Release of kerogen-bound hopanoids by hydropyrolysis

A. N. BISHOP1*, G. D. LOVE2, A. D. MCAULAY2,C. E. SNAPE2 and P. FARRIMOND1

1Fossil Fuels and Environmental Geochemistry, University of Newcastle, Newcastle upon Tyne,NE1 7RU, U.K. and 2Fuel Chemistry Research Group, Department of Pure and Applied Chemistry,

University of Strathclyde, Glasgow, G1 1XL, U.K.

(Received 23 July 1997; returned to author for revision 6 November 1997; accepted 6 July 1998)

AbstractÐHydropyrolysis was used to determine the concentrations of kerogen-bound hopanoid hydro-carbons. The samples characterized were a Recent sediment (Priest Pot, English Lake District) andthree source rock kerogens (Green River shale, Draupne shale and Jet Rock shale). The hydropyroly-zate of a hopanoid producing bacterium (Methylobacterium organophilum) showed that this proceduredoes not overly obscure the original isomer compositions of the samples characterized, although signi®-cant side-chain cleavage does occur, which is inevitable with any pyrolysis method. Notable di�erencesare observed between the composition of pyrolyzates and bitumen extracts. For each of the source rockkerogens, the overall level of hopane isomerization within the pyrolyzate is signi®cantly lower than inthe corresponding extracts. This is particularly the case for the Draupne sample, the pyrolyzate ofwhich contains 17b,21b-hopanes though these are absent in the bitumen. Di�erences in isomeric com-position of the pyrolyzates are in line with the isomer contents of the bitumen extracts. The abundanceof kerogen-derived hopanes is highly variable, both in terms of absolute concentration and also relativeto the concentration of hopanes in the bitumen. This is in part due to maturity di�erences, but deposi-tional environment and diagenetic conditions may also exert a control on kerogen-bound hopane con-centration. These results show that hydropyrolysis provides valuable data on the concentration ofkerogen-bound hopanes, whilst largely preserving their original isomeric composition. # 1998 ElsevierScience Ltd. All rights reserved

Key wordsÐhopane biomarkers, kerogen, hydropyrolysis, pyrolysis, Green River shale, Draupne shale,Jet Rock, Methylobacterium organophilum, Recent sediments, Priest Pot

INTRODUCTION

One of the ®rst studies to document the occurrence

of hopanoid skeletons in a kerogen was presented

by Gallegos (1975), who pyrolyzed a sample of

Green River shale kerogen under vacuum, with the

generated products collected in a cold trap. The

presence of hopanes in kerogen ¯ash pyrolyzates

was recorded by Seifert (1978) and Seifert and

Moldowan (1980). These studies observed that the

hopanes liberated from kerogen are typically

reduced in their level of isomerization compared to

their ``free'' counterparts in the soluble bitumen

fraction. A review of much of this earlier work is

given by Philp and Gilbert (1987). Eglinton and

Douglas (1988) used hydrous pyrolysis to quantify

the kerogen-bound hopanoids in a number of

source rocks, and found that the relative pro-

portions and absolute concentrations of kerogen

and bitumen hopanes were highly variable. In a hy-

drous pyrolysis study of the siliceous and phospha-

tic members of the Monterey Formation, Peters et

al. (1990) found that the yields of kerogen-bound

hopanoids were higher in the phosphatic member,

although absolute concentrations of bitumen

hopanes were higher in the siliceous member.

Bishop and Abbott (1993) and Farrimond et al.

(1996) quanti®ed biomarkers in sediments adjacent

to a dike, and showed that the concentration of

hopanes in the bitumen can increase ®ve fold during

maturation, as hopanoids are released from the

kerogen. Ja�e et al. (1992) have also demonstrated

an increase in hopane concentration during burial

maturation.

Chemical degradation is also commonly applied

to the study of kerogen-bound hopanes. Mycke et

al. (1987) used catalytic hydrogenolysis to show

that hopanoid skeletons were bound into the

Messel shale kerogen by ether linkages. Ruthenium

tetroxide was used by Trilie� et al. (1992) in a

study of hopanes in asphaltenes. At that time it was

believed that this method of chemical degradation

typically degraded aromatic species, and Trilie�'s

data were interpreted as indicating that hopanoids

were bound to the asphaltene structure by aromatic

functionalities. However, Sinninghe Damste et al.

Org. Geochem. Vol. 29, No. 4, pp. 989±1001, 1998# 1998 Elsevier Science Ltd. All rights reserved

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*To whom correspondence should be addressed. PresentAddress: Texaco EPTD, 3901 Briarpark, Houston, TX77042, U.S.A.; Fax: +1-713-9546113; e-mail:bishoan@texaco.com.

989

(1995) have suggested that ruthenium tetroxide oxi-

dation can also cleave ether linkages, casting some

doubt on the importance of aromatic binding.

Ho�man et al. (1992) used lithium in ethylamine to

cleave sulphide-bound hopanoids in asphaltene and

kerogen samples. The use of deuterated reagents to

label the sites of attachment showed that the hopa-

noids were bound by multiple linkages in the side

chain. Similarly, Adam et al. (1993) used deuterated

Raney-Ni to show that C35 hopanoids can be linked

through the side-chain in the macromolecular frac-

tions of sulphur-rich crude oils. The importance of

multiple functionality in the nature of binding was

demonstrated by Richnow et al. (1992, 1993), who

used sequential chemical degradation, involving

both ether and sulphide bond cleavage in combi-

nation with selective deuterium labelling. The

results showed that hopanoid side-chains may be

linked simultaneously by oxygen and sulphur lin-

kages to macromolecular fractions.

