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Source rock and asphaltene biomarker characterization by pyrolysisgas chromatography-mass spectrometry-multiple ion detection
oolh-7077/85/%3.00
Geoch,m,ca n ~‘osmochtmtca A&I Vol. 49, pp. 1421-1432 (r Pergamon Press Ltd. 1985. Printed in U.S.A.
t
.Ml
Source rock and asphaltene biomarker characterization by pyrolysisgas chromatography-mass spectrometry-multiple ion detection R. P. PHILP’ and T. D. GILBERT~
’ School of Geology and Geophysics, University of Oklahoma, Norman, OK 730 19 * CSIRO, Division of Fossil Fuels, P.O. Box 136, N. Ryde, N.S.W. 2113, Australia (Received September
12, 1984; accepted in revised form March 15, 1985)
Abstract-Biomarkets produced by microscale pyrolysis of extracted source rocks, kerogens and asphaltenes have been analysed directly by gas chromatography-mass spectrometry in the MID mode. A series of experiments have been undertaken. These include an investigation into the production of the hopanes at different pyrolysis temperatures and a comparison of results obtained from Curie point pyrolysis and the Chemical Data Systems pyroprobe. Steranes and triterpanes produced by pyrolysis of a series of vitrinites were investigated and correlated with maturity variations. Finally, the biomarkers produced from asphaltenes were compared with those produced from the extract and extracted rock in order to further investigate the theory that asphaltenes are a liquid kerogen. The results of these experiments showed that the biomarkers released by pyrolysis of source rocks have a potential use for source and maturity determinations. The method is advantageous in that it can be said with some certainty that the biomarkers are indigenous to the rock and not present in the rock as a possible result of migration. INTRODUCI’ION IN A PREVIOUS paper we reported preliminary reSukS from the pyrolysis-gas chromatography-mass spectrometry-multiple ion detection (py-MID) characterization of petroleum source rocks and suspected source rocks (PHILP and GILBERT, 1985). The emphasis in that study was placed on using mass spectrometry and multiple ion detection (MID) techniques to determine the distribution of biomarkers, in particular triterpanes, produced by pyrolysis of source rocks. The main reasons for initiating the study were: (a) to determine whether or not biomarkers released by pyrolysis of source rocks could actually be detected in a one-step microscale py-GC-MS analysis; (b) if so, were the results comparable with those obtained in early studies by SEIEERT (1978) and SEIEERT and MOLD~WAN (1980). The studies of the Chevron group comprised a separate large scale pyrolysis step followed by collection and fractionation of the pyrolysis products prior to GC-MS and MID analysis of the aliphatic fraction to determine the biomarker distribution. The biomarker distributions were then used for distinguishing suspected source rock strata and obtaining information on the oil-generating potential of a rock. The multistep procedure is time consuming and the advantages of combining pyrolysis and analytical procedures for the characterization of a large number of source rock samples are obvious. Apart from our earlier paper (PHILP and GILBERT, 1984) the only other paper to have reported, in any detail, on the distribution of hopanes producd by direct py-GC-MS analysis appears to be that of GALLEGOS (1978) in which he described the characterization of five U.S. coals using this technique. An earlier paper by GALLEGOS (1975) also mentioned the production of stemnes and triterpanes by pyrolysis from the Green River Shale.
Here we report results obtained from an extension of our preliminary study. This extended study includes the characterization of a wider range of samples as well as the determination of both steranes and hopanes produced by pyrolysis. In addition, a comparison has been made between results obtained with a Curie point pyrolysis system and a Chemical Data Systems (CDS) pyroprobe. This comparison was undertaken as a result of the necessity to characterize rocks with low organic carbon content. Such characterizations are more readily undertaken by using larger sample sizes with the pyroprobe. The biomarker distributions produced at different pyrolysis temperatures have also been examined. A series of vitrinite concentrates have been analyzed by py-GC-MS-MID (py-MID) to determine variations obtained in the biomarker distributions with changes in vitrinite reflectance or maturity. The results obtained for the hopanes closely parallel those previously reported by GALLEGOS (1978). The sterane distributions change in a manner that could be predicted from changes previously observed in the extractable steranes (MACKENZIE et al., 1982). The results indicate that the technique has potential for source and maturity determinations. It eliminates some of the questions associated with using the extractable fractions, namely whether the fraction is indigenous to the rock or merely contamination from migrating oil. Finally, the asphaltene fractions from a number of source rocks have been investigated by the py-MID technique. The sterane and triterpane distributions produced have been compared with those obtained from solvent extracts and by py-MID of the extracted source rock. BANDURSKI (1982) previously proposed that asphaltenes may be fragments of the original kerogen from which crude oil is derived and may be expelled as part of the crude oil. However little attention has been paid to the distribution of the
1421
R. P. Philp and T. D. Gilbert
1422
biomarkers produced by pyrolysis of asphaltenes. Comparison of the same with those produced by pyrolysis of the source rock and those present in the extracts should provide additional information on the role of asphaltenes in petroleum formation. The results obtained in this way introduce the possibility of using the asphaltene fraction. rather than the source rocks. for makiqg maturity measurements. This would also have the advantage of eliminating any possibility of mineral matrix effects during pyrolysis. Differences in the distributions of hopanes and hopenes produced by py-MID of the asphaltenes compared with the extracted source rocks may also provide important clues to the way in which the hopane precursor, possibly the bacteriohopanetetrol (OURISSON et al., 1984) is attached to the kerogen. EXPERIMENTAL The samples used in this study were extracted by refluxing with chloroform-methanol (I: I) for 48 h prior to pyrolysis using either Curie point pyrolysis or the CDS pyroprobe system. Curie point pyrolysis was performed at 610°C for I5 sets and the pyrolysis products were continually flushed into the injector system of the GC or GC-MS. For CC analysis a HP 5710 gas chromatograph was used with a splitless injector system. The oven temperature was held at 0°C during the pyrolysis and after 2 mins programmed at 4”/min to 300°C. The fused silica WCOT column (50 m X 0.2 mm i.d.) was coated with a BPI phase (SGE Australia). The mass spectrometer was operated with a filament current of 0.25 mA, electron energy 70 eV, and ion source temperature of 230°C. In the full scan mode (py-GC-MS) the scan speed was 1.5 set/scan and the MID mode (py-MID) the dwell time on the ions monitored, normally m/z 191 and m/z 217, was 0.1 sets. Similar conditions were used with the CDS pyroprobe system except the upper temperature was set at 800°C. RESULTS AND DISCUSSION E&ects of variations temperature p.vrolvsis
in Curie poinl
and comparison
and P.vroprohe
p_vrol~vsi.s
twtwecn
pyro1.v.h
Curie
point
results
In our preliminary study (PHILP and GILBERT. no systematic attempt was made to investigate the effects of variations in pyrolysis temperatures on the distribution of biomarkers. Furthermore it was of interest to ascertain whether the biomarkers produced were formed as a result of pyrolysis and not merely released from “traps” or occlusions in the kerogen or associated mineral matrix. as a result of increased temperature. Information of relevance to these two questions was obtained in this study from an examination of an organic rich sample at four different Curie point temperatures, namely 358°C. 480°C. 610” and 770°C. The particular sample selected for this experiment had a very characteristic hydrocarbon extract dominated by the 22R and S epimers of the CX, homohopane. Py-MID of this unextracted sample at 358°C produced the pyrogram in Fig. la which was dominated by this pair of C,,homohopane epimers and virtually identical to the distribution of hopanes observed in the hydrocarbon 1984)
fraction of the soluble extract. (Peak identities for components in all the chromatograms shown in this paper are listed in Table I.) It is clear from Fig. I h that at 480°C some pyrolysis of the sample had occurred since a number of peaks identified as hopenes, from full scan studies. are present in the chromatogram. Changes in the distribution of the saturated hopanes relative to the Cz,-homohopanc epimers can be observed in Fig. I b compared to Fig la. In this sample the C,, epimers can be thought of an indicators of the relative contribution from the soluble fraction since py-MID of the kerogen produces only minor quantities of the CJI-epimers (see below and Fig. 2). As the pyrolysis temperature is increased to 610” and 77O’C the major change observed in the pyrograms (Figs. Ic and Id) is the additional increase in the relative concentrations of the pyrolyticallyproduced hopanes and hopenes relative to the C‘,I homohopane epimers. Minor changes in the ratios of various hopanelhopene pairs can also be observed when these pyrograms (Figs. 1c and Id) are compared with Fig. I h. For each carbon number pair. i.c~ C?;. C29 and Cv,, the concentration of the hopenc (peaks 2. 9 and I I. respectively) has increased relative to the corresponding hopane (peaks 3. 6. and IO. respectively). Since hopanes and hopenes are produced by pyrolysis of the kerogen (see Fig. 2) it is proposed that this change in relative concentration reflects a faster rate of production for the hopenes at the higher temperatures. The main effect of increasing the pyrolysis temperature appears to be an increase in the yield of the pyrolysis products. It appears unlikely that the biomarkers are simply released from traps in the kerogen, or mineral matrices, as a result of increased pyrolysis temperatures. If this was the case it is unlikely that hopenes would be detected since no hopenes were found in the soluble extract from this sample. Biomarker distributions produced by pyrolysis ot the kerogen isolated from the sample used in the previous experiment are summarized in Fig. 2. No hopanes were produced from the kerogen at a pyrolysis temperature of 358°C but at 480°C. 610°C and 770°C the pyrolytically produced hopanes and hopenes were virtually identical (Fig. 2). Pyrolysis appeared to be virtually complete by 610°C and the 77O’C pyrogram is not shown in Fig. 2. The major difference between the pyrograms is quantitative rather than qualitative. Although no accurate measurements were made, comparison of signal intensities from the three analyses suggested a two-fold increase in yield of hopanes and hopenes at 770°C over that at 480°C assuming starting sample sizes were approximately equal. A similar observation was made with the unextracted sample, where yields could be gauged against the relative intensity of the C,, homohopane epimers. A comparison between results obtained with the CDS pyroprobe and the Curie point pyrolysis system was made with the unextracted rock and kcrogen
Source
rock and asphalkne
biomarkers
4
6
-“----I 4
R. P. Philp and T. D. Gilbert
1424 Table
I.
