Chapter 14

Source Rocks and Petroleum Geochemistry

Knut Bjørlykke

As discussed in Chap. 1, petroleum is generated from

organic matter which accumulates in sedimentary

basins. Only a small fraction of the organic matter

produced in the photic zone in the ocean becomes

trapped in sediments (Fig. 14.1). Most of the organic

matter is formed by photosynthesis producing algae

(bacteria) and higher organisms which feed on algae.

There there may also be a supply of organic matter

from land. Most of the organic matter is oxidised in the

water column or on the seafloor and the nutrients are

released into the water and become available for new

organic production near the surface during upwelling.

Most source rocks are black shales like the Upper

Jurassic Kimmeridge Clay and its equivalents in the

North Sea basin (Fig. 14.2). Formation of rich source

rocks requires that the organic matter is not diluted too

much by deposition of clastic material or by precipita-

tion of carbonates.

The organic matter is transformed into kerogen

which consists of very large and complex molecules.

We do not usually apply the term kerogen to fresh

organic material, but to material which is somewhat

dehydrated after burial to about 100 m or more. Kero-

gen is formed gradually within the upper few hundred

metres of the sediment column after deposition from

precursor products like humus, and humic and fulvic

acids. The organic matter may be derived from marine

organisms, mostly algae, or from plants derived from

land.

The transformation of amino acids, carbohydrates,

humic acids and other compounds into kerogen is

achieved by the removal of functional groups such as

acid groups, aldehydes and ketones. This involves a

loss of oxygen from the organic material, also of

nitrogen, water and CO2.

Kerogen therefore has higher H/C, and lower O/C,

ratios than the initial compounds (Fig. 14.3).

Kerogen may also include organic particles of mor-

phologically recognisable biological origin such as

vitrinite (derived from woody tissues and liptinite

materials, e.g. algae spores, cuticles, etc.). Because

of its resistance to strong oxidising acids kerogen can

be recovered from sedimentary rocks by dissolving

most of the rock away with HCl or HF.

It is also possible to separate kerogen by a density

method, using heavy liquids, because kerogen is ligh-

ter than minerals. The resulting concentrate of kerogen

can be studied microscopically using transmitted and

reflected normal light, to identify the biological origin

and the degree of thermal alteration. These phases of

altered organic material are called macerals. Algal

material has a dull appearance, while wood and mate-

rial from higher plants is called vitrinite. Vitrinite

becomes increasingly shiny when exposed to higher

temperatures and by measuring the amount of

reflected light in the microscope we obtain an expres-

sion for the degree of thermal alteration (Vitrinite

index).

It is also useful to use ultraviolet light microscopy,

since certain components, i.e. liptinites, display char-

acteristic fluorescence colours. Infra-red spectroscopy

(IR) or nuclear magnetic resonance (NMR) spectros-

copy can be used to investigate the chemical composi-

tion and structure of kerogen.

K. Bjørlykke (*)

Department of Geosciences, University of Oslo, Oslo, Norway

e-mail: knut.bjorlykke@geo.uio.no

K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,DOI 10.1007/978-3-642-34132-8_14, # Springer-Verlag Berlin Heidelberg 2015

361

Being a complex of very large molecules (poly-

mer), kerogen is difficult to analyse, but upon heating

to 350–450�C in an inert atmosphere (pyrolysis) it will

break down into smaller components which can then

be analysed by means of gas chromatography and

mass spectrometry.

Kerogen has a wide range of compositions, depen-

dant on the original organic composition, but may be

classified into 3 main types which may be plotted as a

function of the H/C ratio and the O/C ratio (Fig. 14.3)

Type I sapropelic kerogen is formed from organic

material with a high content of lipids with long ali-

phatic chains. It consists of spores and planktonic

algae, as well as animal matter, which have been

broken down microbially after deposition in the sedi-

ment. Sapropelic material which consists of fats, oils,

Sea level

Ox.Red.

Photosynthesis phytoplankton, zooplankton(consume phytoplankton)

Plankton eaten by higher marine organisms

Formation of pellets

Breakdown of organic materialoxygen minimum

Adsorption of dissolved organic materialonto clay minerals

Accumulation of pellets and other organic matter

Oxidation

Breakdown by burrowing organisms and bacteria

Preserved organic matter in sediment

Sea floor

Fig. 14.1 Formation of source rocks. Only a small fraction of the organic matter is preserved. The formation of organic-rich source

rocks requires restricted water circulation and oxygen supply

Fig. 14.2 Draupne shale (Kimmeridge shale) cores from northern North Sea (Block 34/7)

362 K. Bjørlykke

waxes, etc., has a high H/C ratio, usually between 1.3

and 1.7. This kind of kerogen is often called Type I,

and contains little oxygen (O/C <0.1). It will provide

mainly oil, with less gas (CH4 and CO2). Type I

kerogen is typical of oil shales, especially in freshwa-

ter basins like the Green River basin in Colorado,

Wyoming and Utah, but is also found in marine basins.

