Trimethylnaphthalenes in crude oils and sediments: Effects of source and maturity

Geochimica et Cosmochimica Acla Vol. 52, pp. 1255-1264 Copyright 0 1988 Pergamon Press plc.Printed in U.S.A.

Trimethylnaphthalenes

0016-7037/88/$3.00

+ .oO

in crude oils and sediments: Effects of source and maturity

MICHAELG. STRACHAN,ROBERT ALEXANDERand ROBERT I. KAGI Petroleum Geochemistry Group, School of Applied Chemistry, Curtin University of Technology, Box U1987 Perth, Western Australia, Australia (Received June 4, 1986; accepted in revisedform February 26, 1988)

Abstract-Three

sedimentary sequences and a number of crude oils have been analysed for trimethylnaphthalenes (TMNs) using gas chromatography. The sediments were found to contain enhanced relative abundances of 1,2,5-TMN and 1,2,7-TMN in samples of Cretaceous age and younger; this was especially pronounced in lower maturity sediments containing type III organic matter. Older samples containing type III organic matter were found to contain enhanced relative abundances of 1,2,5-TMN and lower relative abundances of 1,2,7-TMN. 1,2,7-TMNappears to be derived directly from the structural degradation of oleanane-type triterpenoids present in angiosperms, and as such is proposed as a marker for this class of plant and indirectly, therefore, also for samples deposited since the Early Cretaceous. The crude oils have been classified according to their trimethylnaphthalene compositions. This classification enables crudes of higher plant origin to be identified and to be further classified according to whether the organic matter predated or post-dated the evolution of angiosperms. rings A to E of @-amyrin would eventually yield 2,9-dimethylpicene. CHAFFEE et al. (1984) have reported a series

INTRODUCTION

SOMEOF THE aromatic compounds found in crude oils and sediments are believed to have been derived from modification of bio!ogically produced compounds such as steroids and terpenoids (BENDORAITIS,1974; LAFLAMMEand HITES, 1979): steroids give rise to substituted phenanthrenes (DOUGLAS and MAIR, 1965) and terpenoids appear to produce alkylnaphthalenes (BREGER, 1960; ISHIWATARI and FUKUSHIMA, 1979; STREIBLang HEROUT, 1969). Although terpenoid potential precursors of petroleum alkylnaphthalenes have been reported to occur both in micro-organisms and in higher plants (TISSOT and WELTE, 1984), the most abundant and widespread compounds of this type in the sedimentary environment are the bacterially derived hopanoids (OURISSONet al., 1979) and the Cl3 bicyclics which also have microbial origins (NOBLEet al., 1986). Compounds of higher plant origin are also common in sediments, and include hydrocarbons derived from sesquiterpenoids, diterpenoids, and triterpenoids. Triterpanes of higher plant origin have been reported in crude oils (SMITHet al., 1970; WHITEHEAD,1974; PYM et al., 1975; EKWEOZORet al., 1979a; HOFFMANN et al., 1984), and recent sediments and coals have been shown to contain partially and fully aromatised compounds which bear an obvious structural relationship to the higher-plantderived triterpenoids of the oleanane, ursane and lupane types. (GREINER et al., 1976, 1977; LAFLAMMEand HITES, 1979; WAKEHAMet al., 1980a,b; BARNESand BARNES,1983; SPYCKERELLE,1975; WHITE and LEE, 1980; CHAFFEE and JOHNS, 1983; TAN and HEIT,198 1; GARRIGUES et al., 1986). The processes by which higher plant triterpenoids in sediments are converted into aromatic hydrocarbons have been proposed to commence with loss of the C-3 oxygen functionality, followed by sequential aromatisation from the A ring through to the E ring (LAFLAMMEand HITES, 1979; WAKEHAM et al., 1980a; CHAFFEE and JOHNS, 1983): the ultimate products of this process would therefore be tetracyclic and pentacyclic aromatic hydrocarbons. For example, Pathway A, Fig. 1 shows how successive aromatisation of

of compounds with one to three aromatic rings (e.g., VIII and IX, Fig. 1) in the solvent-extractable material from a Victorian brown coal. They suggested that these compounds were derived by aromatisation of the A, B, D and possibly the E rings and cleavage of the C-ring 8( 14) bond in pentacyclic triterpenoids, either photochemically or by acid-catalysed processes which occur during diagenesis. An important structural feature ofthese tetracyclic compounds is the bridging ethylene group, which has been shown to be the preferred site of cleavage in thermolysis of such systems (CRONAUER et al., 1978). If this bond is cleaved to give two benzylic free radicals, then methyl-substituted naphthalenes would be formed, together with alkyltetralins or alkylindans in the case of compounds with a five-membered E ring. The particular

products formed would be dependent on the parent triterpenoids. For example, ,&amyrin may yield 1,2,5-trimethylnaphthalene (the A and B rings) and 1,2,7-trimethylnaphthalene (the D and E rings) via Pathway B of the scheme outlined in Fig. 1. Trimethylnaphthalenes (TMNs) derived from such aromatisation processes are of special interest, since they are potentially useful as indicators of depositional environment. The formation of particular TMNs should be favoured in acidic depositional environments such as coal swamps, whereas deltaic and marine transgressive environments should be less favourable to aromatisation and result in preservation of the higher plant triterpenoid skeletons. This latter point is illustrated by reports of 18a(H)-oleanane in crude oils and sediments derived from higher plant organic matter deposited in such environments (SMITH et al., 1970; WHITEHEAD, 1974; PYM et al., 1975; EKWEOZORet al., 1979a,b, HOFFMANNet al., 1984). As part of a continuing study of geochemical applications of aromatic compounds (ALEXANDERet al., 1984a; 1985a) we have examined the variations in relative concentrations in sedimentary organic matter of specific TMNs with the nature of the source and the depositional environment. Fur1255

