F O R E W O R D

Marine contamination by petroleum, whether by natural seepage or by spills from ships at sea, by ac­cidents in harbour or at offshore installations or by atmospheric or terrigenous input is by no means a new or rare phenomenon. In recent years however, the problems have been highlighted not only by the increas­ed utilisation and marine transport of oil but also by a number of spectacular accidents which have raised questions about possible effects on the ecosystem. A number of detailed studies have been carried out in an attempt to answer these questions. The demands for such knowledge have been further increased by the various questions raised as a result of expansion of off­shore exploration and exploitation for oil, particularly in environments hostile to these operations, in regions as far apart as the northern North Sea and the coast of Alaska.

Consequently, diverse aspects of the problem are being studied in several parts of the world by chemists and biologists who are often asking the same questions but using different approaches and sometimes pro­ducing conflicting views. Against this background, it seemed timely therefore to bring together a group of scientists from university, industry and government, actively engaged in such work, to examine and discuss common problems relevant to petroleum hydrocarbon contamination of the marine ecosystem and so a Work­

shop was sponsored by the International Council for the Exploration of the Sea, and held in Scotland at Aberdeen in September 1975.

The Workshop considered methodology, occurrence and fate in the environment, and effects on the eco­system of petroleum hydrocarbons in the sea. Most of the papers presented and updated where necessary, are brought together in the present volume together with an edited version of the recorded discussion that fol­lowed each session. Of necessity, the reportage of the discussion is very brief although the proportion of time available for discussion compared favourably with that set aside for formal presentation of the papers. In pre­paring the discussion reports, the editors were assisted in particular by Dr R. Hardy, Dr R. Johnston, Mr P. R. Mackie and Dr I. C. White, and by comments from several contributors.

No attempt was made to produce specific recom­mendations but a study of the papers in this volume does give a clear indication of several lines of research which must be followed up before an adequate under­standing can be reached of the effects of petroleum in the sea and it is evident that widespread monitoring operations will be fully effective only when the basis of our knowledge has been thus extended.

A list of participants to the workshop may be found in Appendix I.

A. D. M c I n t y r e

K.J . W h i t t l e

Section 1. In tro d u c to ry reviews 7

R app. P.-v. Réun. Cons. int. Explor. M er, 171: 7-16. 1977.

T H E C O M P O S I T I O N O F P E T R O L E U M

J. POSTHUMA

Shell Research BV, Postbox 60, Rijswijk, T he Netherlands

Petroleum has a complex chemical composition. I t consists mainly o f hydrocarbons; compounds containing oxygen, nitrogen and sulphur are usually of m inor importance, while metals are even less abundant.

T he hydrocarbons vary widely in boiling point, m olecular size and structure. Although relatively few individual compounds have been identified, several types o f compounds can be distinguished.

Crude oils differ considerably in composition, depending on the source m atter, the therm al history of the source m atter and alteration processes in subsurface reservoirs.

IN T R O D U C T IO N

The composition of petroleum is extremely complex (Speers and Whitehead, 1969). I t would go beyond the scope of this paper to give a complete review of what is known in this field. The paper will cover only the most important aspects of the composition of petro­leum and also try to illustrate how this composition came about by discussing briefly the origin of petro­leum and the alterations that may take place in sub­surface accumulations.

The main elements in petroleum are carbon and hydrogen, while sulphur, nitrogen and oxygen usually occur in subordinate quantities. The elemental com­position of most crude oils generally varies between the following limits (% weight) : carbon 80—87, hydrogen 10—15, sulphur 0—10, nitrogen 0—1 and oxygen 0—5.

Apart from the above elements, crude oils often con­tain a wide range of trace metals such as V, Ni, Fe, Na, Ca, Cu and U.

The elemental composition already suggests that hy­drocarbons are the principal constituents, usually ex­ceeding 75 %. They will therefore be discussed first.

HY DROCA RBONS

The hydrocarbons present in crude oils are generally divided into four groups based on their chemical struc­tures (Fig. 1): a. normal alkanes (normal paraffins), b. branched alkanes (branched paraffins), c. cyclo- alkanes (cycloparaffins or naphthenes), d. aromatics.

