The petroporphyrins of a Cretaceous oil

Chemical Geology, 15 (1975) 193--208 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

THE PETROPORPHYRINS OF A CRETACEOUS OIL

B. DIDYK, Y.I.A. ALTURKI*, C.T. PILLINGER and G. EGLINTON

Organic Geochemistry Unit, School of Chemistry, University of Bristol, Bristol (Great Britain) (Received October 21, 1974; revised and accepted February 27, 1975)

ABSTRACT

Didyk, B., Alturki, Y.I.A., Pillinger, C.T. and Eglinton, G., 1975. The petroporphyrins of a Cretaceous oil. Chem. Geol., 15: 193--208. The petroporphyrins of a Cretaceous crude oil, La Paz, from western Venezuela are shown to be a mixture of etio and DPEP homologues (C2~--C39) maximising at C30 and C31, respectively. Minor amounts of rhodoporphyrins (C30--C39) are also present. Thin-layer chromatography afforded fractions which have been shown by mass spectrometry to contain up to 80% of a single-molecular-weight species. Oxidative degradation of La Paz petroporphyrins to maleimides and mass-spectrometric study of t.l.c, fractions indicate that some of these porphyrins are to a great extent incompletely substituted. Dealkylation reactions have presumably played an important role in their geologic history. Furthermore, the relative simplicity of the alkyl substitution pattern of the La Paz petroporphyrins suggests that transalkylation reactions have not taken place to any significant extent. The fraction of petroporphyrins isolated from the asphaltenes contains a higher proportion of the DPEP homologues than do the total petroporphyrins isolated from the original crude oil.

INTRODUCTION

The detection of porphyrins in geologic materials (Treibs, 1934) and the proposal of a scheme (Treibs, 1936) that correlated the petroporphyrins to biological precursors, led to an increasing interest in the study of the potentialities of these fossil pigments as biological markers (Baker et al., 1967; Boylan et al., 1969; Alturki et al., 1972). These studies have demonstrated that petroporphyrins isolated from geological materials are mixtures of homologous series (etio-, DPEP- and rhodoporphyrins**) covering a wide molecularweight range (C2~--C39). Additionally, the existence of porphyrin polymers was suggested by Blumer and Snyder (1967), who reported the isolation by gel permeation of porphyrins with molecular weights extending up to 20,000. * Present address: University of Riyadh, Saudi Arabia. ** Throughout the text the term rhodoporphyrins is used in the same context as described by Baker et al. (1967), who proposed an alkylbenzoporphyrin structure for these compounds.

194

This was further substantiated by the later detection of dimeric porphyrin aggregates by Blumer and Rudrum (1970). The complexity of the naturally occurring petroporphyrin mixtures is emphasized by evidence suggesting the presence of structural petroporphyrin isomers (Blumer and Rudrum, 1970; Alturki, 1972). In order to explain the generation of a large range of isomeric petroporphyrins from a relatively small number of biogenic precursors, it has been suggested that porphyrins are subjected to transalkylation reactions in the geological environment (Baker, 1966; Alturki, 1972). Different studies have demonstrated that transalkylation reactions occur in vitro in a range of simulated geological conditions (Yen et al., 1969; Casagrande and Hodgson, 1971, 1974; Bonnett et al., 1972), but no direct evidence of their occurrence in the geological environment has been provided. Structural characterization of naturally occurring petroporphyrins would result in a better understanding of the nature and geochemical fate of their precursors. Several techniques have been applied to obtain structural information on petroporphyrins; they include degradation (Inhoffen et al., 1966; Hodgson et al., 1972), mass spectrometry (Jackson et al., 1965; Boylan, 1970) and gas chromatographic analysis of the volatile TMSi Si(IV) porphyrin complexes (Boylan et al., 1969; Alturki, 1972). A full structural characterization has not yet been achieved, though the relative abundance of the structural series (etio, DPEP and rhodo), their molecular weight range and the variety and relative abundance of the alkyl substituents can be established. Complete structural characterization of individual petroporphyrins is rendered difficult by the presence of a large number of homologues of the different structural series which occur in geological materials. Partial separations of these structural series have been achieved by column chromatography (Howe, 1961; Baker, 1966) and thin-layer chromatography (Thomas and Blumer, 1964}. Alturki et al. (1972) suggested that structural differences affect the polarity of the petroporphyrins as revealed by t.l.c. Gel permeation permitted the isolation of high-molecular-weight porphyrins but was not effective for compounds with molecular weights under 1,000 (Blumer and Snyder, 1967; Blumer and Rudrum, 1970). We report here the detailed analysis of the petroporphyrins of a Cretaceous crude oil, La Paz, from western Venezuela. This oil has been previously studied by Gransch and Eisma (1970), who established the total vanadyl porphyrin contents of a number of Venezuelan crude oils and sediments. The high vanadyl porphyrin content was used as a criterion for establishing source- rock--crude-oil relationships in western Venezuela.