1256

M. G. Strachan. R. Alexander and R. I. Kagi EXANDER et al., 1985a) to obtain: (a) the saturated hydrocarbon fraction, (b) a crude aromatic fraction; (c) a hetero-compound fraction. Prior to further fractionation, the crude aromatic hydrocarbon fractions were concentrated to volumes of 100-200 ~1, using a KudernaDanish apparatus. 2. Isolation of dinuclear and trinuciear aromatrc Jiaction. The aromatic fractions from the organic extracts or, in the casz of crude oils. the whole sample, were subjected to preparative thin layer chromatography on alumina plates (Merck Alumina G; 0.6 mm, activated at 120-14O’C for at least 12 h). The plates were developed with nhexane, and the required band (RI = 0.3-0.7), comprising dinuclear and trinuclear aromatics, was located using short wavelength UV light. Following extraction of the alumina bands with dichloromethane (20 ml), m-chloroperbenzoic acid (2 mg) was added to each solution to oxidize any sulphur-containing compounds. After 10 minutes at ambient temperature, the mixture was passed through a column of basic alumina (5 g, Woelm W200 Basic) to remove the resultant sulphones and residual acidic material. Excess solvent was removed in a Kudema-Danish apparatus, leaving a sample volume of 200-300 11. ready for GC analysis. .Qnalysis using gas chromatography

Pathway

A

FIG. 1. Proposed pathways for the aromatisation of oleanane-type triterpenoids during diagenesis (Based on schemesshown in CHAFFEE and JOHNS,1983; CHASE et al.. 1984).

ther, the application of 1,2,7-TMN as a chemical marker indicating the contribution of angiosperms to sediments and petroleum is reported. EXPERIMENTAL Samples 1. Sediments. The sediment samples studied were obtained as either cores or cuttings from four Australian exploration wells, namely Volador # 1 (Gippsland Basin), Jupiter # 1 (Exmouth Plateau), Madeleine # 1 (Carnarvon Basin) and Matches Springs # 1 (Canning Basin). The sequences were selected to represent different source types and geological ages. The main geological features and some of the geochemical properties of these sediment samples are presented in Table 1. 2. Oils. The crude oils examined were obtained from reservoirs ranging in age from Miocene to Ordovician, and are from a variety of locations. The samples chosen span a range of geological ages and thermal maturities. The classification of samples as having been sourced from higher plant organic matter, or otherwise, is based upon published reports (e.g. for some Gippsland Basin oils see SMITHand COOK, 1984; SHANMLJGAM, 1985; for some Cooper-Eromanga Basin oils see KANTSLERet al., 1983, 1984; VINCENTet al.. 1985; for some Camarvon Basin oils see VOLKMANet al., 1983; for some Canning Basin oils see ALEXANDERet al., 1984b, 1985b; for some Indonesian oils see HO~ANN et al., 1984, for some Nigerian oils see EKWEOZOR et al., 1979a, and for some North Sea oils see MACKENZIEet al., 1983). Additional information about source types and maturities of the oils studied here was obtained from sterane biological marker data, using the approach outlined by SEIFERTand MOLDOWAN(198 1) and MACKENZIEet al. (1980). Geochemical data for the crude oils are presented in Table 2. 3. Shale oil. A sample of shale oil (boiling range up to 400°C) was obtained from Mr. Tim Harvey. The oil was produced from Rundle oil shale by the Lurgi-Ruhm retorting process (HARVEYet aI., 1984). Isolation offractions from organic extracts and crude oils 1. Isolation and initial fractionation oj’organic extracts. Cuttings and core samples (40-80 g) were treated as previously described (AL-

Gas chromatography (GC) was performed using a Hewlett-Packard (HP) 5880A chromatograph, fitted with a split-splitless injector and a 50 m X 0.2 mm I.D. fused silica column coated with 5% crosslinked phenylmethyl silicone (BP-5, SGE). In a typical analysis, hydrogen was used as carrier gas at a linear velocity of 30 cm kc-‘, detector (FID) and injector temperatures were 320” and 28O”C, respectively. the sample was injected in the splitless mode, and the oven temperature was programmed from 70°C to 19O’C at 1“C min-‘, then from 19O’C to 300°C at 10°C min-’ and. finallv. held constant for 10 minutes. Peak areas were automaticall; integr&d using the associated HP data terminal. The trimethylnaphthalenes were assigned by combined co-chromatography and comparison of relative retention times with those of authentic isomers (ROWLANDet al., 1984). The C-2 biphenyls elute in the same region of the chromatogram as the TMNs. however under the gas chromatography conditions used, all required peaks were well resolved and were shown to be due only to trimethylnaphthalenes by analysis using CX-MS under identical GC conditions and the MS conditions reported earlier (ALEXANDERet al., 1985a). Heating experiments

A dinuclear and trinuclear aromatic hydrocarbon fraction rich in l-2.5-TMN was isolated from a crude oil. This was adsorbed on aluminum montmorillonite (1% w/w), and samples of the mixture (ca. 10 mg) were sealed in evacuated glass amp&les. After heating at 2OOT for 0.25 h or 1 h. the samks were cooled and extracted with dichloromethane. Excek dichloiomethane was removed by careful distillation to give residues (200 CCL) which were then analysed using capillary gas chromatography. In all experiments at least 75% of the mass of the starting material was accounted for as dimethylnaphthalenes and trimethylnaphthalenes.

RESULTS AND DISCUSSION Samples

Information about the sediment samples is presented in Table 1. The samples represent sediments from maturity zones usually associated with petroleum formation: vitrinite reflectance values range from 0.36 to 1.05%, and the ethylcholestane epimer ratio (20S/20R) values range from 0.36 to 1.02. The sediments contain different types of organic matter as evidenced by differences in kerogen types and the C2&, sterane values. Samples from the Volador # 1 and Jupiter # 1 wells contain organic matter of higher plant origin, indicated by the kerogen type and relative abundance of etbylcholestane (STAINFORTH, 1984; BARBER, 1982; COOK et al., 1985). In contrast, the Madeleine # 1 and Matches Springs # 1 samples

Trimethylnaphthalenes in oils and sediments Table

Sample

Depth

Location

(m)

Volador

Jupiter

3550 3645 3673 3691 3820 3950 4039 4145 4265 4526 4536 4554

Xl

2400 2555 2700 2910 3150 3450 3600 3770 3900 4050 4220 4500 4672 4815

il

Madeleine

tl

Matches Spring tl

a -

Gippsland Basin, Australia

&mouth Plateau Australia

1.