A combination of pyrolysis and chemical degra-

dation approaches has been employed by

Koopmans et al. (1996), who conducted hydrous

pyrolysis on a sulphur-rich marl sample (Gessoso-

sol®fera Formation, Vena del Gesso Basin). The

pyrolyzate hopanoid sulphur species and polar frac-

tion sulphur-bound hopanoids were also character-

ized, in addition to the saturated hydrocarbons.

Signi®cant generation of hopanes from the kerogen

was noted, but more intriguingly, the concentration

of the saturated hydrocarbons in the pyrolyzates

was minimal in comparison with the sulphur con-

taining hopanoids and those released from the

polar fractions. The authors suggest that the func-

tionality of the generated products re¯ects the mode

of binding of the kerogen-bound hopanoids.

Pyrolysis and chemical degradation each have ad-

vantages and disadvantages in the analysis of kero-

gen-bound hopanoids. The hydrous pyrolysis and

contact metamorphism studies show that kerogen-

bound hopanoid concentrations are highly variable,

this variability being dependent on both source and

maturity. However, these methods of pyrolysis

heavily modify the resulting hopane compositions.

The 17b,21b-hopanes are typically only found in

hydrous pyrolyzates of immature samples following

low temperature experiments (i.e. <3008C; e.g.

Koopmans et al., 1996). Similarly, side-chain clea-

vage is an inevitable consequence of any pyrolysis

procedure, its e�ect being related to the temperature

employed. Chemical degradation studies have shed

light on the mechanisms of binding of hopanes into

kerogen and other sedimentary macromolecular

fractions. However, yields are typically lower than

those obtained during pyrolysis (due in part to re-

agent access in the case of kerogen studies), and an

incomplete understanding of the mechanisms

behind these procedures can lead to a miscompre-

hension of the mode of binding.

The present study employs an alternativeapproach to both conventional pyrolysis and chemi-

cal degradation. Fixed-bed pyrolysis, with a dis-persed molybdenum catalyst, at high hydrogenpressures (hydropyrolysis) has been shown to

release strongly bound kerogen constituents, withhigh liquid product yields and minimal isomeriza-tion of bound biomarkers. In terms of biomarker

content, Love et al. (1995) reported pyrolyzateyields of 17b,21b-hopanes three times greater thanin the corresponding bitumen extract for GoÈ ynuÈ k

oil shale. Signi®cant yields of hopanes were alsofound in the hydropyrolyzates of three U.K. coals(Love et al., 1996). In hydropyrolysis, as the pyrol-ysis reactor containing the kerogen is continually

swept by high pressure hydrogen, generated pro-ducts are quickly removed to a cold trap, minimiz-ing secondary reactions. Consequently,

hydropyrolysis provides high yields of molecularproducts, without signi®cant con®gurational altera-tion.

The aim of this study is to use hydropyrolysis todetermine the absolute concentrations and originalisomeric compositions of kerogen-bound hopanoids.

By comparing hopane concentrations between threedi�erent kerogens, a Recent sediment and a hopa-noid-producing bacterium (assumed to contain onlyC30 and C35 functionalized hopanoids with the bio-

logical 17b,21b-stereochemistry), we assess (i) whate�ect the hydropyrolysis procedure has on the com-position of kerogen derived hopanes, (ii) how the

isomeric composition of hopanes in the kerogenpyrolyzate compares to that of the bitumenextracts, and (iii) whether compositional di�erences

in kerogen-bound hopanoids between samples arerelated to their environment of deposition and/ormaturity, or whether such di�erences are con®nedto hopanes in the bitumen extracts.

EXPERIMENTAL

Three kerogen samples were analyzed: Green

River Shale (Parachute Creek Member, Duchesne,UT; Lat. 398 53' 120 N, Long. 1108 44' 580 W); JetRock Shale (Jet Rock Formation, Port Mulgrave,

U.K.); Draupne Shale (Tampen Spur, 34/7-23S,2644.1 m, Norwegian Sector of the North Sea). TheRecent sediment sample studied was taken from

Priest Pot, a small lake in the English Lake District,and represents a composite of sediment from 0 to20 cm deep. General descriptions and TOC data forthe samples are listed in Table 1. A sample of a sol-

vent extracted hopanoid-producing bacterium(Methylobacterium organophilum) was also run totest the e�ect of the pyrolysis conditions on the iso-

merization and carbon number distribution ofhopanoid compounds released by this procedure.The source rock samples were crushed in a disc

mill (Tema) before being soxhlet extracted for 72 h

A. N. Bishop et al.990

with 400 ml of an azeotropic solvent mixture of

dichloromethane/methanol (93:7 v/v). Kerogen iso-

lation was performed on the extracted sediment,

HCl (36%) treatment followed by HF (40%). The

acid was removed by exhaustive washing with dis-

tilled water. The isolated kerogens were sub-

sequently re-extracted using the same conditions as

before, resulting in the isolation of the bitumen-II

fraction. The Recent sediment was extracted in a

Soxtherm apparatus with 150 ml of a azeotropic

solvent mixture of chloroform/methanol (2:1 v/v).