195 EXPERIMENTAL

Extraction

The La Paz crude oil from well P-187-Z was supplied by Shell, Holland. This oil contains a high concentration (628 ppm) of metalloporphyrins, present predominantly as vanadyl complexes (Table I). Minor amounts, 10 ppm, of nickel complexes have also been detected. The asphaltenes were isolated by diluting 30 g of oil with 450 ml of n-hexane: the precipitated asphaltenes (5%) were separated by centrifugation (2,500 r.p.m.) washed by sonication in n-hexane, vacuum dried (60 m m Hg) and stored under nitrogen. The metal-free petroporphyrins were isolated by a demetallation-extraction procedure consisting of treating the oil with methane sulphonic acid as described by Alturki et al., (1972). The extraction of the porphyrins contained in the asphaltene fraction was carried out by treatment of the asphaltenes in methylene chloride with methane sulphonic acid. Oxidative degradation

La Paz petroporphyrins (2 mg) were submitted to controlled oxidative degradation to form their respective maleimides according to the method described by Ellsworth and Aronoff (1968). The maleimides so obtained in 5--10% yield were analysed by gas-chromatography--mass-spectrometry, which allows their characterization at the microgram level. The maleimides were identified by comparison with spectra of authentic standards and published spectra (Ellsworth, 1970; Hodgson et al., 1972). Fractionation

The analytical thin-layer separations were performed on silica gel H (0.3 mm, TABLE I

Characteristicsof La Paz oil OiP Source

La Paz Lake Basin (western Venezuela)

Hydrogen Sulphur

Age Well Depth Boiling below 200 °C1 Carbon

Cretaceous P-187-Z 9,350---10,467 ft. 11.6% 84.7%

Asphaltenes2 MetalloporphyrinI VO porphyrins Ni porphyrins Demetallated petroporphyrins isolated from total oil 3 Demetallated petroporphyrins isolated from asphaltenes 3

12.2% 3.1% 5% 628 ppm 618 ppm 10 ppm 435 ppm 1,340 ppm

The sample of La Paz oil was kindly supplied by the late Dr. J.A. Gransch, together with the information indicated. 2 Determined by n-hexane precipitation. 3 Petroporphyrins were isolated by methane sulphonic acid demetallation.

196

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197

activated for 2 h at 110°C) with methylene chloride as developing solvent. After development a group of five red fluorescent bands with R/, from 0.16 to 0.75 was observed. Two additional bands, one at R f > 0.85 and the other at the origin, were n o t further analysed as they did not contain porphyrins. The five porphyrin-containing zones were removed from the t.l.c, plate, eluted with acetone and rechromatographed separately under the same conditions.

Mass spectrometry and gas-chromatography--mass-spectrometry Mass spectrometric analyses of petroporphyrins were performed by direct insertion into a Varian MAT CH-7 mass spectrometer. The sample was independently heated and its temperature precisely programmed and monitored by a temperature control system. The mass spectrometric conditions were, in all cases: source temperature 200°C, pressure ~ 10 -6 tort, filament current 300 pA, ionisation energy 70 eV. The probe temperature was programmed at 20°C/min and the mass spectra and the total ion current were monitored continuously. All the spectra were recorded at a temperature of 210--220°C. Gas-chromatographic--mass-spectrometric analyses of the maleimides were performed using a CH-7 mass spectrometer linked to a Varian-1400 gas chromatograph by a Watson-Biemann separator and a "line of sight" inlet. Conditions were: column, 8' × 1/16" stainless-steel tubing, packed with 3% Dexsil 300 G C on Gas Chrom Q, 1 0 0 - 2 2 0 mesh, He carrier gas flow rate 5 ml/min; temperatures, injector 320°C, column programmed from 100--220°C at 8°C/min, separator 260°C, "line of sight" 280°C. Mass spectrometric conditions for GC--MSanalysis: ion source temperature 180°C, emission 100 ~A, ionization potential 70 eV, mass range scanned 12--850 m/e. RESULTS