Ceochemical

Data for Sediment

Geological

Kerogena

Age

Type

Cretaceous

Triassic

II-III

II-III

2982

c - Data

d - Ratio

c g

(20R)h

0.53 N.D.

2.46 N.D. 1.76 1.71 1.75 N.D. 1.70 1.34 1.95 N.D. 0.93 1.41 N.D. 0.90

0.36 N.D. 0.38 0.49 0.57 N.D. 0.63 0.67 0.83 N.D. 0.77 0.86 N.D. 1.05

0.36 N.D. 0.42 0.38 0.48 N.D. 0.83 0.84 0.96 N.D. 0.91 0.93 N.D. 1.02

0.67 0.71 0.78 0.84 0.91 0.94 1.00

N.D. N.D. 0.78 0.91 0.96 0.88 N.D.

N.D.

1.0

upper Jurassic

II

3015

Canning Basin, Australia

Ordovician

I-II

0.50

111 and Madeleine _et _., 1985b) al

tl (Alexander

0.67

_ et _., al

1986

(C27) for 2OR Sol(H),14.x(H),17~(H)-isomers.

1) Volador #l (Stainforth, 1984); 3) Madeleine #l (Brikke, 1982).

of 20s to 20R

0.61

N.D. 0.90 N.D. 0.90 N.D. N.D. N.D. 0.87 0.85

Basin Australia

for:

Bd 20R

0.71 0.72 0.82 0.76 0.94 0.89 0.85 0.83 0.90 0.89 0.89

Carnarvon

(C29) to cholestane

Bo %

10.4 N.D. 6.25 N.D. 7.4 N.D. 4.8 N.D. N.D. N.D. 2.2 1.6

N.D. N.D. 0.55 0.71 0.80 0.77 N.D.

qf ethylcholestane 8ource

Samples

3188 3407 3634 3859 3945 4161

Data source for : 1) Volador tl (Stainforth, 1984); 2) Jupiter and Cumhers e a., 1987) and 3) Matches Springs 11 (Alexander

b - Ratio

1257

2) Jupiter

tl (Barber,

1982);

5a(H),14cc(H),17a(H)-ethylcholestane diastoreomers.

ND - NOT DETEBMINED

contain organic matter more typical of marine environments (CROSTELLA and CHANEY, ~~~~;ALEXANDERetal,, 1984b, 1985b, 1986; CUMBERS et al., 1987). Samples range in age from Ordovician to Cretaceous and were selected to represent times before the evolution of higher plants (Ordovician) as well as those which predate (Triassic and Jurassic) and postdate (Cretaceous) the evolution of angiosperms. Fractions containing the trimethylnaphthalenes (TMNs) were isolated from the samples and subjected to analysis by capillary gas chromatography. A partial chromatogram from

a typical analysis is shown in Fig. 2, and the results obtained for the relative abundances of the TMN isomers for each sediment sample are presented in Table 3. Data for the Jupiter #I samples have been reported previously (ALEXANDER et al., 1985a), however partial overlap by a peak due to an unidentified compound during GC analysis caused some uncertainty in the values for the concentrations of 1,2,5-TMN in some samples. A similar co-elution problem was also encountered with 1,2,4-TMN in a sample from another well, Cape Range #2 (ALEXANDER et al., 1985a). In the present

M. G. Strachan, R. Alexander and R. 1. Kagi

1258 Table

Geochemical

2.

ample NO.

Group

1 Af

; 4 5

Data

for Oil

Samples

Offshore Gippsland BUi", Australia South Sumatra

W.%b content

Source=

Reservoir

Location

Eocene cretaceous cretaceous (7)

CPId

s

(20fuC

H H

1.08 1.09

5.1 4.x

H H H

1.08 1.13 1.17

3.1 2.9 0

2ose ZOR 0.85 0.9R 1 0.95

00

-1

1.I0

Miocene

Tertiary

N

1.30

1.5

0.7%

Jurassic Permian

H H H

1.12 1.15 1.09

1.5 1.5 1.5

1.20 0.95 0.90

Basin, Indonesia

Bg

Ch

6

cooper-

7 8 9

Eromanga Basin, Australia

Jurassic

10 11

c*?Z"a?Xo* Basin

12

Australia

Jurassic Jurassic Triassic

Jurassic Jurass I c Triassic

H L L

1.05 N.A. N.A.

1.i 0.5 5.0

1.05 1.00 O.SO

::

Canning Basin Australia North Sea

Ordovician Devonian Jurassic

Ordovician Devonian Jurassic (7)

L L L

1.00 1.00 1.07

0.6 0.7 0.6

0.90 1.00 3.5;

N.A.

; i

1.00

Tertiary

Tertiary

Miocene

Tertiary

1.05

0.9

0.5'1

15

Permian

Denmark 16

central. Sumatra

17

Basin Indonesia Nigeria

Di

(?I L

-_ a-

Probable age of souxe cf. references b - Wax content is defined as: (1) high, alkane fraction is-rich in naphthenes (Cz3 + c25 c - CPI

-

+ c*7 2 x

+ C2g)

+

w*5+

(C24 + C26

to oils given in experimental if the alkane fraction is rich and light paraffins (
+ C2*

+ C29

section. in "C2* -D'C~~ components

(2) Low,

1: are

+ C3li

+ C,O)

d - Ratio of ethylcholestane (C29) to cholestane (C271 for 2OR-So(H), 14a(H), epimers. e - Ratio of 20s to 20R 5a(H), 14a(H), 17a(H) - ethylcholestana f - Oils derived from post-Cretaceous higher plant organic matter. g - Oils h - oils i - Oils

and

derived from,pre-Cretaceous higher plant organic derived from material containing no higher plant containing ltla(H) - oleanane.

study, the 1,2,5-TMN and 1,2,4-TMN peaks have been unambiguously resolved, and the more reliable data are presented here. Crude oil samples were subjected to a similar procedure, and the TMN distributions are given in Table 4. Sources of trimethylnaphthalenes It is apparent that the relative abundance of 1.2,5-TMN is much greater in samples containing organic matter derived

FIG. 2. Partial capillary gas chromatogram of the TMN fraction of crude oil number 1.

