2-Methylretene in sedimentary material: a new higher plant biomarker

Organic Geochemistry 32 (2001) 1211–1217 www.elsevier.com/locate/orggeochem

2-Methylretene in sedimentary material: a new higher plant biomarker Trevor P. Bastow*, Raj K. Singh, Ben G.K. van Aarssen, Robert Alexander, Robert I. Kagi Australian Petroleum CRC/Centre for Petroleum and Environmental Organic Geochemistry, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia

Abstract 2-Methylretene has been synthesised, characterised and identified in sedimentary material from a range of locations, source types and ages. 2-Methylretene was observed only in samples of Permian to Tertiary age and can be associated with specific higher plant precursors that also yield retene. Laboratory dehydrogenation of simonellite yielded 2-methylretene as the major product. Based on this we suggest that 2-methylretene forms from the aromatisation of diterpenoid type natural products with the abietane and phyllocladane skeletons, similar to those that form simonellite and suggest it can be used as a biomarker for higher plant input. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: 2-Methylretene; 1,2-Dimethyl-7-isopropylphenanthrene; Retene; Simonellite; Aromatisation; Alkylphenanthrene biomarkers; Abietane; Phyllocladane; Diterpenoid

1. Introduction The formation of several aromatic compounds in petroleum is thought to result from chemical transformations of naphthenic and/or olefinic natural product precursors (Radke, 1987). The aromatisation of these natural product precursors can alter their structure due to processes such as alkylation, dealkylation, isomerisation, ringopening and migration of gem dimethyl groups (Garrigues et al., 1986; Pu¨ttmann and Villar, 1987; Radke, 1987; Loureiro and Cardoso, 1990; Heppenheimer et al., 1992, Alexander et al., 1992; Ellis et al., 1996). Diterpenoids form a large group of natural products, which are widely distributed in the plant and animal kingdoms. Of these, tricyclic/tetracyclic compounds with the abietane (I) and phyllocladane (II) carbon skeletons commonly occur in both angiosperms and gymnosperms. Aromatic biomarkers derived from compounds with these carbon skeletons include simonellite (III) and retene (IV) (Simoneit, 1977), which have been used in numerous geochemical studies as indicators of higher plant input (Simoneit, 1977; Heppenheimer et al., 1992; Otto et al.,

* Corresponding author. E-mail address: [email protected] (T.P. Bastow).

1997). A diterpenoid such as abietic acid (V) can form retene (IV) upon aromatisation, but cannot form simonellite (III), due to the fact that it does not possess a gem dimethyl group in its structure. In contrast, diterpenoids that do contain gem dimethyls cannot only form simonellite (III) but also retene (IV), therefore, discriminating between different sources for simonellite (III) and retene (IV). In this paper, we identify a new diterpenoid derived biomarker, i.e. 2-methylretene (VI) in crude oils and rock extracts. Its structure suggests it is formed from the aromatisation of diterpenoid natural products with the abietane (I) or phyllocladane (II) skeleton containing unfunctionalised A-ring gem dimethyl groups, similar to simonellite (III). It may therefore be potentially useful in characterising samples containing higher plant derived material.

2. Experimental 2.1. Samples Table 1 lists the samples used in this study, their location and source rock age, as well selected biomarker parameters.

0146-6380/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0146-6380(01)00085-7

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2.2. Reference compounds Simonellite (III) was isolated from a crude oil through a combination of liquid chromatography and molecular sieving techniques. Simonellite (III) was identified on the basis of its mass spectral characteristics. 2.2.1. Preparation of 2-methylretene 2-Methylretene (1,2-dimethyl-7-isopropylphenanthrene) (VI) was prepared according to the method described by Wood and Mallory (1964). Briefly, the Grignard reagent (20 mmol in 50 ml of ether) from 1-bromo 3-isopropyl benzene was added to 2,3-dimethylphenylacetaldehyde (18 mmol in 20 ml ether). The resultant tertiary alcohol (65% yield) was recovered in pure form after subjecting the reaction mixture to liquid chromatography procedures using silica gel with hexane and dichloromethane as solvents. The tertiary alcohol was converted into a stilbene by dehydration using a mixture of sulphuric and acetic acids (75% yield). Subsequently, it was cyclised