The petroporphyrins The direct demetallation-extraction of La Paz oil resulted in the isolation of 435 p p m of demetallated petroporphyrins (Table II). The absorption spectrum, in addition to the Soret band, showed the four absorption bands characteristic of demetallated petroporphyrins. The relative intensities IV > I!I > II > I (Table II) indicated a mixture of etio and DPEP series with etio t y p e predominating. Absorption band I had a shoulder at 630 nm, indicative of the presence of a small a m o u n t of rhodoporphyrins (Baker, 1966). Mass spectrometric analysis confirmed that the petroporphyrins were a mixture of etio and DPEP homologues (Fig.l}, with a DPEP/etio ratio of 0.56. The molecular ions of the predominant etioporphyrins, representing 64% of the total ion current d u e to molecular ions, form an envelope with m/e values at (310 + n14) where n varies from 7 to 19 (408--576) with a maximum at 450 (n = 10; C30) (Table II). The molecular ions of the DPEP-porphyrins consist of a homologous series with m/e at 308 + n14 with n varying from 7 to 19,

198 Ioo

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Fig.1. C o m p o s i t i o n o f d e m e t a l l a t e d p e t r o p o r p h y r i n s isolated f r o m La Paz oil. Molecular ions o f t h e series ( 3 0 6 + n 1 4 ) a n d ( 3 0 8 + n 1 4 ) w i t h i n t e n s i t y > 3% are s h o w n . Mass s p e c t r o m e t r i c c o n d i t i o n s are given in t h e s e c t i o n E x p e r i m e n t a l . N o r m a l i z e d t o t h e m o s t a b u n d a n t ion. Full line: e t i o h o m o l o g u e s ; dash line: DPEP h o m o l o g u e s , a. F r o m t o t a l c r u d e oil. b. F r o m a s p h a l t e n e fraction.

(406--574); this series has a maximum at 462 (n = 11; C31) which is one CH2 equivalent higher than the maximum of the etio series. This bimodal distribution, with the relative maxima of both series differing by one CH2 equivalent is typical of the petroporphyrins detected in a wide range of geological materials. The carbon-number range of the distribution, C~--C~, is similar to that reported for porphyrins from other crude oils (Baker et al., 1967). In the porphyrins of La Paz oil a small a m o u n t of rhodoporphyrins was also detected, covering the range C30--C39. Their presence is confirmed by mass spectrometric analysis of t.l.c. Fraction 5 (see below), where these minor components are concentrated.

Asphaltenes It has been demonstrated previously that asphaltenes contain higher concentrations of porphyrins than the crude oils from which they have been isolated (Erdman and Harju, 1962). Previous studies (Thomas and Blumer, 1964; Baker, 1966; Baker et al., 1967; Blumer and Rudrum, 1970) have been concerned with the analysis of petroporphyrins isolated by precipitation of the asphaltene fraction. For the purpose of comparison, the asphaltenes from La Paz oil were isolated and analysed for their petroporphyrin content; an equivalent of 1,340 ppm of demetaUated petroporphyrins was isolated (Table II). The UV absorption spectrum indicated a mixture of etio and DPEP h o m o ° logues which was confirmed by mass spectrometric analysis (Fig.l). The relative concentration of the DPEP homologues was significantly higher than in the oil, with the C31 DPEP homologue (m/e 462) being the most abundant component. The increase in the DPEP content is reflected by

199 a higher DPEP/etio ratio (0.96) indicating that the asphaltenes preferentially concentrate the DPEP homologues. In contrast to the oil petroporphyrins, the molecular-weight distribution of the asphaltene porphyrins covers only the range C2~--C3s. These differences could be attributed to the fact that the petroporphyrins are incorporated into the asphaltic host by forming ~--~ molecular complexes with the asphaltene aromatic fractions (Vaughan et al., 1970) and that the incorporation of petroporphyrins in the asphaltic host is affected by their polarity and results in the preferential inclusion of the DPEPporphyrins. Petroporphyrins isolated from geological materials with high asphaltene content, such as Gilsonite (a naturally occurring asphaltite), Athabasca tar sands and the Green River shale, have been demonstrated to have high DPEP/ etio ratios (1.5--5.0) and narrow carbon-number-petroporphyrin distributions, C29--C3s, (Baker et al., 1967; Alturki, 1972). Due to the preferential concentration of DPEP-porphyrins in the asphaltenic fractions, any process which produces a change in the asphaltene content, such as migration, natural deasphalting, expulsion from source rock at different stages of maturation etc. could produce a significant change in the petroporphyrin content and the relative abundance of the DPEP and etio series. Fractiona tion