Type II kerogen is a mechanically and chemically

complex mixture of algae and other marine organisms

and plant debris. The composition varies considerably,

depending on the initial organic precursor materials

which again may be linked to depositional facies.

Type II kerogen represents a composition midway

between types I and III but it does not represent a

mixture of these end members. It has relatively high

H/C, and low O/C, ratios, but contains more oxygen-

containing compounds (ketones and carboxyl acid

groups) than Type I. Esters and aliphatic chains are

also common. This is the usual type of kerogen found

in marine basins where mixtures of phytoplankton,

zooplankton and micro-organisms have accumulated

under reducing conditions, sometimes along with

land-derived plant material. This type of kerogen is

the most common source of oil (Fig. 14.3).

Type III humic kerogen is derived from organic

matter from land plants, such as lignin, tannins and

cellulose. This type has a low initial H/C ratio, and a

high initial O/C ratio, reflecting the composition of the

precursor plant matter. In maturing (through the effect

of temperature) this kerogen, which is often called

Type III (Tissot and Welte 1984), generates abundant

water, CO2 and methane (CH4). Most coals have a

composition and structure similar to Type III

kerogens. Coal generates mostly gas but some coals

may also generate some oil.

14.1 Transformation of Kerogen withBurial and Temperature Increase

Source rocks compact mechanically as a function of

effective stress similar to mudstones and the porosity

and void ratio which is defined by the volume of solids

(Kerogen and minerals). When the kerogen gradually

becomes mature some of the solid kerogen becomes

fluid petroleum, thus changing the void ratio. This is

an important factor in the expulsion of oil and gas. The

transformation of kerogen is mainly a function of

temperature even if the pressure may have some

effect.

With increasing temperature the chemical bonds in

these large molecules (kerogen) are broken and kero-

gen is transformed into smaller molecules which make

up oil and gas. This requires that the temperature must

be 100–150�C over long geological time (typically

1–100 million years).

The conversion of kerogen to oil and gas is thus a

process which requires both higher temperatures than

one finds at the surface of the earth and a long period

of geological time. Only when temperatures of about

80–90�C are reached, i.e. at 2–3 km depth, does the

conversion of organic plant and animal matter to

hydrocarbons very slowly begin to take place. About

120–150�C is the ideal temperature range for this

conversion of kerogen to oil, which is called matura-

tion. This corresponds to a depth of 3–4 km with a

normal geothermal gradient (about 30–40�C/km). In

volcanic regions organic matter may mature at much

lesser depths due to high geothermal gradients (e.g.

high heat-flow areas). In large intracratonic sedimen-

tary basins or along passive margins, however, the

geothermal gradient may be only 20–25�C/km and

2.0

1.5

1.0

0.5

0.1 0.2O/C

Metagenesis Dry

gas

Katagenesis

Diagenesis

CO2

H2O

CH 4

IIIHumickerogen

OilWet gas

Sapropelic

kerogen

II

I

H/C

Fig. 14.3 Diagram (Van Krevelen diagram) showing the pri-

mary composition of the different types of kerogen and the

changes as a function of heating (maturation) during progressive

burial

14 Source Rocks and Petroleum Geochemistry 363

the minimum overburden required to initiate petro-

leum generation will be correspondingly greater

(4–6 km).

In general one can say that petroleum can not be

generated near the surface except locally through the

influence of hydrothermal and igneous activity. Shal-

low deposits of oil and gas which we find today were

actually formed at great depths and either the overbur-

den has been removed by erosion or the hydrocarbons

have migrated upwards considerable distances. As we

have already seen, however, large amounts of natural

gas, chiefly methane (CH4), may be formed near the

surface by biochemical processes.

Temperature increases with increasing overburden,

causing the carbon-carbon bonds of the organic

molecules in the kerogen to rupture. This results in

smaller hydrocarbon molecules. When kerogen matu-

ration reactions are completed, the kerogen’s

“organic” components, which may be derived from

lipids, fatty acids and proteins, have been converted

into hydrocarbons.

As the temperature rises, more and more of the

bonds are broken, both in the kerogen and in the

hydrocarbon molecules which have already been

formed. This “cracking” leads to the formation of

lighter hydrocarbons from the long hydrocarbon

chains and from the kerogen. The removal of gas,

mainly CH4, leaves the residual kerogen relatively

enriched in carbon. At the outset kerogen (Type I

and II) has an H/C ratio of 1.3–1.7. Humic kerogen

(Type III), which has high initial oxygen contents,

gives off mostly CO2 gas and so its oxygen/carbon

ratio gradually diminishes. This diagenetic alteration

begins at 70–80�C and as water and CH4 are removed

the H/C ratio will fall to about 0.6 and the O/C ratio

will become less than 0.1 at about 150–180�C.In the North Sea basin, most of the oil is generated

at temperatures around 130–140�C, which equates

with a depth of about 3.5 km. If temperatures higher

than 170–180�C persist for a few million years, all the

longer hydrocarbon chains will already have been

broken (cracked), leaving us only with gas – mainly

methane (dry gas). The kerogen composition will

gradually be depleted in hydrogen and move towards

pure carbon (graphite) (H/C!0).

