High-pressure liquid chromatography of petroporhyrins: evaluation as a geochemical fingerprinting method by principal components analysis

Chemical Geology, 37 (1982) 229--237 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

229

HIGH-PRESSURE LIQUID CHROMATOGRAPHY OF PETROPORHYRINS: E V A L U A T I O N AS A G E O C H E M I C A L F I N G E R P R I N T I N G M E T H O D BY PRINCIPAL COMPONENTS ANALYSIS

M.E. HOHN .1, S.K. HAJIBRAHIM*: and G. EGLINTON Organic Geochemistry Unit, School of Chemistry, University of Bristol, Bristol BS8 1TS (Great Britain) (Received August 12, 1981; revised and accepted February 9, 1982)

ABSTRACT Hohn, M.E., HajIbrahim, S.K. and Eglinton, G., 1982. High-pressure liquid chromatography of petroporphyrins: evaluation as a geochemical fingerprinting method by principal components analysis. Chem. Geol., 37: 229--237. High-pressure liquid chromatographic traces of petroporphyrins can be compared on the basis of areas under selected peaks. Principal components analysis of such data obtained from a suite of twelve crude oils (McKittrick Field, California; Tertiary age) shows inter-sample relationships similar to those observed in a multivariate statistical analysis of published hydrocarbon distributions in the oils. Both groups of compounds appear to reflect the operation of processes in the geologic environment which have been inferred elsewhere to be migration and maturation.

INTRODUCTION Published w o r k o n o c c u r r e n c e a n d c h a r a c t e r of p e t r o p o r p h y r i n s in the g e o s p h e r e has c e n t e r e d a r o u n d correlating gross t y p e s and a b u n d a n c e s o f these p i g m e n t s t o geological f a c t o r s o f h o s t - e n v i r o n m e n t s . The c o m p l e x i t y o f p e t r o p o r p h y r i n m i x t u r e s and the lack o f effective analytical m e t h o d s a c c o u n t f o r a d e a r t h o f r e p o r t s o n the a p p l i c a t i o n o f p e t r o p o r p h y r i n distrib u t i o n s at t h e m o l e c u l a r level t o oil--oil and o i l - - s e d i m e n t correlations. The a d v e n t o f high-pressure liquid c h r o m a t o g r a p h y (HPLC) for f r a c t i o n a t ing and fingerprinting p e t r o p o r p h y r i n m i x t u r e s ( H a j I b r a h i m et al., 1 9 7 8 ; Q u i r k e et al., 1 9 7 9 ; E g l i n t o n et al., 1 9 8 0 ; H a j I b r a h i m , 1 9 8 1 ) offers a solut i o n t o difficulties in using these c o m p o u n d s as g e o c h e m i c a l markers. This p a p e r r e p o r t s the a p p l i c a t i o n o f m o l e c u l a r p e t r o p o r p h y r i n d a t a o b t a i n e d f r o m H P L C traces to the investigation o f m a t u r a t i o n and m i g r a t i o n o f p e t r o l e u m . Objective c o m p a r i s o n o f H P L C profiles requires q u a n t i f i c a t i o n Present addresses: ,1 West Virginia and Economic Survey, P.O. Box 879, Morgantown, WV 26507079, U.S.A. *2University of Riyadh, Department of Chemistry, Faculty of Sciences, P.O. Box 2455, Riyadh, Saudi Arabia. 0009-2541/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

230 'FABLE I Geologic setting of crude oils from McKittrick Field, California*' Oil No. C1 C2 C3 .2 C4 C5 .2 C6 C 7* s P8 P9 P10 O11 012 *2

Formation

Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Carneros Carneros Carneros Carneros Carneros Carneros Carneros Phacoides Phacoides Phacoides Oceanic Oceanic

ifi~,~

Ira)

5,655 5.872--5,915 5~950--5,990 7~194--7,225 6,425--6,463 6,480--6,513 5,950---5,990 7,828--8,630 8,310--8,341 9,058--9,074 8,858 8,834--8,916