Very small concentrations of olefinic hydrocarbons have been reported in some crude oils, but definite con­firmation is lacking. For these reasons they will not be discussed here. The hydrocarbons in petroleum cover

a wide range of molecular sizes and boiling points. The number of carbon atoms in the molecule ranges from Ci to Ceo and in some cases compounds up to C78 have been reported (Ludwig, 1965; Denekas et al, 1951), with molecular weights up to about 900.

Until about 20 years ago, determination of the che­mical composition of oil was extremely difficult be­cause the analytical techniques were limited. Never­theless, by 1927 the American Petroleum Institute had started the famous project 6, a detailed investigation of one crude oil, the Ponca City crude, that was chosen as a typical, average crude. I t took more than 30 years to identify 169 individual hydrocarbons (Rossini and Mair, 1959), and in 1965 a total of 256 hydrocarbons and 8 non-hydrocarbons had been identified, represent­ing about 60 % of the crude’s weight (Mair, 1965).

I t turned out that petroleum was much simpler than was expected on the basis of the many theoretically possible compounds. This is clearly demonstrated in Table 1. Only in the lower molecular weight range have most of the possible isomers been found. Never­theless, petroleum remains extremely complex especially in the higher boiling fractions, where separation be­comes increasingly difficult.

Table 1. Alkane isomers possible and identified

Isomers C arbon atoms in molecule

Ci C 2 Cs Cä Cs C« C 7 Cs Co Cio C 11 C 12 C 13

Possible Identified . .

1 1 1 2 . . 1 1 1 2

3 5 9 18 35 75 159 355 3 5 9 18 30 12 1 1

8021

After Smith (1966)

8 J . Posthuma

CH4

n -A L K A N E S

C H 3 - C H 3 CH3 - ( C H 2 ) n - C H 3

ISO A LK A N ES ( H Y D R O G E N O M I T T E D )C C C C

C - C - C - CIC

ISOBUTANE

C - C - C - C - Ci iC C

ISOOCTANE

C - C - C 3- C - C 3- C - C 3 -C -C

PRISTANE ( ISOPRENOID )

CYCLOALKANES (NAPHTHENES.SIDE CHAINS AND HYDROGEN

u O C O

O M ITTE D )

CYCLOPENTANE CYCLOHEXANE DECALINE

T R I , T E T R A AND PENTACYCLIC NAPHTHENES

AROMATICS ( INCLUDING NAPHTHENO AROMATICS,HYDROGENOMITTED)

5 0BENZENE N A P T H A L E N E A N T HR A CE N E

% Wt IN SATURATED HYDROCARBONS

10

CRUDE DERIVED FROM

MARINE SOURCE MATTER

1 1 1 1 1 1 . .10 )5

CARBON NUMBER

20 25 30 35

<0 -

5 -

CRUDE DERIVED FROM

TERRES TRIAL SOURCE MATTER

)0 )5 20CARBON NUMBER ------

25 30 35PYRENE INDANE FLUORENE

Figure 1. Some basic structures o f hydrocarbons in petroleum. Figure 2. n-Alkane distributions o f two crudes o f different origin.

NORMAL ALKANES

n-Alkanes occur in most petroleums as the most abundant series of hydrocarbons usually in the con­tinuous homologous series of CH 4, CH 3-CH 3, CHs- (CH 2 )n-CH3. The maximum number of carbon atoms (n + 2) is usually around 40, although alkanes with carbon numbers > Coo have been reported (Ludwig, 1965; Denekas et al, 1951).

Two typical n-alkane distributions are shown in Figure 2.