Source rock can in some cases be mapped out on

seismic. Source rocks have in most cases lower density

and velocity (low AI) than sandstones, limestones and

shales. When the kerogen becomes mature, generation

of petroleum changes the solid/fluid ratio and particu-

larly gas will reduce velocity and density. Mapping

out source rocks and their maturity has become a new

and important exploration technique in some sedimen-

tary basins (Løseth et al. 2011). Seismic data may also

be used as indications of the degree of maturity.

14.2 What Factors Influence theMaturation of Kerogen?

The term “maturity” refers here to the degree of ther-

mal transformation of kerogen into hydrocarbons and

ultimately into gas and graphite. The conversion of

kerogen into hydrocarbons is a chemical process

which takes place with activation energies of around

50–60 kcal/mol. This energy is required to break

chemical bonds in the kerogen which consists of

very large molecules (polymers) so that smaller hydro-

carbon molecules can be formed.

It has been assumed that formation of oil is a first

order reaction, the rate of which is an exponential

function of time. Understanding the factors which

influence the rate of this reaction is of great interest.

Four factors are thought to contribute:

1. Temperature

2. Pressure

3. Time

4. Minerals or other substances which increase the

rate of reaction (catalysts) or which inhibit

reactions (inhibitors).

Temperature is clearly the most important factor,

and hydrocarbons can be produced experimentally

from kerogen by heating it (pyrolysis). This reaction

is time-dependent and in laboratory experiments,

where time is more limited than it is in nature, fairly

high temperatures (350–550�C) have to be used in

pyrolysis. This is the case when oil and gas is pro-

duced from oil shales by pyrolysis of immature kero-

gen in ovens after quarrying (see chapter 21). Pressure

appears to play a minor role but increasing pressure

should reduce the rate of the reaction because of the

increase in volume involved in the formation of

hydrocarbons (Le Chatelier’s rule).

There is a relatively small volume increase when

kerogen becomes oil, even though oil is lighter than

kerogen. This is due to the residual (coke) which

remains unaltered.

364 K. Bjørlykke

When kerogen is converted directly into gas, or

from oil which has been formed first, there is a marked

volume increase. This should lead to slower reaction

rates under high pressure in a closed system and retard

generation of gas. Generation of petroleum, particu-

larly gas, may contribute to the formation of overpres-

sure but in a sedimentary basin the pressure will for the

most part be controlled by the flow of water which is

the dominant fluid phase. In the source rock however

overpressure is likely to develop, causing

hydrofracturing which helps to expel the generated

petroleum. The main cause for overpressuring is, how-

ever, not only the increase in fluid volume but the

transformation of solid into fluids. When solid kerogen

is transformed into fluid oil or gas the ratio between

the solid phase and the fluid phase is changed, as

expressed by the porosity and the void ratio.

Temperature is however the most important factor

controlling petroleum generation.

It has long been suspected that minerals, particu-

larly clay minerals, might affect the rate of hydrocar-

bon generation. A number of laboratory experiments

have been carried out in which kerogen is mixed with

various minerals but the results have not been

conclusive.

The conversion of organic matter begins at

70–80�C, given long geological time. Between 70

and 90�C the transformation of kerogen proceeds

very slowly, and it is only in ancient, organic-rich

sediments that significant amounts are formed. Most

of the maturation process occurs between 100 and

150�C. Here the degree of kerogen transformation is

also a function of time. This means that rocks which

have been subjected to 100�C for 50 million years are

more mature than rocks which have been exposed to

this temperature for 10 million years. As the organic-

rich sediment (source rock) is buried in a sedimentary

basin, it will normally be subjected to increasing tem-

perature as a function of increasing burial depth. If we

know the stratigraphy of the overlying sediment

sequence and the geothermal gradient and the subsi-

dence curve, we can calculate the temperature as a

function of time.

At low degrees of maturity we find more of the

alkenes (olefins) and cykloalkenes (naphtenes),

which have high H/C ratios, while with greater matu-

rity there is an increase in the proportion of aromates

and polyaromates (low H/C ratio). Oil thus acquires

increasing gas content with increasing maturity.

During this transformation of organic matter, water

and oxygen-rich compounds are liberated first, then

compounds which are rich in hydrogen. This conver-

sion results in enrichment of carbon and the colour of

the residual kerogen changes from light yellow to

orange, brown and finally black. These gradations

can best be registered by measuring light absorption

of fossil pollen and spores (palynomorphs). It is also

possible to analyse colour changes in other kinds of

fossils, for example conodonts.

For application in exploration a rapid semi-

quantitative method has been developed whereby

these colour changes are estimated from smooth

spores examined under transmitted light and compared

with a standard colour scale. This parameter is called

the “Thermal alteration index” (TAI) and will give a

rough idea of the thermal maturity of the sediments

and their temperature history.

