Novel family of hexacyclic hopanoid alkanes (C32C35) occurring in sediments and oils from anoxic paleoenvironments

Org. Geochem. Vol. 11, No. 2, pp. 103-113, 1987 Printed in Great Britain. All rights reserved

0146-6380/87 $3.00+0.00 Copyright © 1987 Pergamon Journals Lid

Novel family of hexacyclic hopanoid alkanes (C32--C35) occurring in sediments and oils from anoxic paleoenvironments J. CONNAN and D. DESSORT Elf Aquitaine, DRAG-GESED, Av. Prdt Angot, 64018 Pau, France (Received 5 February 1986; accepted I0 November 1986)

Al~traet--Four novel hexacyclic alkanes, fairly common in crude oils and rock extracts from evaporitic series, have been tentatively identified on the basis of GC/MS data as C32, C33, C~ and C35 hexahydrobenzohopanes. These structures, only recorded in carbonate-anhydrite sequences, i.e. very anoxic paleoenvironments, tend to concentrate when pristane to phytane ratios increase. Changes in their relative concentration to ¢tfl hopanes appear to be more related to variations in source and/or environmental conditions than to maturity. The ratio of the novel hexacylic C35hopane to C35~fl hopane, compared with other biomarker ratios, suggests that hexacyclic alkanes and hopanes are byproducts of the same hopanoid precursors via different chemical reactions. In addition the novel hexacyclic alkanes are bacterially resistant and may serve as a useful family to define the paleoenvironment (viz. very anoxic) of the parent source rock of a drastically biodegraded oil. Key words: alkane biomarker, evaporitic environments, crude oils, sediments, carbonates, anhydrites, hopanoid hexacyclic alkanes, mass spectrometry, microorganisms, biodegraded, anoxic sediments

INTRODUCTION

Since the early works by Kimble et al. (1974) and Van Dorsselaer (1975) on hopane-hopene families, several other chemically related series of hydrocarbons have been identified in sediments and petroleums. These subsequently discovered structures comprise aromatic [octahydro- and tetrahydrochrysene derived from triterpenes, Spyckerelle et al. (1977a,b); D ring 8,14 secohopanoids, Hustler et al. (1984a) Hussler (1985); benzohopanes, Hussler et al. (1984a,b)] and sulfur-bearing hopanoids (Valisolalao et al., 1984). All these hopanoid geomarkers are derived from a bacterial functionalized hopanoid precursor as emphasized in review articles published by Ourisson et al. (1979, 1982, 1984). The purpose of this paper is to report on four novel hexacyclic hopanoidal alkanes (C32-C35) which have been discovered in oils and rock extracts from carbonate-anhydrite paleoenvironments. EXPERIMENTAL

The reference sample

The sedimentary section in which the new family of biomarkers was first observed is a thick (1300m) evaporitic sequence within the Oligocene of the Camargue Basin (Ste C~cile 1 well, South France). This interval comprises marls, massive anhydrites, fractured bituminous carbonates (calcitic and dolomitic facies), black argillaceous carbonates. Several facies were analyzed (Table 3) but we will limit ourselves to discussing detailed geochemical