matter. Organic

17o(H)

-isomers

matter.

from higher plants than in samples without a higher plant input. A very high abundance occurs in the Volador # 1 samples, especiahy those from shallower depths, while the Jupiter # 1 samples have intermediate abundances and the Madeleine #l samples and the Matches Springs #l sample have low abundances. It is therefore likely that higher plants have been a significant source of compounds which can serve as precursors for 1,2,5-TMN. The 1,2,5-TMN found in sediments may have formed from several natural product precursors found in higher plants. Bicylic diterpenoids, which occur widely in conifers, readily undergo aromatisation reactions to produce 1,2.5-TMN under laboratory conditions. One example is agathic acid which yields agathalene (1,2,5-TMN) when heated with selenium (RUZICKA and HOSKING, 1930). Similar aromatisation reactions probably occur in sediments, so the very high abundance of 1,2,5-TMN in the Volador #l samples, may arise from degradation and aromatisation of the bicyclic diterpane. labdane, which has been shown to be present in these sediments (ALEXANDER et al., 1987). The Triassic samples from Jupiter # 1 predate the evolution of conifers (STEWART, 1983). They therefore do not contain conifer-derived aromatics, and the 1,2,5-TMN in these sediments may have been formed from residues of other higher plants. Figure 1 shows proposed reaction schemes for the aromatisation of plant-derived pentacyclic triterpenoids with P-amyrin used as an example. The products which form viu reaction pathway B are 1,2,5-TMN and 1.2,7-TMN. It is ap-

Trimethylnaphthaienes Table 3:

Distributions of Trimethylnaphthalene

sample

Depth(m)

in oils and sediments

1259

Isomers and Related Isomer Ratios for Sediment Samples.

1.3,7

1,3‘6

1,4,6 + 1,3,5

2,3,6

1,2,7

1,6,7

1‘2,b

1,2,4

1,2,5

1,2,51 1,3,6

1,2,?/ 1,3,?

Volador #I

3550 3645 3673 3691 3820 3950 4039 4145 4265 4526 4536 4554

2.2 2.2 2.9 3.4 3.6 14*9 5.1 6.3 9.2 10.4 10.3 12.0

4.1 4.1 5.1 5.6 7.3 9.6 8.3 12.5 14.1 14.7 14.3 16.1

5.5 5.4 6.9 7.1 7.4 9.8 8.7 12.5 14.8 17.2 15.4 17.5

2.8 2.8 3.3 4.2 4.2 7.7 6.2 8.8 8.1 9.6 10.5 10.4

29.6 8.2 8.7 10.3 18!6 14.6 15.3 9.6 10.7 5.3 5.3 4.1

3.4 3.3 4.2 4.8 5.4 6.0 6.7 .4.8 7.2 9.7 10.6 6.7

5.2 4.3 5.6 6.0 6.2 6.5 7.6 8.1 8.9 6.8 7.0 5.6

3.0 1.3 2.3 2.2 1.9 4.0 2.7 3.1 1.6 5.7 4.8 1.9

44.2 68.4 61.0 56.4 45.4 36.9 39.4 34.3 25.4 20.6 21.8 23.7

10.78 16.68 11.96 10.07 6.22 3.84 4.75 2.74 1.80 1.40 1.52 1.31

13.45 3.73 3.00 3.03 5.17 2.98 3.00 1.52 1.16 0.51 0.51 0.34

Jupiter #I

2400 2555 2700b 2910b mob 3450b 3600 3770b 4050 4220b 4500 4b72b 4815

13.1 10.5 13.6 6.8 11.2 13.5 11.3 11.3 10.6 12.0 12.3 10.4 15.2 13.8

21.8 18.5 17.7 18.7 21.0 21.2 18.5 19.4 19.5 17.5 18.5 13.5 19.4 14.6

21.9 23.4 17.2 19.7 18.8 18.9 15.6 15.7 15.6 15.5 16.2 12.9 16.0 11.7

9.7 10.0 10.9 9.0 9.3 9.9 10.0 10.5 11.4 11.2 12.1 11.6 10.4 14.3

3.1 4.3 4.6 3.7 4.3 5.1 3.8 6.1 6.6 4.2 5.1 4.5 5.4 2.7

10.3 10.0 15.2 13.4 12.2 12.0 9.6 12.4 12.2 12.5 12.7 12.5 10.0 10.4

4.0 5.0 5.2 6.7 7.0 7.0 8.5 9.1 10.9 9.2 9.7 7.6 6.3 5.5

1.9 1.0 1.0 0.9 0.5 1.0 3.9 0.6 0.5 0.8 0.6 3.5 2.0 1.2

14.2 17.3 14.8 21.1 15.7 11.4 18.5 14.9 12.7 17.1 12.8 23.5 15.3 25.8

0.65 0.94 0.84 1.13 0.75 0.54 1.00 0.71 0.65 0.98 0.69 1.74 0.79 1.78

0.24 0.41 0.34 0.54 0.38 0.38 0.34 0.54 0.62 0.35 0.41 0.43 0.36 0.20

2982 3188 3407 3634 3859 3945 4161

16.2 15.9 16.9 13.4 20.2 20.2 22.2

37.2 26.6 24.4 33.7 26.2 27.9 29.5

25.8 21.2 20.6 26.2 21.1 20.3 17.9

1.6 5.7 8.4 4.5 9.8 12.0 11.5

0.7 2.0 3.7 1.0 2.2 1.8 1.4

2.9 5.8 6.1 4.0 6.5 6.3 5.5

1.4 5.4 8.0 2.3 5.7 5.3 3.7

11.6 10.2 6.5 10.6 3.8 2.7 5.2

1.9 7.5 5.5 4.2 4.4 3.5 3.3

0.05 0.28 0.23 0.12 0.17 0.13 0.11

0.04 0.13 0.22 0.07 0.11 0.09 0.06

3015

14.9

20.4

18.0

10.8

2.2

12.5

8.0

5.2

7.9

0.39

0.15

3900b

Madeleine #l

Hatches Spring tl

a - Expressed IS percentages of the total trimethylnaphthalenea b - Data from Alexander & a., 198%

