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hopane ratios in crude oils derived from Tertiary source rocks
Org. Geochem. Vol. 9, No. 6, pp. 293-304, 1986 Printed in Great Britain
0146-6380/86 $3.00+ 0.00 Pergamon Journals Ltd
Sterane isomerisation and moretanelhopane ratios in crude oils derived from Tertiary source rocks P. J. GRANTHAM Koninklijke/Shell Exploratie en Produktie Laboratorium, Postbus 60, 2280 AB Rijswijk (Z.H.), The Netherlands (Received 22 October 1985; accepted 26 March 1986) Abstract--The extent of sterane isomerisation reactions and the moretane/hopane ratios of 234 crude oils, taken world wide, from a wide variety of source rocks of differing geological ages, have been measured. This data indicates that in 78 crude oils derived from Tertiary source rocks, sterane isomerisation reactions as determined by the 20S/(20S + 20R) ratio of the C~ 5~(H), 14~(H), 17ct(H) normal-steranes and the C29 iso/(iso + normal) ratio [iso = 5~(H), 14fl(H), 17fl(H)] are mainly incomplete and sometimes considerably so. In addition, the same crude oils have 17fl(H), 21~t(H)-moretane/17~(H),21fl(H)-hopane ratios which are significantly greater (predominantly in the range 0.10-0.30) than those of crude oils derived from older, mature source rocks (mainly less than 0.1). This data, for crude oils, lends support to the hypothesis, proposed by Mackenzie and McKenzie (1983) for source rock extracts, that the time/temperature constraints of sterane isomerisation reactions are such that the time available for isomerisation in Tertiary sediments is generally insufficient,despite generation of crude oil at relatively high temperatures. An alternative hypothesis is that the incomplete sterane isomerisation of Tertiary crude oils may be due to generation of these crude oils from their deltaic, land plant-containing source rocks under low heating conditions. A third hypothesis proposes that the Tertiary crude oils may have picked up the incompletelyisomerised steranes from immature sediments during migration. Although possible in particular instances, such a mechanism does not appear to be generally applicable since, in that case, the phenomenon would then appear to be restricted to the Tertiary. The higher moretane/hopane ratios of the Tertiary crude oils could suggest that constraints, similar to those applying in sterane isomerisation, also operate in the conversion of moretane to 17ct(H)-hopane. Key words: steranes, moretane, hopane, Tertiary, Cretaceous, time of oil generation, isomerisation, crude oil, source rocks
INTRODUCTION The search for reliable geochemical tools for the determination of the maturity of a source rock or crude oil is a major concern of geochemistry. In recent years an increasing number of studies on the effects of maturity on steranes and triterpanes have been described. In these publications the isomerisation of steranes and the reactions determining the relative concentrations of moretanes and hopanes have received much attention. Two main sterane isomerisation reactions have been described (Mackenzie et al., 1980, 1981, 1982a, b, 1983, 1984; McKenzie et al., 1983; Mackenzie and McKenzie, 1983). The first is the isomerisation of "normal" 5ct(H), 14~(H), 17~t(H)-20R steranes which occur as the most prominent steranes of very immature sediments (see Fig. 1 for structures). As the maturity increases, isomerisation at the chiral carbon-20 takes place and, compared to the 20R isomer, the 20S isomer increases in concentration. This increase continues until an equilibrium condition is reached [C~ 20S/(20S + 2 0 R ) ~ 0.55] after which the relative isomer concentrations remain the same even though source rock maturity may continue to increase. O . G 9/6-- B
The second sterane isomerisation reaction concerns the increase in 5~t(H), 14fl(H), 17fl(H) "iso" steranes relative to the "normal" steranes as source rock maturity increases (Fig. 1). Again, isomerisation proceeds until an equilibrium condition is reached [C29 iso/(iso + normal) ~ 0.75] whereby the iso-steranes are more stable than the normal steranes. Beyond equilibrium no further change is reported to take place even though, again, the maturity of the source rock may continue to increase. Moreover, the relative stabilities of the isomers of the normal steranes, and the stabilities of the iso- with respect to the normal steranes, observed in nature, are in agreement with their calculated thermodynamic stabilities using molecular mechanics (van Graas et al., 1982). Time appears to play an important role in sterane isomerisation reactions. Mackenzie and McKenzie (1983) and McKenzie et al. (1983) have published data which indicates that: (a) Sterane aromatisation is strongly dependent on source rock temperature and has a higher activation energy compared to sterane isomerisation. (b) sterane isomerisation requires time to proceed to equilibrium (there is a slow rate determining step), despite high temperatures.