The analytical thin-layer-chromatographic separation of La Paz petroporphyrins resulted in the isolation of five distinct bands. The bands were designated as Fractions 1--5 in order of increasing Rf, and their characteristics are summarized in Table III. The absorption spectra indicate that Fraction 1 was composed predominantly of DPEP-type porphyrins, with relative intensities of the absorption bands: IV > II > I I I :> I. Fractions 2, 3 and 4 had etio-type spectra, the relative intensities of the absorption bands being IV > III > II > I. Fraction 5 also had an etioporphyrin-type spectrum but' the absorption bands II and I were split into doublets with secondary maxima IIa and Ia at longer wavelengths, indicative of the presence of a small amount of rhodoporphyrins. The mass spectra of Fractions 1--5 (Fig.2) indicate that a good structural-type separation had been achieved. The DPEP series was isolated complete in Fraction 1, and exhibits the maximum at C31 (m/e 462) and the same carbon number spread as detected previously in the unfractionated porphyrins. The etioporphyrins, free of the DPEP homologues, are distributed in Fractions 2--5 with an average molecular weight which increases with Rf. Fractions 2 and 3 show high concentrations of a single-molecular-weight component (80%) and a very narrow carbon-number distribution of homologues, corresponding to 3 and 4 methylene equivalents, respectively. Fraction 4 contained the major part of the etioporphyrins with a carbon-number distribution (C27---C39) similar to that of the total oil, but with the maximum (C3,) shifted one methylene equivalent to higher carbon number, probably as a consequence of the concentration of lower molecular-weight homologues into Fractions 2 and 3.

200

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Fig.2. Composition of the demetallated petroporphyrins from La Paz oil, Fractions 1--5. Conditions as for Fig.1. Full line: etio homologues; dash line: DPEP homologues; and dash-stop line: rhodo homologues.

The etioporphyrins from Fraction 5 (C21--C~) with a maximum at m/e 436 (C29) showed a higher concentration of the high-molecular-weight porphyrins (C33 plus). A second porphyrin series (m/e 444--570) with molecular ions 6 mass units lower than the corresponding etioporphyrins was also observed, thereby confirmingthe presence in La Paz oil of a small amount of rhodoporphyrins. These compounds have been reported to occur in a variety of geological materials always coexisting with the more abundant etio- and DPEPporphyrins. Baker et al. (1967) proposed an alkylbenzo porphyrin structure for these rhodo-type compounds and suggested that they are formed by a process of ring closure and aromatization during post-depositional maturation.

202

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203 No direct biological precursors which could be directly linked with these rhodo-type petroporphyrins have yet been identified.

Oxidative degradation La Paz petroporphyrins were submitted to controlled oxidative fragmentation with chromic acid. The alkyl maleimides so obtained are expected to retain the substitution pattern of the corresponding pyrroles of the porphyrin and give information on the alkyl substituents present. This technique has been widely used for structural studies of chlorophylls and related compounds {Fischer, 1911; Purdie and Holt, 1965; Ellsworth and Aronoff, 1968). The oxidation resulted in a series of five maleimides; monomethyl; monoethyl, methyl ethyl, methyl n-propyl and ethyl n-propyl maleimides {Table IV), with the methyl ethyl homologue being the most abundant component. Other maleimides such as the dimethyl, diethyl and methyl isopropyl derivatives were absent. This indicates that the alkyl substituents of the La Paz petroporphyrins are limited to methyl, ethyl and n-propyl groups and that unsubstituted positions are present. Similar results were obtained by Hodgson et al. (1972) by oxidative degradation of petroporphyrins from several geological sources. They reported that the most extensive series detected consisted of the same five maleimides observed in the present study, with the methyl ethyl homologue being the most abundant species. DISCUSSION The chromatographic behaviour of the petroporphyrins indicates that structural changes significantly affect their polarity, and in such a way that the different structural types decrease in polarity in the series DPEP > etio > rhodo. Within the etio series the polarity decreases with increasing molecular weight. This chromatographic behaviour has been discussed previously by Alturki et al. (1972). For example, Fraction 2, R V= 0.29 contains a high concentration of a Ca etioporphyrin, m/e 436, while etio-type porphyrins with the same molecular weight are also present in Fractions 4 and 5, which have a significantly lower polarity, Rf = 0.53 and 0.75 respectively. Overlapping must be minimal because an intermediate fraction, Fraction 3, contains only a very small amount of C29 etioporphyrin. The presence in this naturally occurring petroporphyrin mixture of compounds of the same structural type and molecular weight but with different polarities, suggests that these isomeric C29 etioporphyrins differ in their substitution pattern. A similar situation will, most likely, occur for other porphyrins of this oil. Isomeric petroporphyrins have been reported to occur in oil shales (Blumer and Rudrum, 1970) and in an asphaltite, Gilsonite (Alturki et al., 1972). Their occurrence in a crude oil has been shown as a result of the present study. This suggests that the presence of isomers among alkyl porphyrins isolated from geological materials may be a common occurrence and that they are the