Another way of assessing palaeotemperatures at

which alteration took place in sedimentary rocks is to

record the degree of carbonisation of other plant

remains which are usually present. Vitrinite, which

originally was fragments of woody tissue, is a com-

mon component of coal but is also found in smaller

amounts in marine source rocks. This material is

analysed by measuring the amount of light it reflects.

It becomes shinier and reflects light better as the

degree of carbonisation increases. By measuring the

reflectivity of vitrinite particles under a reflected light

microscope an exact value is obtained for this maturity

parameter, expressed by the reflectivity coefficient R0

(% vitrinite reflectance). If R0 is less than 0.5% in a

shale it can not have generated much oil and is classed

as immature. Shales with R0 ¼ 0.9–1.0 have been

exposed to temperatures corresponding to maximum

oil generation. R0 ¼ 1.3 represents the upper limit for

oil generation, above which the shale will only pro-

duce condensate (light oils) or gas.

For certain source rocks the ratio between extract-

able alkenes (paraffins) with an even number of carbon

atoms per molecule and those with an odd number

may also be an expression of maturity. In plant mate-

rial and in marine algae, one finds a higher abundance

of alkenes with an odd number of carbon atoms than in

transformed organic matter like waxes and fatty acids.

The decrease in this predominance of odd over even in

source rocks with increasing maturity is due to the

dilution of the original biologically derived n-alkanet

mixture with a newly generated mixture which has a

14 Source Rocks and Petroleum Geochemistry 365

regular carbon number distribution. This odd/even

ratio is normally expressed by means of an index

called the Carbon Preference Index (CPI):

CPI ¼ n-alkenes oddð Þ=n-alkenes evenð Þ

It is based on analyses of alkenes with carbon

number between 25 and 33 (C25 and C33) in oils.

However, organisms also start off with different

CPI index values, and land plants have high ratios

between odd and even carbon numbers. Bacteria

have a predominance of even carbon numbers.

The maturation process will cause a shift in the

carbon number distribution towards smaller

molecules, particularly in the range C13–C18.

Oil that comes from carbonate source rocks often

has a low CPI index, while oil derived from plants has

a high index value. With increasing temperature the

CPI index goes towards 1, that is to say, equal even

and odd carbon numbers.

The isotopic ratio also changes because the bonding

between hydrogen and 13C, and between hydrogen and12C, are not equally stable. Light gases such as meth-

ane, are enriched in the light isotope 12C, and the

hydrocarbons that remain will therefore have an

increasing 13C/12C ratio with increasing temperature.

There is isotopic fractionation of carbon, and when

kerosene is releasing petroleum, this phase is some-

what enriched in 11C corresponding to the precursor

kerosene. Gases, particularly methane, normally have

lighter carbon isotopes than kerosene and oil. When

methane is formed from larger hydrocarbon molecules

by thermal cracking, the 12C–12C bond is less stable

than the 13C–12C bond and the product becomes

enriched in 12C (low δ13C).

14.3 Modelling of Petroleum Generation

The rate of petroleum generation can be calculated and

modelled.

We assume that the rate (ki) of petroleum genera-

tion follows an Arrhenius function:

kið Þ ¼ A exp �Ei=RTð Þ

where R is the gas constant, T is the temperature and Ei

is the activation energy. The activation energy mostly

varies between 50 and 80 kcal/mol or about 200 kJ/

mol. A is an exponential constant dependant on the

type of kerogen.

Since the temperature changes during the burial

history the effect of temperature has to be integrated

over the range of temperatures that the kerogen is

exposed to. This may be expressed by the Time Tem-

perature Index (TTI)

TTIð Þ ¼Z tx

t0

2F Tð Þdt

This is the integrated temperature (T) over time

from the time of deposition (t0) to the present day

(tx). F is a factor dependant on the activation energy

for the reactions. The reaction rates may approxi-

mately double for each 10�C increment. The matura-

tion of kerogen to petroleum can be successfully

modelled (Makhous and Galushkin 2005) but the

results depend very much on the input data with

respect to the temperature history. The activation

energy may also vary significantly for different types

of source rocks. If the geothermal gradient can be

assumed to have been constant throughout the relevant

period, this is relatively simple. However, if the geo-

thermal gradient has varied considerably through time

it is a lot more complicated.

A theoretical maturity parameter (P) can be calcu-

lated by integrating temperature with respect to time:

P ¼ ln

Z t

0

2T=10 � dT

t, geological time (million years); T, temperature (�C).We see that a doubling of the reaction rate for every

10�C is built into this expression (Geoff 1983). This is

an expression which is very similar to Lopatin’s Time-

Temperature Index (TTI) (Waples 1980) which

integrates the temperature the source rock is subjected

to with respect to the burial time.

When the temperature rises above about

130–140�C, maturation proceeds very rapidly, and

then the time factor is less crucial.

There are differing views as to how much emphasis

should be placed on time in relation to temperature in

the matter of maturation. Oil companies use different

formulae for calculating these temperature factors and

the 10-degree rule is now found not always to be valid,

particularly for very young sedimentary basins with

high geothermal gradients.