data on one sample, namely a vacuolar oil-stained dolomite (CE/TOC = 81%, Table 1) used as a reference in this study. The chloroform extract of this sample exhibits basic properties of heavy nonbiodegraded oils from evaporitic carbonate series: occurrence of even n-alkane predominance (n-C24, n-C26, n-C2s, n-C30, Fig. 1), low pristane to phytane ratio (0.49, Table 1), high sulfur content (15%), minute amount of n-alkanes (2.3%) associated with high concentration of polar compounds (asphaltene: 34%, NSO's: 49%). The branched and cyclic alkanes (Fig. 1) show the following molecular features based on mass spectral data: unusual concentration of C21-C23 steranes (Restlt, 1983) as already noted in other hypersaline environments (Messinian evaporitic basin, Italy, Ten Haven et al., 1985), occurrence of non-ubiquitous steranes (5~t, 14fl androstane, C20), of rather common C26 steranes (Moldowan et al., 1985), of C24 tetracyclic terpanes, C25 regular isoprenoid and regular C2s ~fl hopane. [29, 30-bisnor-17~t(H)-hopane tentatively assigned by Seifert et al. (1986) in oil seeps from Greece]. The oil-impregnation, choosen as reference, is indigenous to the evaporitic series which has produced a sulfur-rich (8%) heavy oil (13 ° API) within the 2340-2367m interval. According to both kerogen properties (Tm~ = 407-435°C on decarbonatedpreextracted rocks) and molecular characteristics of extract [no 18~t(H)-22-29-30-trisnorhopane II, i.e. Ts, C29a~,S/C29et~R steranes=0.5, % C29/~fl steranes = 62, Table 2], one may consider that the oil analyzed is immature to marginally mature. 103

104

J. CONNAN and D. DE&SORT

- - Total atkanes

=0 t'~

. '~nC16

n-C15.~,

C ~ tO .~ ~=N,=,~=~

IPHYTANE ~ Z ~

n-C14, ,.: [ i . _ i

:

~0 =~ ~ N

~

eli

=

~

T2

.C32a#Hopane

i/

Lil

,

I! --

t

i~l,,,_ ..~<~.<.~ L. c

.

C34a#Hopane

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....

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. . . .

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~

=1

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d.,

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Fig. 1. Gas chromatograms of total and branched-cyclic alkanes from the Ste C ~ i l e 1 reference sample (C.5, 2340 m, Oligocene, Camargue Basin, oil-stained vacuolar dolomite). Chromatographic conditions: 50 m glass capillary column, i.d. 0.25 mm, stationary phase OV1, split 1/60, temperature programmed from 100 to 300°C at 1.7°C/min.

mode. Mass spectrometer conditions were: scanning rate at 1 amu/2msec in full spectrum acquisition mode. Different analytical conditions were used for other samples. They have been listed for each sample in Tables 3 and 4.

Analytical conditions The branched and cyclic alkanes were analyzed using a FINNIGAN 4500 combined gas chromatograph/quadrupole mass spectrometer equipped with an INCOS data system. Gas chromatographic conditions were: capillary column (bonded SE 52, fused silica, 50 x 0.25 mm i.d.). The column oven was programmed from 100 to 300°C at 3°C/min. Helium (30 psi) was used as cartier gas. A Grab-type injector was employed in split

RESULTS AND DISCUSSION

Assessment of molecular structures Rinaldi (1985) has recently reported on the occurrence of a novel C31 alkane (tool. wt 424, abbreviation

Table 1. Basic geochemical data (lithology, TOC, % chloroform extract, sulphur in % chloroform extract, pristane/phytane, n-C~7/pristane, n-Cis/phytane, 613C/(%o/PDB) on whole extract) of the reference sample where the novel hexacyelic family was originally discovered

w

R S COM$~(~I~ION OF CE

m

_

~-~<,,=

el-

456

o

C.5 8 2340m

,.,

~-

=

.~x

' I " U,,l

,~ >-

"

~o

-

~.

