in crudes serves as an important tool for

correlation of oils of similar geologic ages. This property

depends on the amount of cholesterine substances in the oil.

There prevails a transformation of the triterpane and sterane

hydrocarbons into polycyclic naphthenes thus, causing a

gradual depletion of the concentration of the cholestorine

substances with geologic age. Optical rotations of younger

crude oils are usually greater than those of older rocks.

Amosov (1951) measured the optical rotation of several

crudes in USSR, and arrived at a conclusion that crude oils

of Tertiary age have optical rotations above 1

, while those

of Mesozoic and Paleozoic oils were below 1

Table VII shows the change in optical rotation of crude oils

with Age. Trace metals are used in geologic interpretation

on the basis of the vanadium-to-nickel ratio. Nickel

substances in crude oils are more stable than those of

vanadium. These metals enter the oil substance very early in

its history and leave the substance at unequal rates as the oil

matures in its reservoir rock. This gives rise to a net change

of the vanadium-to-nickel ratio toward a lower value as the

oil becomes older. Hodgson (1954) noted a uniformity in

the trend of variations of vanadium nickel, sulphur, resins

and increasing asphaltene content in the crude oils of

Western Canada. He regarded this trend as a product of

alteration. All these characteristics of trace metals in crude

oils earn them a recognition as sensitive indicators of

alteration and geologic age.

The sulphur isotopic ratio (S

) of crude oils help in the

correlation of oils of different source rocks and those of

different stages of maturation. Though there occurs some

loss of sulphur during maturation and weathering of the

crude oils, sulphur isotope ratios show little or no change.

Thus, the presence of high sulphur isotopic ratios and

generally low sulphur content is indicative of alteration.

Very viscous crude oils may suggest some degree of

weathering in highly fractured basins.

Criteria for Economic Significance

Some physical and chemical properties of crude oils

determine their economic value. Such properties are the

A.P.l gravity, wax content, sulphur content and the type of

dominant hydrocarbon group (or fractions) of the crude oil.

Furthermore, the economic significance of a crude oil also

depends on the immediate requirements of the consumer or

country. For example light paraffinic (of high A.P.I

gravities) oils yield high amount of gasoline and, kerosene

as the refinary products. Such countries as India desiring

greater supply of kerosene, for domestic source of heat have

high demand for crudes that give maximum percentage of

kerosene on refining. Crude Oils of moderate gravities

(mixed and naphthenic base crudes) are of high demand by

advanced countries. This is because of great need for

lubricating (industrial) oils and gasolines in such countries

for example; U.S.A. makes high demand for both naphthenic

and light paraffin oils. Generally however, there is usually a

great demand for oils of A.P.I gravities ranging from 20° to

45°. Very light oils (of high A.P.l gravities) and heavy oils

(of low A.P.I gravities) can be blended to meet the

requirements of the consumers.

The amount of paraffin wax contained in a crude oil

influences its economic evaluation. Waxy crude oils are

characterized by low calorific values, thus a low capacity to

Paper ID: ART20174603

DOI: 10.21275/ART20174603

1518

International Journal of Science and Research (IJSR)

ISSN (Online): 2319-7064

Index Copernicus Value (2015): 78.96 | Impact Factor (2015): 6.391

Volume 6 Issue 6, June 2017

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generate heat. This limitation therefore earns any waxy

crude oil a relatively low price.

Occurrence of sulphur in crude oils is of negative economic

significances. This is because; high content of sulphur in a

crude creates corrosive effects on refining columns. And

any installation of special sulphur-resistant steel columns is

usually very expensive. The processes of removal of sulphur

from crude oils are also expensive and time consuming.

Moreover, refining of sulphur bearing crude oils

create

strong environmental pollution. All these amount to a

decrease in demand for, or subsequent reduction in the price

of sulphur bearing crude oils.

Broadly speaking however, the prices of typical crude oils of

all the member countries of OPEC countries are determined

by the Organization of Petroleum Exporting Countries

(O.P.E.C).