366 K. Bjørlykke

The rate of sediment heating, i.e. temperature

increase per unit time, can be expressed as dT/dt.

The rate of subsidence is dZ/dt, and the geometrical

gradient dT/dZ. We then get:

Rate of heating dT=dtð Þ ¼ rate of subsidence

dZ=dtð Þ � geothermal gradient dT=dZð Þ:

In basins like the North Sea the rate of heating is

typically only 1–2�C/million years but during rapid

subsidence and sedimentation in the late Cenozoic

the heating rate was higher. During the deposition of

1–1.5 km of upper Pliocene and Pleistocene sediment

in 2–3 million years, the gradient was lowered but the

heating rate must have been about 10�C/million years.

The maturity of source rocks can now be calculated

with the help of basin modelling integrating tempera-

ture over time. The subsidence curve for the source

rock is determined from the stratigraphic age and

thickness of the overlying sequence.

By estimating a geothermal gradient, depth can be

converted to temperature. This way we obtain a curve

that shows the temperature history of the kerogen

through geological time. Integration of this curve

enables the maturity to be calculated (Fig. 14.4).

If we know the stratigraphy of the overlying

sediments and the geothermal gradient, temperature

can be calculated as a function of time.

Calculating the maturity is important not only for

predicting where the source rocks are sufficiently

mature to produce oil and gas (or perhaps over

mature). It is also important for estimating the timing

of the generation and migration of oil and this type of

basin modelling has become an important part of oil

exploration. The most important input into the calcu-

lation is the subsidence history as derived from the

stratigraphic record, and the estimated geothermal

gradient as a function of geological time. The success

of the basin modelling depends as always very much

on the quality of the input data.

14.4 Rock-Eval Analyses

Rock-Eval is a standard routine analysis of source

rocks, usually shales, to establish how much of the

kerogen has been transformed into petroleum and how

much can be transformed at a higher temperature.

The sample of shale is crushed and heated to

300�C, at which point one measures the amount of

hydrocarbons that are already formed in the source

rock but have not migrated out. The content of hydro-

carbon with carbon numbers between C1 and C25, is

called S1. It is measured as the area beneath the peak

S1 (Fig. 14.5).

On further heating from 300 to 550–600�C, newpetroleum is formed in the laboratory from the kero-

gen by heating (pyrolysis), and this amount is called

S2. This is a measure of how much oil and gas could

have been generated if the source rock and been buried

deeper. The reason it requires such high temperatures

is that the heating in the laboratory lasts just a few

minutes or hours, instead of some millions of years.

During heating from c. 300 to 550�C, CO2 is also

formed and is collected and measured separately as the

S3 peak (Fig. 14.5). Most of the CO2 groups dissociate

from the kerogen between 300 and 390�C. The gener-ation of petroleum varies with temperature and

reaches a peak corresponding to S2 (Fig. 14.5). This

temperature, which gives the maximum petroleum

generation, is called Tmaks and is typically in the

range 420–460�C.

A

B

Time

DepthTemperature

C

Fig. 14.4 The maturity of a source rock is a function of the

time – temperature index (TTI). This can be calculated from the

burial curve if the geothermal gradients are known. The geo-

thermal gradients are most critical during the deepest burial

because the maturation is an exponential function of the tem-

perature. At a certain depth and temperature the maturity may

vary greatly and burial curve C will produce the highest maturity

at this depth. Source rocks buried following curve A and B are

less mature because their exposure to greater burial depth

(higher temperatures) has been much shorter

14 Source Rocks and Petroleum Geochemistry 367

The ratio between the amount of petroleum

generated (S2) and the total content of organic material

(TOC) is known as the Hydrogen Index (HI).

The quantity of CO2 which is formed (S3), is lim-

ited by the oxygen content of the kerogen. The S3/

TOC ratio is the Oxygen Index (OI).

The ratio between the quantity of free oil already

formed (S1) and the total amount of petroleum (S1 +

S2), is an expression of how much petroleum is still

left in relation to how much has already been

generated. The S1/(S1 + S2) ratio is the Production

Index (PI). Good source rocks have a high production

index.

When we analyse source rocks, we must however

take into account that some of the oil which has been

generated has migrated out of the source rock: the S1peak represents the remaining petroleum.

Good source rocks are the first prerequisite for

finding oil and gas in a sedimentary basin. If they are

not present, one can save oneself the trouble of further

prospecting. However the quality of the source rock

can vary through the basin and the type of source rock

determines the composition of the oil.

The timing of the oil and gas generation is also very

important. It also determines the timing of oil

migration with respect to the formation of traps.

Once we know with reasonable certainty when oil

generation and migration took place, we can attempt

to construct maps to show the structures and faults at

that time. For example, if a trap was formed by folding

after the oil had migrated, it can not have captured any

of the oil, but perhaps the gas which is formed later.