1,724 1,790--1,803 1,814 - 1,826 2,193--2,202 1,958--:1,970 1,975 1,985 ] ,814---1,826 2,386- -2,630 2,53 3-- 2,542 2,761--2,766 2,700 2,693--2,7t8

Rese~'voir age Miocene Miocene Miocene Miocene Miocene Miocene Miocene Early Miocene Miocene Early Miocene Oligocene Oligocene

,1 Samples were kindly provided by Dr. W.K. Seifert (COFRC); data are from Seifert a~d Moldowan (1978, table 4)~ ,2 Sample not included in petroporphyrin study. ,3 Sample not included in study of Seifert and Moldowan (1978 }. o f individual p e a k s , a t a s k m a d e d i f f i c u l t b y the p r e s e n t level o f r e s o l u t i o n (e.g., Fig. 1). D e s p i t e this p r o b l e m , t r e n d s a m o n g s a m p l e s o b s e r v e d in multivariate statistical analysis o f p e t r o p o r p h y r i n p e a k areas parallel t r e n d s observed f r o m i n d e p e n d e n t criteria, t h u s establishing t h e g e o c h e m i c a l m e a n i n g fulness o f t h e H P L C fingerprints. S e v e n t e e n p e a k s w e r e q u a n t i f i e d in H P L C traces o f a suite o f twelve p e t r o l e u m s f r o m t h e M c K i t t r i c k Basin o f California, U.S.A. A p r e v i o u s l y p u b l i s h e d s t u d y o f t h e m o n o a r o m a t i c h y d r o c a r b o n s , steranes and t e r p e n e s f r o m a n e a r l y - i d e n t i c a l g r o u p o f s a m p l e s (Table I) p r o v i d e s an i n d e p e n d e n t c r i t e r i o n f o r assessing t h e g e o c h e m i c a l usefulness of p e t r o p o r p h y r i n s (Seifert and M o l d o w a n , 1 9 7 8 ) . A suite o f oil a n d shale s a m p l e s f r o m the M a r a c a i b o L a k e Basin o f V e n e z u e l a p r o v i d e s a f u r t h e r c h e c k on the c o n s i s t e n c y o f c o r r e l a t i o n s a m o n g t h e H P L C p e a k s ( H a j I b r a h i m , 1978). H P L C p e a k areas f o r t h e M c K i t t r i c k s a m p l e s and h y d r o c a r b o n d a t a f r o m Seifert a n d M o l d o w a n ( 1 9 7 8 ) w e r e c o m p a r e d t h r o u g h the m u l t i v a r i a t e statistical m e t h o d o f p r i n c i p a l c o m p o n e n t s analysis. EXPERIMENTAL Isolation o f p e t r o p o r p h y r i n c o n c e n t r a t e s

T o a k n o w n q u a n t i t y o f oil ( 5 - - 1 5 g) or t h e organic e x t r a c t o f shale in t o l u e n e ( o i l - - t o l u e n e , 1 : 3 w / v ) was a d d e d a n h y d r o u s m e t h a n o l (5 × t w i c e

231 t h e v o l u m e o f t o l u e n e used) and the m i x t u r e was s o n i c a t e d (~ 3 min.); cent r i f u g a t i o n (~ 3 0 0 0 r.p.m.: 5 min.) a f f o r d e d an u p p e r layer (wine-red) containing t h e c r u d e m e t a l l o p e t r o p o r p h y r i n s w h i c h was r e m o v e d , filtered t h r o u g h pre-washed (MeOH) c o t t o n w o o l t o r e m o v e s u s p e n d e d cellulose particles, and e v a p o r a t e d t o d r y n e s s using a r o t a r y e v a p o r a t o r (40 ° C). The c o n c e n t r a t e was d e m e t a l l a t e d b y t r e a t m e n t w i t h m e t h a n e s u l p h o n i c acid, and t h e free bases r e c o v e r e d as o u t l i n e d b y A l t u r k i et al. (1972).