204

result of normal mechanisms of incorporation and transformation of tetrapyrrolic pigments in the geological environment. Fractions 2 and 3 {Table III) contained only etioporphyrins with a very narrow distribution of homologues and a high concentration {approx. 80%) of a single-molecular-weight component. Hence, the overlapping of the fragmentation patterns of the different porphyrin homologues is minimized (Fig.3). In each case the pattern of fragmentation ions may be distinguished from that of the molecular ions. The most important fragmentation is the loss of 15 mass units, (M -- 15) +/> 40%, corresponding to the ~ cleavage of ethyl substituents. It has been demonstrated {Jackson et al., 1965; Boylan, 1970) that the relative intensities of fragment ions generated by ~ cleavage of the alkyl-side chains in porphyrins are directly proportional to the number of substituents in the molecule capable of undergoing that/3 cleavage. Comparison under identical mass spectrometric conditions of the relative intensities of the (M -- 15) ÷ fragmentation of the major components of Fractions 2 and 3 with the fragmentation of the tetramethyl tetraethyl porphyrin, etioporphyrin III (Fig.3) indicates that these components will, similarly, have several ethyl substituents. The etio homologue (m/e 436) of Fraction 2, has 9 methylene equivalents for allocation among the substituents, but the presence of several ethyl groups requires the existence of unsubstituted positions on the pyrrole rings of this

11°t

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2

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z

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2

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,.

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Fig.3. Partial mass spectra ( m / e > 400) of petroporphyrins of Fractions 2 and 3 f r o m La Paz oil and of standard etioporphyrin. Molecular ions (M ÷) and primary fragmentation ions from the high molecular weight region are shown, normalized to M ÷. Conditions as for Fig.1.

205 petroporphyrin. A similar situation is apparent for the etio homologue (m/e 450) of Fraction 3 and suggests that unsubstituted positions occur in other petroporphyrins of this oil. The oxidative degradation of the total petroporphyrins resulted in the generation of two mono-substituted maleimides: the monomethyl and monoethyl homologues. The relative abundance of the monoethyl product (26%) confirms the abundant occurrence of unsubstituted positions on the pyrrole units. This suggests that dealkylation processes have played an important role in the diagenetic transformation of La Paz petroporphyrins, assuming the conventional chlorophyll origin for these pigments. Dealkylation and cracking reactions, catalyzed by clay minerals, are known t o occur in the geological environment and have been postulated as an important contributory process for the generation of homologous series of geolipids of decreasing molecular weight (Henderson et al., 1968). The most favorable conditions for the occurrence of cracking will exist in the later, more severe, stages of the diagenetic process (Johns and Shimoyama, 1972). An alternative, or contributory process by which unsubstituted positions are introduced into the petroporphyrin nucleus can take place early in the postdepositional history of tetrapyrrolic pigments. Thus, protoheme compounds when degraded by microorganisms lose their two vinyl substituents and are converted into deuteroporphyrins having no alkyl substituents at positions 2 and 4 (Falk, 1964, p. 17). A dealkylation process of this sort would contribute to the generation of isomers and the lower-molecular-weight petroporphyrins detected. The origin of higher-molecular-weight (> C~) petroporphyrins has also been discussed: Transalkylation has been postulated by Baker (1966), and experimental evidence provided which proves that this is feasible under simulated laboratory conditions (Yen et al., 1969; Casagrande and Hodgson, 1971, 1974; Alturki, 1972; Bonnett et al., 1972). On the other hand, the oxidative degradation of the La Paz petroporphyrins shows that the range of their alkyl substituents is quite small with only methyl, ethyl and n-propyl groups being detected. Furthermore, only certain combinations of substituents occur (monomethyl; monoethyl; methyl ethyl; methyl n-propyl and ethyl n-propyl). Transalkylation would have been expected to have produced a wider range of substituents and also a larger number of combinations of substituent pairs on the pyrrole nuclei. Hence, transalkylation does not appear to have occurred to any significant extent in the La Paz petroporphyrins. An alternative origin for the higher-molecular-weight petroporphyrin h o m o , logues could be a series of biogenic precursors with a basic carbon skeleton of more than 32 carbon atoms as suggested by Baker et al. (1967). Indeed, chlorobium chlorophylls with an extended carbon skeleton range (C33--C~) have been isolated and characterized by Holt et al. (1966), and could give rise to the substitution patterns observed among petroporphyrins, except that no isobutyl has yet been detected. The chlorobium chlorophylls are produced abundantly (ca. 3.3% dry weight) as light-harvesting pigments by green photosynthetic bacteria, Chlorobium thiosulphatophilum (Cruden and Stanier, 1970). Green bacteria also contain bacteriochlorophyll a, a photosynthetic chlorophyll,