a

~

~

!~*-

~: i~_-e

~

78 1.91 1.55 81 2.3 14.1 49.5 34.1 15 0.49 1.9 0.7 -23.E

Hexahydrobenzohopanes in anoxic paleoenvironments

105

Table 2. Sterane and terpane ratios on the reference sample

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62

ee 456

2340m

1.1

0.5

1.7

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0.06 0.43 0.93 5.9

54

0.05

Abbreviations: 29¢t~S/29~tctR: 5a(H), 14a(H), l%t(H),20 S C29 sterane/5ct(H), 14ct(H), 17~t(H), 20 R C29 sterane. 29flflS/29¢tctR: 5a(H), 14fl(H), 17fl(H),20 S C29 sterane/5ct(H), 14~t(H), l%t(H), 20 R C29 sterane. 21 St/22 St: C2~ sterane/C22 sterane. 22 4Me St/22 St: C22 4-methylsterane/C22 sterane. 23/3/21 St: C23 tricyclic terpane/C2j sterane. C29H/C30H: I%t(H), 21fl(H)-norhopane/17a(H), 21fl(H) hopane. TT/ST: Triterpane to sterane ratio = ratio of m/z 191 integral to m/z 217 integral for C27-C35terpanes and C27-C-z9steranes. % 20 S-: % 20S/20R-+20S-24ethyl-5ct(H), 14ct(H), l%t(H)-cholestane. %flflC29: % 5~t(H), 14fl(H), 17fl(H)-/5~t(H), 14/](H), 17fl(H)-+ 50t(H), 14~t(H), 17~(H)-C29 steranes. % 22 S C32:22 S-17ct(H), 21fl(H)-bishomohopane/22 S-17ct(H), 21fl(H)-bishomoh o p a n e + 2 2 R-17a(H), 21fl(H)-bishomohopane. fl~/ctfl: 17fl(H), 21~(H)-hopane/17~t(H), 21/3(H)-hopane. Analytical conditions: electron energy 70 eV, (1) SE 52, 50 x 0.25 mm, split or (2) OV I, 50 x 0.25 mm, split or (3) DB5, 60 x 0.25 mm, on column or (4) SE 50, 50 x 0.25 mm, splitless.

C31/6, Fig. 2) in Paleozoic petroleums. On the basis of fragmentation pattern similarities, he proposed a hopane-related structure formed through a cyclization process of the extended hopanoid side chain during diagenesis (Fig. 2). It should be noted that this structure is presumably demethylated at position C-18. Simultaneously we found, associated with this C3~/6 structure four other compounds thought to be also new hexacyclic alkanes: abbreviation C32/6, C33/6, C34/6 and C35/6 (Fig. 3), molecular weight (438, 452, 466, 480) and suggested structures in Fig. 4. These molecules, however, appear to have a non-demethylated five ring hopane skeleton according to the following features:

well as their chemical ionization mass spectra (Fig. 5) show fragmentation patterns similar to ~tfl hopanes. (2) These compounds were not removed from the branched and cyclic alkanes after a purification step on TLC plates using AgNO3-SiO2. This excludes hop-22(29)enes, which were shown to display a similar mass spectral fragmentation pattern (Van Dorsselaer, 1975). (3) The aromatic fraction of the same sample contains closely related C32, C33, C~, C35 monoaromatic benzohopanes (Hussler et al. 1984a,b; Hussler, 1985, Fig. 6). (4) The lack of doublets (R and S isomers) suggests a short side chain (CH3, C2H5, C3H7, Fig. 4) attached on ring F.

(1) Their electron impact mass spectra (Fig. 4) as

In order to determine the definite molecular

100.

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400

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500

i.

550

Fig. 2. Structure proposed by Rinaldi for the C31/6hexacyclic alkane. The mass spectrum reproduced here is identical to the original mass spectrum given by Rinaldi (personal communication).

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CORUCUTTIG/TES

GEOGRAPHICAL LOCATION

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2 C35/6 C35a~Hop. R+S 29~z,~Hop. + 30~,B H o p . 35a/~ Hop. R+S 33~,~ Hop. R + S ANALYTICAL CONDITIONS

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OUARTZ "/=

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PRISTANE/PHYTANE l"t

WHOLE O I L OR E X T R A C T SULPHUR %

WHOLE OIL OR EXTRACT TRITERPANES STERANES

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mm

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SAMPLE ELF A Q U I T A I N E REFER.