-16

Boscan

9 K 3 Oil 14-

~

v

15 17

6

i

23

~

5

7

9

1

I0

I ,/ '

0

Forced / / '

I

'

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15

I

30

'

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4.5

MIN

Fig. 1. Example HPLC trace showing the numbering of petroporphyrin peaks, quantification method and typical resolution. Operating conditions: column: Partisil®-5 silica (24 cm × 4.6 mm I.D.); mobile phase: A -- hexane + 10% toluene;B = toluene--CHC13 (1 : 1 v/v); gradient: 20% B to 100% B in 40 rain., curve 9 (concave); flow: 1.5 ml/min. Detection: visible a 400 nm X 0.1 AUFS. Sample = Boscan 9K3 oil.

232

High-pressure liquid chromatography HPLC analyses were carried out using Partisil ® -5 silica column and gradient elution. The HPLC column packing method (e.g., balanced density) was described by HajIbrahim et al. (1978). Solutions of samples were intro. duced into the column, using the stop-flow technique, from a 10-/A syringe via a septum inlet port. HPLC grade solvents (Rathburn Chemical Ltd., Scotland, U.K.), were employed as mobile phases. Operating conditions are given in the legend for Fig. 1.

Assignment and quantification of the HPLC peaks Trapped peaks in the HPLC trace of Boscan (K3 petroporphyrins (Fig. 1 ) ! were designated as 1 to 17. Petroporphyrins giving rise to these peaks (Table II) were then used to assign (by coinjection) peaks in the HPLC traces of TABLE II Chromatographic and mass spectral characteristics of the major peaks in the liquid c h r o m a togram of Boscan 9 K 3 demetatlated petroporphyrins Peak No. *l

1 2 3 4 5 6 7 8 9 10 11 12 I3 14 15 16

17

Rt*: (min.)

R t * ~' (min.)

Area .4 (%)

Molecular ions* s ................................................ major minor component(s), (%)*~ component

8.00 9.00 9.50 11.00 12.50 14.25 16.00 18.00 20.75 23.75 26.25 29.00 30.50 33.00 35.00 37.00 39.00

7.50 8.50 9.50 10.50 12.75 14.50 16.50 18.25 21.50 24.25 27.00 29.50 31.00 33.25 35.50 *7 37.50 40.00

0.8 1.0 1.5 4.8 1.7 5.2 3.4 2.3 6.5 3.1 1.1 4.4 8.4 22.1 5.5 23.4 7.6

C32E

CzvE C3,E Ca0E C~E C:gE C31E C2~E C~0E C~0D C:9E CaID C~D C32D C3~D C3tD C30D

C3oE(65); C:9E(55); C3~E(4() ) C3tE(79); C3:E(72) C~8E(26)

Ca4D(74 ) C33D(71); C30D(53 ) C30D(60)

,i F r o m Fig. 1; in increasing elution order. ,2 Absolute retention time; from Fig. 1. Reproducibility = + 1 rain. ,a Absolute retention time; individual peaks were trapped and rechromatographed under same conditions as for Fig. 1. ,4 Calculated by triangular method; expressed as percentage of total area of peaks 1- 17 ,s F r o m the E1 mass spectra; expressed as carbon number. E = etio type; D = D P E P type. *~ Relative to the major ion (= 100). S o m e of the petroporphyrins in peaks / and 2 m a y be the same compounds. ,v For the major peak.

233 petroporphyrins from the McKittrick oils. This m e t h o d of peaks assignment appears to be most valid with respect to peaks 4--11, 14, 16 and 1 7, and less so for peaks 1--3, 12, 13 and 15 as shown by mass spectrometric analysis of these peaks trapped for petroporphyrin samples including one McKittrick oil (C1) (HajIbrahim, 1978). Quantification of peaks 1--1 7 was carried out by the measurement of their areas by triangulation using a forced baseline (Fig. 1). Peak area percentage was calculated with regard to the total areas of peaks 1--1 7. Area measurem e n t for severely overlapping peaks is usually complicated by the difficulty in choosing a correct baseline. In the present study, the m e t h o d used was chosen because geochemical correlations based on the calculated peak areas were in good agreement with those based on other, well-established parameters (see p. 234).