This solvent method is used to e�ciently remove

the ``free'' hopanoid compounds in the sediment,

including the bacteriohopanepolyols (Innes et al.,

1997). The hopanoid-producing bacterium,

Methylobacterium organophilum strain XX (NCIMB

11278), was grown in a methanol salts medium

(Medium 121, NCIMB Catalogue of Strains, 1994)

containing 0.5% methanol as sole source of carbon

and energy. Cultures (1 liter) were grown in 2 liter

Erlenmeyer ¯asks and incubated at 258C in a rotary

incubator (200 rpm). Cells were harvested by cen-

trifugation (8,000 rpm, 20 min) in a Hermle ZK380

refrigerated centrifuge and the cell pellet was freeze

dried. The resulting biomass was soxhlet extracted

for 72 h with 400 ml of an azeotropic solvent mix-

ture of dichloromethane/methanol (93:7 v/v).

For the hydropyrolysis procedure, the extracted

kerogens, sediment and bacterium samples (typi-

cally ca. 1 g sample) were impregnated with an aqu-

eous/methanol solution of ammonium

dioxydithiomolybdenate [(NH4)2MoO2S2] to give a

nominal molybdenum loading of 1%. After being

vacuum dried, the samples were then mixed with

sand (1:2 w/w) and placed in the reactor tube. The

kerogens and sediment were ®rst heated from ambi-

ent to 1008C at a rate of 108C/min, and then at

58C/min to a ®nal temperature of 5208C. The gener-

ated volatiles are quickly swept out of the heated

reactor tube by the ¯ow of high pressure H2, and

collected in a cold trap. A more detailed method

description is documented by Love et al. (1997).

Because biomass samples tend to soften in the ap-

paratus leading to the leakage of unreacted material

from the reactor tube, the bacterium was heated at

a much higher heating rate, i.e. from ambient to

1008C at a rate of 108C/min, and then at 508C/min

to a ®nal temperature of 5208C.The extracts and pyrolyzates were fractionated

using silica gel thin layer chromatography. Gas

chromatography-mass spectrometry (GC-MS) of

the aliphatic hydrocarbon fractions was performed

on a Fisons 8060GC using on-column injection

(2508C) linked to a Fisons Trio 1000 MS (electron

voltage 70 eV; ®lament current 4.1 A; source current

2000 mA; source temperature 2008C; multiplier vol-

tage 500 V; interface temperature 3008C). Data ac-

quisition was controlled by a TVM 486 computer

(Masslab software) in selected ion mode (30 ions,

40 ms dwell). Separation was performed on a HP-1

fused silica capillary column (0.25 mm ®lm thick-

ness, 30 m� 0.25 mm i.d.). The GC oven was pro-

grammed from 508C to 1758C at 108C/min, from

1758 to 2258C at 68C/min and then from 2258C to

3008C at 48C with a ®nal hold time of 32 min. An

internal standard [(20R)-5a,14a,17a-[2,2,4,4-d4]-cho-lestane] was added to the extracts and pyrolyzates

prior to fractionation. Concentrations were deter-

mined on the basis of integrated compound peak

areas in relation to the peak area of the known con-

Table 2. Peak assignments used inFigs 1±3 and 5 and Table 5

a C27 hop-13(18)-eneb Tsc Tmd C27 17be C29 hop-13(18)-enef C29 17a,21bg C29 Tsh C30 hop-17(21)-enei C29 17b,21aj C30 17a,21bk C29 17b,21bl C30 17b,21am C31 (22S)-17a,21bn C31 (22R)-17a,21bo Gammaceranep C30 17b,21bq C31 17b,21ar C32 (22S)-17a,21bs C32 (22R)-17a,21bt C32 (22S)-17b,21au C32 (22R)-17b,21av C31 17b,21bw C33 (22S)-17a,21bx C33 (22R)-17a,21by C33 (22S)-17b,21az C33 (22R)-17b,21aaa C32 17b,21bab C34 (22S)-17a,21bac C34 (22R)-17a,21bad C34 (22S)-17b,21aae C34 (22R)-17b,21aaf C33 17b,21bag C35 (22S)-17a,21bah C35 (22R)-17a,21bai C35 (22S)-17b,21aaj C35 (22R)-17b,21aak C34 17b,21bal C35 17b,21b

Table 1. Descriptions of samples used in this study

Sample Locality Age Environment TOC

Priest Pot lakesediment Priest Pot Recent Lacustrine 20.0Green River Shale Uinta Basin, Utah Eocene Lacustrine 23.2Draupne Shale North Sea Upper Jurassic Marine 8.1Jet Rock Shale Port Mulgrave, North Yorks., England Toarcian Marine 13.5

Release of kerogen-bound hopanoids by hydropyrolysis 991

centration of the internal standard (without correc-

tion for response factors). Concentrations of mol-

ecular components are expressed as mg compound

per gram TOC (i.e. mg/g TOC).

RESULTS AND DISCUSSION

Bitumen and bitumen-II extracts

The hopane distribution of the Priest Pot sample

is shown in Fig. 1(a). Compound assignments are

Fig. 1. m/z 191 mass chromatograms of the bitumen extract aliphatic hydrocarbon fractions isolatedfrom: (a) Priest Pot sediment; (b) Green River Shale; (c) Draupne Shale; (d) Jet Rock Shale.

Compound assignments listed in Table 2.

A. N. Bishop et al.992

listed in Table 2. This extract is unusual for aRecent sediment, given the ``mature'' 22S/

(22S+ 22R) values of the C31+homohopanes(Table 3). The presence of 17b,21b-hopanes showsthat this sample is a blend of natural products and

anthropogenic contamination, the likely source ofwhich is runo� from a road running ca. 50 m from

the lake. The elevated concentration of C31 (22R)-17a,21b-hopane may be related to the sphagnum

mosses which grow around the margins of PriestPot, which are known to be associated with anoma-

lously high levels of this compound (Quirk et al.,1984). As this sample was not demineralized, to pre-

vent alteration of the protokerogen, a bitumen-IIfraction has not been characterized.