parent that 1,2,5-TMN can arise from the A/B ring moiety and 1,2,7-TMN from the D/E ring moiety of the triterpenoid. Since the A/B ring substitution pattern is the same for many of the plant-derived triterpenoids, 1,2,5-TMN is a common product of aromatisation by this route. We therefore suggest that aromatisation of plant-derived pentacyclic triterpenoids via pathway B contributed to the 1,2,5-TMN observed in these samples. The samples from the Madeleine # 1 and Matches Springs # 1 wells contain organic matter with little or no contribution from higher plants. The Matches Springs # 1 sample predates the evolution of higher plants, while, in the case of the Madeleine #I samples, the data for kerogen type and sterane dist~bution (Table 1) suggest that the samples from this well are typical marine shales. The low relative abundances of 1,2,5-TMN in samples from these two wells (Table 3) is attributed to the absence of higher-plant terpenoid material in the organic matter of the sources: the 1,2,5-TMN which is

present in these samples might have been derived from aromatisation of compounds such as the ubiquitous Crs bicyclic alkanes, which have been shown to occur in sediments which predate the evolution of higher plants (NOBLE etal., 1986). An alternative explanation, namely that 1,2,5-TMN has been derived from the A/B rings of hopanes by a process analagous to that shown in pathway B (Fig. l), in unlikely. These compounds do not contain the oxygen fun~ion~ity at the C-3 position required to initiate this process. In fact, in these systems aromatisation commences in the D ring and proceeds into the A ring (GREINERetal., 1977). It must also be recognised that low levels of 1,2,5-TMN could occur as soon as isomerisation reactions of TMNs commence in a reversible system, even if there had been no direct input of 1,2,5-TMN or suitable precursors into the sediments. Another feature apparent from Table 3 is the elevated levels of 1,2,7-TMN present in samples from the shallower sections of the Volador # 1 well. The sequence labelled Pathway B in

1260

M. G. Strachan, R. Alexander and R. 1. Kagi

Table 4.

Distributions of himethylnaphtbalene

Group

Sample NO.

Iscmsrs and Related Ratios for Crude Oil Samples.

TrimethyLnaphthalenes

1,3‘7

B

C

D

(#ja

2,3,6

1,2,(7

1,6,7

1,2,6

1,2,4

1,2,5

1,2,5/ 1,.3,6

i,2,7/ x,3,7

2 3 4 5

13.6 13.4 13.5 11.7 9-8

10.4 17.0 19.0 ;16.5 18.9

17.1 15.4 16.1 14.4 13.6

11.2 10.3 11.0 8.4 9.1

6.. 2 6.7 5.6 8.3 13.0

10.7 9.7 9.5 8.7 9.2

6.5 6.7 6.1 8.0 9.8

4.1 3.7 4.0 3.1 3.4

20.1 17.1 15.2 19.6 12.2

1.93 1.01 0.80 I.19 O‘G5

u.46 0.50 0.41 0.7X :.3g

6 7 8 9

3.0 14.5 11.1 15.5

6.1 20.2 15.5 20.8

9.0 15.2 13.0 14.6

2.9 11.2 7.7 12.2

1.3 3.4 3.0 2.5

4.0 10.9 8.6 9.5

3.2 6.4 6.9 5.7

0.8 3.1 4.5 2.7

67.9 15.2 29.7 16.5

x.13 OJS 1.92 0.79

a.34 a.23 0.27 0.16

10 11 12 13 14 15

16.7 LG.3 16.7 15.2 15.3 10.4

22.8 22.4 20.5 25.3 21.7 38.2

17.5 14.1 15.4 16.5 19.7 15.7

14.6 16.3 12.6 13.3 17.7 8.1

3.0 2.4 3.8 4.4 2.2 3.3

10.8 9.9 10.0 9.2 11.7 11.8

5.0 5.0 6.0 6.4 3.8 5.5‘

4.0 6.4 6.9 3.0 3.5 21.5

5.6 7.2 8.0 6.7 4.4 5.5

0.25 0.32 O,j9 0.26 0.20 0.30

G.18 G.15 0.23 0.29 0.14 0.32

16 17

14.6 17.3

18.4 27.1

16.6 17.0

17.9 10.1

2.1 4.4

13.4 7.1

4.8 8.2

3.7 1.1

2.7 7.7

0.15 0.2R

0.14 0.25

1

A

1,3,6

,1,4,6 + 1,3,!i

a - Expressed as percentages of the tobal trimethylnaphthalenes

Fig. 1 shows how 1,2,7-TMN may be derived from the D/E ring moiety of triterpenoids with the substitution pattern typical of the oleanane skeleton. Only triterpenoids with the oleanane skeleton will yield 1,2,7-TMN from the D/E ring moiety, and the very high abundance of this isomer may be related to the high abundance of triterpenoid precursors with the oleanane skeleton. It therefore appears that 1,2.7-TMN may serve as a marker for input into sediments of triterpenoids of the oleanane type. Reports on the occurrence of oleanane skeleton derivatives in modem plants indicate that they occur only in angiosperms (DAS and MAHATO, 1983; PANT and RASTOCX,1979). Compared with the levels observed in the Volador #l well, the lower abundances of 1,2,7-TMN in sediments of Triassic age from the Jupiter # 1 well, which predate the evolution of angiosperms (STEWART, 1983), suggests that the ancient nonangiosperm plants contributing to the organic matter in these sediments contained little or no triterpenoid material of the oleanane type. In contrast, the sediments of Cretaceous age from the Volador # 1 well contain plant material from a period when angiosperms were abundant, and these contain comparatively high levels of 1,2,7-TMN. The identities of the natural product precursors of the TMNs in sediments which do not contain an input from higher plants have yet to be determined, however 1,2,4-TMN may provide the key to this question. This isomer has two tu-substituents, and is therefore unstable relative to most other TMN isomers. The shallowest sample from the Madeieine # 1 well contains an unexpectedly high abundance of the unstable (reactive) 1,2,4-TMN, and 1,2,4-TMN comprised 22.1% of the TMN components of the oil obtained from pyrolysis of a typical immature oil shale (Rundle oil shale, Queensland, Australia, HARVEY et al., 1984). Further evidence that 1,2,4-TMN may be an important primary TMN

in samples containing no higher plant organic matter is provided by the observation that 1,2,4-TMN comprises 2 1.4% of the TMNs in crude oil 15 (Table 4), which is a crude from a typical marine shale of low maturity (2OS/2OR = 0.57). Which natural product(s) gave rise to the high relative concentrations of 1,2,4-TMN in these samples is unclear: one of the few natural products with the approprate pattern of methyl substituents and which has been reported to be present in recent marine sediments is Lu-tocopherol (RRASSELLand EGLINGTON, 1986), however this compound would have to undergo reduction, further cyclisation, and loss of the isoprenoid side chain to produce 1,2,4-TMN.