Statistical analysis Data for hydrocarbons in McKittrick oils (Table I) were taken from tables in Seifert and Moldowan (1978). Variables (Table III) include percentages and ratios among peak areas measured by integration of reconstructed mass fragmentograms provided by gas chromatographic--mass spectrometric analysis. Each data set was subjected to principal components analysis. This method calculates principal components as linear transformations of variables; the components are uncorrelated with each other. Although the number of components equals the number of variables, usually many of the components contain little variance, that is, a small number of principal components account for most of the total variance of the data. Each c o m p o n e n t is an eigenvector of an array of correlation coefficients among the variables, and has associated with it an eigenvalue. An eigenvector consists of loadings of variables on a principal component. Loadings may be treated as Cartesian coordinates for graphing to elucidate relationships among the variables. Each sample has a score on each principal component, and these scores may be graphed to express chemical similarities among the samples. Because the eigenvalues equal the variance of the scores along respective principal components, they are used to evaluate the relative importance of each principal component, a n d to express the goodness-of-fit of principal components to the original data. In summary, principal components analysis reduces a set of variables to a smaller set of composite variables. JSreskog et al. (1976) describe the mathematics and interpretation of principal components in detail. Multivariate analyses were run on the ICL ® system 4-75 of the University of Bristol, using a programme written in GENSTAT, a statistical programming language. Each variable was standardized to zero mean and unit standard deviation before analysis.

234 TABLE III Variables from Seifert and Motdowan (1978) used in principal components analysis in this paper Molecular parameter

Description

Mml/Mm 2

ratio of two aromatic compounds

Mm/Ms

ratio of maturable to stable monoaromatics

Tricyclic terpanes

percentage of total terpanes

1 7a(H)-hopanes

percentage of total terpanes

(6 + 7 + 9)/T s Tm/lOa

ratios among specific terpanes

lla/11b Tm/Ts 5~-C~8/5a-C29

ratio of 5a(H)-24-methylcholestane to 5~(H)-24ethylcholestane

5~-C27/5a-C2~

ratio of 5{3(H)-cholestaneto 5a(H)-cholestane

5~'C28/5~-C~8

ratio of 5~(H)-24-methylcholestane to 5~(H)24-methylcholestane

5~-C~9/5a-C29

ratio of 5~(H)-24-ethylcholestane to 5~(H)-24 ~ ethylcholestane

Rearranged/[5a-(C27 + C~8 + C29)J

ratio of rearranged steranes to 5a(H )-cholestane, -24-methylcholestane and -ethylcholestane

5¢-steranes/1 7a (H)-hopanes

ratio of 5~-steranes to C29--C3~ 1 7a(H)-hopanes

RESULTS Plots o f principal c o m p o n e n t scores f r om p e t r o p o r p h y r i n s (Fig. 2A) and f r o m h y d r o c a r b o n s (Fig. 2B) show distinct separation between the group of oils f r o m the Carneros F o r m a t i o n (C) and the group from the Phacoides F o r m a t i o n (P). Samples f r om the Oceanic F o r m a t i o n (O) plot farther from the Carneros oils when the h y d r o c a r b o n data are analyzed than when the p e t r o p o r p h y r i n data are analyzed, pr obabl y because of the availability of only one sample f r om this f o r m a t i o n and the low weight given a single sample in the calculations. Dissimilarities bet w een the Carneros and Phacoides oils dictate largely the placement of the principal c o m p o n e n t s in multivariate space. Nevertheless, a third c o m p o n e n t calculated from the p e t r o p o r p h y r i n data clearly separates Oceanic and Carneros oils. In b o t h principal c o m p o n e n t s analyses, the first two c o m p o n e n t s express m ost of the variance in respective data sets (Table IV), although the first two or three c o m p o n e n t s a c c o u n t for a smaller p r o p o r t i o n of the total variance in the case of the p e t r o p o r p h y r i n analysis. The higher n u m b e r of c o m p o n e n t s

235

31

~

oCI

,c~ e011

O 0.