The source rock samples vary in both source and

maturity characteristics, which is re¯ected by theirbitumen extract hopane distributions. The Green

River Shale hopane distribution (Fig. 1(b); par-ameters listed in Table 3) is dominated by high C30

17a,21b-hopane and gammacerane concentrations.Due to the very low relative abundance of the>C30 17a,21b-hopanes, it is not possible to deter-

mine a 22S/(22S+ 22R) value for this sample.However, the presence of 17b,21b-hopanes attests

to its immaturity. The Draupne extract is moremature than the Green River Shale, with the

absence of 17b,21b-hopanes and a C31 22S/(22S+ 22R) value of 00.40 (Fig. 1(c); Table 3).

The elevated extended hopane content of theDraupne sample is indicative of a highly reducing

depositional environment, associated with high sul-phide concentrations (Peters and Moldowan, 1991).

The Jet Rock Shale is the most mature sample,with a C31 22S/(22S+ 22R) value of 0.58 (Table 3).

The bitumen-II hopane compositions show some

di�erences from those exhibited by the bitumenextracts. For the Green River Shale, the relative

proportions of C30 17a,21b-hopane and gammacer-ane are lower, particularly with respect to the17b,21b-hopanes (Fig. 2(a)). The main di�erence

between the Draupne bitumen and bitumen-IIextracts is the relative proportion of the extended

hopanes, which is marginally higher with respect toC30 hopane in the bitumen-II (Fig. 2(b)). The Jet

Rock Shale bitumen-II hopanoid composition(Fig. 2(c)) is broadly equivalent to that of the bitu-men extract.

Hydropyrolyzates

In order to use the hopanoid composition ofhydropyrolyzates to provide insight into the natureof kerogen-bound hopanoids, we must understand

the e�ects of the pyrolysis procedure upon thehopanoids released, particularly with regard to iso-merization and carbon number distributions. To do

this, we have taken advantage of the biologicalspeci®city of bacterial hopanoids to act as a controlon the starting composition, and conducted hydro-pyrolysis on soxhlet extracted cells of

Methylobacterium organophilum. Rohmer et al.(1984) have reported that this species of bacteriumcontains diplotene (350 mg/g), diplopterol (5100 mg/g) and bacteriohopanepolyols (2500 mg/g). However,the concentration of the hopanoids is a�ected bythe culture conditions, thus the data of Rohmer et

al. (1984) do not necessarily directly relate to thecomposition of the bacterium culture utilized in thisstudy. Composite hopanoids linked to other polar

moieties, such as amino acids, sugar derivatives andnucleosides, are also important hopanoid constitu-ents of this bacterium (Rohmer et al., 1992).Although the extraction procedure used in the pre-

sent study is e�cient for diplotene and diplopterol,bacteriohopanepolyols are only poorly extracted bysoxhlet with dichloromethane/methanol (Helen

Innes, personal communication), due to theirstrongly amphiphilic nature. The poor extraction ofbacteriohopanepolyols (including the composite

hopanoids) means that the hopanoids we see in thepyrolyzate should be derived primarily from C35

hopanoids, all with the biological (22R)-17b,21b-con®guration. However, a contribution from more

tightly bound C30 hopanoids cannot be discounted.The m/z 191 chromatograms of the hydropyroly-

zates of the bacterium, Priest Pot sediment and the

source rock kerogens are shown in Fig. 3. Thehopane distribution of the bacterium (Fig. 3(a)) isdominated by 17b,21b-hopanes, although the

17b,21a-hopanes are also major components, with

Table 3. Selected hopane molecular parameters

C31 bb/(ab+ ba + bb)

C31 17ab22S:22R

C32 17ab22S:22R

C32 17ba22S:22R

C29 ba/(ba+ ab)

C27 17b/(17b+ 17a) Ts/Ts + Tm

HomohopaneIndex

Bacterium Pyrolyzate 0.75 0.30 0.28 0.03 0.65 0.83 <0.01 nmPriest Pot Bitumen 0.16 0.26 0.64 0.63 0.18 0.44 0.23 0.04

Pyrolyzate 0.61 0.24 0.24 0.09 0.31 0.55 <0.01 nmGreen River Bitumen 0.69 nm nm nm 0.41 0.81 0.10 nm

Bitumen-II 0.49 0.33 nm nm 0.38 0.80 0.10 nmPyrolyzate 0.59 0.16 0.35 0.20 0.62 0.47 0.08 nm

Draupne Bitumen nm 0.40 0.55 0.39 0.21 0.11 0.42 0.21Bitumen-II nm 0.42 0.40 0.39 0.26 0.26 0.29 0.10Pyrolyzate 0.28 0.25 0.27 0.24 0.33 0.32 <0.01 0.11

Jet Rock Bitumen nm 0.58 0.60 nm 0.11 0.35 0.40 0.06Bitumen-II nm 0.57 0.65 nm 0.09 nm 0.41 0.07Pyrolyzate nm 0.45 0.47 0.33 0.29 0.25 <0.01 0.04

nm = not measurable.