ISO-ALKANES

Iso-alkanes are present in most oils in relatively large concentrations. Molecules with methyl groups as side chains are most abundant especially those with methyl groups at 2, 3 and 4 positions. Below C10 many isomers have been isolated from Ponca City crude (Mair, 1964 a). Above C13 the most important group of iso- alkanes is the series of isoprenoid hydrocarbons that

have the isoprene skeleton repeated in their structure (Fig. 1). Various isoprenoid hydrocarbons with more than 13 carbon atoms have been identified in petro­leum (Dean and Whitehead, 1961; M air et al, 1962; Bendoraitis et al, 1963) with pristane (C 1 9) and phy- tane (C2 0 ) as the most abundant. This series of iso- alkanes is now known to be very common in petroleum and sometimes constitutes the predominant hydrocar­bons in the Cio—C20 range. These iso-alkanes are be­lieved to originate from plant pigments especially from the phytol side chain of chlorophyll (Bendoraitis et al, 1963) and their presence thus supports the biogenic origin of petroleum.

CYCLOALKANES

The cycloalkanes are found in all crudes in quanti­ties ranging from 30—60 % and are present in the frac­tions from Cs upwards. The vast majority of cyclo- alkanes consists of cyclopentane and cyclohexane deri-

The composition of petroleum 9

vatives. Normally the mono- and bicyclic show the highest concentrations, but in some asphalts 4 and 5 ring naphthenes are predominant. Like the isoprenoids, these polycyclic naphthenes are assumed to be related to specific precursors in animal and plant life (steranes and triterpanes) and therefore considered as evidence for the biogenic origin of petroleum (Hills and White­head, 1966; Mair, 1964 b).

AROMATIC HYDROCARBONS

On the definition of aromatic hydrocarbons in petro­leum there is no general agreement. I t is customary to define aromatics as compounds with one or more aro­matic rings in the molecule including those that have both aromatic and naphthenic rings e.g. tetralin. Some investigators put the latter compounds in a separate group, the naphtheno-aromatic hydrocarbons. In this paper the first definition will be used.

The aromatics in petroleum occur in various struc­tures. In light oils the monocyclic compounds are pre­dominant. Rossini and M air (1960) have identified all possible Ce—Cs benzene derivatives in several crudes. Bicyclic aromatics are mainly methyl and ethyl naph­thalenes and biphenyl, while tricyclic are anthracene, phenanthrene and their simple alkylated forms. Fluorene structures are sometimes also present in significant quantities. Polycyclic aromatics with more than three rings are comprised of mainly pyrene, chrysene, ben- zanthracene, perylene and benzofluorene structures. In the higher boiling fractions the naphtheno-aromatics increase. The total number of rings in this type of com­pound may be as high as six. Members of the follow­ing series have been identified : indane, tetralin, fluorene and cyclopentanophenanthrene.

NO N -H Y D RO CA R BO N S

The non-hydrocarbons (Fig. 3) are generally con­sidered to be minor constituents of petroleum but in heavy oils they may sometimes become predominant. For example in the Wilmington crude (California) they account for one third (Ball et al, 1959) and in the Boscan crude (W. Venezuela) for about two thirds of the oil (Bestougeff, 1967). This is well illustrated in Figure 4, where the distribution of the major groups of compounds in a light and a heavy oil is shown.

The following groups of non-hydrocarbon com­pounds can be distinguished: a) sulphur compounds, b) nitrogen compounds, c) oxygen compounds, d) por­phyrins, e) asphaltenes, f) trace metals.

SULPHUR COMPOUNDS

Sulphur compounds form the most important group of non-hydrocarbon constituents with respect both to

SULPHUR COMPOUNDS

C -C -S H c -c -s -c -c c -c -s -s -c -c

ETHANETHIOL 3 THIAPENTANE 3 ,4 D ITHIAHEXANE

o o'S ' 'S " 'S 'THIACYCLOHEXANE THIOPHENE DIBENZOTHIOPHENE

NITROGEN COMPOUNDS

u

PYRIDINE QUINOLINE BASIC

' N ' ^

ACRIDINE

'N' O gPYRROLE INDOLE

NON BASIC

NCARBAZOLE

OXYGEN COMPOUNDS

C -C )6- C < 0H1-C 0 0 H 0

IIC -C -C -C -C -C

STEARIC ACID CYCLOPENTANE METHYL-n-BUTYL

n

CARBOXYLIC ACID KETONE

FLUORENONE PHENOL

Jl II'O'