Identification
of by
Produced
Triterpanes
_ Peak
M/z
Number
and
Steranes
py-MID _____
..__ _
_ ..__.--___
Identification
191
(Triterpanes) 18C%(H)-Trisnorhopane 2
c
3
~~~~~~~~~~~~*=h~p~*~ 176(H)-Trisnorhopane
4
Unidentified
5 6
17cr(H)-Norhopane
7
Unidentified
8
17B(H),2la(H)-Normoretane
9
c
-H0pene I?&H),ZlBfH)-Hopane C -Hopene i~f3(R,,2lu(H)-Moretane
10 11 12
17R(H),ZtBfH)-Narhopaoe
I3
17a(H)
14 15 16
?
and
S epimers)
GalWWKI?rXle 17nfH),218(H)-Bishomohopanes
18
M/z
,2 I@(H)-Homohopanes
178(H),21B(H)-Hopane
17 I9
(22R
3
f22R ___~___
and
S epimers) ____-A.“..._----
217
(Steranes) I’
14a(H),I7a(H)-cholestane
(20s)
2’
Iba(H),17a(H)-cholestane
(20R)
3’
24-Methyl-I4a(H),17a(H)-cholestane
(20s)
4’
24-Methyl-f4~(Hf,l7~fHf-cholestane
(ZOS+ZOR)
or
24-Methyl-5B(H),14c((H)17u(H)-cholestane
5’
%4-Methyl-fSn(Hf,l7afH)-cholestane
6’
24-Ethyl-
7’
17acHf-cholestane
24-Ethyl-148(H),l78(H)-cholestane or
a'
14afHf,
(2OR) (205) (20S+20R)
24-Ethyl-5B(H),l4a(H),l7cr(H)-cholestane
24-Ethyl-i4a(H),l7a(H)-cholestane
used in the experiments above. It was necessary to make this comparison since it was anticipated that many of the samples to be examined in this and future studies would have relatively low values of organic carbon. Use of the CDS pyroprobc enables larger samples to be pyrolyzed than the Curie point pyrolysis system. The best operating conditions for the pyroprobe were found to have a final pyrolysis temperature setting of 800°C. The distributions of hopanes and hopenes produced at this temperature were virtually identical, from a qualitative point of view to those produced by Curie point pyrolysis at 610°C. Similar observations were made with both the unextracted rock and the kerogen. Despite the uncertainty concerning the final and actual pyrolysis temperature the remainder of the analyses described in this study was undertaken using the pyroprobe. The final temperature was always set at 800°C and then repr~ucible results were obtained. In most samples it was possible to determine the distributions of both hopanes and steranes formed during the pyrolysis of extracted rocks and kerogens. In our previous paper only the hopane distributions were reported but examination of the steranes provides additional info~ation on source and maturity of the organic material in a sample. In the following section,
(20R)
the application of the py-MID to a series of vitrinitt: concentrates is described as a means of ohtaining source information and observing variation in biomarker distributions with increasing maturit?
MACKENZIE et cl/. (1981. 1983) and SEIW,R I and MOLDOWAN (1980. 1981) have previously observed and discussed a number of trends in the biomarker dist~butions produced by pyrolysis that correlate with changes in maturity. In order tit further investigate these observations over a wider range uf maturities and to eliminate the problem of source vatiations, a series of vitrinite concentrates of increasing maturity were examined by py-MID and variations in sterane and triterpane distributions determined. GALLEGOS (t978) briefly commented on variations in the hopane distributions produced by pyrolysis ot five U.S. coals but did not discuss them in any detail, and no mention was made of variations observed in the sterane distributions. The vitrinites used in the present study were hand picked concentrates From Sydney Basin (AustmIia) coals and have bcen described in more detail elsewhere (EVANS ri cd I984).
1425
Source rock and asphaltene biomarkers EXTRACTED
KEROGEN
6
480”
C
610°C
FIG. 2. Curie point pyrolysis of kerogen isolated from same sample as used for Fig. 1. Chromatograms obtained by SIM of ion at m/s 191. Peak identifications given in Table I.
A summary of the hopane distributions produced by py-MID are shown in Fig. 3. The sample with R. = 0.3 is excluded since the hopane distribution is virtually identical to that of the sample with R. = 0.39. The pyrograms in Fig. 3 show a number of variations with increasing maturity. The main features that can be summarized are a decrease in intensity of the Car-hopene (peak 2) relative to the 17a(H)trisnorhopane (3) along with a noticeable decrease in the 17@H)-C&risnorhopane (4) relative to the 17a(H)-trisnorhopane (3). The concentration of the Caa-hopane (10) increases relative to the Czg-norhopane (6) and the CZ9 and CjO hopenes (9,11) decrease relative to their saturated counterparts (6,lO) with increasing maturity. The overall concentration of the biomarkers produced from the vitrinite concentrate
with R. = 0.93 was low. The resultant chromatogram has a high noise level and contains a number of additional peaks that are not hopanes and are not visible in the other normalized chromatograms. At a reflectance of R,, = 1.2 the pyrolytically produced hopanes were virtually absent. As the maturity level increases there is also a decrease in the relative moretane concentrations and a gradual increase in the relative concentration of the C3,-22R and Shomohopanes at the expense of the 17/3(H),2lp(H)homohopane. At the same time the concentration of the 22s epimer (14) increases at the expense of the 22R epimer (15). These latter effects are similar to those previously reported by SEIFERT and MOLDOWAN (1980, 198 1) although their separate pyrolysis step did apparently eliminate the production of hopenes. This in turn could reflect differences arising from the
R. P. Philp and T. D. Gilbert
1426 VITRINITE M/f
CONCENTRATES
191
Ro 0.39
b i
8 Ii
Ro 0.5
Ro 0.69
FIG. 3, Variation in hopane/hopene distributions MID from six vitrinite concentrates isolated from given in Table 1.
longer pyrolysis times and slower heating rates in their study. However, the results from this study demonstrate that the one step method can be used to characterize the biomarkers produced pyrol~ically from source rocks. The dist~butions obtained can be used to observe and monitor maturity differences. One possible disadvantage is production of the hopenes which were not observed in the earlier work of SEIFERT and MOLD~WAN (1978) or SEIFERT and MOLDOWAN ( 1980). Examination of the steranes also revealed a number of variations that could be correlated with increasing maturity (Fig. 4). The results are simplified somewhat by the fact that the pyrolysis products are dominated by Cz9 steranes. At the lowest level of reflectance (& = 0.3 and 0.39) only two steranes are produced, namely the &Y(H), 14a(H), 17cu(H)-20R-CZ9 sterane (8’) and the 5&H), I4cu(H), 17cufH)-Czs sterane (7’). (These identifications are based on comparison of
Ro 1.20
(m/z 191) with increasing maturity produced bk I?Sydney Basin (Australia) coals. Peak identiticatiw~
retention times with a sterane fraction from an immature oil shale where the only components present have been identified as the same two isomers.) At a reflectance value of 0.5 the Sa(H). 14cu(H), 17n(,tii2OS-C2+ epimer (6‘) appears although it is still present in lower concentrations than the 58 isomer. ht K,, = 0.69 there is a further increase in the concentration of the SLY(H). 14ar(H), 17cr(H)-20S-CZo epimcr relative to the .5@-isomer. A significant change is observed between the chromatogram of R0 := 0.69 and X,, = 0.93. At RQ = 0.69 it is assumed, with a high degree of confidence that peak 7’ represents only the S/‘ZLC~~ sterane. However at R0 = 0.93 it is clear that the 14@(H), 17@(H)-20R and 20s epimers are now predominant in that region of the chromatogram. It was not possible to ascertain whether or not the 58Ci9 sterane was still present in the pyrolysis products. The remaining vitrinite samples with R. =r~ I.Oland R0 = 1.20 show only minor variations compared with
1427
Source rock and asphaltene biomarkers
VITRINITE
CONCENTRATES
M/Z 217 6’
Ro 0.93
a’
7’
6’
8’
Ro 1.01
Ro 0.5
8’ I Ro 0.69
Ro
1.20
6’
FIG. 4. Variations in sterane distribution (m/z 217) produced from the six vitrinite used for Fig. 3, by py-MID. Peak identifications given in Table I.