As depth and maturity increase down a sedimentary sequence, the relative concentrations of DMNs and TMNs with methyi substituents in the cr-positions (1,4,5 or 8) decrease relative to those of isomers with methyl substituents in the P-positions (2,3,6 or 7), and this effect has been attributed to isomerisation reactions (ALEXANDERet al., 1985a, and references therein). In the case of l,Z,%TMN, the most favourable isomer&&ion reaction is likely to involve a t,2 methyl shift of the methyl at C-5 to give 1,2,6-TMN. In order to show that this reaction does occur, an experiment was carried out in which the dinuclear and trinuclear aromatic fraction of a crude oil rich in I ,2,5-TMN was heated at 200°C in sealed tubes with aluminum mo~tmo~ilonite for 0.25 h and f h. The partial. gas chromat~ms in Fig. 3 show that the relative abundances of 1,2,5-TMN and 1,2,6-TMN change markedly with heating time. The initial increase in 1,2,6-TMN at the expense of 1,2,5-TMN supports a I,2 methyl shift isomerisation reaction. Further heating results

Trimethylnaphthalenes in oils and sediments

80

70

min. FIG. 3. Partial capillary gas chromatograms of the dinuclear and trinuclear aromatic fraction of a crude oil (A) and the organic extract obtained after heating the aromatic fraction with aluminum montmorillonite at 200°C for 0.25 h (B) and 1 h (C).

in a decrease in 1,2,6-TMN relative to the 1,3,6-TMN, 1,3,7TMN and 2,3,6-TMN isomers. This again is consistent with an isomerisation reaction involving successive 1,2 methyl shifts and leading to the more stable P-substituted isomers. A proposed reaction scheme to account for these laboratory observed changes is shown in Fig. 4. By an analogous argument, 1,2,7-TMN would be expected to undergo a 1,2-methyl shift of the methyl at C-2 to give the more stable 1,3,7-TMN which in turn could rearrange to the more stable 2,3,6-TMN. From the values shown in Table 3 for samples from the Volador #l well, it is apparent that the relative abundances of the 1,2,5-TMN and 1,2,7-TMN isomers are dependent upon the maturity of the sediments, and that the relative concentrations of these two isomers decrease with increasing maturity. This is shown more clearly by the values for the isomer pairs 1,2,5-TMN/1,3,6-TMN and 1,2,7_TMN/1,3,7TMN. We have elected to use 1,3,6-TMN in preference to 1,2,6-TMN in the first of these reactant/product pairs because the relative concentration of 1,2,6-TMN is usually not high in mature sediments (ALEXANDERet al., 1985a); presumably this isomer is less stable with respect to 1,3,6-TMN in sediments. Values for both ratios show a general decrease with increasing depth as maturity increases. In contrast with the Volador #l samples, where the relative concentrations of many of the isomers show comparatively smooth and very marked changes with depth (maturity), samples from the Jupiter #l well do not show such marked changes. The Jupiter #l samples range from immature to mature (vitrinite reflectance 0.36 to 1.05% and 20S/20R 0.361.02), so one might expect to see changes similar to those found in the Volador #l sediments, however this is not the

1261

case. The TMN complexion of Jupiter #l is quite different from that of Volador #l, the striking point being that even the less mature samples show values for relative concentrations of some isomers which are similar to those found in the most mature Volador #l sediments. One explanation for these differences is that the two sedimentary sequences have been subjected to very different heating rates. The Jupiter #1 sediments have experienced a heating rate over the past 80 Ma of only 0.14”C Ma-’ (BARBER, 1982), while the heating rate experienced by the Volador # 1 sediments over the past 20 Ma has been 7.4”C Ma-’ (STAINFORTH,1984). The effect of a low heating rate is to cause reaction to occur at a lower temperature (ALEXANDERet al., 1986). These results suggest that the reaction causing depletion of the 1,2,5-TMN is more susceptible to changes in heating rate than are the reactions which cause the other maturity indicators to change. Changes in relative abundances of TMN isomers may be caused by either reversible or non-reversible processes. The fact that changes commence at low values of maturity and yet the more reactive less stable isomers persist even at high values of maturity indicates that the processes are reversible in nature, and accordingly, as sediment maturity increases the isomer ratios should approach equilibrium values. In order to obtain an estimate of the values of the ratios at equilibrium, we have used three very mature samples from deep in the Cape Range #2 well which have vitrinite reflectance values of 1.62%, 2.03% and 2.56% (data published previously in ALEXANDERet al., 1985a). In these deep samples, the ratios 1,2,5-TMN/1,3,6-TMN and 1,2,7-TMN/1,3,7-TMN reached minimum values in the ranges 0.50-0.20 and 0.43-0.29, respectively, and we have chosen to use the upper limits of these values (i.e. 0.50 and 0.43) which are from the deepest sample (vitrinite reflectance value of 2.56%) as benchmarks in the present study. This approach indicates that even in the deepest Volador # 1 samples, the ratio 1,2,5-TMN/ 1,3,6TMN has not decreased to the levels found in very mature sediments. A logarithmic plot of the ratio 1,2,5-TMN/1,3,6-TMN against the ratio 1,2,7-TMN/1,3,7-TMN for the sediment samples is shown in Fig. 5. The dotted lines indicate the equilibrium values for the two variables selected for comparison purposes (most mature Cape Range #2 sample), and

FIG.

4. Proposed reaction scheme for conversion of 1,2,5-TMN

into other TMN isomers.