293
294
P.J. GRANTHAM
S H
H
,5=(H), 14= (H), t7==(HJ - 20 _R 24-ETHYL ' N O R M A l : CHOLESTANE
~)a( H),t4(z ( H ), t7a( H ) - L:'O~ 24-ETHYL ' N O R M A l ' CHOLESTANE
F O R M A T I O N OF 2OS ' N O R M A L ~S T E R A N E S
S
i
H
5(=(H),~I~(H),I7~(I-I)-201~_ AND20~} 24-ETHYL'1SO' CHOLESTANES
5a(H),I4a(H),ITa(H)-20R AND 2OS 24.ETHYL"NOBMA[ CHOLESTANES FORMATION OF'ISO'
STERANES
,,H
i7¢=(H),2t/~(H)- HOPANE
i?/3(H),21(=(H)-MORETANE
Fig. 1. Structures of the biomarkers discussed in this report.
Thus, in young Tertiary basins, which have been rapidly buried and have rapidly reached a high temperature, sterane aromatisation has proceeded faster than sterane isomerisation, which remains incomplete. In older sediments which have never reached a high temperature, sterane isomerisation is more advanced than sterane aromatisation. The differences in the rates of steroid isomerisation and aromatisation reactions have been demonstrated by the use of aromatisation/isomerisation (AI) diagrams which plot the ratios of triaromatic/ (triaromatic + monoaromatic) steroid hydrocarbons against the ratio of 20S/(20S + 20R) C29 normal steranes (Mackenzie and McKenzie, 1983). Another reaction which has received attention as a possible maturity tool is the relative concentrations of 17#(H),21a(H) moretanes and 17ct(H),21#(H) hopanes (Seifert and Moldowan, 1980~see Fig. 1 for structures). These workers found that in extracts of low mature source rocks (apparently below % Ro levels of 0.62-0.69) there was no correlation of C30 moretane/hopane ratios (generally high, 0.25-0.60) with increasing source rock maturity. However, the ratio decreased in pyrolyzates of the same source rocks as the maturity became higher, and, in extracts of mature source rocks, and in crude oils, the ratio was low (<0.1).