R. = 0.93. It should be noted that rearranged
steranes are not produced during pyrolysis as previously observed by SEIFERT (1978). This observation supports the claim that the steranes are indeed pyrolysis products and not simply products released from occlusions in the vitrinite upon heating. Examination of the soluble fractions from these vitrinite concentrates show the presence of both rearranged and regular steranes. The changes in the sterane distributions observed here are similar to those that would be expected in the sterane distribution of the soluble extract apart from the absence of rearranged steranes. In other words as the maturity increases, so do the relative concentrations of the 14a(H), 17a(H)-CZs20s sterane and the 14&H), 17P(H)-C2,-20S+20R steranes (MACKENZIE et cd., 1982). The sterane distributions produced by this py-MID technique can be used. therefore, for making maturity determinations in the same way as the extractable steranes, but again
concentrates.
also
without the problem of trying to determine whether or not they are actually indigenous to the sample as is sometimes the problem with the extractable material. Furthermore the steranes distributions obtained in this way are also useful as source indicators.
The results described above. and similar results previously described by SEIFERT (1978), raise a number of interesting questions and suggest further areas to be investigated. These include the need to determine why such biomarker distributions are observed, how are the biomarkers attached to the kerogen and what is the nature of the precursor molecules? One approach to obtain partial answers to some of these questions is to examine the asphaltene fractions isolated from source rock extracts. Previous work by BANDURSKI (1982) has indicated that the oil gener-
R. P. Philp and T. D. Gilbert
1428
ating portions of kerogens and petroleum asphaltenes may have similar structures and the asphaltenes may be expelled as part of the crude oil. In this study asphaltenes have been isolated from four Chinese shales, previously described by JI-YANG rl ~11.( 1982). and two potential source rocks from the Cooper Basin, Australia. The asphaltene fractions have been characterized by py-MID and the resulting biomarker distributions compared with those produced from the corresponding kerogens and soluble extract. The hopane distributions produced and determined by py-MID for the soluble extract with asphaltenes removed (i), the asphaltenes (ii) and the extracted rock (iii) from two samples of suspected source rocks from the Cooper Basin, Australia, are shown in Figs. 5a and b respectively. The hopanes in the py-MID chromatograms of the deasphaltened soluble extracts are virtually identical to those obtained by GC-MS analysis of the aliphatic fractions for the same samples.
COOPER
BASIN
This observation is not surprising since the chromatograms shown in Figs. 5a(i) and b(i) represent hopanes determined in the total soluble extract by thermal distillation. The presence of small amounts of the C2,-hopene (peak 2 in 5a(i) and Qi)) may result from incomplete removal of asphaltene materiai from the soluble extract. The asphaltene fractions from the two samples produce a significant number of hopenes by py-MID as shown in Figs. Sa(ii) and h(ii). The relative concentrations of the saturated hopanes (i.e. peaks 1,3,4,6,8,10,12,14,15) in the asphaltene fraction are virtually identical to those observed in the py-MID chromatogram of the extracted rock (Figs. Sa(iii) and Qiii)). This observation supports BANDURSKI’S (1982) theory that there are structural similarities between the kerogen and asphaltene fraction. It is of interest to note a number of variations II-I relative distributions of the hopanes and hopenes
(AUSTRALIA)
(a)
M/Z
)
191
SOLUBLE i4
FRACTION
( ii
ROCKS_
(b)
M/Z 191
(i)
SOURCE
ASPHALTENES
(i)
(ii)
FRACTION
ASPHALTENES
c
(i ii)
EXTRACTED
3
ROCK
FIG. 5. Distribution of hopanes/hopenes (m/z 191) produced by py-MID from the deasphaltened soluble extract (i), asphaltene (ii) and extracted rock samples (iii) of two suspected source rocks (a and 0) from the Cooper Basin, Australia. See Table I for peak identifications.
Source rock and asphaltene between the soluble extract, the asphaltenes and extracted rocks. The hopanes in the soluble extract (Figs. 5a(i) and b(i)) are dominated by the C30-hopane whereas the asphaltenes and extracted rocks produce chromatograms dominated by the CZ7 and &-hopanes. It can also be observed that the ratio of the C3,-homohopane 22s and R epimers (peaks 14 and 15 respectively) for the soluble extract (Figs. 5a(i) and b(i)) is >l but
COOPER
(a)
M/Z
BASIN
ates, involved in the formation of hopanes in the asphaltenes compared to the extracted rocks. These may be functionalized precursors such as alcohols, acids or bacteriohopanetetrol which produce the hopenes upon pyrolysis. It is unlikely that the hopenes themselves are present in the asphaltenes since they would be removed in the soluble fraction during isolation of the asphaltene fraction. Steranes produced by py-MID of the same samples are shown in Figs. 6a and b, respectively. A number of observations can be made from these chromatograms. First the sterane chromatogmms for the soluble extracts (Figs. 6a(i) and 6b(i)) contain both regular and rearranged steranes, the asphaltenes and extracted rock chromatograms do not. This is similar to the observation previously made by SEIFERT (1978) that only regular and not rearranged steranes are produced during pyrolysis. The ratio of the 20S/20R-CZ9 steranes (peak 6’/peak 8’) in the soluble extracts indicates that
(AUSTRALIA)
SOURCE
(b)
217
1429
biomarkers
ROCKS
M/Z
217 a 6' 7'
(ikOLUl3LE FRACTION
SOLUBLE
ASPHALTENES
3,
i ii)
EXTRACTED
)
FRACTION
6’ ,,
ASPHALTENES
(ii)
(i
II
6'
n
EXTRACTED
FIG. 6. Sterane distributions (m/z 217) produced by py-MID from the deasphaltened soluble extract (i), asphaltene (ii) and extracted source rock sample (iii) of two suspected source rocks (a and b) from the Cooper Basin, Australia. See Table 1 for peak identification.
cii)
R. P. Philp and T. D. Gilbert
1430
sample a (Fig. 6a(i)) is less mature than sample b (Fig. 6b(i)). A similar conclusion can be reached if these ratios are measured using the 20S/20R-C,, sterane ratios from the asphaltene and extracted rock pyrograms (Figs. 6a(u-m) and 6b(u-111) respectively). However for both rocks the value for this ratio is higher in the extract than it is in the asphaltene OI extracted rock. A similar investigation has been performed using the asphaltenes isolated from four Chinese shale samples previously described by JI-YANG cl al.(1982). Figure 7a(i-iii) comprises the hopanes from the soluble fraction with asphaltenes removed, the asphaltenes and the extracted rock produced by py-MID of the Chinese shale sample, referred to as Yi-21 in the original paper (JI-YANG et u/.. 1982). As with the Cooper Basin samples described above there is a higher concentration of hopenes in the asphaltene fraction than the pyrogram of the extracted rock.
CHINESE ____-.
(a)
M/Z
191
10
SHALE
lb
However, sample Yi-21 has been shown by JI-~'ANC, et ul. (1982) to have a high concentration of gammacerane (peak 16) in the hydrocarbon fraction and py-MID of the soluble extract shows a similar result (Fig. 7a(i)). Py-MID also shows the absence of gammacerane from the products of the extracted rock (Fig. 7a(iii)) but its presence in the pyrolysis products of the asphaltene fraction (Fig. 7a(ii)). .4dditional purification of the asphaltene fraction showed that the gammacerane was not present as the free hydrocarbon in the asphaltene fraction. This raises the question of whether the gammacerane was ever attached to the kerogen and subsequently released 01 is it present as the more polar precursor tetrahymanol in the asphaltene fraction and never bonded to the kerogen.