1262

M. G. Strachan. R. Alexander and R. I. Kagi Application of trimethylnaphthalene crude oil characterization

0.5.

0.0.

.

I

-1.6 -1.5

-1.0

-0.5

0.0

0.5

1.0

-_

FIG. 5. A plot showing the relative abundances in sediments and crude oils of 1,2,5-TMN and 1,2,7-TMN, expressed as logarithms of the ratios of isomer pairs: 1,2,5/1,3,6 and 1,2,7/1,3,7. Sediment samples are indicated by solid symbols [Jupiter #I (A), Matches Springs #l (O), Madeleine #l (m), Volador #I (O)] and the crude oils by circles in which their group classification [Table 21is indicated. The broken lines represent benchmark values for the two ratios, derived from values found in the deepest and most mature sample from the

Cape Range #2 well.

the point of intersection of these lines should indicate the values for the two ratios which one might expect to find in very mature samples; that is, the value of the ratios in samples of increasing depths (maturities) in a sequence should approach this point. It is apparent that by using this plot, the sediments can be classified into three groups. Samples in the first group, represented by the Volador #l samples, contain relative excesses of 1,2,7-TMN and 1,2,5-TMN, and as maturity increases the ratios approach the mature values from high in the top right quadrant. The second group is represented by the Jupiter #I samples. Although these appear in the top two quadrants they lie predominantly in the top left sector and close to the 1,2,7-TMN/ 1,3,7-TMN benchmark line. The values for samples from the Jupiter #I well occupy a position on the plot representing a slight to moderate excess of 1,2,5-TMN over that found in very mature Cape Range #2 samples. The Madeleine # 1 and Matches Springs # 1 samples are in positions which occupy the bottom left quadrant of the plot: compared with the terrestrially derived hydrocarbons, these samples show a much more even distribution of TMNs, are relatively deficient in 1,2,5-TMN and 1,2,7TMN, and contain relatively higher proportions of 1.3,7TMN, 1,3,6-TMN, and 1,4,6-TMN + 1,3,5-TMN. It is therefore apparent that, although the relative abundances of 1,2,5-TMN and 1,2,7-TMN are aIfected appreciably by thermal maturation of the sediments, this treatment enables an assessment of the nature of the input of TMNs to sediments even when those sediments are comparatively mature.

isomer distributions to

Details of the crude oils selected for this study are shown in Table 2. They have been classified into four groups designated A to D. Group A crude oils were sourced from sediments of Cretaceous age or younger (STAINFORTH,1984; KINGSTON, 1977). These oils appear to have been derived from organic matter containing a significant amount of higher plant material, because at least two of the following conditions were satisfied: high wax content, CPI > 1, dominance of ethylcholestane over cholestane. Using the same criteria, one concludes that the sources of the Group B crude oils also contained higher plant material, however this material was deposited prior to the widespread appearance of angiosperms in Cretaceous times (KANTSLERet al., 1983, 1984: VINCENT et al., 1985). The crude oils in Group C were derived from sediments of various ages and show no evidence of higher plant contribution, although crude oils 10 and 12 have unusually high levels of ethylcholestane (VOLKMAN, 1986). Group D oils contain the higher-plant-derived triterpane 18a(H)-oleanane. Such crudes are typically associated with higher plant organic matter deposited in deltaic environments (SMITH et al., 1970; WHITEHEAD, 1974; PYM et al., 1975; EKWEOZOR,1979a,b; HOFF-MANNet al., 1984). Values ofthe ethylcholestane epimer ratio (2OS/20R) are also shown in Table 2 and may be used to assess the maturity of the sediments from which the crude oils were derived. The distributions of TMNs in each of the crude oils is shown in Table 4, together with values for the ratios of 1.2.5TMN/l.3.6TMN and 1,2,7-TMN/l,3,7-TMN. Comparison of the abundances of 1,2,5-TMN and 1,2,7TMN in Group C with their abundances in other groups shows that crude oils from Group A are character&d by a relatively high abundance of 1,2,5-TMN and 1,2,7-TMN, whereas, because their source rocks predated the evolution of the angiosperms, Group B oils contain lower amounts of 1,2,7-TMN and are not derived from compounds of the oleanane skeletal type. Group D crude oils are similar to those in Group C; however the Group D crude oils are of special interest because, although they contain l&x(H)oleanane, they do not contain enhanced levels of 1,2,5-TMN or 1,2,7-TMN as do the Group A and Group B crude oils. It appears that the organic matter from which Group D oils were derived was deposited under conditions very different to those which gave rise to oils in Groups A and B. Conditions in the source sediments of Group A crude oils favoured aromatisation and cleavage processes to such an extent that none of the compounds with oleanane skeletons were preserved, but instead were converted into 1,2,5-TMN and 1,2,7-TMN. In contrast, conditions in the source sediments of Group D crude oils were such that aromatisation and cleavage were comparatively minor processes, and reduction to the saturated hydrocarbon, resulting in formation of 18a(Hfoleanane, was the dominant process. It appears that the marine deltaic environments associated with the formation of Group D crude oils are more similar to the open marine environments associated with the Group C crude oils than the more restricted aquatic environments associated with Group A and Group B crude oils.

