Nature and geochemistry oI".high molecular weight hydrocarbons
(above CojO) in oils and solid bitumens
J.
e. DEL RIO,· R. P. PmLP
and J. ALLEN
School of Geology and Geophysics, The University of Ol::lahoma. Norman, OK 73919, U.S.A.
A......,.-Series of hydrocarbons rangins up lo c" ha"" been detccted by high temperature gas
chromatosraphy (HTGC) in a number of ...,.,hemical samples, incIuding oil. aOO waxes precipitated in
driII stem pipes. A saturated hydrocarbon f....ction isolated from a fossil bitumen, ozocerite, and enriched
in lxanchedfcyclic compounds, was anaIyoed by direct insertioa probe mass spectrometry (DIPfMS) and
showed Ihe presence of cyclic structures in !he high molccular weight region (> C .. ). The asphaltene
fractions isolated from Ihe various samples were analyoed by Hash pyrolysis-high temperature gas
chromatography (Py-HTGC). Similar distributions ofhigh molecular weight hydroearbons were produced
frona !he asphaltencs of bo!h !he oil. and waxcs. In vilro simulation experiments of!he asphaltene fraelions
produced a similar distribution of high molecular wcighl hydrocarbons as obscrved from Py-HTGC,
sugpting thal !hennal breal::down of asphallencs may be responsible for Ihe production, or release, of
sorne naturaUy occurring high molecular weight hydrocarbons.
Key words---oiJs, waxcs, asphaltenes, high molecular weight hydrocarbons (C.tO + ). high temperature gas
chromatography (HTGC), Hash pyrolysis-HTGC
lNTRODUcnON
Traditionally, organic geochemistry has been concerned with the characteñzation of organic compounds ranging from el to e.. present in the
geosphere since such compounds are widely
distñbuted and easily detected by conventional gas
chromatography-mass spectrometry (GC-MS) techniques. However, the presence of Iipids up to e lOO and
higher (mainly polyisoprenyl alcohols) have been
widely reported in living organisms (Sasak and
Chojnacki, 1973; Hemming, 1983; Lehle and Tanner,
1983; Chojnacki and Vogtman, 1984; ehojnacki
et al., 1987; Swiezewska and ehojnacki, 1988. 1989;
Suga el al., 1989; Liaeen-Jensen, 1990), and it can be
anticipated that these compounds, or their diagenetic
products, should be present in the sedimerÍt8.ry
record. A few papers have presented data descñbing
the occurrence of extended señes of acyclic isoprenoids and tricyclic terpanes beyond e .. , and
suggested polyprenols as the biological precursors for
these compounds (Albaiges, 1980; Moldowan et al.,
1983).
Tbe study of high molecular weight hydrocarbons
has been overlooked in the past for two main reasons.
Tbe first being the lack of appropñate analytical
techniques. Tbe second, particularly in the case of
oiIs, being that the higher molecular weight components are often absent from oils collected at the
well-head. Rather these components precipitate out
in the dñll stem pipe or remain in the reservoir rocks
due to their low mobility.
Recently, with the development of columns suitable for high temperature gas chromatography
(HTGC) (Lipsky and Duffy, 1986a, b), and the introduction of supercritical fluid chromatography (SFe),
(Hawthome and Miller, 1987, 1989; Smith et al.,
1987), it has been possible to extend the carbon
number range of the compounds analysed and identified. Tbese HTGC columns have a1so been successfully coupled with mass spectrometers (Smith et al.,
1987; Hawthome and Miller, 1987; Blum et al., 1990;
Olesik, 1991). Kohnen et al. (1990), reported the
analysis of a novel series of alkylthiophenes ranging
up to e .. using bigh temperature gas chromatography. Van Aarssen and de Leeuw (1989) descñbed
the identification of e., hydrocarbons, thought to be
tñmeñc cadinanes, in crude oils and sediments from
South East Asia, using fused silica capillary columns.