Additional
studies are required
SAMPLE-Yi-21
(b)M/Z
2’
i (i)
SOLUBLE
b
FRACTION
I
to answer
The absence of gammacerane from the pyrolysate extends the range of biomarkers present in the extract but not the pyrolysate. Previous studies
these questions.
i
I [
(i)
217
SOLUBLE FRACTION
1~
1
(j_i)ASPHALTENES
(iii)
EXTRACTED
i j
i il)
ASPHALTENES
i' ( i ii) EXTRACTED
I
ROCK
FIG. 7. Distribution of (a) hopanes/hopenes (m/z 191) and (b) steranes (m/z 217) produced by py-Mil) from the deasphaltened soluble extract (i), asphaltene (ii), and extracted rock (iii) of a Chinese shale sample (Yi-21). The sample contains a relatively high concentration of gammacerane ( 16) and has hecn previously described by JbY4NG TV a/. (1982). Peak identifications are given in Table I
1431
Source rock and asphaltene biomarkers have noted this phenomenon with the diasteranes (SEIFERT, 1978), monoaromatized steroids (MACKENZIE et al., 1981) and 28,30-bisnorhopane (MOLDOWAN rt al., 1984). The sterane distributions produced by py-MID for the soluble extract, the asphahene. and extracted rock for the same sample are similar (Fig. 7b(i-iii)) to those published by JI-YANG Ed al. (1982) for the extractable steranes present in this shale. The results for the sterane dist~butions provide another example to show that the source and maturity information obtainable from the py-MID results of the asphaltenes and extracted rock is similar to that obtained from the soluble extract. The results from a second Chinese shale sample are shown in Fig. 8 to illustrate that in shale samples of fairly low maturity there is quite a strong similarity in the pyMID dist~butions for the hopanes and steranes of the extracted rock and asphahene fractions. The extracted rock and asphaltene both show high con-
CHINESE (a)
M/Z
centrations of the Cl,-hopene (peak 2) and 178(H)trisnorphopane (4) and similar hopane/moretane ratios (Fig. 8a(ii and iii)). This leads to the suggestion that as the maturity level increases the precursors responsible for the hopenes are cleaved and transferred to the asphaltene fraction possibly still functionalized to some degree. Thus at higher levels of maturity the extracted rock produces no hopenes upon pyrolysis whereas the asphaltene fraction still has the potential to produce hopenes. it is also of interest to compare the differences in the sterane distributions for the asphaltenes and extracted rock for the two samples .. . . . shown in Fig. 7b(11-111)and Fig. Sb(~~-~~~). The sample used for Fig. 7 shows relatively high concentrations of the C+& steranes whereas that used for Fig. 8 produces only the C2, steranes. These differences are related to source differences and agree with the results previously published by J&YANG ( 1982) for the soluble extracts. However, the results demonstrate the poten-
SHALE
(LOW RANK)
10
191
(b)M/Z
2’
217
6 SOLUBLE (if
FRACTION
(i)
SOLUBLE FRACTION
4
(ii)
iii)
ASPHALTENES
EXTRACTED
1’
ASPHALTENES
EXTRACTED
ROCK
8 9
FIG. 8. Dist~bution of (a) ho~nes/ho~nes (m/z 19 1) and (b) steranes (m/z 2 17) producedby m-MID from the deasphaltened soluble extract (i), asphaltene {ii) and extracted rock (iii) of a low rank Chinese shale previously described by JI-YANG et a/. (I 983). Peak identifications given in Table 1.
(ii)
(ii
i)
R. P. Philp and T. D. Gilbert
1432
tial of using the sterane distributions from the asphaltene and extracted rock to obtain source information on a sample. No attempt has been made in this paper to discuss the nature of the linkages by which the hopanes and steranes are bonded to the kerogen. This forms the basis of a separate study. but the possibility of a similarity in the type of linkages between these compounds and the kerogens, with those found in the long chain isoprenoids and archaebacteria cannot bc ignored. In summary it would appear that the products from py-MID of the asphaltene fractions of shales and source rocks show many similarities to those obtained from the extracted rock samples. This would support the idea previously put forward by BANIXJKSKI ( 1982) that the asphaltene fraction can be thought of as a liquid or mobile kerogen fraction. Furthermore. it introduces the possibility of using the asphaltenc fraction rather than the extracted roch in py-MID studies to obtain source and maturity information. This would eliminate any problems associated with mineral matrix effects during pyrolysis and would also make it easier to increase the sample size and yield of biomarkers during the py-MID study. It does have the disadvantage that the asphaltene fraction may not be indigenous to the suspected source rock but may represent migrated hydrocarbons. Future studies will be directed at determining how the hopene concentrations vary in the asphaltene fraction and to see whether this is related to maturity or the nature of the precursors. CONCLUSIONS This paper has demonstrated the potential value of using py-MID as a one step method for charactcrization of biomarkers in source rocks. It has shown that the results obtained are similar to those obtained by the processes involving a separate pyrolysis step, followed by time-consuming fractionation steps. In addition it has demonstrated that the steranes produced can be used both as source and maturit! indicators in the same way as the extractable fraction but with the advantage of knowing that they represent the indigenous organic material in the rock. Py-MID of a series of vitrinite concentrates has illustrated the variations in hopane and sterane distributions with maturity changes. Particularly significant were the changes in hopene and 17/j(H)-trisnorhopane concentrations. Changes in stcrane isomers were similar to those previously observed in extractable fractions. Comparison of pyrolysis products from asphaltene fractions and extracted rocks has provided further insight into the possible nature of asphaltene as an intermediate in the production of crude oil from kerogen. It has also provided evidence to suggest that certain biomarkers. or their precursors. such as gammacerane may not be bonded to the kerogen. Further studies must now be directed at obtaining information on the way in which the hopane and
sterane precursors are linked to the kerogen. The dist~butions of the hopanes produced bb pyrolysis differ from those in the total extracts suggesting additional catalytic reactions occur to the hopancs after their in-.sift formation. Py-MID has not hren extensively applied to biomarker characterization previous to this paper, but it has now been shown that a great deal of information can be obtained II? this one step process. .Iclino,~/~~~rtnc,n(-We would likr LOthank DI-. %I Ji-1’ang for the Chinese shale samples.
REFERENCES BANDUKSKIE. (1982) Structural similanties between oilgenerating kerogens and petroleum asphalrenc\. Ettcvu~ Sonnrs 6( l/2), 47-66. EVANS E. J.. BATTSB. D. and SMrrH J. W. (lYX4) Determination of the prospectivity of sediments h\ hydrogenation. 4PE.4 .1. 23, 83-91. CiALLEGos E. J. ( 1975) Terpane-sterdne release from kerogen by pyrolysis gas chromalography-mass spcctrctmct;?,. ltrai. I%cm. 47, 1524-1528. I;AL t EGOSE. J. (1978) Analysis ol’tice U.S. coals. Pyroi\si+ gas chromatography-mass spectrometry-colnputrr and thermal gravimetry-mass spectometry-computer method%. In il.f‘S ,4&s. rn C%em .G+\. i7lf (eds. i’ f‘ Lirx pS. SIGGIAand H. G. JENSLN).pp. 3-30. Jr-YANG S.. MACKENZIEA. S.. AI I-SAN~~K R ~:c~Ii’~tox G.. GOWAK A. P., Wol.r-r. G. .4. and MA,XWCI.L J. K. ( 1982) A biological marker Investigation &’ petroleum and shales from the Shengli olllield. The Peoples Repubhc of China. C/lc,rn. fieol. 35, l-3:. MA~KENZIL,A. S., LEWISC. A. and MAVW~1L ,I Ii i iYXi! Molecular parameters of maturation in thr Ioarcian Shales. Paris Basin. France-IV. Laborator! thermal alteration studies. Geochitrt i “mtt~crcfritr~. lurr 45. l?hQ2376. MACKENZE A. S., HRASS~L.I s. c ., t:bLiwc,*. Ci. snd MAXWEU J. R. (I 982) Chemlcat fossils: The geological fate of steroids. Scienc,e 217, 49 I-504. MACKENZIEA. S., REN-WEIL.. MAxwtxt. J. R.. film.~w% 1.k J. M. and SEIFERTW. K. ( 1083)Molecular i~~l)venlen~s of thermal maturation of Cretaceous shale5 from the Overthrust Belt. Wyoming, U.S.A. In .~ldvutw~ it! Iltyyort~. Geochetni.str~ I YHI. (eds. M. BJORC)I’(‘I
Geoch,m,ca n ~‘osmochtmtca A&I Vol. 49, pp. 1421-1432 (r Pergamon Press Ltd. 1985. Printed in U.S.A.