The generation of oil and gas lends itself to mathe-

matical modelling, and there are good programs which

make this easier. The most important data which have

to be fed in are the subsidence rate, the geothermal

gradient and the activation energy of the kerogen.

The activation energy varies according to the type

of kerogen and is an important factor when it comes to

calculating the timing of the oil generation.

In nature natural processes cause fractionation of

carbon which is reflected in the isotopic composition

of CO2 (Fig. 14.6). Note that the CO2 released by

bacterial fermentation has a positive δ13C while the

carbon from maturation of kerogen (thermal decarb-

oxylation) has negative values.

Re-Os dating of petroleum has made it possible to

date the age of the source rock for oil found in

rerservoirs. Rhenium (Re) and Osmium(Os) are metals

that are concentrated under reducing conditions in

organic matter and also in sulphides. They are released

under oxygenated conditions during weathering. Their

relative concentrations change as a function of geo-

logic time relecting different conditions in the hydro-

sphere and lithosphere. The isotope 187Re decays to187Os and as a result the ratio of 187Os /188Os increases

with geologic time. The age of of organic matter and

sulphides in black shale can the be determined from an

isochron defined by the 187Re/188Os ratio and the187Os/188Os ratio (Hannah et al. 2012). This method

is now being developed to include dating the age of the

source rocks for the oil in oil reservoirs.

14.5 Composition of Petroleum

Naturally occurring petroleum has a very complicated

chemical composition. The most important

hydrocarbons occurring in oil are alkanes (paraffins,

CnH2n+2). Important gases are methane (CH4), ethane

(C2H6), propane (C3H8) and butane (C4H10). Paraffins

from carbon numbers 5, pentane (C5H12) to 15,

pentadecane (C15H32) occur mainly as liquids at

room temperature. These paraffins are an important

300 400 500 600 C

S2S1

S3

Generation ofhydrocarbons

Fig. 14.5 Rock-Eval analyses. A rock sample representing a

possible source rock is heated gradually to about 550�C while

the amount of hydrocarbons generated is measured. At about

300�C oil and gas which has already been generated in the

source rock is expelled and measured as the S1 peak. The peak

at about 400–460�C represents the amount of hydrocarbons

generated from the kerogen in the sample. The temperature of

peak HC generation is called the Tmax. The Hydrogen Index

(HI) ¼ S2 (S)/TOC (total organic carbon) is a measure of the

potential of the source rock to generate petroleum. The total

amount of CO2 generated is measured as the S2 peak. The

Oxygen Index (OI) ¼ P3/TOC

368 K. Bjørlykke

part of the gasoline fraction of oil. Higher paraffins are

waxy and almost a solid phase at room temperature but

become less viscous at higher temperature.

Naphthenes (cycloparaffins) are a series of cyclic,

saturated hydrocarbons with the general formula

(CH2)n. Cyclopentane (C5H10) and cyclohexane

(C6H12) are important members of the naphthene

series which are found in all types of petroleum.

Naphthenes with lower carbon numbers, C3H6 and

C4H8, are gases. The percentage of naphthenes in

petroleum varies from 7 to 31. Petroleum rich in

naphthenes is called asphalt-based crude, because

many of the more complex naphthenes have high

boiling points and high viscosity.

Olefins (alkenes) are hydrocarbons with the same

chemical composition as alkanes, but contain one or

more double bonds. Hydrocarbons of this type do not

normally occur in crude oil. Aromatic hydrocarbons

are unsaturated, cyclic, and have the general formula

CnH2n�6. For example benzene, C6H6, and toluene,

C6H5CH3, are important ingredients of car petrol on

account of their high octane numbers. Aromatic

hydrocarbons have a strong odour and a lower boiling

point than aliphatic hydrocarbons with the same num-

ber of carbon atoms. Aromatic compounds make up

10–39% of crude oil. Refinement of crude oil with a

high content of aromatic compounds results in a high

octane product.

Asphalt is brown to black, solid or highly viscose

petroleum consisting of hydrocarbons with high

molecular weights. Asphalt is rich in aromatic

compounds and naphthenes, and is enriched in sul-

phur, nitrogen and oxygen.

The sulphur content of crude varies from 1 to 5–6%

and occurs as pure sulphur, hydrogen sulphide (H2S)

and organic sulphur compounds. Components in oil

that contain sulphur, nitrogen and oxygen, are called

NSO compounds.

The density of crude oil is measured using the unit

API gravity where “gravity” is expressed as an inverse

value in relation to density as defined by the American

Petroleum Institute.

API gravity ¼ 141:5=ρ� 131:5

HC

O3

–

CH

4

HC

O3

–

SO

42–+15

(–60 to –90)(methane)

CH4

–25

–25

–10 to –25

0 (approx.)