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AQUITAINE BASIN

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GEOGRAPHICAL LOCATION

(S.W. FRANCE)

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TRITERPANES STERANES

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Tm Ts Ts Tm +Ts 2 C35/6 C 3 5 a ~ Hop. R + S 2 9 a B H o p . + 3 0 a ~ Hop. ~ p . R +S 3 5 a ~ HOp. R + $ 33,=~ H0p. R ÷ S A N A L Y T I C A L CONDITIONS

o m

, < ~ = . =r~= z <, m ~m

Z

m

Z =

=

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sluotuuoa!Auaooied o[xoue u! souedoqozuaqoap/~qexoH

108

J.

CONNAN

and

D.

DESSORT

C30=pH

100.1

C29a~H

~

:=

191

S 1

®

100.0

TOTAL

A ,,~

.;,

t, ,,

, , tl,

t

100.(~

ee 3700 61:40

3800 63:26

3900 65:00

. .'~ . .

©

TOTAL

3600 60.'00

-

4000 66:40

4100 68:20

e 4200 70:00

4300 Th40

4400 SCAN 73:20 TIME

Fig. 3. Diagnostic mass chromatograms showing the four new hexacyclic hopanoid alkanes (C32/6, Co3/6, C34/6, C35/6). Analytical conditions: 70 eV, SE 52, 50 x 0.25 ram, split. (A) m/z 191 mass chromatogram. (B) m/z 398-482 summed (every 14 ainu): summed mass chromatograms of the pentacyclic hopanes molecular ions, (c) m/z 424-480 summed (every 14 ainu): summed mass chromatograms of the hexacyclic hopanoids molecular ions. structures which are very probably hexahydrobenzohopanes, synthesis of reference molecular structures should be undertaken. These structures, described in one reference sample from the Ste C6cile 1 well, are also present in several other samples, all belonging to very anoxic environments, The geochemical significance of this new class of alkane will be discussed in the next section.

Occurrence and geochemical significance of the novel series of hexacyclic alkanes A preliminary review of our data suggests that the C31 hexacyclic hopanoid alkane (C31/6) discovered by Rinaldi (1985) seems more ubiquitous than the four hexacyclic hopanoid alkanes (C32/6--C3s/6) reported here. These new structures are, however, fairly common in sediments and oils from evaporitic basins and especially in sabkha-typ¢ deposits (Tables 3 and 4), but they apparently do not exist in all anoxic environments, as they were found lacking in oils (offshore Santa Maria) and sediments (Santa Cruz, Point Arena, Eel River basin) from the Monterey formation in California (unpublished data our laboratory). They are, however, definitely present in:

Sulfur-rich heavy oils (G.9 and G. 1 wells, Table 3) and rock extract (Ste C_~ile 1 well, Table 3) from the Camargue basin (South of Nimes, France). The Ste C6cile 1 samples are selected in various lithofacies: marls, bituminous carbonates, anhydrites, oil-stained vacuolar dolomites, etc. within a thick Oligocene evaporitic section. Heavy unaltered oils (MRN.1, Aire 1, GAQ.I, VBH.2, Table 4) as well as heavy biodegraded oils (Lee 1, Connan and Van der Weide, 1978; Restl6, 1983; CL.6, Peyrelongue 1, Table 4) from the Aquitaine Basin (S.W. France) Heavy non biodegraded oil and related source rock from the MAR 101 well in the Al6s basin (S. France, Table 3). On both samples we have previously reported new sulfur-bearing structures namely thiamonocyclanes (Connan et al., 1983). Some basic geochemical data about the above mentioned samples are given in Tables 3 and 4. Lithology data indicate that all sets of samples originate from dominantly carbonate-anhydrite series. Corresponding paleoenvironments are very anoxic as shown by pristane to phytane ratios varying within the 0.1-1.0 range. In most samples the high concentration of the ~fl hopane family (TT/ST = 1-14,