In thls paper a number of samples, including a
microcrystalline wax, Fischer-Tropsch wax, and a
solid ozoceñte bitumen from. the Uinta Basin, have
been analysed using aluminum coated high temperature capillary columns. Some Waxes and their associated asphaltenes tbat frequently occur in the dñll
stem pipes of oil wells have also been analysed and
their hydrocarbon distñbutions compared with their
co-occurñng oils collected at the weIl-head. The
aspbaltenes isolated from the oils were analysed by
flash pyrolysis-HTGC and selected samples have
been analysed by direct insertion probe mass spectrometry in both the electron impact (El) and chemical ionization (el) modes of ionization.
EXPERIMENTAL
A bitumen sample,ozocerite, from the Uinta basin,
Utah, a microcrystalline wax (Micro 195) and a
synthetic wax obtained by Fischer-Tropsch synthesis
(FT H-I) were selected for the initial part of this
study. (Samples of the Micro wax-195 and FT wax
H-I were provided by Dr Hawthorne). Two wax
deposits (wax SB and wax H) collected from different
drill stem pipes along with the oils produced from
the corresponding wells (oil SB and ojl H). an
asphaltene-type deposit (FRL 9170) collected from
anolher pipeline. along with its respective oil
(FRL 9169) were also analysed. Tbe saturate frac-
tions of the oils were isolated by alumina column
chromatography and elutíon with cyclohexane. Tbe
branched/cyclic fractions were ísolated by removal
of n-alkanes under reftux with 5 Á molecular
sieve in iso-octane for 15 h. The asphaltenes were •
isolated from the oils by precipitation with excess .
n-pentane and subsequent centrifugation followed by
re-precipitation with n-pentane to purify the asphaltenes.
High temperature gas chromatography analyses
were peñormed using a Carlo Erba gas chromatograph equipped either with a short (3 m) or long
(25 m) aluminum cIad capillary colurnn coated
with the HT-5 liquid film. The conditions were:
e...,
Micro
WQX
195
Wox FT H-1
4
9
13
18
22
27
32
36
41
50
Minutes
Fil. 1. Capillar}' HTGC (3 m) of (a) a microcrystalline wax (wax 195) and (b) a FISCher-Tropsch synthetic
wax (wax FT H-I). OC oven temperature: lOO-44O°C al 8°Cjmin and SOmin isothermal.
a high temperature aluminum ciad capillary column
coated with HT-5 liquid film. The asphaltenes were
pyrolysed at 800°C using a platinum ribbon CDS
pyroprobe and the oven was programmed from 60 lo
400"C at gOC/min. Helium was used as the carrier gas
and the GCMS transfer Jine was set at the maximum
temperature of 350°C.
The artificial simulation, or maturation, experiments with the asphaltenes were carried out by
heating them in glass tubes, seaJed underN2 • at 350°C
overnight. Mter cooling. the tubes were opened, the
products extractcc1. and the saturate fractions isolated
and analysed by HTGC using the conditions described aboye.
splitfsplitless ínjector and ftame ionization detector at
400°C and the cotumn was heated from 60 to 440°C
at arate of 8°C/min with 50 min final hold time. The
samples were dissolved in warm p-xylefl$: before
injection.
The mass spectra were obtained in both the El and
CI mode by direct insertion peobe (DIP) using a
Finnigan TSQ 70 triple stage quadrupole mass spectrometer. Chemical ionization was performcd using
eitber methane 01" ammonia as Iagent gas at a
pressure of 0.7 tOrf.
Flash pyrolysis-gas chromatograpby-mass spectrometry (py-GC-MS) of the asphaltenes was perforroed with a Finnigan GCMS system equipped with
Oil H
Czo
I
eX!
I
~~~~ll,~,~.u~~~.~.~~~.l~l.l~I_~._______________
C40
\
WoxH
<;o
¡so
I
L
•
.IJ
l.
tI1 ti I J 1~
J
1
.