206 which is always present as a minor c o m p o n e n t accounting for only 2--6% of the total chlorophylls (Jensen et al., 1964). Anaerobic photosynthetic bacteria require special environmental conditions for their growth. In a typical non-stratified water column, the bulk of the organic matter productivity will be accounted for by phytoplanktonic material (Lorenzen, 1968; Taylor, 1972). In such an environment, chlorobium-type chlorophylls will be absent or account only for a minor fraction of the chlorinoid pigments. Organic matter of photosynthetic origin from different aquatic environments has been reported (Swain et al., 1964) to present a high concentration of chlorophylls (up to 300 mg of chlorophyll/g carbon). In all cases the predominant chlorophyll is chlorophyll a, which outweighs all other chlorinoid pigments. The great abundance of chlorophyll a in the organic matter produced in these environments, contrasts strongly with the low levels of chlorophyll a and its direct degradation products detected in the associated sediments (Swain et al., 1964; Hodgson et al., 1968), where this abundance is mainly controlled by environmental conditions. The significantly lower chlorinoid pigment content of Recent sediments indicates that in many cases only a minor fraction of the pigments from the water column is incorporated into the sediments and/or preserved in them. A completely different situation exists in certain aquatic environments where stratification of the water column occurs. In these environments, in the metalimnion, green photosynthetic sulphur bacteria can reach very high concentrations (Gophen et al., 1974). During the stratification, the biomass of photosynthetic bacterial origin is up to 4.5 times greater than the phytoplanktonic biomass produced in the oxic zone of the water column. The chlorobium chlorophyll concentrations may then reach levels up to 900 mg/m 3 and exceed the amount of pigments from the oxidative zone by a factor of 10 (Takahashi and Ichimura, 1968). The chlorinoid pigments from organisms growing in the anaerobic section of the water column might be expected to stand a significantly better chance of incorporation into the sediment. Thus, pigments from photosynthetic anaerobic organisms could have a significantly more important role, as precursors of petroporphyrins, than their relative abundance in the biosphere would suggest. Recent sediments from the Black Sea, where stratification of the water column occurs, have been reported by Peake et al. (1974) to contain a very high concentration of chlorinoid pigments (1,600 ppm on dry sediment basis). Furthermore, the pigments of the top layers of the sediment were present as homologous series with extended carbon skeletons, suggesting the significant contribution to the sediment of chlorobium-type chlorophylls. The observed differences in chlorinoid pigment content among Recent sediments would suggest that the occurrence of high concentrations of porphyrins in ancient geological materials would also be mainly the result of favourable paleo-environmental conditions of deposition. The direct correlation of petroporphyrins of extended carbon skeleton with precursors of extended homologous series, indicative of specific environmental conditions,

207

should provide a useful paleoenvironmental criterion. A study of this nature, on the basis of a detailed structural characterization of petroporphyrins from different geological materials is presently taking place in this laboratory. ACKNOWLEDGEMENTS

We thank the Natural Environment Research Council (NERC GR/3/634) and the National Aeronautics and Space Administration (NGL 05-003-003) for the facilities employed in the present study; the Empresa Nacional del Petroleo, Chile, and the Ford Foundation for the financial support (B.M.D.); the University of Riyadh, Saudi Arabia for financial support (Y.I.A.A.); the British Steel Corporation for a fellowship (C.T.P.) and also the late Dr. J.A. Gransch (Shell Exploratie en Produktie Laboratorium, Rijswijk, Holland) for the sample of La Paz crude oil.

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