Hexahydrobenzohopanes in anoxic paleoenvironments

109

I

i

,,

I '

i

~o

o

'O

.to

Loo e~

:N

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[8 :O

ro -O

6

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to

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110

J. CONNANand D . ~ R T C3S/6 hexahydrobenzohopaneM.W.-480

m

479 t [M-H'I +

190

287

273 205

259

' [I 219

11

465

231

3°~1327,3s~ 383 .^^ 43T

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+

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C35 (~'~' S) homohopane M.W. 48:~

I 'I'"'T''l

+09

550

481 [M-HI +

191 289

205

M / Z 150

Z00

261 I 275

250

300

350

400

450

500

550

Fig. 5. Chemical ionization mass spectra of C35/s hcxahydrobenzohopanc and C35 22S homohopanc showing stricking similarities in fragmentation patterns. The data come from the I..¢¢ I well (C.2, 1680.2 m, Fig. 9). Analytical conditions: CI, isobutan¢, 0.20 Tort, DBS, 60 x 0.25 ram, on column injection. Tables 3 and 4) suggests that both oils and rock extracts originate from a sulfur-rich kerogen in which bacterial remains represent a significant contribution. In an attempt to understand what important f a c tors govern the hexacyclic alkane concentration, some selected biomarker ratios have been calculated (Tables 3 and 4). Hexacyclic alkane concentration has been evaluated by reference to ~tfl hopanes. Thus the C35/6compound has been compared to the C35 =fl hopanes (R + S). This ratio is a measurement of the

relative importance of hexacyclic alkanes and has been plotted against several other biomarker ratios: pristane/phytane and Ts/Ts + Tm (Fig. 7), triterpanes/steranes and C29+C30 g// hop. R + S (Fig. 8). In Fig. 7(b), it should be noted that if a hexacyclic alkane enrichment follows a pristane/phytane increase no relationship is seen in Fig. 7(a) by changing Ts/Ts + Tin, i.e. there is no maturity dependence. Drastic variations in hexacyclic concentrations along

111

Hexahydrobenzohopanes in anoxic paleoenvironments

the Ste C6cile well at low stage of maturity [Ts/Ts + T m = 0 , Fig. 7(a)] is probably a source and/or an environmental effect. The environmental conditions influence the hexacyclic alkane concentration according to the following rule: increase of hexacyclic alkanes with an increase in pristane/phytane, i.e. less anoxic deposits [Fig. 7(b)]. Changes in depositional conditions generally entail variations in source input and consequently in biomarker ratios. Enrichment in hexacyclic alkanes with both triterpane to sterane and C29 ~fl hop. + C30 ~tfl hop. to C35 ~fl hop. (R + S) ratio (Fig. 8, non-

Fig. 6. Structure of the benzohopane family (after Hussler al., 1984a,b).

et

TS

PRISTANE/ PHYTANE

fs + fm 0.4

I.iii1 O~O

1.0

oLII1

0.9-

0.7-

• PEYRELONGUE**

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0.6.

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014

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glMAR 101// O.4- 121~.,f 4 8 0 2 / /

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~"J) . . . .

1:5 . . . .

0.1.

2 C35/6

C35.#Hop:R + S

2.0-

0.5

1.0

Fig. 7.(a) T~/Ts + Tm vs 2 C3s/6/C3s~tflhop. (R + S). (b) Pristane/phytane vs 2C3s/6/C35ctflhop. (R + S). • Camargue Basin, Ste C~ile well (455, 500, 456, 501,479, 502, 457), • Aquitaine Basin. • Al6s Basin. *Slightly biodegraded, **moderately biodegraded; ***severely biodegraded.