D IB E N Z O F U R A N

Figure 3. Some basic structures o f S ,N ,0 compounds in petroleum.

quantity and to the problems they cause in refineries. Sulphur and its compounds also have a considerable bearing upon the origin of petroleum. The sulphur content of crudes varies widely from almost zero in e.g. the Far East crudes to about 5 % in heavy Western Venezuelan crudes. In extreme cases sulphur contents of almost 15 % have been recorded in asphalt seeps. The general trend is that the sulphur content, especial­ly in one area, increases with the specific gravity of the oils. Most sulphur is present as organically-bound sul­phur but sometimes elemental sulphur has been detect­ed in concentrations of more than one percent (Rail et al, 1972; Gransch and Posthuma, 1973). One of the most recent and complete reviews on sulphur com­pounds in crude oils is USBM report on the API pro­ject 48 summarising 20 years of research (Rail et al,1972).

Crudes contain four main classes of organic sulphur

10 J. Posthuma

1 n o rm a l p a ra f f in hydrocarbons 2 3 .3

2 i so -p a ra f f in hydrocarbons

3 cy c lopara ff in s

4 a ro m a t ic s

5 a r o m a t ic cyc lopara ff in s

resins aspha ltenes

to ta l

Number of carbon atoms

1 n o rm a l p a ra ff in hydrocarbons 0 .9 5

2 ,iso -p a ra f f in hydrocarbons 3. 20

3 cyc lopara ff in s 19. 20

3 4 a ro m a tic s 9 .1 5

5 a ro m a t ic cyc loparaffins

+ sulfur com pounds 27 .9 0Öid

6 h e te ro c y c l ic s (resins) 2 3 .1 0T3I 2 . aspha ltenes 16 .5 0u to ta l 1 0 0 . 00

ÏÏ&a- 1

5

6^/S

S’

° 5 10 20 30 40 50Number of carbon atoms

Figure 4. D istribution of the m ajor groups o f compounds in a light and a heavy crude oil. (After Bestougeff, 1967).

compounds (Fig. 3): (1) thiols, (2) disulphides, (3) sulphides, (4) thiophenes.

The thiols and disulphides are only present in the low boiling fractions (< 200°C) and all possible struc­tures below C7 have been identified (Smith, 1966). Most sulphur is found in the higher boiling fractions as cyclic sulphides and thiophenes.

Up till now very few molecules containing two sul­phur atoms have been identified although mass spectra indicate that they are present (Rail et al, 1972).

The origin of sulphur in petroleum has been the subject of much discussion for many years and for this reason a separate paragraph is devoted to this problem later.

OXYGEN COMPOUNDS

The majority of oxygen compounds in petroleum (Fig. 3) are fatty acids and naphthenic acids. In the carbon number range Ci—C 9 both straight chain and branched acids have been reported. In the C 1 4 — C20

range the odd-numbered members appear to be absent while isoprenoid acids have been encountered in a Californian crude. The naphthenic acids are by far the most abundant especially in naphthenic crudes. In addition to acids, various phenols have been detected whereas small concentrations of aliphatic ketones, fluorenones and dibenzfurans also occur. In the higher boiling fractions and distillation residues, compounds with two oxygen atoms or oxygen plus nitrogen and/or sulphur also exist (Snyder, 1970).

NITROGEN COMPOUNDS

All crude oils contain some nitrogen but the maxi­mum is about 1 %. In terms of nitrogen compounds this will probably vary roughly between 0.5 and 15 %. Practically all nitrogen compounds are concentrated in the higher boiling fractions and about 90 % of them in the distillation residues. They are generally divided into basic and non-basic compounds (Fig. 3). The basic compounds such as pyridines, quinolines, benzoquino- lines and acridines have been studied most extensively because they are the easiest to isolate. The non-basic nitrogen compounds are derivatives of pyrroles, indoles, carbazoles and benzcarbazoles.

PORPHYRINS

Although the porphyrins belong to the group of nitrogen compounds they are discussed here separately because they have an important significance in the origin of petroleum. They have therefore received much attention for several years, being first discovered by Treibs (1934).