Ttimethylnaphthalenes

The ratios 1,2,5-TMN/1,3,6-TMN and 1,2,7-TMN/l,3,7TMN shown in Table 4 have also been plotted in Fig. 5, together with the values for the sediments. The position of a crude oil on such a plot is dependent primarily upon the type of organic matter in the source rocks, modified by secondary maturity effects. Thus, the quadrant in which an oil sample is located is determined by the type of organic matter from which it was formed. Further, the proximity of an oil in Group A to the crossover point of the lines selected as equilibrium values for mature samples reflects the thermal history of the crude oil sample. Group A crude oils have been sourced from organic matter which contained both 1,2,5-TMN and 1,2,7TMN and their precursors in high relative concentrations, although maturation of the source rocks has no doubt resulted in some depletion of these isomers. Similarly, it is apparent that crude oils from Group C and Group D are located in the quadrant where 1,2,5-TMN and 1,2,7-TMN are in low relative concentrations. These crude oils were apparently derived from source materials which, initially, were even more depleted in 1,2,5-TMN and 1,2,7-TMN and their precursors. Crude oils from Group B are located in the top left quadrant which represents source material with high relative concentrations of 1,2,5-TMN and its precursors, and with 1,2,7TMN comparatively depleted in concentration relative to benchmark values for the ratio 1,2,7-TMN/l,3,7-TMN. In summary, a plot such as that shown in Fig. 5 enables crude oils to be character&d according to the nature of their source material and the environment in which this material was deposited. Group A crude oils were derived from organic matter containing higher-plant-derived compounds deposited under conditions conducive to aromatisation reactions. The presence of 1,2,7-TMN, which indicates the presence of precursors with the oleanane skeleton, suggests that the source organic matter was of Cretaceous or younger age, and because aromatisation has proceeded, one infers that it was deposited in a restricted aquatic environment, such as a coal swamp. Group B crude oils were sourced from organic matter of higher plant origin, but this material did not contain compounds with the oleanane skeleton, and it was deposited under conditions similar to those described for the Group A case. Group C and Group D oils were sourced from organic matter deposited under conditions which did not favour aromatisation processes. These were probably typical marine environments. In the case of Group D crude oils, the source material contained type skeletons.

some triterpenoid

compounds

with oleanane-

CONCLUSIONS 1,2,5-TMN and 1,2,7-TMN occur in enhanced relative abundances in samples of Cretaceous age and younger, es-

pecially when the samples were from lower maturity sediments which contain type III organic matter. Samples deposited since the Early Cretaceous have higher relative abundances of 1,2,7-TMN, and this observation is attributed to this isomer being formed from the structural degradation of oleanane-type triterpenoids present in angiosperms which first appeared at this time. The crude oils can be classified according to their trimethylnaphthalene isomer compositions.

in oils and sediments

1263

This classification enables crudes of higher plant origin to be identified, and to be further classified according to whether the organic matter predated or post-dated the evolution of angiosperms. Acknowledgements-The gift of a sample of shale oil by Mr. Tim Harvey and technical assistance from Mr. Steve Fisher is gratefully acknowledged. This work was supported by the National Energy Research Development and Demonstration Programme of Australia. Editorial handling: J. Rullkijtter REFERENCES ALEXANDERR., MAGI R. I. and SHEPPARDP. N. (1984a) 1,8-Dimethylnaphthalene as an indicator of petroleum maturity. Nature 308,442-443. ALEXANDERR., CUMBERSK. M. and KAGI R. I. (1984b) Geochemistry of some Canning Basin crude oils. The Canning Basin WA., Proc. Symp. On Canning Basin WesternAustralia,pp. 353-358, Frank Daniels Pty. Ltd., Perth. ALEXANDERR., KAGI R. I., ROWLAND S. J., SHEPPARDP. N. and CHIRILAT. V. (1985a) The effects of thermal maturity on distributions of dimethylnaphthalenes and trimethylnaphthalenes in some ancient sediments and petroleums. Geochim. Cosmochim. Aeta 49, 385-395. ALEXANDERR., CUMBERSK. M., HARTUNG B. and KAGI R. I. (1985b) Petroleum geochemistry of the Canning Basin. Western AustralianMining and PetroleumResearch Inst. Rept. N. 20, I 19~. ALEXANDERR., STRACHANM. G., MAGIR. I. and VAN BRONSWIJK W. (1986) Heating rate effects on aromatic maturity indicators. In Advances in Organic Geochemistry, 10 (eds. D. LEYTHAEUSER and J. RULLK~T~ER),pp. 997-1004. Pergamon Press, Oxford. ALEXANDERR., KAGI R. I. and NOBLE R. A. (1987) Fossil resin biomarkers and their application in oil to source-rock correlation, Gippsland Basin, Australia. APEA J. 27, 63-72. BARBERP. M. (1982) Palaeotectonic evolution and hydrocarbon genesis of the Central Exmouth Plateau. APEA J. 22, 13 l- 144. BARNESM. A. and BARNESW. C. (1983) Oxic and anoxic diagenesis of diterpenes in lacustrine sediments. In Advancesin OrganicGeechemistry, 1981 (eds. M. BJO&Y et at.), pp. 289-298. J. Wiley & Sons. BENIXRAITISJ. G. (1974) Hydrocarbons of biogenic origin in petroleum-aromatic triterpenes and bicyclic sesquiterpenes. In Advancesin OrganicGeochemistry,1973 (eds B. TISSOTand F. BIENNER), pp. 209-224. Editions Technip. BRASSELLS. C. and EGLINTONG. (1986) Molecular geochemical indicators in sediments. In Organic Marine Geochemistry (ed. M. L. SOHN).DD. 10-32. Amer. Chem. Sot.. Washinaton. D.C. BREGERI. A. (1960) Diagenesis of metabolites and a d&&on of the origin of petroleum hydrocarbons. Geochim. Cosmochim.Acta 19,297-308. BRIKKEI. (1982) Geochemical interpretation of some oils and condensates from the Dampier sub-Basin of Western Australia. APEA J. 22, 179-187. CHAFFEEA. L. and JOHNS R. B. (1983) Polycyclic aromatic hydrocarbons in Australian coals. I. Angularly fused pentacyclic tri- and tetraaromatic components of Victorian brown coal. Geochim. Cosmochim. Acta47,2141-2155. CHAFFEEA. L., STRACHANM. G. and JOHNSR. B. (1984) Polycyclic aromatic hydrocarbons in Australian coals. II. Novel tetracyclic components from Victorian brown coal. Geochim. Cosmochim. Acta 48,2037-2043. COOK A. C., SMITH M. and VOS R. G. (1985) Source potential of upper Triassic fluviodeltaic systems of the Exmouth Plateau. APEA J. 25,204-2 15. CRONAUERD. C., JEWELLD. M., SHAH Y. T. and KUESERK. A. ( 1978) Hydrogen transfer cracking of dibenzyl in tetralin and related solvents. Ind. Eng. Chem. Fundam. 17, 291-297. CROSTELLAA. and CHANEYM. A. (1978) The petroleum geology of the outer Dampier Sub-Basin. APEA J. 20, 13-22.

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