JIJ j
IÁ1A
Ceo
,
I
6
f2
19
25
32
38
44
5t
57
64
70
Minutes
Fig. 2. Capillary HTOC (25 m) of tite oiI (00 H) ud the wax deposit (wax H) oollected from drill stem
pipe of the same weU. OC oven temperature: SO--44O°C at 4°C/min and SO min isothermal.
aluminum coated capillary column and the chromatograros obtained are shown in the Fig. l. Tbe
Micro-l95 wax showed a distribution of n...aJkanes
up to ~. with maximum around C 40 and a marked
even over odd carbon number predominance. The
wax FT H-l showed an n-alkane distribution from
~ to C n without any pronounced even/odd predominance as may be expected from a synthetic product
oC Fischer-Tropsch synthesis. Prevíous studies on
these samples by supercriticaJ fluid chromatography
(SFC) have shown a similar distribution of hydrocarbons, although the FT H-l showed hydrocarbons extending up to CIOO (Hawthorne and Miller,
1987).
RESVL1'8 AND DISCVSSION
Microcrystalline wax-195 and wax FT H-I
Microcrystalline waxes are products derived Crom
the crystalline residues left after distillation oC petroleum waxes which, when purified, yield a wax
dominated by n-alkanes. Fischer-Tropsch waxes are
produced from coal vía the FlSCher-Tropsch synthesis
of n-alkanes. SampJes of both the microcrystaUine
wax Micro-195 and the synthetic wax FT H-l had
been prevíously analysed by supercritical fluid chromatography-mass spectrometry (SFC-MS). In the
present study, both sampJes were analysed by high
temperature gas chromatography using the short
Oí' S8
C40
\
Wox S8
I
6
,
12
j
••
I
2S
j
32
I
;,e
I
«
I
51
I
57
I
64
10
Minutes
F"¡g. 3. Capillary HTGC (25m) oftbe 00 (00 S8) and tbe wax deposit (wax S8) col1eeted Crom driD stem
pipe oC tbe same -n. (femperature conditions as for F"¡g. 2.)
Wax precipitates
Tbe saturate fractions of tbe waxes (wax H and
wax S8) taken from the oil well drill sterq pipes and
their corresponding oils (oil H and oil .S8) were
analysed by HTGC using the long (25 m) aluminum
coated capillary column (Figs 2 and 3). Whilc tbe oils
produced a fairly typical ,.-alkane distribution cangina up to C 3S • the waxes showed a bimoda1 distribution of n-allcanes canpag up to
witb maxima
al C 17 and around C..-c41- No signifialnt odd/even
n-alkane predominance could be obscrved Cor the
n-alkane distributions in cither the low- or high
molecular weight regioll fOl' the wax deposits apart
Crom the C~40 regíon of wax SB. Despite tbe major
di~rences in tbe n-alkane distributions. the saturate
Cractions of the waxes posscssed tbe same biomarker
e.
100
fingerprint as tbe oils (for example see the mIz 191
distributions Cor oil H and wax H in Fig. 4). Tbe
presence of high concentrations oC tbese high molecular weight bydrocarbons in the wax deposits collected
from the drill stem pipes is not altogethec surprising.
but the results demonstrate that high molecular
weigbt bydrocarbons are absent, or plescnt in very
low concentrations. Crom oíls collectcd. at the wellhead. Tbese components precipitate out in the drill
stem pipes or rcmain in the RSCrVoir rocks duc to
their low mobility. Scvcral bnmched/cydic bydrocarbons can be obscrved in thc chromatopams in the
regían above C 40 • Furthcr attempts to ooacentrate the
high molecular weight branched/CYclic fraction by
molecular sieving Cailed. probably due to thc Cact tbat
the structure oC these compounds is maiIlIy comprised
oC long linear aliphatic chains. witb few branching
mIz 191
Hopones
OilH
80
60
;e
Cs.
Tricyclic triter pones
!..
40
C
\
.JlJU
20
100
3a
\
Cn
mIz 191
C3<)
Wax H
Ca
80
Cs.
60
;e
!..
en
\
40
\3a Css
\ eS.
\
20
Time
Fig. 4. A comparisoa oC the mass chromatograms (miz 191) Coc the saturated fractiODS oC oil H and wax
H. GC oven temperature: 4O-320°C at 2°C/min and 30 min isothermal.
points or ring structures. Direct analyses of the total
saturate fraetions of the waxes by HTGC-MS were
not very satisfaetory due to problems experieneed
with heating the GC-MS interface to a suitable
temperature.