photochemically in cyclohexane in the presence of air and iodine to yield 2-methylretene (VI) and 1,2-dimethyl 5-isopropylphenanthrene (VII). These two compounds were separated using column chromatography techniques. The mass spectrum of 2-methylretene (VI) obtained from this synthetic procedure is shown in Fig. 1. 2.3. Nuclear magnetic resonance (NMR) data All measurements were carried out in a solution of chloroform-d using a Varian Gemini-200 spectrometer operating at 200 MHz for 1H (ppm relative to CDCl3 at 7.27 ppm) and 50 MHz for 13C (ppm relative to CDCl3 at 77.7 ppm). 1 H NMR data for 2-methylretene (VI): d 1.39, d, J=7.0 Hz, 6H, CH(CH3)2; 2.54, s, 3H, s, 3H, Ar(C2)– CH3; 2.67, s, 3H, Ar(C1)–CH3; 3.13, septet, J=7.0 Hz, Ar(C7)–CH(CH3)2; 7.46, d, J=8.2 Hz, 1H, Ar(C3)–H; 7.54, dd, J=8.6, 1.8 Hz, 1H, Ar(C6)–H; 7.70, d=J=2.1 Hz 1H, Ar(C8)–H; 7.73, d, J=9.8 Hz, 1H, Ar(C9)–H;

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T.P. Bastow et al. / Organic Geochemistry 32 (2001) 1211–1217 Table 1 Locations, source rock age and geochemical data for samples Sample

Country

Basin

Probable age of source rocks

Pr Ph

20S 20S+20R

C29 C27

CPI(2)

Oils Baram-8 Iron Duke Shengli Kenmore-1 Moorari-4 Rough Range-1 Dongara-8 Flylake-1 Watson South Nieber Dome Blina-1 East Dollarhide McManus

Sarawak Brunei China Australia Australia Australia Australia Australia Australia Wyoming, USA Australia Texas, USA Australia

– – Zhanhua Eromanga Eromanga Carnarvon Perth Cooper Cooper – Canning – MacArthur

Tertiary Tertiary Tertiary Jurassic Jurassic Jurassic Triassic Permian Permian Carboniferous Devonian Cambro.-Ordiv. Proterozoic

4.7 3.4 0.6 2.8 6.9 3.7 1.3 6.1 5.1 0.6 0.6 – –

0.43 0.35 0.45 0.49 0.39 0.53 0.47 0.43 0.43 0.50 0.50 – –

3.3 1.7 0.8 3.4 3.9 1.6 1.0 0.7 3.7 0.7 1.1 – –

1.0 1.0 1.2 1.0 1.3 1.0 1.0 1.1 1.1 1.0 1.0 1.1 1.0

Sediments DH Loy Yang Kerosene Shale PD130A Acacia-1

Vietnam Australia Australia Australia Australia

– Gippsland Sydney Canning Canning

Tertiary Late Cretaceous Permian Devonian Ordovician

4.4 2.1 0.5 0.3 1.7

0.05 – 0.38 0.32 0.5

5.5 – 3.8 4.5 0.6

3.0 5.8 0.9 1.2 1.0

Definitions and methods of measurements: Pr/Ph, pristane/phytane (TIC); 20S/(20S+20R), 20S and 20R diasteromers of 5a(H),14a(H),17a(H)-ethylcholestane (m/z 217); C29/C27, (20R)-5a(H),14a(H),17a(H)-ethylcholestane/(20R)-5a(H),14a(H),17a(H)cholestane (m/z 217); CPI(2) calculated using n-alkanes (TIC) using carbon numbers from 23 to 29 ([(C23+C25 +C27)+(C25+C27+C29)]/2  (C24+C26+C28)) (Marzi et al., 1993). – absent or not measurable.

Fig. 1. Mass spectrum and molecular structure of 2-methylretene.

8.00, d, J=9.2 Hz, 1H, Ar(C10)–H; 8.60, d, J=8.6 Hz, 1H, Ar(C5)–H; 8.61, d, J=8.4 Hz, 1H; Ar(C4)–H. 13 C NMR data for 2-methylretene (VI): d 15.73, Ar(C2)CH3, 21.59, Ar(C1)CH3; 24.76, Ar(C7)CH

(CH3)2; 34.39, Ar(C7)CH(CH3)2; 120.72, 123.40, 123.49, 125.79, 126.48, 127.33, 129.65 Ar(C3, C4, C5, C6, C8, C9, C10); 129.46, 130.70, 131.34, 131.96, 132.90, 134.57, 147.33, Ar(C1, C2, C4a, C5a, C7, C8a, C10a).