t
.Ml
Source rock and asphaltene biomarker characterization by pyrolysisgas chromatography-mass spectrometry-multiple ion detection R. P. PHILP’ and T. D. GILBERT~
’ School of Geology and Geophysics, University of Oklahoma, Norman, OK 730 19 * CSIRO, Division of Fossil Fuels, P.O. Box 136, N. Ryde, N.S.W. 2113, Australia (Received September
12, 1984; accepted in revised form March 15, 1985)
Abstract-Biomarkets produced by microscale pyrolysis of extracted source rocks, kerogens and asphaltenes have been analysed directly by gas chromatography-mass spectrometry in the MID mode. A series of experiments have been undertaken. These include an investigation into the production of the hopanes at different pyrolysis temperatures and a comparison of results obtained from Curie point pyrolysis and the Chemical Data Systems pyroprobe. Steranes and triterpanes produced by pyrolysis of a series of vitrinites were investigated and correlated with maturity variations. Finally, the biomarkers produced from asphaltenes were compared with those produced from the extract and extracted rock in order to further investigate the theory that asphaltenes are a liquid kerogen. The results of these experiments showed that the biomarkers released by pyrolysis of source rocks have a potential use for source and maturity determinations. The method is advantageous in that it can be said with some certainty that the biomarkers are indigenous to the rock and not present in the rock as a possible result of migration. INTRODUCI’ION IN A PREVIOUS paper we reported preliminary reSukS from the pyrolysis-gas chromatography-mass spectrometry-multiple ion detection (py-MID) characterization of petroleum source rocks and suspected source rocks (PHILP and GILBERT, 1985). The emphasis in that study was placed on using mass spectrometry and multiple ion detection (MID) techniques to determine the distribution of biomarkers, in particular triterpanes, produced by pyrolysis of source rocks. The main reasons for initiating the study were: (a) to determine whether or not biomarkers released by pyrolysis of source rocks could actually be detected in a one-step microscale py-GC-MS analysis; (b) if so, were the results comparable with those obtained in early studies by SEIEERT (1978) and SEIEERT and MOLD~WAN (1980). The studies of the Chevron group comprised a separate large scale pyrolysis step followed by collection and fractionation of the pyrolysis products prior to GC-MS and MID analysis of the aliphatic fraction to determine the biomarker distribution. The biomarker distributions were then used for distinguishing suspected source rock strata and obtaining information on the oil-generating potential of a rock. The multistep procedure is time consuming and the advantages of combining pyrolysis and analytical procedures for the characterization of a large number of source rock samples are obvious. Apart from our earlier paper (PHILP and GILBERT, 1984) the only other paper to have reported, in any detail, on the distribution of hopanes producd by direct py-GC-MS analysis appears to be that of GALLEGOS (1978) in which he described the characterization of five U.S. coals using this technique. An earlier paper by GALLEGOS (1975) also mentioned the production of stemnes and triterpanes by pyrolysis from the Green River Shale.
Here we report results obtained from an extension of our preliminary study. This extended study includes the characterization of a wider range of samples as well as the determination of both steranes and hopanes produced by pyrolysis. In addition, a comparison has been made between results obtained with a Curie point pyrolysis system and a Chemical Data Systems (CDS) pyroprobe. This comparison was undertaken as a result of the necessity to characterize rocks with low organic carbon content. Such characterizations are more readily undertaken by using larger sample sizes with the pyroprobe. The biomarker distributions produced at different pyrolysis temperatures have also been examined. A series of vitrinite concentrates have been analyzed by py-GC-MS-MID (py-MID) to determine variations obtained in the biomarker distributions with changes in vitrinite reflectance or maturity. The results obtained for the hopanes closely parallel those previously reported by GALLEGOS (1978). The sterane distributions change in a manner that could be predicted from changes previously observed in the extractable steranes (MACKENZIE et al., 1982). The results indicate that the technique has potential for source and maturity determinations. It eliminates some of the questions associated with using the extractable fractions, namely whether the fraction is indigenous to the rock or merely contamination from migrating oil. Finally, the asphaltene fractions from a number of source rocks have been investigated by the py-MID technique. The sterane and triterpane distributions produced have been compared with those obtained from solvent extracts and by py-MID of the extracted source rock. BANDURSKI (1982) previously proposed that asphaltenes may be fragments of the original kerogen from which crude oil is derived and may be expelled as part of the crude oil. However little attention has been paid to the distribution of the
1421
R. P. Philp and T. D. Gilbert
1422
biomarkers produced by pyrolysis of asphaltenes. Comparison of the same with those produced by pyrolysis of the source rock and those present in the extracts should provide additional information on the role of asphaltenes in petroleum formation. The results obtained in this way introduce the possibility of using the asphaltene fraction. rather than the source rocks. for makiqg maturity measurements. This would also have the advantage of eliminating any possibility of mineral matrix effects during pyrolysis. Differences in the distributions of hopanes and hopenes produced by py-MID of the asphaltenes compared with the extracted source rocks may also provide important clues to the way in which the hopane precursor, possibly the bacteriohopanetetrol (OURISSON et al., 1984) is attached to the kerogen. EXPERIMENTAL The samples used in this study were extracted by refluxing with chloroform-methanol (I: I) for 48 h prior to pyrolysis using either Curie point pyrolysis or the CDS pyroprobe system. Curie point pyrolysis was performed at 610°C for I5 sets and the pyrolysis products were continually flushed into the injector system of the GC or GC-MS. For CC analysis a HP 5710 gas chromatograph was used with a splitless injector system. The oven temperature was held at 0°C during the pyrolysis and after 2 mins programmed at 4”/min to 300°C. The fused silica WCOT column (50 m X 0.2 mm i.d.) was coated with a BPI phase (SGE Australia). The mass spectrometer was operated with a filament current of 0.25 mA, electron energy 70 eV, and ion source temperature of 230°C. In the full scan mode (py-GC-MS) the scan speed was 1.5 set/scan and the MID mode (py-MID) the dwell time on the ions monitored, normally m/z 191 and m/z 217, was 0.1 sets. Similar conditions were used with the CDS pyroprobe system except the upper temperature was set at 800°C. RESULTS AND DISCUSSION E&ects of variations temperature p.vrolvsis
in Curie poinl
and comparison
and P.vroprohe
p_vrol~vsi.s
twtwecn
pyro1.v.h
Curie
point
results
In our preliminary study (PHILP and GILBERT. no systematic attempt was made to investigate the effects of variations in pyrolysis temperatures on the distribution of biomarkers. Furthermore it was of interest to ascertain whether the biomarkers produced were formed as a result of pyrolysis and not merely released from “traps” or occlusions in the kerogen or associated mineral matrix. as a result of increased temperature. Information of relevance to these two questions was obtained in this study from an examination of an organic rich sample at four different Curie point temperatures, namely 358°C. 480°C. 610” and 770°C. The particular sample selected for this experiment had a very characteristic hydrocarbon extract dominated by the 22R and S epimers of the CX, homohopane. Py-MID of this unextracted sample at 358°C produced the pyrogram in Fig. la which was dominated by this pair of C,,homohopane epimers and virtually identical to the distribution of hopanes observed in the hydrocarbon 1984)
fraction of the soluble extract. (Peak identities for components in all the chromatograms shown in this paper are listed in Table I.) It is clear from Fig. I h that at 480°C some pyrolysis of the sample had occurred since a number of peaks identified as hopenes, from full scan studies. are present in the chromatogram. Changes in the distribution of the saturated hopanes relative to the Cz,-homohopanc epimers can be observed in Fig. I b compared to Fig la. In this sample the C,, epimers can be thought of an indicators of the relative contribution from the soluble fraction since py-MID of the kerogen produces only minor quantities of the CJI-epimers (see below and Fig. 2). As the pyrolysis temperature is increased to 610” and 77O’C the major change observed in the pyrograms (Figs. Ic and Id) is the additional increase in the relative concentrations of the pyrolyticallyproduced hopanes and hopenes relative to the C‘,I homohopane epimers. Minor changes in the ratios of various hopanelhopene pairs can also be observed when these pyrograms (Figs. 1c and Id) are compared with Fig. I h. For each carbon number pair. i.c~ C?;. C29 and Cv,, the concentration of the hopenc (peaks 2. 9 and I I. respectively) has increased relative to the corresponding hopane (peaks 3. 6. and IO. respectively). Since hopanes and hopenes are produced by pyrolysis of the kerogen (see Fig. 2) it is proposed that this change in relative concentration reflects a faster rate of production for the hopenes at the higher temperatures. The main effect of increasing the pyrolysis temperature appears to be an increase in the yield of the pyrolysis products. It appears unlikely that the biomarkers are simply released from traps in the kerogen, or mineral matrices, as a result of increased pyrolysis temperatures. If this was the case it is unlikely that hopenes would be detected since no hopenes were found in the soluble extract from this sample. Biomarker distributions produced by pyrolysis ot the kerogen isolated from the sample used in the previous experiment are summarized in Fig. 2. No hopanes were produced from the kerogen at a pyrolysis temperature of 358°C but at 480°C. 610°C and 770°C the pyrolytically produced hopanes and hopenes were virtually identical (Fig. 2). Pyrolysis appeared to be virtually complete by 610°C and the 77O’C pyrogram is not shown in Fig. 2. The major difference between the pyrograms is quantitative rather than qualitative. Although no accurate measurements were made, comparison of signal intensities from the three analyses suggested a two-fold increase in yield of hopanes and hopenes at 770°C over that at 480°C assuming starting sample sizes were approximately equal. A similar observation was made with the unextracted sample, where yields could be gauged against the relative intensity of the C,, homohopane epimers. A comparison between results obtained with the CDS pyroprobe and the Curie point pyrolysis system was made with the unextracted rock and kcrogen
Source
rock and asphalkne
biomarkers
4
6
-“----I 4
R. P. Philp and T. D. Gilbert
1424 Table
I.