Analysis of lhe asphaltenes iso/ated from the oils
The asphaltenes isolated from the oils H and SB
were characterized initially by flash pyrolysis-gas
chromatography using a fused silica capillary
column. (Note: It is important to ernphasize here that
the asphaltene fraetions were prepared in the classical
sense by precipitation with pentane and purifieation
by reprecipitation with additional n-pentane. It is
entirely possible that during this isolation process,
higher molecular weight hydrocarbons aIso precipitate due to laek of solubility in pentane. Despite not
being ehemically bound within the asphaltene structure these hydrocarbons are by definition still a part
of the so-called asphaltene fraetion). The pyrograms
(not shown here) consisted of series of n-aIkanes and
n-alk-I-enes, dominated by n-alkanes extending beyond C 40 , with a predominance of the higher molecu·
lar weight hydrocarbons. Prist-I-ene, a commor
constituent of many asphaltene pyrolysates, was no
observed in any of the pyrograms. The flash pyrol
ysis-GC experiments were repeated using high tern
perature gas chromatography and the aluminim
c..,
\
Oí! H
O¡I S8
10
16
22
28
34
40
46
58
64
70
Minutes
Fig. 5. Flash pyrolysis-HTGC of the aspbaltenes isolated from the oils H and SB. Pyrolysis temperatu
800°C. Oven temperature: 60-400"C at 8°C¡min and 50 min isotbermal. Interface at 350°C and detect.
al 400°C.
coated columns and programming the oven temperature to 400°C. The results of these analyses showed
that the pyrolysis products were dominated by nalkanes extending to C6(J, witb a predomi~ce of the
higher molecular weight hydrocarbons (Fig. 5). In
vitro maturation of asphaltenes isolated from oils SB
and H were performed in glass tubes, sealed under
N z , at 350°C ovemight. Analysis of tbeir thermal
dcgradation products by HTGC showed a hydrocarbon distribution ranging up to c,. (Fig. 6). similar
to that observed from the flash pyrolysis analyses.
On the basis of tbe results described above, ít is
proposed tbat !be waxes precipi1ate as a result of tbe
decreasing temperature and pressure experienced by
the oil as it is brought to the surface. The productíon
of high molecular weight hydrocarbons during the
pyrolysis of the asphaltenes isolated from the oils
leads us lo suggest that the asphaltenes may also
undergo thermal breakdown at the high temperatures
and pressures existing in reservoirs 10 produce higb
molecular weight hydrocarbons which are soluble in
the oíl under reservoir conditions.
Asphaltene deposit
The so-called "asphaltene" pipeline deposit. FRL
9170, collected from an oíl productíon pipeline was
extracted aOO tbe resulting saturate fraction had a
bydrocarbon distributíon similar to that of tite oi)
collected from the well-head of the same pipeline
(not shown bere), with no indication of any higb
OilH
c""
1
Oil 56
c..,
C lO
\
I
2
7
12
22
28
33
38
43
I
48
53
Minutes
Fig. 6. HTGC analyses for !he bydrocarbons produced after in vitro artificial maturation of tbe asphaltenes
isolated from the oils H and SB. (Chromatographic conditions as for Fig. 2.)
molecular weight hydrocarbons. However, py-HTGC
of the asphaltene fractions isolated from both oil
FRL 9169 and the pipeline deposit, FRL 9170,
showed a very different hydrocarbon distribution.
Whereas the pipeline deposit, FRL 9170, showed a
distribution of n-alkanes ranging only up to C 40 • the
asphaltenes isolated from the oíl FRL 9169 showed
the presence of bigh molecular weigbt hydrocarbons
ranging up to at least C 60 (Fig. 7). In vitro artificial
maturation experiments of the asphaltenes isolated
from the oil FRL 9169 and subsequent analysis of the
products by HTGC produced a hydrocarbon distribution ranging up to at least C ro (Fig. 8). A possible
explanation for tbis difference in the pyrolysis products of the asphaltenes may be that the oil did not
reach high enough pressures and/or temperatures in
CO(J
I
FRL 9169
FRL 9170
2
7
12
I
I
I
I
I
17
22
28
33
38
i
43
I
48
Minutes
Fig. 1. Flash pyrolysis-HTGC oC the aspbaltcne Craction isolatcd Crom (a) the oil FRL 9169 ~ (b) the
asphaltcn pipeline deposit FRL 9110. (Pyrolysis and chromatographic conditions as Cor Flg. 4).