Release of kerogen-bound hopanoids by hydropyrolysis 993

lesser amounts of the 17a,21b- isomers. Some iso-

merization at the C-22 position has also taken placefor the 17a,21b and 17b,21a- isomers (e.g. C31

17a,21b-hopane 22S/(22S+ 22R) = 0.30; Table 3).Because of the high heating rate employed for this

particular sample, the isomerization observed as aresult of the hydropyrolysis procedure will be more

severe than that which has a�ected the source rock

kerogens and the Recent sediment. The pyrolyzateof the Recent sediment contains a greater pro-

portion of extended hopanes than that of the bac-terium (Fig. 3(b)). It also includes hopenes, which

are absent in all of the other pyrolyzates. The

Green River shale pyrolyzate has a higher pro-portion of moretanes and 17b,21b-hopanes than the

extracts (Fig. 3(c)), though the proportion of17b,21b-hopanes is less in this samples than it is for

the bacterium and the Recent sediment. The

Draupne shale pyrolyzate also contains 17b,21b-

hopanes (Fig. 3(d)), despite their absence in theextracts. The proportion of (22S)- relative to (22R)-

is lower in the pyrolyzate of this sample, and aswith the Green River shale sample, the proportionof moretanes is higher. The distribution of hopa-noids in the Jet Rock shale pyrolyzate follows the

same pattern as the other source rock kerogens(Fig. 3(e)), with higher amounts of moretanes and alower proportion of (22S)-, however, 17b,21b-hopanes are absent. In each of the source rockkerogen pyrolyzates, the carbon number distri-bution is shifted towards lower homologues.

Isomerization

Figure 4 shows a comparison of the relative pro-

portions of 17b,21b-, 17b,21a- and 17a,21b-hopaneisomer concentrations for all of the samples andfractions studied. As would be expected, the pyroly-

zate of the Recent sediment has a similar isomeric

Fig. 2. m/z 191 mass chromatograms of the bitumen-II extract aliphatic hydrocarbon fractions isolatedfrom: (a) Green River Shale; (b) Draupne Shale; (c) Jet Rock Shale. Compound assignments listed in

Table 2 (peak marked with an asterix is an unidenti®ed contaminant).

A. N. Bishop et al.994

Fig. 3. m/z 191 mass chromatograms of the aliphatic hydrocarbon fractions isolated from the hydropyr-olyzates: (a) hopanoid producing bacterium (Methylobacterium organophilum); (b) Priest Pot sediment;(c) Green River Shale; (d) Draupne Shale; (e) Jet Rock Shale. Compound assignments listed in Table 2.

Release of kerogen-bound hopanoids by hydropyrolysis 995

composition to the bacterium, although the relative

proportion of 17b,21b-hopanes is higher in the

Priest Pot sample at the expense of the 17b,21a-hopanes. This di�erence is probably related to the

high heating rate employed in the hydropyrolysis of

the bacterium (see experimental).

Each of the samples shows a signi®cant decrease

in the level of isomerization in the pyrolyzate com-

pared with the bitumen (as observed in previous

studies using other methods of pyrolysis, e.g.

Seifert, 1978; Seifert and Moldowan, 1980).

Furthermore, the level of nuclear isomerization of

the pyrolyzate varies for the source rock samples

studied. The Green River Shale kerogen-derived

hopanes are signi®cantly more immature than those

in the Draupne kerogen, which are in turn less

mature than their Jet Rock counterparts. As with

isomerization at C-17 and C-21, the proportion of

(22S)-hopanes is also less in the pyrolyzate com-

pared to the extracts, both for the 17a,21b- and the

17b,21a-hopanes (Table 3). The 22S/(22S+ 22R)

index also increases in the pyrolyzate of the more

mature samples, as was noticed for isomerization at

C-17 and C-21. Consequently, although the absol-

ute isomerization levels of the pyrolyzates are low,

the relative di�erences between the samples are

comparable to those of the bitumen extracts. Both

bitumen and pyrolyzate compositions, therefore,

have responded to the e�ects of maturation in all of

these samples.

Carbon number distributions

Table 4 lists the normalized carbon number dis-

tributions for the sum of all isomers and stereoche-mistries, for each of the samples and fractions

studied. The bacterium pyrolyzate contains thecomplete range of hopane homologies (C27 andC29±C35) which shows that signi®cant side-chain

cleavage of C35 and C30 precursors has occurred.However, there is a noticeable elevation of the C35

hopane concentration, resulting from the elevated

concentration of the C35 17b,21b-hopane (Fig. 3).Comparison of the carbon number distributions of

the di�erent isomers shows some important fea-tures. In the rC30 homologues, the distributions ofthe 17b,21a- and the 17a,21b-hopanes are reason-

ably comparable. In contrast, the distribution of the17b,21b-hopanes is markedly di�erent. Instead ofthe relative enrichment of the C32 homologue, as

shown by the 17b,21a- and 17a,21b-isomers, the17b,21b-distribution maximizes at C31.

Additionally, the C29 to C35 17b,21a- and 17a,21b-hopanes have a lower average carbon number thanthe corresponding 17b,21b-hopane isomers. These

observations suggest that there may be a linkbetween side-chain cleavage and the extent of17b,21b- to 17b,21a- and 17a,21b-hopane isomeriza-

tion.In each of the extracts, the C30 hopanoids rep-

resent the most abundant carbon number, with theexception of the Priest Pot sample which maximizesat C31 due to the contribution of C31 (22R)-

Fig. 4. Comparison of overall isomer variability for the fractions characterized for each sample(M.o. = Methylobacterium organophilum; B = bitumen; B2 = bitumen-II; Py = pyrolyzate).