U-ll

,c6

oc7

P9

oP10 -

35

-4

-

3

-2

®

3 COMPONENT

4

1

3E i i

c~ ,c2

iC1 .c3

o,~2

Z o

o*

U.

,c~

P8 P9 °eP1C

,c5

®

5

4

3

2

1 o COMPONENT

~

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~ -~

1

Fig. 2. Scores of McKittrick oils on principal components calculated from: (A) petroporphyrin HPLC relative peak areas; and (B) hydrocarbon distributions reported by Seifert and Moldowan (1978). Samples are numbered as in Table I. TABLE IV Largest eigenvalues and percentages of total variance in principal components analyses of McKittrick crude oils Eigenvalue

Percent of variance

Hydrocarbon variables: (1) 9.529 (2) 2.971 (3) 0.787

68 21 6

Petroporphyrin variables: (1) 7.465 (2) 5.992 (3) 2.148

37 30 11

236 needed to account for a given proportion of variance could mean that the petroporphyrins reflect a larger number of geochemical processes than do the hydrocarbon parameters. On the other hand, the petroporphyrin data may be subject to greater analytical error in quantification relative to the hydrocarbon parameters; this error leads to a large number of components being necessary to account for total variance because there will exist both geochemical and analytical components of variation. Seifert and Moldowan (1978) use mass fragmentograms and parameters computed from peak areas (Table III) to infer relative m a t u r i t y and effects of source and migration on the McKittrick oils. They conclude that the Oceanic and Phacoides oils are more mature than the Carneros oils, that 0 1 2 migrated farther than the other Oceanic oils, and that C5 and C6 migrat~d farther than the other Carneros oils. Point distributions in Fig. 3 express similarities and differences resulting from maturation and migration effects~ Therefore, the visual similarity between Fig. 2A and B suggests that petro.porphyrin peak areas also express relative maturity and extent of migratiorl. Seifert and Moldowan (1978} attribute some differences among the McKittrick oils to source effects, but because these effects cannot be separated from maturation and migration effects in the statistical analysis or" the hydro~ carbon parameters, tile source concept can receive little attention here. The most mature oils, namely, the Phacoides and Oceanic oils, manifest the largest areas for peaks, 5, 8, 10, 12 and 14. This maturation effect has also been observed in statistical analysis of petroporphyrins in oils from the Maracaibo Basin, Venezuela (HajIbrahim, 1978). DISCUSSION Seifert and Moldowan (1978) stress the importance of both within-group and among-group comparisons when assessing significance of organic geochemical processes and parameters that correlate with such processes. Principal components analysis provides a means for visualizing the relationships among the McKittrick oils, and for comparing two sets of variables. This statistical m e t h o d has the desirable property of partitioning sample variation between geologically-meaningful variation and analytical error; this partition is based on the premise that total variation caused by geological factors is greater than statistical noise, and that error is nonsystematic, i.e. not correlated among variables. The second premise might be violated in analysis of HPLC profiles because placement of a baseline can influence measured areas of adjacent peaks. However, results suggest that this violation is not severe, and that principal components analysis gives a meaningful, graphic summary of chromatographic fingerprints in the presence of errors in quantification. Principal components analyses of the two sets of variables show: (1) The arrangement based on petroporphyrin HPLC profiles of points representing oils resembles that obtained from a previously well-studied