e
I
53
Moturation pro.ducts
fro.m aspholtenes
0.1 o.il FRL 9169
10
16
22
28
34
52
70
64
Minutes
Fig. 8. HTGC Analyses of the hydrocarbons produced by in vitro artificial maturation of the asphaltenes
isolated from the oil FRL 9169. (Chromatographic conditions as for Fig. 2.)
the reservoir for breakdown of the asphaltenes to
occur and produce high molecular weíght hydrocarbons. Instead, only partial fragmentation occurred
leading to the formation of the pipeline deposit
having very different solubility and mobility behaviour with respect to the original material. and
which subscquently precipitated in the productíon
pipetine and not the drill stem pipe.
OztICeTite
The analysis of the total saturate fraetíon from the
fosal bitumen, ozocerite, by high temperature gas
chromatography showed a distributíon of n -alkanes
ranging up to C n with maxima at C 32 and another
around C 42 and no even/odd predominance (Fig. 9).
The saturate fmetíon was comprised mainly of
n -alkanes and relatively small amounts of
branched/cyclic hydrocarbons, which after molecular
sieving were found to be concentrated in the C SO--C7S
n -alkane region.
An El mass spectrum for the total saturate and
branehed/cyelic fractions from the ozocerite was obtained using the direct insertion probe. Molecular
ions of hydrocarbons ranging up to
were observed
in the spectrum (not shown here) ofthe total saturate
hydrocarbons. The mass spectrum (DIP-EIMS) of
the high molecular weight branehed/eyclíe fraetíon
isolated from the ozocerite is shown in Fig. 10.
c..
Ozocerite
Soturote froctíon
eso
I
2
I
6
I
11
I
15
I
19
24
28
36
41
I
45
Minutes
Fig. 9. Capillary HTGC (3 m) analysis of tbe saturate fmetion isolated from tbe ozocerite bitumen.
(Chromatographie conditions as for Fig. 1.)
A1thougb the mass spectrum is from the mixture and
not individual compouncls. some significant information can be obtaíned from it. The mass spectrum
oC the branched/cyclic fraction showed a base peak at
mIz 97 and a series oC peales at mIz 97 + 14n. revealing the presence of alkyl side-chain structures. The
miz 97 fragment ion is probably due to the C 7 H¡';
methylcyclohexyl ion after the bond between the
methylcyclohexane ring and the alkyl side chain has
been cleaved. (n-Alkylthiophenes also give a major
Cragment ion at miz 97 but examination of this
sample by GC-FPD did not reveal the presence oC
any sulphur compounds.) The EI-MS spectrum also
showed a series of even-numbered fragments at mIz
C"H 2It _ 2 • typical of long chain dialkylcyclohexanes.
This fragment is due lo a hydrogen transfer rrom the
ring (or the another side chain) to the side chain
group which is cleaved to produce a neutral n -alkane
moiety. Dialkylcyclopentanes cannot be totally
exc1uded as a possible structure-type. Therefore.
cycloalkyl systems appear to be an important component of the branched/cyclic fraction and comprise
mainly single units rather than fused polycyclic systems. Another series oC even mass Cragments corresponding to C"H,.. fragments were also detected. at
lower intensities. but are characteristic oC branched
r100
97.2
100.,
4~.s
*11'011
*E +05
Fig. 10. DIP-(EI)MS of the enriched branchedjcyc1ic fraction from the ozocerite bitumen. (Note that the expanded insert was obtained from a second DIP analysis of the same sample.
Hence there is a slight variation in the position of the maxima in the mjz 480-530 region of the spectrum.)
alkanes and result Crom Cragmentation oC the molecule adjacent to the branching point. Under EI-MS
conditions. no Curther inCormation on these structures could be obtained due to significant molecular
Cragmentations. The mass spectrum obtained in the
CI(CH4 ) mode Crom the branched/cyclic Craction (not
included bere) showed a high degree oCCragmentation
and was very similar to that obtained in the El mode.