A. N. Bishop et al.996

17a,21b-hopane (Fig. 4). The carbon number distri-

bution of the pyrolyzates is shifted slightly towards

lower homologues in the pyrolyzate (typically C29

maximizes) which is probably a feature of the high

temperatures employed (as observed in hydrous py-

rolysis). The source rock kerogen hydropyrolyzates

share a number of similar features. The overall car-

bon number distribution consistently follows thesame pattern, i.e. C29>C30>C27>C31>C32>C33>

C34>C35. However, the relative proportion of the

extended 17a,21b-hopanes (i.e. rC31) varies signi®-

cantly between the kerogen samples studied: the

Green River Shale having the lowest, and the

Draupne the highest. In each sample, the carbon

number distributions of the 17b,21a- and 17a,21b-hopane isomers are broadly equivalent. As with the

bacterium, the 17b,21b-hopane carbon number dis-

tribution di�ers markedly from the other isomers,

although none of the source rock kerogen hydro-

pyrolyzates exhibits the enhancement of C35

17b,21b-hopane. The similarity between the hydro-

pyrolyzates of the bacterium and the Priest Pot

sample suggests that much of the hopanoid content

of this Recent sediment is still in the form of bio-

mass, and has not yet been incorporated into a

thermally stable geopolymer (Fig. 4 and Table 4).

Di�erences in the proportion of kerogen-derived

extended hopanes in the source rocks may be at-

tributable to either di�erences in the side-chain

lengths of bound hopanoid species, or the type of

side-chain binding (e.g. sulphide vs ether linkages).

Table 4. Hopane abundances for each sample and fraction, normalized to total hopane concentration, and the abundance of each isomer

27 29 30 31 32 33 34 35

Methylobacterium organophilumPyrolyzate all isom. 59 100 55 60 56 32 11 27

17b21b- na 68 33 100 61 33 18 6117b21a- 100 92 61 19 43 22 2 ÿ17a21b- 41 100 41 23 32 25 8 ÿ

Priest PotBitumen all isom. 49 93 99 100 27 12 5 4

17b21b- na 56 100 81 ÿ ÿ ÿ ÿ17b21a- 100 82 88 34 14 ÿ ÿ ÿ17a21b- 30 86 82 100 32 16 6 6

Pyrolyzate all isom. 69 88 89 59 100 77 21 8717b21b- na 29 49 45 75 59 24 10017b21a- 86 79 100 28 50 40 ÿ ÿ17a21b- 60 100 48 35 60 41 ÿ ÿ

Green RiverBitumen all isom. 9 11 100 3 ÿ ÿ ÿ ÿ

17b21b- na 100 97 45 ÿ ÿ ÿ ÿ17b21a- 28 10 100 3 ÿ ÿ ÿ ÿ17a21b- 2 5 100 ÿ ÿ ÿ ÿ ÿ

Bitumen-II all isom. 22 25 100 22 2 ÿ ÿ ÿ17b21b- na 58 100 60 10 ÿ ÿ ÿ17a21a- 82 26 100 20 ÿ ÿ ÿ ÿ17a21b- 7 15 100 12 ÿ ÿ ÿ ÿ

Pyrolyzate all isom. 64 100 92 41 29 7 1 117b21b- na 94 38 100 31 12 6 417b21a- 46 77 100 16 19 7 ÿ ÿ17a21b- 100 90 64 23 29 ÿ ÿ ÿ

DraupneBitumen all isom. 27 32 100 26 17 14 12 19

17b21b- na ÿ ÿ ÿ ÿ ÿ ÿ ÿ17b21a- 35 69 100 67 41 47 31 5917a21b- 16 14 100 24 16 12 10 17

Bitumen-II all isom. 43 58 100 65 32 24 14 1217b21b- na ÿ ÿ ÿ ÿ ÿ ÿ ÿ17b21a- 43 65 100 57 25 21 ÿ ÿ17a21b- 31 46 100 67 33 25 17 16

Pyrolyzate all isom. 47 100 90 44 44 27 14 1317b21b- na 100 29 78 28 12 7 ÿ17b21a- 36 69 100 19 29 19 8 717a21b- 55 100 81 43 50 31 18 18

Jet RockBitumen all isom. 31 48 100 55 34 31 14 8

17b21b- na ÿ ÿ ÿ ÿ ÿ ÿ ÿ17b21a- 72 35 100 84 ÿ ÿ ÿ ÿ17a21b- 16 33 100 51 38 34 15 9

Bitumen-II all isom. 40 74 100 79 51 41 14 1317b21b- na ÿ ÿ ÿ ÿ ÿ ÿ ÿ17b21a- ÿ 46 97 100 ÿ ÿ ÿ ÿ17a21b- 27 61 100 77 57 46 16 14

Pyrolyzate all isom. 55 100 75 34 30 19 7 317b21b- na ÿ ÿ ÿ ÿ ÿ ÿ ÿ17b21a- 48 100 100 26 25 14 5 ÿ17a21b- 58 100 65 37 32 20 7 4

Release of kerogen-bound hopanoids by hydropyrolysis 997

Total hopane yields

Table 5 lists the hopane concentrations in the

bitumen and bitumen-II extracts and the pyroly-zates. The proportion of kerogen-derived hopanes

relative to those in the extracts is highly variable.The sample with the highest relative proportion of

kerogen-derived hopanes is the Recent sediment

from Priest Pot (075%). However, Innes et al.(1997) have found bitumen extract concentrations

of functionalized hopanoids in Priest Pot sedimentsas high as 4500 mg/g. Bacteriohopanepolyols and in

particular the composite hopanoids represent the

dominant hopanoid molecular species. In contrast,the extract saturated hydrocarbons were found to

represent less than 5% of the total extract hopanoidcontent (Helen Innes, unpublished data), and as dis-

cussed earlier, a signi®cant proportion of this

hopane fraction is sourced from anthropogenic con-tamination. Although functionalized hopanoid data

for the particular sample employed in the presentstudy was not obtained, comparison with other

samples from Priest Pot suggest that the kerogen-

derived hopanoids also represent less than 5% ofthe total hopanoid content. This may be a re¯ection

of the young age of these sediments (<20 years),with much of the functionalized component yet to

be incorporated in to kerogen or degraded.