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Fig. 2. Partial m/z 248 mass chromatograms showing 2- and 9-methylretene in Moorari-4 crude oil.

Fig. 3. Total ion chromatogram showing the reaction products from heating simonellite with Pt/C.

2.4. Sample preparation

2.5. Isolation of aromatic hydrocarbon fractions

Rock samples were air dried and crushed to a fine powder using a Tema mill and extracted ultrasonically with dichloromethane/methanol (95:5 v/v). The solvent extract was recovered by filtration and the solvent was carefully removed to yield the soluble organic matter (SOM).

In a typical separation, the aromatic hydrocarbons from a crude oil or sediment extract were isolated as follows. Glass columns (10 cm 5.7 mm i.d.) were packed with activated silica gel (0.6 g, Merck, particle size 0.063–0.200 mm for chromatography) as a slurry in n-pentane. The sample (10–20 mg) in n-pentane was

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introduced to the top of the column to produce a concentrated band. Aliphatic hydrocarbons were eluted under gravity with n-pentane (2 ml), followed by aromatic hydrocarbons eluting with n-pentane/dichloromethane (7:3 v/v, 2 ml). After careful removal of the solvent the aromatic hydrocarbon fraction was dissolved in hexane to provide a sample ready for GC–MS analysis. 2.6. Analysis by gas chromatography-mass spectrometry (GC–MS) A Hewlett-Packard 5973 MSD, interfaced with a 6890 gas chromatograph was fitted with fused-silica open tubular columns coated with either ZB-5 stationary phase (Phenomenex, 60 m0.25 mm i.d., 0.25 mm film thickness) or HP-1ms stationary phase (Hewlett Packard, 50 m0.20 mm i.d., 0.25 mm film thickness). The GC oven temperature was programmed from 40 to 300  C at 3  C min 1. Samples for analysis were dissolved in hexane and injected on-column using a HP 6890 autosampler. Helium was used as carrier gas at a constant flow of 1.0 ml min 1 (ZB-5) and 0.9 ml min 1 (HP-1ms). Typical MSD conditions were: ionisation energy 70 eV; source temperature 230C; electron multiplier voltage 1800 V. 2.7. Dehydrogenation experiments Simonellite (III) (0.1 mg) and Pt/C (1 mg, 5%) were placed in small Pyrex glass ampoules (1 ml). The ampoules were flushed with dry nitrogen, evacuated, sealed and then heated at 300  C for 30 h. The contents were recovered in dichloromethane (2 ml) after which the solvent was removed by careful distillation. An aliquot was then dissolved in n-hexane and subjected to GC–MS analysis.

3. Results and discussion 3.1. Identification of 2-methylretene in crude oils and sediments 2-Methylretene (VI) was identified in crude oils and sediment extracts using GC-MS by comparing retention behaviour and mass spectral characteristics of the component with those of the synthesised reference compound. The latter co-eluted with the compound present in the crude oils and sediment samples, on two different capillary columns. A partial mass chromatogram of an aromatic fraction of a crude oil, obtained using a ZB-5 capillary column is shown in Fig. 2. 2-Methylretene (VI) is shown as a dominant peak in this chromatogram. Also shown is 9-methylretene (VIII), which previously has been suggested to be a geosynthetic methylation product of retene (IV) (Alexander et al., 1995). Retention indices relative to PAH standards for these compounds and other compounds used in this study are listed in Table 2.