15 C29a.p Hopane+C30~ p Hoparne C35o ~HopaneR+$ 14

15: ~TRITERPANES/STERANES

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112

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Fig. 9. Gas chromatograms of branched and cyclic alkanes from biodegraded oils (Lee 1 well, Aquitaine Basin) showing the increased concentration of hexacyclic alkanes in the most biodegraded oil (Lee 1, C.4, 1930.8m). Hexacyclic hopanoid alkanes are bacterially resistant for their amount increases in the branched and cyclic alkanes when the ~//hopanes are bacterially consumed. biodegraded samples from the Camargue Basin) is possibly reflecting such a prevailing source influence. It is noteworthy that the Ca5/6compound increases as the C35 hopanes decrease. Such a relationship suggests that both series of structures may be issued from a common precursor (i.e. bacteriohopanetetrol) via competing chemical pathways, viz. reduction and cyclization. Cyclization reactions seem to be easily performed under less reducing conditions. Within the scope of this geochemical survey we have also examined the resistance of hexacyclic alkanes to bacterial attack. As expected on the basis of molecular criteria these structures do survive in drastically biodegraded oils. Figure 9 comparing branched and cyclic alkanes of two severely biodegraded oils from the Aquitaine basin shows that the hexacyclic alkanes may even become prominent structures when ~# hopanes have been extensively

altered (Lee 1, C.4, 1930.8 m, Fig. 9). Hence the significant enrichment of hexacyclic alkanes in mildly to moderately biodegraded oils (e.g. in Lee 1 well) may be used to prove that the incipient blodegradation of ~fl hopanes has started. Moreover, it should be emphasized that the occurrence of hexacyclic alkanes in the severely biodegraded Lee 1 oil (both alkanes and aromatics are drastically altered) still allows us to recognize that this oil has been generated in a very anoxic source rock. In the present case the source rock is located within a sabkha-type paleoenvironment (Dolomie de Mano Fm, PortlandJan to Kimmeridgian). CONCLUSIONS

Four novel hexacyclic alkanes (C32/6, C33/6, C~/6, C35/6), fairly common in crude oils and rock extracts

Hexahydrobenzohopanes in anoxic paleoenvironments from evaporitic series have been tentatively identified as probable C32, C33, C34 and C35 hexahydrobenzohopanes. These structures found in hypersaline carbonate environments have not been recorded in oils and rock extracts from the anoxic Monterey formation in California. The importance of hexacyclic alkanes relative to ~// hopanes appears more related to paleoenvironments (increase with pristane/phytane) and source (increase with triterpane to sterane and C29 0~fl hop. + C30 ~ hop. to C35 ~tfl hop. ratio) than to maturity

(rJrs + Tin). Hexacyclic alkanes concentrate in bacterially altered oils for they are particularly resistant to bacterial attack. Their occurrence in severely biodegraded oils where numerous alkane and aromatic biomarkers have been destroyed still allows us to conclude that the altered oils originate from a very anoxic source rock within an evaporitic sequence. Acknowledgements--We are indebted to J. M. Moldowan of

Chevron Oil Field Research Co. and to J. W. Farrington of Woods Hole Oceanographic Institution, for their review and constructive criticisms of the manuscript. The management of Soci6t6 Nationale Elf Aquitaine (Production) kindly gave permission for this paper to be published. Note added in proof--In a paper on hexacyclic mono-

aromatic hydrocarbons of petroleum, sent recently to us by Dr Petrov, Ostroukhov et al. (1983) show a mass spectrum of a C35 alkane obtained by hydrogenation of the C35 benzohopane (180°C, H2, 10 MPa, nickel Raney as catalyst). This mass spectrum, which should represent a characteristic mass spectrum of one C35 hexahydrobenzohopane stereoisomers, is very much alike those of the C35compound shown in Fig. 4 (base peak: m/z 191, other diagnostic ions: m/z 480 (M+), 465, 259). Consequently Ostroukhov et al.'s hydrogenation experiments provide additional data favoring the hexahydrobenzohopanes as likely structures for the novel hexacyclic hopanoid alkane family described in our paper. REFERENCES Connan J. and Van der Weide B. M. (1978) Thermal evolution of natural asphalts. In Bitumens, Asphalts and Tar Sands (Edited by Chilingarian G. V. and Yen T. F.), Developments in Petroleum Science 7, pp. 27-55. Elsevier, Amsterdam. Connan J., Grondin J. L., Colin J. P., Hussler G. and Albrecht P. (1983) Non-biodegraded heavy oils in some carbonate basins, llth International Meeting in Organic Geochemistry, The Hague.