The detailed chemistry is extremely complex. How­ever, the essential part of the molecules consists of four condensed pyrrole rings of the structure:

NH

in which the pyrrole hydrogen atoms can be replaced by metals. In petroleum the porphyrins occur almost ex-

The composition of petroleum 11

COOCH, 0

AT OR JUST BELOW , ,CH LOR OPHYLL (o) ------------S E D IM E N T P H EO PH YT IN (o)

WATER INTERFACE

_______ vo**__-C ,0 H,,OM l ™ * TO l)

RÏDUCTION, AHOMATlSATïON DECARBOXYLATION

EARLY

DIAGENESIS

Figure 5. Conversion of chlorophyll into vanadylporphyrin.

clusively as complexes of vanadium (vanadyl) and nickel. The metal porphyrin complexes are the only compounds that can be observed easily from the ab­sorption spectra of crude oils. In high concentrations the red colour betrays their presence.

The porphyrin content of petroleum varies from 1— 3500 mg/litre. The highest concentrations are found in sulphur-rich asphaltic oils, while light oils contain practically none (Gransch and Eisma, 1966). I t is now generally accepted that porphyrins are derived from chlorophyll molecules and thus provide evidence for the biogenic origin of petroleum. The conversion of chlorophyll into porphyrins is assumed to occur by metal exchange, decarboxylation and hydrogenation as is schematically shown in Figure 5.

ASPHALTENES

Apart from the heteroatomic compounds already mentioned, many oils contain a fraction of high mole­cular weight material containing nitrogen, sulphur, oxygen and metals that precipitates on addition of low-boiling hydrocarbons e.g. propane or pentane. This fraction, which varies somewhat with the precipitating agent used, is called “asphaltenes” and consists of mole­cules with an average molecular weight somewhere be­tween 1000 and 10 000, although there is no general agreement on these numbers. The chemical structure has not been fully established, but it is certain that asphaltenes are highly aromatic, partly heterocyclic structures composed of 10—20 fused rings (Fig. 6), with aliphatic and naphthenic side chains, forming stacks of about 4—5 (Yen et al, 1961). The asphaltene content

VANADYL-DEOXOPHYLLOERYTHROETIO PORPHYRIN

of crude oils ranges from 0—20 % and in natural as­phalts percentages of 30—40 % are not uncommon.

TRACE METALS

The trace metals mentioned in the introduction are all present in the asphaltene fraction of petroleum. Vanadium and nickel are usually the most abundant, sometimes reaching concentrations of thousands of mg/ litre. They are not only present in the porphyrins; the total content of these metals is sometimes 5—10 times higher than that complexed in the porphyrins. Due to the small amounts involved the study of the metals in petroleum is hampered by the possibility of contamina­tion.

T H E O R IG IN O F PE T R O L E U M

In a paper on the composition of petroleum, a brief discussion on its origin can hardly be avoided since the composition of a crude strongly depends on its origin. However, the discussion is restricted to two important

( C 7 9 H 9 2 N 2 S j 0 ) 3

mol.wt. 3449Figure 6. H ypothetical structure for asphaltenes from Venezuelan

crude oil. (After W inniford and Bersohn, 1962).

12 J. Posthuma

HYDROCARBONS GENERATED N.ALKANES CYCLOALKANES AROMATICS

0 -[ BIOCHEMICAL CH4

toUJ<ri-LÜ

g 2C-ATOM NUMBER RING NUMBER C-ATOM NUMBER <

OILi1-a.UJ ,û '

30 33

GAS

4

Figure 7. General scheme of hydrocarbon generation. (After Tissot et al, 1974).

aspects, namely the effect of the origin on some general characteristics of crude oils and the origin of sulphur.

It is almost generally accepted that petroleum is of biogenic origin. I t contains many compounds which point to such an origin, namely, isoprenoids, por­phyrins, carotanes and steranes which have their na­tural precursors in organic-rich sediments; indeed pe­troleum in nature is predominantly associated with sedimentary organic matter.