237 g r o u p o f variables, h y d r o c a r b o n a b u n d a n c e s . Visual similarities b e t w e e n the two principal component plots mean that both groups of compounds r e s p o n d in similar w a y s t o c h e m i c a l p r o c e s s e s in t h e geologic milieu. T h e s e p r o c e s s e s have b e e n i n f e r r e d in p r e v i o u s w o r k (Seifert and M o l d o w a n , 1 9 7 8 ) to be migration and maturation. (2) With b e t t e r c h r o m a t o g r a p h i c r e s o l u t i o n and s t r u c t u r a l i d e n t i f i c a t i o n of petroporphyrins, more detailed information about relationships among oils a n d t h e n a t u r e o f g e o c h e m i c a l p r o c e s s e s s h o u l d be f o r t h c o m i n g . T h e m u l t i v a r i a t e a p p r o a c h is r e l e v a n t t o t h e p r o b l e m o f i d e n t i f y i n g s o u r c e r o c k s w h e n a subset o f s a m p l e s r e p r e s e n t s e x t r a c t s f r o m s u s p e c t e d s o u r c e rocks. T h e h y d r o c a r b o n d a t a h a v e b e e n used to e v a l u a t e p e t r o p o r p h y r i n s as geoc h e m i c a l m a r k e r s . D i s c r e p a n c i e s b e t w e e n the t w o s a m p l e d i s t r i b u t i o n s o n p r i n c i p a l c o m p o n e n t s suggest t h a t p e t r o p o r p h y r i n s will e v e n t u a l l y supplem e n t h y d r o c a r b o n - d e r i v e d p a r a m e t e r s c u r r e n t l y in use. This i n f e r e n c e s e e m s r e a s o n a b l e on m o l e c u l a r g r o u n d s in view o f t h e d i f f e r e n c e s to be e x p e c t e d in p h y s i o c h e m i c a l b e h a v i o r b e t w e e n h y d r o c a r b o n s a n d p e t r o p o r p h y r i n s . ACKNOWLEDGEMENTS T h e oil s a m p l e s w e r e k i n d l y p r o v i d e d b y W.K. Seifert. T h e N a t u r a l Env i r o n m e n t R e s e a r c h C o u n c i l ( G R / 3 / 2 4 2 0 ) p r o v i d e d t h e H P L C s y s t e m . We are g r a t e f u l to Dr. W.K. Seifert a n d t h e m a n a g e m e n t o f C h r e v o n Oil Field R e s e a r c h C o m p a n y f o r financial aid t o S.K.H. a n d t h e N a t i o n a l A e r o n a u t i c s a n d S p a c e A d m i n i s t r a t i o n f o r financial aid to M.E.H. t h r o u g h I n d i a n a University (NGL 15-003-118).

REFERENCES Alturki, Y.I.A., Eglinton, G. and Pillinger, C.T., 1972. The petroporphyrins of gilsonite. In: H.R. von Gaertner and H. Wehner (Editors), Advances in Organic Geochemistry, 1971. Pergamon, Oxford, pp. 135--150. Eglinton, G., HajIbrahim, S.K., Maxwell, J.R. and Quirke, J.M.E., 1980. Petroporphyrins: structural elucidation and the application of HPLC fingerprinting to geochemical problems. In: A.G. Douglas and J.R. Maxwell (Editors), Advances in Organic Geochemistry, 1979. Pergamon, Oxford, pp. 193--203. Hajlbrahim, S.K., 1978. Application of petroporphyrins to the maturation, migration and origin of crude oils. Ph.D. Dissertation, University of Bristol, Bristol. HajIbrahim, S.K., 1981. Development of high pressure liquid chromatography (HPLC) for fractionation and fingerprinting of petroporphyrin mixtures. J. Liq. Chromatogr., 4: 749--764. HajIbrahim, S.K., Tibbetts, P.J.C., Watts, C.D., Maxwell, J.R., Eglinton, G., Colin, H. and Guiochon, G., 1978. Analysis by HPLC of carotenoid and porphyrin pigments of geochemical interest. Anal. Chem., 50: 549--553. JSreskog, K.G., Klovan, J.E. and Reyment, R.A., 1976. Geological Factor Analysis. Elsevier, Amsterdam, 178 pp. Quirke, J.M.E., Eglinton, G. and Maxwell, J.R., 1979. Petroporphyrins, I. Preliminary characterisation of the porphyrins of gilsonite. J. Am. Chem. Soc., 101: 7693--7697. Seifert, W.K. and Moldowan, J.M., 1978. Application of steranes, terpanes and monoaromatics to the maturation, migration and source of crude oils. Geochim. Cosmochim. Acta, 42: 77--95.