Both the Draupne and the Jet Rock samplesretain signi®cant proportions of hopanes in their

kerogen pyrolyzates (047% and 040% respect-

ively), despite their higher maturity. Most surprising

perhaps is the ®nding that the lowest proportion of

kerogen-derived hopanes is for the Green River

Shale sample (025%), which is the most immature

of the ancient sediments studied. However, despite

the relatively low yield of pyrolyzate hopanes in

comparison to those in the extract, the Green River

Shale pyrolyzate contains the highest absolute

abundance of kerogen-derived hopanes, being twice

that of the Priest Pot sample, and an order of mag-

nitude greater than those of the Draupne and Jet

Rock samples. None of the bitumen-II extracts

accounts for more than 2% of the total hopane

content for any of the samples.

Figure 5 shows a stacked plot for each sample,

comparing the relative proportions of bitumen

extract, bitumen-II extract and pyrolyzate concen-

trations for selected hopane compounds (gammacer-

ane is also included for the Green River Shale

sample). These data show that the ratio of kerogen-

derived compounds to those in the bitumen extract

is highly variable. In the Priest Pot sample, some of

the compounds associated with the anthropogenic

contamination and sphagnum mosses are found

predominantly in the extract (e.g. Ts, (22R)- and

(22S)-C31 17a,21b-hopanes). Although the majority

of the hopanes in the Green River shale are present

in the bitumen extract, some compounds are pre-

Table 5. Concentrations of hopane hydrocarbons quanti®ed for each of the samples and their respective fractions (expressed as mg com-pound per g carbon)

a b c d e f g h i j k l m n o p q r s t

Priest Pot Bitumen 0 2 7 6 0 21 0 0 5 20 3 5 6 18 0 6 2 5 3 0Pyrolyzate 12 0 17 21 9 53 0 22 24 18 18 28 3 11 0 47 9 7 23 2

GreenRiver

Bitumen 0 4 40 170 0 87 0 0 61 1642 106 615 0 0 1171 102 21 0 0 0

Bitumen-II 0 0 0 1 0 1 0 0 0 5 1 2 0 0 4 2 0 0 0 0Pyrolyzate 0 9 100 89 0 90 0 0 148 64 70 191 4 19 89 28 30 10 19 7

Draupne Bitumen 0 4 6 1 0 6 6 0 2 40 0 2 4 6 0 0 1 4 3 0Bitumen-II 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0Pyrolyzate 0 0 8 4 0 14 0 0 7 11 4 10 1 5 0 1 2 2 5 1

Jet Rock Bitumen 0 8 11 6 0 23 12 0 3 71 0 8 21 15 0 0 7 16 11 0Bitumen-II 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0Pyrolyzate 0 0 22 7 0 38 0 0 15 25 0 15 6 8 0 0 4 6 7 1

u v w x y z aa ab ac ad ae af ag ah ai aj ak al sum%total

Priest Pot Bitumen 0 5 2 1 0 0 0 2 0 0 0 0 1 0 0 0 0 0 122 18.5Pyrolyzate 16 38 4 12 4 12 64 0 6 0 10 46 0 0 0 0 0 0 537 81.5

GreenRiver

Bitumen 0 48 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2898 73.5

Bitumen-II 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 14 0.4Pyrolyzate 29 75 0 0 2 12 23 0 0 0 0 9 0 0 0 0 4 3 1033 26.2

Draupne Bitumen 1 0 2 3 0 1 0 2 3 0 0 0 3 4 1 1 0 0 105 51.9Bitumen-II 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 0.8Pyrolyzate 2 3 1 3 0 2 1 1 2 0 1 0 1 2 0 1 0 0 96 47.3

Jet Rock Bitumen 0 0 15 9 0 0 0 7 4 0 0 0 4 2 0 0 0 0 254 59.4Bitumen-II 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0.1Pyrolyzate 3 0 4 4 1 1 0 1 1 0 1 0 1 1 0 0 0 0 173 40.5

sum = total concentration of all hopane hydrocarbons quanti®ed for that fraction.%total = the total hopane concentration for each fraction expressed as a percentage of the total hopane hydrocarbon concentration for

that sample (i.e. sum bitumen extract + bitumen-II + pyrolyzate).

A. N. Bishop et al.998

ferred by the kerogen, including Tm, C29 and C31

17b,21a-hopane and C31 17b,21b-hopane. This is in

contrast to Ts and the C30 + 17a,21b-hopaneswhich are consistently biased towards occurring in

the extracts for each of the source rocks studied.

However, it must be borne in mind that the hopane

distribution of the pyrolyzate is inevitably partially

an artefact of the hydropyrolysis method, as is any

pyrolyzate. The hopane composition observed will

not directly relate to that which is bound in the

Fig. 5. Stack plot showing a comparison of the relative abundances of bitumen extract, bitumen-IIextract and pyrolyzate (kerogen-derived) hopanes for selected compounds: (a) Priest Pot sediment; (b)

Green River shale; (c) Draupne shale; (d) Jet Rock shale. Compound assignments listed in Table 2.