6. Some Characteristics of Nigerian Crude Oils

A.P.I Gravity

The A.P

I gravities of crude oils of the Niger delta do not

exhibit a very consistent trend. The A.P.I gravities of

Nigerian crude oils vary from 30

to 40

A.P.I. They change

abruptly after a certain depth called the critical depth. The

trend of variation of A.P.l gravities of the oils corresponds

with the direction of change from medium A.P.I gravity

naphthenic oils to light paraffinic oils, as the depth of burial

increases. The interface between the heavier and lighter oils

is often described as the level of transformation. It is

characterized by temperatures of about 60 - 80

According to Evamy et al (1978), the paraffinic oils below

the level of transformation were produced by late migration

from the source rock.

The accumulation of lighter paraffinic oils at greater depths

of burial is as a result of the combination of source and

reservoir maturation processes, associated with the high

geotemperature conditions at such depths, (which ranges to

about 100

- 115

C) , and other katagenetic processes.

In some of the oil fields of the Niger delta, extensively

faulted reservoirs are thought to have created leakage

channels through which inorganic agents such as oxygen and

water drain into the reservoir.

These inorganic agents cause anomalous weathering of

crude oils in the reservoir at high temperatures. Fig. 11

shows the faulted nature of the Niger Delta sediments.

Pour points

The pour points of crude oils from different oil fields of the

Niger Delta show significant variation. This variation

reflects a change in the most abundant group of

hydrocarbons contained in the crudes. For example,

paraffinic waxy oils of the Niger delta possess pour points

ranging from 20 to 90

F. But naphthenic nonwaxy oils of

Niger Delta have pour points generally lower than - 25

(Frankl and Cordvy, 1967).

Paraffin wax Content

Most Nigerian oil fields depict a close intercalation of high

wax and low wax oils. Some of the paraffin wax bearing

crude oils produce as high as 5% wax content.

These waxes are believed to be derived from terrigenous (or

land plants) materials of the source rock

Chukwuemeka and

Okoye (1980) supported this view by their successful

investigation of the presence of humic, vitrinite and liptinite

particles in the Niger Delta source rock. These particles

being indicators of terrestrial lithotypes. The widely

supported view is that the source beds of the Niger Delta

crudes are deeply buried paralic to marine Akata Shale, A

schelaitic representation of the Niger Delta structures is

shown in figure 12.

Influence of Geotemperatures

The nature of the crude oils in the Niger Delta is being

controlled by geotemperatures, thermal maturation and

depth of burial. Variation in these elements of katagenesis

results in the production of paraffinic-rand paraffin-

naphthenic crude oils. API gravities and pour points are low

at cooler depths of the reservoir. At relatively higher

subsurface temperatures, the crudes are light and

characterized by pour points higher than 20

F. The

significance of geoteraperatures on the crude oils of the

Niger Delta is not only reflected on the A.P.I gravity, but

also in their viscosities and boiling point fractions.

The irregular variation in the properties of crude oils of

multiple reservoirs of Niger Delta can also be attributed to

random sand bodies that are interbedded with shales, and the

influence of tectonic activity, high rates of subsidence and

deposition of its source rocks.

Generally speaking, the Nigerian crude oils are light

paraffinic, and paraffin-naphthene base crudes. They yield

significant amounts of gasoline, kerosene, lubricating oils,

and provide naphtha which can be the raw material for

fertilizer industries. Some representative properties of

Nigerian crudes are shown on Table VIII. Nigerian crudes

are characterized by little or no occurrence of sulphur. This

saves the expense of purchasing sulphur-resistant steel for

refining installations, and also saves the time and expensive

processes of removal of sulphur in crudes. These favourable

factors make the Nigerian crude oils attractive in advanced

countries.