It has been established that petroleum is formed by thermal cracking from organic matter deposited in an­cient seas, lakes and lagoons after this material is buried to a sufficient depth. Not all organic matter is suitable for the generation of oil. The nature of the organic m atter considered to be the source matter for oil has been discussed recently (Lijmbach, 1975).

O ur present concept is that oil source rocks i.e. rocks containing a reasonable amount of hydrogen-rich (> ~ 5 % H) organic m atter are deposited only under special conditions. Normally in aquatic environments an abundant life is present in the upper water layers. Photosynthetic plankton synthesise organic matter from CO 2 and water. In most cases the organic debris re­sulting from this photosynthetic life is completely oxid­ised to CO 2 and water again by aerobic bacteria and consequently is almost completely recycled. However, under certain conditions, which include an unusually high photosynthetic activity and stagnant water, an anaerobic situation will develop below the water sur­face. Only under these conditions will significant con­centrations of organic matter be preserved and incor­porated into the sediments. This organic matter has been reworked to a large extent by aerobic and an­aerobic bacteria and will consist only of these rework­ers and the microbially resistant part of the original detritus including pollen, spores, resins and waxes sup­

plied from elsewhere. When this organic matter is buried a sequence of processes will take place resulting in the formation of oil and gas (Fig. 7). In the first stages of burial the organic m atter is converted into an insoluble complex polymer (kerogen) by polymerisation and condensation reactions. The initial hydrocarbon content remains low at first but at a certain depth, usually 2000—3000 m dependent on temperature and time, new hydrocarbons are generated by thermal cracking of the kerogen. This is the principle stage of oil generation (Vassoyevich et al, 1969). At still greater depth only gas will be generated both from the kerogen and the oil retained (zone of gas generation). Even­tually only a highly carbonised residue will be left be­hind in the oil source rock.

I t seems evident that the composition and the amount of petroleum generated in a source rock de­pends on the composition of the kerogen. Laboratory heating experiments (Lijmbach, 1975) have provided evidence that the bacterial mass which reworked the organic detritus during deposition seems to be the main constituent of most kerogens. Heating of it would ge­nerate a common basis for petroleum with a carbon number distribution as indicated in Figure 8. Contri­butions of other material to the kerogen would be super­imposed on this general distribution. For example, resins would generate C 1 5 - and Can-naphthenes and plant waxes would yield «-alkanes in the C 2 5 — C 3 3 range (Fig. 8, Fig. 2).

Various studies have shown that other constituents of crude oils can be explained as being products of thermal degradation of specific precursors in the kero­gen, like the porphyrins and isoprenoid hydrocarbons originating from chlorophyll and related compounds. Discussion of these studies is beyond the scope of this

tozu.U-<oc<0.cu_o

ALGAEto\-zZ>O2<UJ>

•yy'ÀxEsiBÀCTËRIA

<—IUJo:

0 5 10 ^5 20 25 30 35

NUMBER OF CARBON ATOMS PER MOLECULE

Figure 8. Contribution of the various types o f organic m atter to the n-alkane distributions of crude oils. (After Lijmbach, 1975).

The composition of petroleum 13

% S u lp h u r in c ru d e s

0.75 0.80 1.000.90

% S u lp h u r in c r u d e s

6

0.75 0.80

S p e c i f i c g r a v i t y o f c r u d e s

0.90 100

S p e c i f ic g r a v i t y o f c r u d e s

Figure 9. Relationship between sulphur content and specific gravity in high sulphur areas (W Venezuela, M iddle East).

paper. Only one aspect, the origin of sulphur, will be discussed in detail, because the presence of sulphur in crude oils has such an important effect on their gravity, chemical composition and economic value.

T H E O R IG IN O F SU L P H U R IN PE T R O L E U M

There has never been much doubt that most non­hydrocarbon constituents like nitrogen, oxygen and trace metals originate from the oil source rock. The origin of sulphur, however has caused much contro­versy in the past. There have been two main theories. The first assumes that crudes are originally poor in sulphur and that most of the sulphur in high-sulphur crudes (> 0-5 % S) was introduced by the action of sulphate reducing bacteria after the oil accumulated in subsurface reservoirs.