Carbon isotope composition(δ13 C PDB) of CO2(and CH4)

Rate of supply of CO2derived from organiccarbon

Concentration (mM/L)

10 20 30 40 50

Marinecarbonate

Bacterialoxidation

0

10–2

101

2–3.103

Thermaldecarboxylation

Bacterialsulphatereduction2CH2O + SO4

2–

2CO2+ 52– + 2H2O

→

Bacterialfermentation

2 CH2O

CH4O + CO2

→

CH2+O2 CO2+H2O→

→

CH4

SO42–

HCO3–

→

Dep

th in

m b

elow

sea

floo

r

Due to oxidation by Fe3+ reduction

Due to thermal decarboxylation

Isotopically light CO2

Isotopically heavy CO2

→

Fig. 14.6 Natural fractionation of carbon isotopes in CO2

generated in sediments during burial. During oxidation by oxy-

gen or sulphate reduction the δ13C values are highly negative

while bacterial fermentation produces positive values. Thermal

decarboxylation also produces negative δ13C values

14 Source Rocks and Petroleum Geochemistry 369

Here ρ is the density of the petroleum at 60�F(15�C).

We see from the formula that a light oil with a

density (ρ) of 0.8 g/cm3 has a gravity close to 45,

whereas a heavy oil with a density of 1.0 g/cm3 has a

gravity of 10. Oil with API gravity of about 15–20 or

lower is considered heavy oil.

Crude with low API gravity, contains more sulphur

than the light crudes.

Pure (native) sulphur also occurs in great quantities

in some oil fields, and most of the world’s sulphur

production comes from sources of this type. In most

oil reservoirs, however, sulphur is an unwelcome com-

ponent, since it causes corrosion and problems in the

refining process. Burning oil of this type releases SO2

into the atmosphere. Much of it can be removed during

refining, but that increases the cost.

The NSO compounds include resins and

asphaltenes which are enriched in oxygen, nitrogen

and sulphur. In addition hydrocarbons contain a num-

ber of trace metals derived from the organic matter in

the source rock. The most important are Ni and V.

API gravity and density of some hydrocarbons

Hydrocarbon

Molecule

type Formula

H/C

ratio API

Density

gravity g/

cm3

Hexane Paraffin C6H14 2.3 82 0.6594

Cyclohexane Naphthene C6H12 2.0 50 0.7786

Benzene Aromatic C6H6 1.0 29 0.8790

Hydrocarbons with a high H/C ratio tend to have

lower densities.

Classification of crude oil:

1. Low shrinkage oil: Oil which after production and

separation at ordinary pressure and temperature

contains a high percentage of fluid (>80%) and

little gas.

2. High shrinkage oil: Oil which after production and

separation at ordinary temperature and pressure

contains a smaller percentage of liquid (less than

70%) and a large amount of gas.

3. Condensate: Gas which turns into a liquid with

pressure reduction. Condensates have low densities

(about 40–60� API).4. Dry gas: This is gas that does not form a liquid

phase at normal temperature. This is especially

methane and other molecules with low carbon num-

bers (less than 6, hexane).

5. Wet gas: This is gas that in addition to methane also

contains significant amounts of alkanes with high

carbon numbers. It may be in a gaseous phase in the

reservoir, but after production and separation may

form a good deal of liquid at normal pressure and

temperature.

6. Undersaturated crude: Oil which contains less gas

than is potentially soluble at reservoir temperature

and pressure.

7. Saturated crude: Oil which contains as much gas

as can be dissolved at reservoir pressure and

temperature.

Oil saturated with gas will form gas bubbles if the

pressure is reduced due to tectonic uplift or during

production in a reservoir.

14.6 Gas

Methane is a thermodynamically stable compound,

even at temperatures of 500–600�C or more. As a

result we may find methane gas at very great depths

in sedimentary basins and also deep in the Earth’s

crust. The limiting factors for economic exploitation

of gas of this type are the low porosity and permeabil-

ity we normally find in reservoirs at these depths

(6–8 km), and the expense of deep drilling. The main

component of natural gas is normally methane (CH4),

but ethane (C2H6), propane (C3H8) and butane (C4H10)

may also be important. In addition we have varying

amounts of CO2, H2O, nitrogen, hydrogen and inert

gases such as argon and helium.

Methane produced by bacterial breakdown of

organic matter at low temperatures (<60�C) is

characterised by a light carbon isotope composition.

Elemental composition of hydrocarbons in percent (after Hunt

1979)

C H S N O

Oil 84.5 13 1.5 0.5 0.5

Asphalt 84 10 3 1 2

Kerogen 79 6 5 2 8

370 K. Bjørlykke

This kind of gas is often referred to as “shallow bio-

genic gas”.

14.7 Biodegradation

Hydrocarbons can be broken down by micro-

organisms (bacteria, yeast and fungi). This is a form

of biological oxidation whereby hydrocarbons are

oxidised to alcohols, ketones and various acids. The

biological breakdown of hydrocarbons proceeds far

more rapidly with smaller molecules (carbon number

<20). When it comes to molecules with the same

carbon number, n-paraffins will break down first,

and then isoparaffins, naphthenes and aromatics.