113

Hussler O. (1985) Marqueurs g6ochimiques en s6ries carbonat6es. Th~se Universit~ Louis Pasteur de Strasbourg 1. Hussler O., Connan J. and Albrecht P. (1984a) Novel families of tetra- and hexacyclic aromatic hopanoids predominant in carbonate rocks and crude oils. Org. Geochem. 6, 39-49. Hussler G., Albrecht P., Ourisson G., Cesario M., Guilhem J. and Pascard C. (1984b) Benzohopanes, a novel family of hexacyclic geomarkers in sediments and petroleums. Tetrahedron Lett. 25, 1179-1182. Kimble B. J., Maxwell J. R., Philp R. P., Eglinton O., Albrecht P., Ensminger A., Arpino P. and Ourisson O. (1974) Tri-and tetraterpenoid hydrocarbons in the Messel oil shale. Geochim. Cosmochim. Acta 38, I 165-1181. Moldowan J. M., Seifert W. K. and GaUegus E. J. (1985) Relationship between petroleum composition and depositional environment of petroleum source rock. AAPG Bull. 69, 1255-1268. Ostroukhov S. B., Aref'yev O. A. and Petrov Al. A. (1983) Hexacyclic monoaromatic hydrocarbons of petroleum. Pet. Chem. U.S.S.R. 23, 53-60.

Ourisson G., Albrecht P. and Rohmer M. (1979) The hopanoids. Pure Appl. Chem. 51, 709-729. Ourisson G., Albrecht P. and Rohmer M. (1982) Predictive microbial biochemistry--from molecular fossils to procaryotic membranes. Trends Biochem. Sci. 7, 235-239. Ourisson G., Albrecht P. and Rohmer M. (1984) The microbial origin of fossil fuels. Sci. Am. 251, No. 2, 44-51. Restl~ A. (1983) Etude de nouveaux marqueurs biologiques dans les p6troles biod~grad6s: cas naturels et simulations in vitro. Th~se Docteur ~s Sciences, Universit6 Louis Pasteur de Strasbourg 1. Rinaldi G. G. L. (1985) Presence of monoaromatic secohopanes and benzohopanes in petroleums. Symposium on Chemical Biomarkers. American Chemical Society Meeting, Miami. Seifert W. K., Moldowan J. M. and Demaison G. J. (1984) Source correlation of biodegraded oils. Org. Geochem. 6, 633-643. Spyckerelle C., Greiner A., Albrecht P. and Ourisson G. (1977a) Aromatic hydrocarbons. Part III Tetrahydrochrysene derived from triterpenes in recent and old sediments. J. Chem. Res. 330-331. Spyckerelle C., Greiner A., Albrecht P. and Ourisson G. (1977b) Aromatic hydrocarbons from geological sources. Part IV. An octahydrochrysene derived from triterpenes in oil shales. J. Chem. Res. 332-333. Ten Haven H. L., de Leeuw J. W. and Schenck P. A. (1985) Organic geochemical studies of a Messinian evaporitic basin, northern Apennines (Italy) I: hydrocarbon biological markers for a hypersaline environment. Geochim. Cosmochim. Acta 49, 2181-2191. Valisolalao J., Perakis N., Chappe B. and Albrecht P. (1984) A novel sulfur containing C3s hopanoid in sediments. Tetrahedron Lett. 25, No. 11, 1183-1186. Van Dorsselaer A. (1975) Triterp6nes de s6diments. Th6se Docteur 6s Sciences. Universit6 Louis Pasteur de Strasbourg.