7. Conclusion

Crude oils are naturally occurring liquid phase of petroleum,

composed principally of hydrocarbon compounds and are

extracted from the earth in the liquid state. The hydrocarbon

compounds in crude oils include paraffins, ranging from

pentane to pentadecane (C

-C

) alkylparaffins, naphthenes,

alkylbenzenes and nuclear aromatics. Other associated

constituents are natural gases, hydrocarbon waxes, and salt

water.

There are three main categories of crude oils viz:

1) Paraffin base crudes: containing higher percentage-

abundance of paraffin hydrocarbons, and yield more

gasoline and kerosene as refinery products.

Paper ID: ART20174603

DOI: 10.21275/ART20174603

1519

International Journal of Science and Research (IJSR)

ISSN (Online): 2319-7064

Index Copernicus Value (2015): 78.96 | Impact Factor (2015): 6.391

Volume 6 Issue 6, June 2017

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Licensed Under Creative Commons Attribution CC BY

2) Naphthene base crudes: containing greater amount of

naphthenes and aromatica, and yield more lubricating

oils and less gasoline and kerosene.

3) The mixed base, which is an intermediate between I and

II.

Crude oils are characterized by some physical and chemical

properties, which to a measurable extent, play important role

in the understanding of their geologic history and

environment of origin. Such a physical property as optical

activity is dependent on hydrocarbon derivaties from organic

cholesterine substances, and are destroyed at high

temperature of about 200

C. They serve as important

criteria for environmental analysis, reconstruction of

temperature history of the crudes, and correlation of crude

oils. Cloud and pour points of crude oils reveal the

influence of low temperatures on crude oils and

simultaneously provide information about the paraffin wax

content of the crudes. Most of the physical properties are

influenced by the reservoir pressures and temperatures,

chemical composition of the crudes, and sometimes, by the

amount of dissolved natural gases.

The chemical properties of crude oils vary in relation to

changes in geotemperatures and pressures, coupled with

some other elements of katagenesis. Such chemical

properties as paraffin wax and perhyrins are complex forms

of hydrocarbons which have genetic relationship with living

organic-life. Occurrence of isoprenoid hydrocarbons, such as

pristanes and phytanes in crude oils, enhances the

construction of the genetic environments of deposition of the

source rocks. Trace metal substances concentrated in crudes

are thought to be derived from sea water as secretions by

marine life.

The economic significance of any crude is influenced by

such properties as paraffin wax content, sulphur content and

A.P.I gravity. Standard prices of crude oils are however

fixed by producers like the Organization of Petroleum

Exporting Countries (OPEC).

The A.P.I. gravities of Nigerian crudes change abruptly after

a critical depth, where there is a marked change in

temperature. Nigerian crudes are generally light, low

sulphur-bearing, and are in great demand by advanced

countries.

Some physical and chemical properties of crude oils have to

some significant extent, given explanations to the genesis of

crudes, and have provided information about the likely

environments of deposition of the crudes source rocks. They

also serve as correlation indices, and indicators of geologic

age and alteration.

References

[1] Amosov, G. A. (1951). Optical Rotation of Petroleum.

New series No. 5 p. 225 – 233.

[2] Baker, E. G. (1967). A geochemical evaluation of

petroleum migration and accumulation. In fundamental

Aspects of petroleum Geochemistry. Elsevier, New

York, P. 299 – 329.

[3] Brunnock, J.U. (1966). Distribution of normal paraffins

in Libyan and Nigerian crude oils. Nature, Vol. 212 No.

5060 p. 385 – 387.

[4] Chukwuemeka, M. E. and Okoye, N.V. (1980).

Petroleum source bed of the Tertiary Niger Delta.

A.A.P.G Bull. Vol. 64 No. 8 P. 1251 – 1258.

[5] Cordell, R. J. (1972). Depths of oil, origin and primary

migration: A review and critique. A.A.P.G Bull. Vol. 56

No. 1 P. 2029 – 2067.

[6] Evamy, B.D., Haremboure, J., Kamerling, P., Knaap,

W.A., Molloy, F.A. and Rowlands, P.H. (1978).