Seifert and Moldowan (1980) account for the above phenomena by a reaction scheme which allows for moretane to hopane conversion at high (oil generation) temperatures. At low temperatures this conversion is not possible and, in extracts, moretane/ hopane ratios do not reflect maturity. It has been noted however by Mackenzie et al. (1980) that in extracts of sediments in the Paris Basin there does appear to be loss of moretane with respect to hopane as maturity increases. As part of our search for reliable maturity parameters amongst biomarkers, the preceding concepts of sterane isomerisation and moretane/hopane ratios have been tested on a large number of crude oils. This paper describes the results of this research with particular emphasis being placed on those crudes derived from Tertiary source rocks. EXPERIMENTAL
Sterane isomerisation reactions
The progress of sterane isomerisation reactions has been monitored by combined gas chromatographicmass spectrometric analysis (GC-MS) of saturated (branched/cyclic) fractions isolated from a crude oil or source rock extract; Use was made of multiple ion detection (MID) of selected fragments. The presence
Sterane isomerisation and moretane/hopane ratios in crude oils of steranes in a geological sample can be monitored using a variety of fragment ions (m/z 217, 218, 259, 232, 149, 151 plus the parent ions m/z 372, 386 and 400--see Seifert and Moldowan, 1979 and references therein). Published fragmentograms of sterane distributions are usually based on the base peak of the Set(H), 14~t(H), 17~t(H) normal steranes i.e. m/z 217 (see for example Mackenzie et al., 1981) whilst the reported ratio of the C:9 20S/(20S + 20R) steranes has been calculated from peak areas using this fragment ion (Mackenzie et al., 1981). However, the ratio of C29 iso/(iso + normal) steranes based on peak area measurement using m/z 217 would discriminate against the actual amount of iso steranes present since the latter have m/z 218 as base peak, not m/z 217 (Djerassi, 1978; Seifert and Moldowan, 1979). Therefore, we have calculated C29 20S/(20S + 20R) and C29 iso/(iso + normal) sterane ratios from peak area measurements based on a fragmentogram which sums the intensities of the fragment ions 217, 218 and 259 (the latter being a fragment ion characteristic of 13fl(H), 17ct(H)-rearranged or -diasterancs--see
100.0
Fig. 2 for examples). The aim has been to calculate relative sterane intensities, from one fragmentogram, which reflect as accurately as possible those in nature and those calculated by others (see results below). As a check on possible co-elution of the C29 steranes with other components in the Tertiary crude oils, three samples have also been analysed using linked scan combined gas chromatography-mass spectrometry (Warburton and Zumberge, 1983). This technique enables only those fragment ions (e.g. m/z 217) which result from the fragmentation of a given stcrane parent ion to be identified. These analyses confirmed the incomplete sterane isomerisation of the analysed samples (see Fig. 3) and indicated that co-elution of the C29 steranes with other compounds is not a serious problem.
Determination of moretane /hopane ratios The relative concentrations of moretane and hopane are monitored using the m/z 191 fragment ion, an approach used by other workers (see for
14
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INCREASING RETENTION TIME u
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v
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Fig. 2. Sterane fragmentogram (m/z 217 + 218 + 259) of a Tertiary source rock derived crude oil compared to that of a crude oil derived from a mature Cretaceous source rock. Gas chromatographic conditions: 22 m (x 0.32 mm i.d.) fused silica capillary column coated with CP Sil 5; helium flow rate 35 cm/sec; temperature programmed from 50-160°C at 20°C/rain, then from 160-290°C at 2°C/min. Isothermal at 290°C for 10 min.
296
P.J. GRANTHAM BARAM-8. SARAWAK
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291
20a_ 20a_ GaS chron~tOgtaphic
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conditions: 25 m {x 0.22 mm i.d3 fused =lica capi,aW column coated with CP Si~ S CB. Temp. p, Ov,~,,,,med from
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Z m/z 217 z 10000.
le0 to 300°C at
2°C/min. /sec.
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Fig. 3. Linked scan fragmentograms of metastable m/z 217 ions originating from parent ions m/z 400 (C29 steranes) in three crude oils from Tertiary source rocks in the Far East.
example Seifert and Moldowan, 1980; Hughes et al., 1985). This paper presents data which demonstrates that crude oils derived from Tertiary source rocks contain, relative to 17~t(H),21fl(H)-hopane, large concentrations of 17fl(H),21~t(H)-moretane. Since the large majority of the 78 Tertiary crude oils are from land-plant-containing deltaic environments it is theoretically possible for the large moretane peak to result from co-elution of moretane with other landplant-derived triterpanes. Kimble (1972) has determined the mass spectra and Kovats retention indices of a large number of triterpanes and her data indicate that the triterpanes most likely to co-elute with moretane are glutane and 17ct(H) moretane (Kovats retention indices: Dexsil 300, 280°C, glutane, 3321, 17~(H)-moretane, 3324, moretane, 3327; OV-101, 250°C, glutane, 3118, moretane, 3112; OV-101, 280°C, 17ct(H)-moretane, 3174, moretane, 3178). The phase used in our analyses, CP Sil-5, is a Silicone gum, similar to Dexsil 300 and OV-101 and similar retention behaviour is expected. However, there are many other cyclic hydrocarbons present in these Far Eastern crude oils, the retention data of which are not known, and these may also co-elute with moretane.