Several Cragments could be identified corresponding
lo carbon numbers up to C 60 , and indicatíng acydic
and cyclic compounds with one, two and maybe even
three degrees oC unsaturation.
Origin of the high molecular weight hydrocarbons
In discussing the origin oC these high molecular
weight compounds, the presence oC natural precursors must be included since polyisoprenyl alcohols
up to e 100 have been reported in several organisms
(Hemming, 1983; Lehle and Tanner, 1983; Chojnacki
and Vogtman, 1984; Chojnacki el al., 1987;
Swíezewska and ehojnacki, 1988, 1989) and bacterial
carotenoid skeletons containing up to eso are well
known (Liaeen-Jensen. 1990). Although detailed
structures have not yet been determined, it is possible
that the higb molecular weight compounds in the
oils could be Corroed by intramolecular cyclisation
oC high molecular weigbt polyisoprenoid aIcohols.
Moldowan et al. (1983) proposed that cyclisation
oC solanesol (a e", polyprenol) could lead to the
extended series oC tricydic terpenoids up to e", and
Albaiges (1980) also proposed polyprenoIs as precurSOIS oC isoprenoids extending up to e",. These aleohols may be converted into high molecular weigbt
isoprenoid alkanes just as the low molecular weigbt
analogues (i.e. phytol) are converted into hydrocarbons by attack at the alcohol moeity, Collowed by
reduction and thermal cracking.
Another possibility is that these high molecular
weight hydrocarbons are di- and trimerization products oC lower molecular weight precursors (del Rio
and Philp. 1992). Previous papers strongly support
this hypothesis. For instance. de Leeuw et al. (1980)
obtained di- and trimerization products oC phytol
alter heating it in presence oC clay minerals. RubinsteiD and Strausz (1979) reported the Cormation oC
diJDerization products of fatty acids when heating
tbem in the presence oC day minerals. More recently,
Van Aarssen and de Leeuw (1989) discovered sorne
C'" hydrocarbons. thougbt to be trimeric cadinanes.
in erude oils and sediments Crom South East
Asia. suggesting that sesquilerpenes may dimerize
or oligomerize under appropriate conditions by an
abiotic process.
CONCLUSIONS
High molecular weigbt
found to be concentrated
frequent1y occur in drill
oíl wells. Pyrolysis oC the
hydrocarbons have been
in the wax deposits that
stem pipes oC producing
asphaltenes isolated Crom
the oils suggest that thermal breakdown under
reservoir conditions may lead to the formation
of high molecular weight hydrocarbons. The presence
of higb molecular weight hydrocarbons has been
observed in the saturated fraction of ozocerile. a
fossil bitumen. Cyclic structures ranging up 10 e8()
could be observed in the ozocerite although no
detailed structures were confirmed for these compounds.
It is suggested that the study of high molecular
weigbt hydrocarbons has been overlooked in the past
for two main rcasons. The first being the Iack of
appropriate analytical techniques. The second reason
is that. particu1arly in the case oC produced oíls. the
higher molecular weigbt components are often absent
in the oil collected al the well-head. Rather these
components precipitate out in the drill stem pipes or
remain in the reservoir rocks due to their low mobility. In view of the results presented herein it is
proposed that continued study oC these high molecular weight fractions will provide additional insigbts
into the origiD and types oC organic source materials
responsible for various types oC oils. Major advances
can be expected in this arca over the next few years
largely because oC improvements in the analytical
techniques that will make it easier to identiCy the
high molecular weigbt compounds on a molecular
leve!.
Traditional gas chromatographic methods are
limited to species with sufficient volatility. High temperature gas chromatography and supercritical fluid
chromatography coupled with mass spectrometry
offer the potential to provide high resolution separation oC very high molecular weight compounds with
selective mass spectrometric analysis and could be the
appropriate techniques to analyse these new jiigh
molecular weigbt compounds. possibly potential bíomarkers, although instrumentation and interfaces are
still being developed. The potential of HTGC-MS
and SFC-MS will continue to grow as HTGC and
SFe techniques are extended by the introduction 01
new mobile and stationary phases, new approache~
for the interfaces are developed and as improvec
mass spectrometric detectors continue to becom(
availab1e.
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•
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