Identification
of by
Produced
Triterpanes
_ Peak
M/z
Number
and
Steranes
py-MID _____
..__ _
_ ..__.--___
Identification
191
(Triterpanes) 18C%(H)-Trisnorhopane 2
c
3
~~~~~~~~~~~~*=h~p~*~ 176(H)-Trisnorhopane
4
Unidentified
5 6
17cr(H)-Norhopane
7
Unidentified
8
17B(H),2la(H)-Normoretane
9
c
-H0pene I?&H),ZlBfH)-Hopane C -Hopene i~f3(R,,2lu(H)-Moretane
10 11 12
17R(H),ZtBfH)-Narhopaoe
I3
17a(H)
14 15 16
?
and
S epimers)
GalWWKI?rXle 17nfH),218(H)-Bishomohopanes
18
M/z
,2 I@(H)-Homohopanes
178(H),21B(H)-Hopane
17 I9
(22R
3
f22R ___~___
and
S epimers) ____-A.“..._----
217
(Steranes) I’
14a(H),I7a(H)-cholestane
(20s)
2’
Iba(H),17a(H)-cholestane
(20R)
3’
24-Methyl-I4a(H),17a(H)-cholestane
(20s)
4’
24-Methyl-f4~(Hf,l7~fHf-cholestane
(ZOS+ZOR)
or
24-Methyl-5B(H),14c((H)17u(H)-cholestane
5’
%4-Methyl-fSn(Hf,l7afH)-cholestane
6’
24-Ethyl-
7’
17acHf-cholestane
24-Ethyl-148(H),l78(H)-cholestane or
a'
14afHf,
(2OR) (205) (20S+20R)
24-Ethyl-5B(H),l4a(H),l7cr(H)-cholestane
24-Ethyl-i4a(H),l7a(H)-cholestane
used in the experiments above. It was necessary to make this comparison since it was anticipated that many of the samples to be examined in this and future studies would have relatively low values of organic carbon. Use of the CDS pyroprobc enables larger samples to be pyrolyzed than the Curie point pyrolysis system. The best operating conditions for the pyroprobe were found to have a final pyrolysis temperature setting of 800°C. The distributions of hopanes and hopenes produced at this temperature were virtually identical, from a qualitative point of view to those produced by Curie point pyrolysis at 610°C. Similar observations were made with both the unextracted rock and the kerogen. Despite the uncertainty concerning the final and actual pyrolysis temperature the remainder of the analyses described in this study was undertaken using the pyroprobe. The final temperature was always set at 800°C and then repr~ucible results were obtained. In most samples it was possible to determine the distributions of both hopanes and steranes formed during the pyrolysis of extracted rocks and kerogens. In our previous paper only the hopane distributions were reported but examination of the steranes provides additional info~ation on source and maturity of the organic material in a sample. In the following section,
(20R)
the application of the py-MID to a series of vitrinitt: concentrates is described as a means of ohtaining source information and observing variation in biomarker distributions with increasing maturit?
MACKENZIE et cl/. (1981. 1983) and SEIW,R I and MOLDOWAN (1980. 1981) have previously observed and discussed a number of trends in the biomarker dist~butions produced by pyrolysis that correlate with changes in maturity. In order tit further investigate these observations over a wider range uf maturities and to eliminate the problem of source vatiations, a series of vitrinite concentrates of increasing maturity were examined by py-MID and variations in sterane and triterpane distributions determined. GALLEGOS (t978) briefly commented on variations in the hopane distributions produced by pyrolysis ot five U.S. coals but did not discuss them in any detail, and no mention was made of variations observed in the sterane distributions. The vitrinites used in the present study were hand picked concentrates From Sydney Basin (AustmIia) coals and have bcen described in more detail elsewhere (EVANS ri cd I984).
1425
Source rock and asphaltene biomarkers EXTRACTED
KEROGEN
6
480”
C
610°C
FIG. 2. Curie point pyrolysis of kerogen isolated from same sample as used for Fig. 1. Chromatograms obtained by SIM of ion at m/s 191. Peak identifications given in Table I.
A summary of the hopane distributions produced by py-MID are shown in Fig. 3. The sample with R. = 0.3 is excluded since the hopane distribution is virtually identical to that of the sample with R. = 0.39. The pyrograms in Fig. 3 show a number of variations with increasing maturity. The main features that can be summarized are a decrease in intensity of the Car-hopene (peak 2) relative to the 17a(H)trisnorhopane (3) along with a noticeable decrease in the 17@H)-C&risnorhopane (4) relative to the 17a(H)-trisnorhopane (3). The concentration of the Caa-hopane (10) increases relative to the Czg-norhopane (6) and the CZ9 and CjO hopenes (9,11) decrease relative to their saturated counterparts (6,lO) with increasing maturity. The overall concentration of the biomarkers produced from the vitrinite concentrate
with R. = 0.93 was low. The resultant chromatogram has a high noise level and contains a number of additional peaks that are not hopanes and are not visible in the other normalized chromatograms. At a reflectance of R,, = 1.2 the pyrolytically produced hopanes were virtually absent. As the maturity level increases there is also a decrease in the relative moretane concentrations and a gradual increase in the relative concentration of the C3,-22R and Shomohopanes at the expense of the 17/3(H),2lp(H)homohopane. At the same time the concentration of the 22s epimer (14) increases at the expense of the 22R epimer (15). These latter effects are similar to those previously reported by SEIFERT and MOLDOWAN (1980, 198 1) although their separate pyrolysis step did apparently eliminate the production of hopenes. This in turn could reflect differences arising from the
R. P. Philp and T. D. Gilbert
1426 VITRINITE M/f
CONCENTRATES
191
Ro 0.39
b i
8 Ii
Ro 0.5
Ro 0.69
FIG. 3, Variation in hopane/hopene distributions MID from six vitrinite concentrates isolated from given in Table 1.
longer pyrolysis times and slower heating rates in their study. However, the results from this study demonstrate that the one step method can be used to characterize the biomarkers produced pyrol~ically from source rocks. The dist~butions obtained can be used to observe and monitor maturity differences. One possible disadvantage is production of the hopenes which were not observed in the earlier work of SEIFERT and MOLD~WAN (1978) or SEIFERT and MOLDOWAN ( 1980). Examination of the steranes also revealed a number of variations that could be correlated with increasing maturity (Fig. 4). The results are simplified somewhat by the fact that the pyrolysis products are dominated by Cz9 steranes. At the lowest level of reflectance (& = 0.3 and 0.39) only two steranes are produced, namely the &Y(H), 14a(H), 17cu(H)-20R-CZ9 sterane (8’) and the 5&H), I4cu(H), 17cufH)-Czs sterane (7’). (These identifications are based on comparison of
Ro 1.20
(m/z 191) with increasing maturity produced bk I?Sydney Basin (Australia) coals. Peak identiticatiw~
retention times with a sterane fraction from an immature oil shale where the only components present have been identified as the same two isomers.) At a reflectance value of 0.5 the Sa(H). 14cu(H), 17n(,tii2OS-C2+ epimer (6‘) appears although it is still present in lower concentrations than the 58 isomer. ht K,, = 0.69 there is a further increase in the concentration of the SLY(H). 14ar(H), 17cr(H)-20S-CZo epimcr relative to the .5@-isomer. A significant change is observed between the chromatogram of R0 := 0.69 and X,, = 0.93. At RQ = 0.69 it is assumed, with a high degree of confidence that peak 7’ represents only the S/‘ZLC~~ sterane. However at R0 = 0.93 it is clear that the 14@(H), 17@(H)-20R and 20s epimers are now predominant in that region of the chromatogram. It was not possible to ascertain whether or not the 58Ci9 sterane was still present in the pyrolysis products. The remaining vitrinite samples with R. =r~ I.Oland R0 = 1.20 show only minor variations compared with
1427
Source rock and asphaltene biomarkers
VITRINITE
CONCENTRATES
M/Z 217 6’
Ro 0.93
a’
7’
6’
8’
Ro 1.01
Ro 0.5
8’ I Ro 0.69
Ro
1.20
6’
FIG. 4. Variations in sterane distribution (m/z 217) produced from the six vitrinite used for Fig. 3, by py-MID. Peak identifications given in Table I.