The second view is that the sulphur originates main­ly from the source rock. O ur recent studies (Gransch and Posthuma, 1973) have shown that this latter con­cept is supported by the experimental data. Analysis of source rocks and oils, particularly in western Vene­zuela, have revealed that many kerogens contain up to10 % organic-bound sulphur. The kerogens thus pro­vide an excellent source of sulphur. Indeed, it was ob­served both in the laboratory and in nature that with increasing thermal metamorphism these kerogens first yield a heavy, sulphur-rich crude, rich in asphaltenes and often also in porphyrins. On further heating the

generated hydrocarbon mixture became gradually light­er and contained less sulphur, asphaltenes and por­phyrins. This observation explains the occurrence of the whole range of sulphur contents in the crudes en­countered in areas with high-sulphur source rocks (Fig. 9).

The formation of kerogen rich in organic sulphur could also be explained. The organic detritus from which kerogen is formed is low in sulphur, but, as al­ready mentioned, anaerobic bacteria rework this detri­tus during sedimentation. In marine environments, where sulphate is always abundantly present, sulphate reducing bacteria are especially responsible for this an­aerobic degradation. During this process the sulphate is reduced to hydrogen sulphide (H 2S). The fate of this H 2S varies with the environment:

(a) If metal ions (e.g. Fe++, Zn++, Cu++) are present (e.g. in clays) it may be trapped as metal sul­phides, mostly pyrite, and will thus not be incorporated into the kerogen. As a result the sulphur content of the kerogen will be low and crudes derived from it will be poor in sulphur. This is usually the case if the source rock has been deposited in a marine siliciclastic envi­ronment like the Niger delta and the Texas Gulf coast.

(b) If metal ions are absent, the H 2S can be chemic­ally or bacteriologically oxidised to elemental sulphur, as is illustrated in Figure 10.

In recent sediments elemental sulphur is often found. On burial, it will react with the kerogen to form a

14 J . Posthuma

LIGHT

C 0 2

PHOTOSYNTHETIC ALGAE AND BACTERIA

AERO BIC ZONE (SAPROPHYTIC BACTERIA)

INTERM EDIATE ZONE [SULPHUR BA CTERIA)

ANAEROBIC ZONE (SAPRO PHYTIC BACTERIA)

S O 4"- REDUCTION

C02 + H2 0 — ORGANIC MATTER

ORGANIC MATTER+O2 — CO2 + H2O

H2S +O 2 — H20 + S (CHEMICAL OXIDATION)

«2S * ORGANIC MATTER + 02 — H2O + CO2* SO4"

— H2O + CO2+S

ORGANIC MATTER

ORGANIC MATTER

-NO3

- SO4 *

C02+ H2O

- C02+ H2O + N02f - ^ N 2

-CH4 +CO2 +H2O

-CO2 + H2O + H 2S

SEDIM ENT

1 RESISTAMT ORGANIC MATTER : ( PARTS OF) MICROBIAL BODIESSPORES I p y iM c cPOLLEN JRESINSWAXESSTEROLS

Figure 10. Role of microbes during sedimentation. (After L ijm bach, 1975).

high-sulphur kerogen. On deeper burial this kerogen will crack and yield a heavy asphaltic, sulphur-rich oil. In later stages of organic metamorphism the generatedoil becomes increasingly lighter and poorer in sulphur, asphaltenes and porphyrins. Both the incorporation of sulphur in the organic matter and the generation of high- and low-sulphur crudes have been demonstrated in laboratory oil generation experiments (Lijmbach, 1975; Gransch and Posthuma, 1973). Figure 11 sum­marises our views on the occurrence of high- and low-

SEDIMENTATION OFORGANIC MATTER

(LOW IN SULPHUR)BRUNEI

SULPHATE REDUCTION

HIGH-S KEROGENLOW-S KEROGEN

BURIAL

HIGH-S CRUDESLOW-S CRUDES AND

LOW-S CRUDESLOW-S CRUDES INCREASING MATURATION

Figure 11. Explanation for the occurrence of high- and low sulphur crudes in nature.

sulphur crudes. It explains why in some areas only low- sulphur crudes occur (Brunei, Nigeria), while in others the sulphur content varies widely with the specific gra­vity of the oil (W. Venezuela).