Hydrocarbon Habitat of tertiary Niger Delta. A.A.P.G

Bull. Vol. 62 No. 1 p.

[7] Frankl, E.J. and Cordry, E.A. (1967). The Niger Delta

Oil provine: Recent Developments Onshore and

Offshore. Proc. 7

World petroleum Congress Mexico,

Vol. 1B P. 195 – 209.

[8] Hedberg, H. D. (1968). Significance of high wax oils

with respect to genesis of petroleum. A.A.P.G Bull.

Vol. 52 p. 736 – 750.

[9] Hodgson, G.W. (1954). Vanadium, Nickel and Iron

trace metals in crude oils of Western Canada. A.A.P.G

Bull. Vol. 38 No. 12 P. 2537 – 2554.

[10] Hodgson, G.W. and Baker, B.L. (1957). Vanadium,

Nickel and Porphyrins in Thermal Geochemistry of

petroleum. A.A.P.G Bull. Vol. 41 p. 2413 – 2426.

[11] Hodgson, G.W., Baker, B.L. and Peake, E. (1967).

Geochemistry of porphyrins. In fundamentals of

Petroleum geochemistry. (Ed. Colombo, U., Navy, B.),

Elsevier, London, P. 177 – 259.

[12] Levinson, A.A. (1974). Introduction to Exploration

Geochemistry. Applied Publishing Ltd., Illinois.

[13] Lijmbach, G.W.M. (1975). Origin of petroleum. Proc.

World petroleum Congress, Vol. 2 p. 356 – 371.

[14] Silvermann, S.R. and Epstein, S. (1959). Carbon

isotopic composition of petroleum and other

sedimentary origin materials. A.A.P.G Bull. Vil. 42 No.

5 p. 1912 – 1921.

[15] Silvermann, S.R. (1960). Carbon isotope evidence on

mechanism of petroleum maturation. A.A.P.G. Bull.

Vol. 44 No. 12 p. 1256 – 1262.

[16] Welte, D.H. (1965). Relation between petroleum and its

source rocks. A.A.P.G. Bull. Vol. 30 No. 11 p. 2514 –

2524.

[17] Zobell, C.E. (1947). Microbial transformation of

molecule hydrogen in marine sediments with particular

reference to petroleum. A.A.P.G Bull. Vol. 31 no. 6 p.

1709 – 1751.

Table 1: Classification of Crude Oils

Types of

Crude

Wax % Asphalt

% Composition Of 200 – 300

Fraction

Light

Paraffinic

0 – 1.5 0 – 6.0 Paraffin Naphthenes Aromatic

46 – 61 22 – 32 12 – 25

Paraffin-

Naphthenic

1.0– 6.0 0 – 6 5 – 42 9 – 38 16 – 20

Naphthenic trace 0 – 6 15 – 26 61 – 76 8 – 13

Aromatic 0 – 5.0 0 – 20 0 – 8 57 – 78 20 – 35

After Levinson, 1974

Paper ID: ART20174603

DOI: 10.21275/ART20174603

1520

International Journal of Science and Research (IJSR)

ISSN (Online): 2319-7064

Index Copernicus Value (2015): 78.96 | Impact Factor (2015): 6.391

Volume 6 Issue 6, June 2017

www.ijsr.net

Licensed Under Creative Commons Attribution CC BY

Table 2: Some properties of Nigerian crude oils

Bomu Oil Field (Onshore) Okan Oil Field (Offshore)

Gravity: 34.4

API Gravity: 34.6

API

Viscosity: 4.4 centipoises at

100

Flash point: below 60

Paraffin wax: 5.1% Paraffin wax:

Sulphur: 0.16% Sulphur: 0.15%

Pour point: + 64

0+F

Pour point: 25

Refractive Index: 1.4816

Colour: Greenish brown

Dissolved Natural: gases 3.56%

+ Heavier Hydrocarbons:

96.44%

(Source: Frankl and Cordry, 1967)

Paper ID: ART20174603

DOI: 10.21275/ART20174603

1521