R. = 0.93. It should be noted that rearranged
steranes are not produced during pyrolysis as previously observed by SEIFERT (1978). This observation supports the claim that the steranes are indeed pyrolysis products and not simply products released from occlusions in the vitrinite upon heating. Examination of the soluble fractions from these vitrinite concentrates show the presence of both rearranged and regular steranes. The changes in the sterane distributions observed here are similar to those that would be expected in the sterane distribution of the soluble extract apart from the absence of rearranged steranes. In other words as the maturity increases, so do the relative concentrations of the 14a(H), 17a(H)-CZs20s sterane and the 14&H), 17P(H)-C2,-20S+20R steranes (MACKENZIE et cd., 1982). The sterane distributions produced by this py-MID technique can be used. therefore, for making maturity determinations in the same way as the extractable steranes, but again
concentrates.
also
without the problem of trying to determine whether or not they are actually indigenous to the sample as is sometimes the problem with the extractable material. Furthermore the steranes distributions obtained in this way are also useful as source indicators.
The results described above. and similar results previously described by SEIFERT (1978), raise a number of interesting questions and suggest further areas to be investigated. These include the need to determine why such biomarker distributions are observed, how are the biomarkers attached to the kerogen and what is the nature of the precursor molecules? One approach to obtain partial answers to some of these questions is to examine the asphaltene fractions isolated from source rock extracts. Previous work by BANDURSKI (1982) has indicated that the oil gener-
R. P. Philp and T. D. Gilbert
1428
ating portions of kerogens and petroleum asphaltenes may have similar structures and the asphaltenes may be expelled as part of the crude oil. In this study asphaltenes have been isolated from four Chinese shales, previously described by JI-YANG rl ~11.( 1982). and two potential source rocks from the Cooper Basin, Australia. The asphaltene fractions have been characterized by py-MID and the resulting biomarker distributions compared with those produced from the corresponding kerogens and soluble extract. The hopane distributions produced and determined by py-MID for the soluble extract with asphaltenes removed (i), the asphaltenes (ii) and the extracted rock (iii) from two samples of suspected source rocks from the Cooper Basin, Australia, are shown in Figs. 5a and b respectively. The hopanes in the py-MID chromatograms of the deasphaltened soluble extracts are virtually identical to those obtained by GC-MS analysis of the aliphatic fractions for the same samples.
COOPER
BASIN
This observation is not surprising since the chromatograms shown in Figs. 5a(i) and b(i) represent hopanes determined in the total soluble extract by thermal distillation. The presence of small amounts of the C2,-hopene (peak 2 in 5a(i) and Qi)) may result from incomplete removal of asphaltene materiai from the soluble extract. The asphaltene fractions from the two samples produce a significant number of hopenes by py-MID as shown in Figs. Sa(ii) and h(ii). The relative concentrations of the saturated hopanes (i.e. peaks 1,3,4,6,8,10,12,14,15) in the asphaltene fraction are virtually identical to those observed in the py-MID chromatogram of the extracted rock (Figs. Sa(iii) and Qiii)). This observation supports BANDURSKI’S (1982) theory that there are structural similarities between the kerogen and asphaltene fraction. It is of interest to note a number of variations II-I relative distributions of the hopanes and hopenes
(AUSTRALIA)
(a)
M/Z
)
191
SOLUBLE i4
FRACTION
( ii
ROCKS_
(b)
M/Z 191
(i)
SOURCE
ASPHALTENES
(i)
(ii)
FRACTION
ASPHALTENES
c
(i ii)
EXTRACTED
3
ROCK
FIG. 5. Distribution of hopanes/hopenes (m/z 191) produced by py-MID from the deasphaltened soluble extract (i), asphaltene (ii) and extracted rock samples (iii) of two suspected source rocks (a and 0) from the Cooper Basin, Australia. See Table I for peak identifications.
Source rock and asphaltene between the soluble extract, the asphaltenes and extracted rocks. The hopanes in the soluble extract (Figs. 5a(i) and b(i)) are dominated by the C30-hopane whereas the asphaltenes and extracted rocks produce chromatograms dominated by the CZ7 and &-hopanes. It can also be observed that the ratio of the C3,-homohopane 22s and R epimers (peaks 14 and 15 respectively) for the soluble extract (Figs. 5a(i) and b(i)) is >l but
COOPER
(a)
M/Z
BASIN
ates, involved in the formation of hopanes in the asphaltenes compared to the extracted rocks. These may be functionalized precursors such as alcohols, acids or bacteriohopanetetrol which produce the hopenes upon pyrolysis. It is unlikely that the hopenes themselves are present in the asphaltenes since they would be removed in the soluble fraction during isolation of the asphaltene fraction. Steranes produced by py-MID of the same samples are shown in Figs. 6a and b, respectively. A number of observations can be made from these chromatograms. First the sterane chromatogmms for the soluble extracts (Figs. 6a(i) and 6b(i)) contain both regular and rearranged steranes, the asphaltenes and extracted rock chromatograms do not. This is similar to the observation previously made by SEIFERT (1978) that only regular and not rearranged steranes are produced during pyrolysis. The ratio of the 20S/20R-CZ9 steranes (peak 6’/peak 8’) in the soluble extracts indicates that
(AUSTRALIA)
SOURCE
(b)
217
1429
biomarkers
ROCKS
M/Z
217 a 6' 7'
(ikOLUl3LE FRACTION
SOLUBLE
ASPHALTENES
3,
i ii)
EXTRACTED
)
FRACTION
6’ ,,
ASPHALTENES
(ii)
(i
II
6'
n
EXTRACTED
FIG. 6. Sterane distributions (m/z 217) produced by py-MID from the deasphaltened soluble extract (i), asphaltene (ii) and extracted source rock sample (iii) of two suspected source rocks (a and b) from the Cooper Basin, Australia. See Table 1 for peak identification.
cii)
R. P. Philp and T. D. Gilbert
1430
sample a (Fig. 6a(i)) is less mature than sample b (Fig. 6b(i)). A similar conclusion can be reached if these ratios are measured using the 20S/20R-C,, sterane ratios from the asphaltene and extracted rock pyrograms (Figs. 6a(u-m) and 6b(u-111) respectively). However for both rocks the value for this ratio is higher in the extract than it is in the asphaltene OI extracted rock. A similar investigation has been performed using the asphaltenes isolated from four Chinese shale samples previously described by JI-YANG cl al.(1982). Figure 7a(i-iii) comprises the hopanes from the soluble fraction with asphaltenes removed, the asphaltenes and the extracted rock produced by py-MID of the Chinese shale sample, referred to as Yi-21 in the original paper (JI-YANG et u/.. 1982). As with the Cooper Basin samples described above there is a higher concentration of hopenes in the asphaltene fraction than the pyrogram of the extracted rock.
CHINESE ____-.
(a)
M/Z
191
10
SHALE
lb
However, sample Yi-21 has been shown by JI-~'ANC, et ul. (1982) to have a high concentration of gammacerane (peak 16) in the hydrocarbon fraction and py-MID of the soluble extract shows a similar result (Fig. 7a(i)). Py-MID also shows the absence of gammacerane from the products of the extracted rock (Fig. 7a(iii)) but its presence in the pyrolysis products of the asphaltene fraction (Fig. 7a(ii)). .4dditional purification of the asphaltene fraction showed that the gammacerane was not present as the free hydrocarbon in the asphaltene fraction. This raises the question of whether the gammacerane was ever attached to the kerogen and subsequently released 01 is it present as the more polar precursor tetrahymanol in the asphaltene fraction and never bonded to the kerogen.