O T H E R FA C TO R S A FFE C T IN G T H E C O M P O S IT IO N

O F PE T R O L E U M

In addition to the composition of the kerogen and its thermal history the composition of petroleum can be affected drastically after it has been trapped in the reservoir rock. Firstly, the oil may become lighter and eventually become destroyed by thermal alteration (maturation) which is the result of deeper burial, thus elevated temperatures. This process is, in fact, a pyro­lysis and tends to lead to chemical equilibrium. It in­volves removal of the heteroelements in the form of water, carbon dioxide, hydrogen sulphide etc., with simultaneous formation of increasingly small saturated hydrocarbons on the one hand and increasingly con­densed, aromatic structures on the other. Eventually it ends up with methane and graphite only. This process of thermal alteration has been documented very recent­ly (Van der Weide et al, 1975).

Petroleum may also become heavier in subsurface reservoirs. Physical processes, like evaporation of light components and solution in water of the most soluble compounds, result in an increase of specific gravity. The most severe effect, however, is caused by bacterial transformation or degradation. The latter process is restricted to relatively shallow reservoirs since it re-

INORGANICPARTICLES

7 7 7 7 7 7 7 7SEDIMENTARY-" / ' AND RESISTANT ̂

ORGANIC M ATTER»

The composition of petroleum 15

M8-99UNALTERED

r n-C

r n-C

M8-97 ALL n-PARAFFINS

MISSING

ORIGINAL 3 DAYS 4 DAYS

R ETEN TIO N T IM E

M 8 - 107 n-PARAFFINS

MISSING UP TO C17

5 DAYS 14 DAYS 21 DAYS

|PHPR

Figure 13. Bacterial degration o f crude oil in the laboratory. (After Bailey et al, 1973).

way by successive removal of n-alkane and branched alkanes. Eventually one and two ring naphthenes and aromatics are also attacked. The result is about the same as weathering of oil at the surface (Blumer et al,1973). A medium-gravity oil may become heavy or even a tar or asphalt.

It has often been suggested that biodégradation can

% Sulphur in crudes

6-• Flank oils

o Cap-rock and super-cap- rock oils

Figure 12. Bacterial degration of crude oil in the reservoir. (After W inters and Williams, 1969).

quires a moderate temperature — 80°C is about the upper limit in which bacteria can survive — and in­flux of meteoric water to supply some oxygen and nu­trients and to remove the metabolic products of the microorganisms.

The process of biodégradation has probably been en­countered by most oil companies in their own fields. The most complete study has recently been published by Winters and Williams (1969) and Bailey et al (1973). Figures 12 and 13 taken from their studies clearly de­monstrate how bacteria in laboratory experiments as well as in field samples attack crude oil in a selective

L.î- —S - / - 0* '* '

0.75 0.80 0.90 1.00

Specific gravity of crudes

Figure 14. Sulphur in crudes associated with salt domes (Texas and Louisiana G ulf Coast).

16 J. Posthuma

change a light low-S crude into a heavy asphaltic sul­phur* rich one. From the preceding it will be clear that biodégradation will increase the sulphur and asphaltene content to a limited extent because it removes that part of the oil that contains no asphaltic material and prac­tically no sulphur thus causing a relative enrichment of the latter compounds. In our studies (Gransch and Posthuma, 1973) we have found no indications that sulphate-reducing bacteria introduce much extra sul­phur into petroleum.

In the classical example of Gulf Coast cap-rock oils associated with salt domes, where biodégradation was detected and elemental sulphur was present as a result of bacterial sulphate reduction, the oils were found to be only slightly higher in sulphur (Fig. 14) and still extremely poor in asphaltenes, compared with the cor­responding (unaltered) flank oils. Thus, although the conditions seemed to be ideal for the introduction of extra sulphur, the H 2S and/or elemental S hardly react­ed with the oils and certainly did not lead to the for­mation of heavy, high-sulphur asphaltic oils as are pre­sent in western Venezuela and the Middle East.

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