Isoprenoids, steranes and triterpanes show the greatest

resistance to biodegradation. Relatively rapid biodeg-

radation depends on a supply of oxygen, usually

dissolved in water or in air. If sulphate is present

hydrocarbons may also be broken down by sulphate-

reducing bacteria which use the oxygen in sulphates

(e.g. gypsum) to oxidise some of the oil. Minerals

containing trivalent iron (Fe3+), such as haematite,

can also contribute to the biodegradation and oxida-

tion of oil deeper in the basin when ferric iron is

reduced to ferrous (Fe2+). The content of haematite

and other minerals with trivalent iron in sediments can

thus also be an oxidant.

Anaerobic biodegradation may occur under reduc-

ing conditions deeper in a sedimentary basin and the

supply of nutrients such as phosphorus in the

sediments may then be critical.

The temperature must however be lower than about

80�C. The bacteria eat the lighter compounds so that

the larger, more asphaltic, compounds are enriched.

Biodegraded oils (tar) have therefore high viscosity.

Biodegradation occurs as soon as the oil flows out

at the surface as oil seepage and the lighter compounds

will evaporate. Bacteria can then use oxygen from the

air and the biodegradation is relatively fast.

Biodegradation may occur in relatively shallow

reservoirs (<1,500–2,000 m, <70–80�C). Very close

to the surface and in contact with groundwater flow,

the biodegradation may be oxic but for the most part is

not possible to supply oxygen from the surface and we

must assume that the biodegradation is anoxic. As a

result the oil is less valuable and difficult to produce.

Injection of steam to heat the oil and reduce its viscos-

ity is the most effective method of increasing

production from reservoirs containing heavy,

biodegraded oil. Increasing oil prices have however

made production of heavy oil and tar sand more prof-

itable. Heating the oil may however require up to 30%

of the energy recoved thus increasing the CO2 emis-

sion (see tar sand and oil shales, Chap. 21). Excavation

and separation of oil from sand also require much

more energy than production of conventional oil.

Reservoirs buried to greater depth (>80–100�C)are sterilised by the heat and may remain without

biodegradation also after uplift to lower temperatures.

It is difficult to introduce new bacteria. Biodegration

and the formation of heavy oil and tar sand occurs

most readily when oil migrates into reservoir rocks

which have not been sterilised at temperatures exceed-

ing about 80�C.When oil forms seeps near the surface,

aerobic biodegradation occurs rather rapidly.

In recent years it has become possible and econom-

ical to produce oil and gas directly from shales that are

source rocks. This is the petroleum which has not been

expelled from the source rocks. This is shale oil

and shale gas which has been generated at greater

depth to become mature but is produced by drilling

at shallow depth in uplifted areas, mostly on land (see

Chaps. 1, 23).

Further Reading

Geoff, J.C. 1985. Hydrocarbon generation and migration from

Jurassic source rocks in the East Shetland Basin and Viking

Graben at the North Sea. Journal of the Geological Society

140, 445–474.

Hannah, J.L. and Stein, H.J. 2012. Re-Os geochemistry. In:

Melezhik, V.A., Kump, L.R., Fallick, A.E., Strauss, H.,

Hanski, E.J., Prave, A.R. and Lepland, A., (eds), Reading

the Archive of Earth’s Oxygenation, Volume 3: Global

Events and the Fennoscandian Arctic Russia – Drilling

Early Earth Project: Springer-Verlag, Berlin, Heidelberg,

pp. 1506–1514. DOI 10.1007/978-3-642-29670-3_10.

Hunt, J.M. 1996. Petroleum Geochemistry and Geology. Free-

man and Co., New York, 743 pp.

Karlsen, D.A., Nedkvitne, T., Larter, S.R. and Bjørlykke, K.

1993. Hydrocarbon composition of authigenic inclusions –

Application to elucidation of petroleum reservoir filling his-

tory. Geochemica et Cosmochemica Acta 57, 3641–3659.

Løseth, H., Wensaas, L., Gading, M., Duffaut, K. and Springer,

M. 2011. Can hydrocarbon source rocks be identified on

seismic data? Geology 39, 1167–1170.

Makhous, M. and Galushkin, Y. 2005. Basin Analysis and

Modeling of the Burial, Thermal and Maturation Histories

in Sedimentary Basins. Editions Technip, Paris, 379 pp.

14 Source Rocks and Petroleum Geochemistry 371

Nadeau, P.H., Bjørkum, P.A. and Walderhaug, O. 2005.

Petroleum system analysis: impact of shale diagenesis

on reservoir fluid pressure, hydrocarbon migration, and

biodegradation risks. In: Dore, A.G. and Vining, B.A.

(eds.), Petroleum Geology: North-West Europe and

Global Perspectives—Proceedings of the 6th Petroleum

Geology Conference, Geological Society, London, pp.

1267–1274.

Tissot, B.P. and Welte, D.H. 1984. Petroleum Formation and

Occurrence. Springer, Berlin, 669 pp.

Wapples, D.W. 1980. Time and temperature in petroleum explo-

ration. AAPG Bulletin 64, 916–926.

372 K. Bjørlykke