Additional
studies are required
SAMPLE-Yi-21
(b)M/Z
2’
i (i)
SOLUBLE
b
FRACTION
I
to answer
The absence of gammacerane from the pyrolysate extends the range of biomarkers present in the extract but not the pyrolysate. Previous studies
these questions.
i
I [
(i)
217
SOLUBLE FRACTION
1~
1
(j_i)ASPHALTENES
(iii)
EXTRACTED
i j
i il)
ASPHALTENES
i' ( i ii) EXTRACTED
I
ROCK
FIG. 7. Distribution of (a) hopanes/hopenes (m/z 191) and (b) steranes (m/z 217) produced by py-Mil) from the deasphaltened soluble extract (i), asphaltene (ii), and extracted rock (iii) of a Chinese shale sample (Yi-21). The sample contains a relatively high concentration of gammacerane ( 16) and has hecn previously described by JbY4NG TV a/. (1982). Peak identifications are given in Table I
1431
Source rock and asphaltene biomarkers have noted this phenomenon with the diasteranes (SEIFERT, 1978), monoaromatized steroids (MACKENZIE et al., 1981) and 28,30-bisnorhopane (MOLDOWAN rt al., 1984). The sterane distributions produced by py-MID for the soluble extract, the asphahene. and extracted rock for the same sample are similar (Fig. 7b(i-iii)) to those published by JI-YANG Ed al. (1982) for the extractable steranes present in this shale. The results for the sterane dist~butions provide another example to show that the source and maturity information obtainable from the py-MID results of the asphaltenes and extracted rock is similar to that obtained from the soluble extract. The results from a second Chinese shale sample are shown in Fig. 8 to illustrate that in shale samples of fairly low maturity there is quite a strong similarity in the pyMID dist~butions for the hopanes and steranes of the extracted rock and asphahene fractions. The extracted rock and asphaltene both show high con-
CHINESE (a)
M/Z
centrations of the Cl,-hopene (peak 2) and 178(H)trisnorphopane (4) and similar hopane/moretane ratios (Fig. 8a(ii and iii)). This leads to the suggestion that as the maturity level increases the precursors responsible for the hopenes are cleaved and transferred to the asphaltene fraction possibly still functionalized to some degree. Thus at higher levels of maturity the extracted rock produces no hopenes upon pyrolysis whereas the asphaltene fraction still has the potential to produce hopenes. it is also of interest to compare the differences in the sterane distributions for the asphaltenes and extracted rock for the two samples .. . . . shown in Fig. 7b(11-111)and Fig. Sb(~~-~~~). The sample used for Fig. 7 shows relatively high concentrations of the C+& steranes whereas that used for Fig. 8 produces only the C2, steranes. These differences are related to source differences and agree with the results previously published by J&YANG ( 1982) for the soluble extracts. However, the results demonstrate the poten-
SHALE
(LOW RANK)
10
191
(b)M/Z
2’
217
6 SOLUBLE (if
FRACTION
(i)
SOLUBLE FRACTION
4
(ii)
iii)
ASPHALTENES
EXTRACTED
1’
ASPHALTENES
EXTRACTED
ROCK
8 9
FIG. 8. Dist~bution of (a) ho~nes/ho~nes (m/z 19 1) and (b) steranes (m/z 2 17) producedby m-MID from the deasphaltened soluble extract (i), asphaltene {ii) and extracted rock (iii) of a low rank Chinese shale previously described by JI-YANG et a/. (I 983). Peak identifications given in Table 1.
(ii)
(ii
i)
R. P. Philp and T. D. Gilbert
1432
tial of using the sterane distributions from the asphaltene and extracted rock to obtain source information on a sample. No attempt has been made in this paper to discuss the nature of the linkages by which the hopanes and steranes are bonded to the kerogen. This forms the basis of a separate study. but the possibility of a similarity in the type of linkages between these compounds and the kerogens, with those found in the long chain isoprenoids and archaebacteria cannot bc ignored. In summary it would appear that the products from py-MID of the asphaltene fractions of shales and source rocks show many similarities to those obtained from the extracted rock samples. This would support the idea previously put forward by BANIXJKSKI ( 1982) that the asphaltene fraction can be thought of as a liquid or mobile kerogen fraction. Furthermore. it introduces the possibility of using the asphaltenc fraction rather than the extracted roch in py-MID studies to obtain source and maturity information. This would eliminate any problems associated with mineral matrix effects during pyrolysis and would also make it easier to increase the sample size and yield of biomarkers during the py-MID study. It does have the disadvantage that the asphaltene fraction may not be indigenous to the suspected source rock but may represent migrated hydrocarbons. Future studies will be directed at determining how the hopene concentrations vary in the asphaltene fraction and to see whether this is related to maturity or the nature of the precursors. CONCLUSIONS This paper has demonstrated the potential value of using py-MID as a one step method for charactcrization of biomarkers in source rocks. It has shown that the results obtained are similar to those obtained by the processes involving a separate pyrolysis step, followed by time-consuming fractionation steps. In addition it has demonstrated that the steranes produced can be used both as source and maturit! indicators in the same way as the extractable fraction but with the advantage of knowing that they represent the indigenous organic material in the rock. Py-MID of a series of vitrinite concentrates has illustrated the variations in hopane and sterane distributions with maturity changes. Particularly significant were the changes in hopene and 17/j(H)-trisnorhopane concentrations. Changes in stcrane isomers were similar to those previously observed in extractable fractions. Comparison of pyrolysis products from asphaltene fractions and extracted rocks has provided further insight into the possible nature of asphaltene as an intermediate in the production of crude oil from kerogen. It has also provided evidence to suggest that certain biomarkers. or their precursors. such as gammacerane may not be bonded to the kerogen. Further studies must now be directed at obtaining information on the way in which the hopane and
sterane precursors are linked to the kerogen. The dist~butions of the hopanes produced bb pyrolysis differ from those in the total extracts suggesting additional catalytic reactions occur to the hopancs after their in-.sift formation. Py-MID has not hren extensively applied to biomarker characterization previous to this paper, but it has now been shown that a great deal of information can be obtained II? this one step process. .Iclino,~/~~~rtnc,n(-We would likr LOthank DI-. %I Ji-1’ang for the Chinese shale samples.
REFERENCES BANDUKSKIE. (1982) Structural similanties between oilgenerating kerogens and petroleum asphalrenc\. Ettcvu~ Sonnrs 6( l/2), 47-66. EVANS E. J.. BATTSB. D. and SMrrH J. W. (lYX4) Determination of the prospectivity of sediments h\ hydrogenation. 4PE.4 .1. 23, 83-91. CiALLEGos E. J. ( 1975) Terpane-sterdne release from kerogen by pyrolysis gas chromalography-mass spcctrctmct;?,. ltrai. I%cm. 47, 1524-1528. I;AL t EGOSE. J. (1978) Analysis ol’tice U.S. coals. Pyroi\si+ gas chromatography-mass spectrometry-colnputrr and thermal gravimetry-mass spectometry-computer method%. In il.f‘S ,4&s. rn C%em .G+\. i7lf (eds. i’ f‘ Lirx pS. SIGGIAand H. G. JENSLN).pp. 3-30. Jr-YANG S.. MACKENZIEA. S.. AI I-SAN~~K R ~:c~Ii’~tox G.. GOWAK A. P., Wol.r-r. G. .4. and MA,XWCI.L J. K. ( 1982) A biological marker Investigation &’ petroleum and shales from the Shengli olllield. The Peoples Repubhc of China. C/lc,rn. fieol. 35, l-3:. MA~KENZIL,A. S., LEWISC. A. and MAVW~1L ,I Ii i iYXi! Molecular parameters of maturation in thr Ioarcian Shales. Paris Basin. France-IV. Laborator! thermal alteration studies. Geochitrt i “mtt~crcfritr~. lurr 45. l?hQ2376. MACKENZE A. S., HRASS~L.I s. c ., t:bLiwc,*. Ci. snd MAXWEU J. R. (I 982) Chemlcat fossils: The geological fate of steroids. Scienc,e 217, 49 I-504. MACKENZIEA. S., REN-WEIL.. MAxwtxt. J. R.. film.~w% 1.k J. M. and SEIFERTW. K. ( 1083)Molecular i~~l)venlen~s of thermal maturation of Cretaceous shale5 from the Overthrust Belt. Wyoming, U.S.A. In .~ldvutw~ it! Iltyyort~. Geochetni.str~ I YHI. (eds. M. BJORC)I’(‘I