The retention indices were calculated using the method described by Lee et al. (1979). 3.2. Formation of 2-methylretene in sediments The formation of 2-methylretene (VI) from a partially aromatised precursor derived from the abietane skeleton (I) was investigated in the laboratory by dehydrogenation of simonellite (III). This yielded two major compounds corresponding to retene (IV) and 2-methylretene (VI) (Fig. 3). Based on this result we propose that 2methylretene (VI) can form from diterpenoid natural product precursors via aromatisation and methyl migration of A-ring gem dimethyl groups. The formation of this compound is likely to proceed via a 1,2methyl shift, which has been postulated to occur upon aromatisation of other compounds containing gem dimethyl groups (Chaffee and Johns, 1983; Pu¨ttmann and Villar, 1987; Alexander et al., 1992) 3.3. Occurrence of 2-methylretene in crude oils and sediments extracts A number of crude oils (13) and sediment extracts (5) were analysed by GC–MS for the presence of 2-methylretene (VI). These samples represent a variety of petroleum producing regions in the world. The geological ages of their source rocks range from Proterozoic to Tertiary and they encompass a range of maturity/source types and depositional environments (Table 1). The crude oils and sediment extracts have been analysed for the presence of land-plant biomarkers (Table 3; previously described by Ellis et al. (1995)). Table 3 lists the relative abundance of 2methylretene (VI) in these crude oils and sediment extracts. Samples without higher plant biomarkers do not contain 2-methylretene (VI), whereas 2-methylretene (VI) is present in all samples with a significant higher plant contribution containing retene, strongly suggesting an higher plant origin. Diterpenoid precursors based on the abietane (I) and phyllocladane (II) carbon skeletons are especially abundant in plant and fossil resin deposits

Table 2 Retention indices for compounds used in this study Compound

Simonellite Retene 9-Methylretene 2-Methylretene

Structure

III IV VIII VI

Retention indices Ia HP-1ms

ZB-5

348.84 358.22 370.31 376.91

345.79 355.93 367.36 374.35

a I=I(naphthalene)+(tR tR(naphthalene))/(tR(phenanthrene) tR(naphthalene))  100. Where I(naphthalene)=200 and tR is the retention time.

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Table 3 Higher plant indicators and relative abundance of 2-methylretene in samples Sample Oils Baram-8 Iron Duke Shengli Kenmore-1 Moorari-4 Rough Range-1 Dongara-8 Flylake-1 Watson South Nieber Dome Blina-1 East Dollarhide McManus Sediments DH Loy Yang Kerosene Shale PD130A Acacia-1

Olea Hopane

Bicad(T) Hopane

Phyllo Hopane

Pimar Hopane

Retene 9-MP

Higher plant contribution

2-MRetene 9-MP

0.96 1.49 – – – –

0.12 0.07 – – – –

– – – 0.18 0.25 –

– – – 0.36 0.27 0.37

– – – – –

– – – – –

0.14 – – – –

0.10 – – – –

– – 0.14 1.68 3.81 0.53 – – 0.82 – – – –

Yes Yes Yes Yes Yes Yes No No Yes No No No No

– – 0.02 0.16 0.23 0.11 – – 0.10 – – – –

0.93 – – – –

– – – – –

0.73 – – – –

0.18 – – – –

1.74 1.16 0.05 – –

Yes Yes Yes No No

2.78 0.04 0.01 – –

Definitions and methods of measurements: Olea/Hopane, 18a(H)-oleanane/C30 17a(H),21b(H)-hopane (m/z 191); Bicad (T)/Hopane, trans-trans-trans bicadinane/C30 17a(H),21b(H)-hopane (m/z 191); Phyllo/Hopane, 16b(H)-phyllocladane (m/z 123)/C30 17a(H),21b(H)-hopane (m/z 191); Pimar/Hopane, isopimarane (m/z 123)/C30 17a(H),21b(H)-hopane (m/z 191); Retene/9-MP, retene (m/z 234)/9-methylphenathrene (m/z 192); 2-MRetene/9-MP, 2-methylretene (m/z 248)/9-methylphenathrene (m/z 192). – absent or not measurable.

(Streibl and Herout, 1969; Thomas, 1969; Douglas and Grantham, 1974), and are therefore potential sources for 2-methylretene (VI) in crude oils and sediment extracts.

4. Conclusions 2-Methylretene (VI) has been synthesised and identified in crude oils and sediment extracts. The formation of 2-methylretene in sedimentary material is suggested to occur via a 1,2-methyl shift during the aromatisation of diterpenoid natural products such as the abietanes (I) and phyllocladanes (II) that contain C-4 gem-dimethyl groups. 2-Methylretene occurs in crude oils and sediments in association with biomarkers characteristic of higher plant input and it is suggested that 2-methylretene is derived from diterpenoid natural products of higher plant origin.

Acknowledgements Geoff Chidlow is thanked for his assistance with GC– MS analysis. Financial support was provided by the

Australian Petroleum Co-operative Research Centre. Dr. Hans Peter Nytoft and an anomous reviewer are thanked for reviewing this manuscript.

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