Release of kerogen-bound hopanoids by hydropyrolysis 999

kerogen. However, as each of the samples were pyr-olyzed under identical conditions, the di�erences

between the samples must re¯ect di�erences in thecomposition and/or mode of incorporation of thekerogen-bound hopanoids.

In an earlier work, Eglinton and Douglas (1988)reported wide ranges in the relative proportions ofkerogen-bound and bitumen extract hopanes in

samples of Kimmeridge Shale, Green River Shale,New Albany Shale and the Monterey Fm. The rela-tive proportions of kerogen-derived hopanes ranged

from 85% for the Monterey sample to 12% for theGreen River Shale. For an immature sulphur-richmarl, Koopmans et al. (1996) have documentedrelative proportions of kerogen-derived hopanes

(for compounds rC31) released by hydrous pyrol-ysis of over 95%. In earlier hydropyrolysis work,Love et al. (1995,) have reported relative pro-

portions of kerogen-bound hopanes (17b,21b-hopanes) in excess of 70% for a Carboniferous coaland the GoÈ ynuÈ k oil shale.

The results of these earlier studies taken togetherwith our own indicate that kerogen-bound hopanecontents are highly variable, both in terms of their

relative proportion with respect to the bitumenextract composition, but also in terms of absoluteconcentration. One reason for such di�erences ismaturity. With increasing levels of maturation,

kerogen-derived hopanes are progressively releasedinto the bitumen. Bishop and Abbott (1993) andFarrimond et al. (1996) have shown that bitumen

hopane concentrations increase dramatically at lowlevels of thermal alteration (i.e. prior to the onsetof the main phase of bitumen generation), where

di�erences in maturity are small, as measured byvitrinite re¯ectance and molecular maturity par-ameters. However, the Green River Shale sample,which has the lowest observed relative proportion

of kerogen-derived hopanes (both in this study andthat of Eglinton and Douglas, 1988), shows thatmaturity is not the only control. Some aspect of the

depositional environment and/or source organicmatter is the likely cause for the high bitumenextract hopane concentrations in the Green River

shale.

CONCLUSIONS

This study demonstrates hydropyrolysis to be auseful method for quantifying the kerogen-boundhopanoid content of ancient source rocks andRecent sediments. Although this method does result

in some isomerization, as shown by the data for ahopanoid producing bacterium (Methylobacteriumorganophilum), this does not obscure the isomeric

composition of the bound hopanoids. Comparisonof the pyrolyzate hopane composition of the PriestPot sample with that of the bacterium suggests that

hopanoid incorporation in this Recent sediment is

still at a very early stage. The very high concen-trations of solvent extracted bacteriohopanepolyols

also suggest that hopanoid diagenesis is far fromcomplete in this sediment.Di�erences between the composition of hopanes

in the pyrolyzates and their corresponding bitumenextracts are observed for all of the source rockkerogens studied. This is thought to be, in part, an

artifact of the pyrolysis process which results in ahigher proportion of lower molecular weightspecies. Additionally, as noted in other pyrolysis

studies, the overall degree of isomerization of thepyrolyzates (in terms of isomerization at C-17, C-21and C-22) is signi®cantly lower than that of thebitumen extracts. This e�ect is more marked with

hydropyrolysis due to the relatively mild e�ectswhich this technique imparts on the generated pro-ducts. However, the observed levels of pyrolyzate

isomerization are not solely related to experimentalartifacts. Maturation is clearly exerting a control onthe composition of the kerogen-bound hopanes, as

di�erences in the level of isomerization between thepyrolyzates is proportional to those observed in thebitumen extract. This observation may have import-

ant consequences for the timing and location ofhopane isomerization reactions.Kerogen-derived hopane concentrations are

highly variable, both in absolute terms and also

relative to the concentration of hopanes in the bitu-men. The Green River shale sample has the highestabsolute concentration of kerogen derived hopanes,

but due to the very high hopane concentration inthe bitumen, the relative abundance of the hopanesin the pyrolyzate was the lowest for all the samples

studied. The observed variability of kerogen-derivedhopanes is in some cases the result of maturitydi�erences, but the Green River shale data suggeststhat depositional environment may also exert a con-

trol.The quantitative results of this study, together

with those of other workers, demonstrate that in

many immature source rocks, the concentration ofkerogen-bound hopanoids is typically greater thanthat of the bitumen extract. More data from a

wider range of samples will ultimately expand ourunderstanding of the signi®cance of hopanes insource rock extracts and oils.

Associate EditorÐA. G. Requejo

AcknowledgementsÐWe are grateful to Saga Petroleuma.s. and Tim Ruble (CSIRO) for their donation of theDraupne and Green River shale samples respectively.Thanks also go to Ian Head (NRG) for providing the cul-ture of Methylobacterium organophilum, Paul Fox (NRG)for sample collection and extraction of the Priest Pot sedi-ment, Helen Innes for provision of unpublished data, andPaul Donohoe (NRG) for GC-MS analyses. This projectwas funded by Saga Petroleum a.s. and the NaturalEnvironment Research Council (GR3/10495). Finally, we

A. N. Bishop et al.1000

wish acknowledge the helpful reviews of A. G. Requejo,C. J. Boreham, and L. I. P. Dzou.

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Release of kerogen-bound hopanoids by hydropyrolysis 1001