As a check on possible co-elution of any components with moretane, four of the Tertiary crude oils were analysed using cyclic scanning of all fragments (rn/z 60-600). In all cases the mass spectrum of the moretane component was in agreement with the published spectrum of moretane and indistinguishable from that of 17~t(H)-moretane (Kimble, 1972). It is therefore possible that 17~t(H)-moretane may also be present in these samples (but see the Results and Discussion below). "[he mass spectrum of glutane contains only a minor m/z 191 fragment ion and hence moretane/hopane determinations based on this ion will not be affected by possible glutane co-elution. Figure 4 shows the m/z 191 fragmentogram of a Tertiary crude oil compared to that of a mature Cretaceous crude oil. The 17~(H), 21fl(H) hopane peak is, however, not pure, being made up of (predominantly) hopane with contributions from the closely eluting 18~t(H)-oleanane, component "J" and the small peak which elutes immediately after the hopane peak. The moretane/hopane ratios have therefore been determined on computer enhanced m/z 191 fragmentograms and in the example shown (Fig. 4) the ratio was determined to be 0.25.
Sterane isomerisation and moretane/hopane ratios in crude oils 30 ,,
J
il I
m/z 19l
TERTIARY CRUDE OIL LEGEND TI 18all TRISNORHOPANE Tm 17all TRISNORHOPANE 2g-¢ ITaH ,2IBH NORHOPANE 30-a 17(IHt21~H HOP&NE 31 -e I~oH ,21~H HOMOHOPANE 32-¢1 17¢IH,2~H IBISHOMOHOPANE 33-a 17aH,21~H TRISHOMOHOPAN[ 34-(I I?aH,Z$~H TETRAHOMOHOP&NE 55-a 170H,21~H PENTAHOMOH0PANE O,..~n) 18GHOLEANANE J COMPOUNDJ
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I'?J~H,ZlGH MORETANE 17~H,21{IH NORMORETANIE
31-o
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297
32°
2400
2600 SCANS
MATURE, CRETACEOUS CRUDE OIL
29-o z 31-0
2o00
S
NS
Fig. 4. Triterpan¢ fragmentogram (m/z 191) of a Tertiary source rock derived crude oil compared to that
of a crude oil derived from a mature Cretaceous source rock. Gas chromatographicconditions: see Fig. 2.
RESULTS AND DISCUSSION
The sterane isomerisation parameters [C~ 20S/(20S + 20R) and iso/(iso + normal)] and the moretane/ hopane ratios of 234 crude oils, generated from a wide variety of source rocks which vary in age from the Tertiary to the Cambrian, have been determined (see "Experimental"). Of these crude oils, 78 have been generated from Tertiary source rocks and are mainly from the Far East (Sarawak, Brunei, Sabah and Indonesia) and Nigeria. The large majority of these crude oils are from deltaic source rocks which contain land-plant derived organic matter. Those from the Far East are frequently characterised by the triterpane 18~(H)-oleanane and the compounds "W", "T" and "R" (Grantham et al., 1981; Cox et al., 1986). Table 1 gives the biomarker maturity parameters of 73 of these Tertiary crude oils plus a number of their bulk maturity parameters. Figures 5, 6 and 7 show the two sterane and the moretane/hopane parameters of all analysed crude oils plotted against the attributed geological age of their source rocks. A number of observations can be made:
(i) Crude oils from mature (% Ro above 0.65) Cretaceous or older source rocks tend to give C29 20S/(20S + 20R) sterane isomer ratios of between 0.5 and 0.6 (mean 0.56) which is in agreement with published values (Mackenzie and McKenzie, 1983) for mature source rocks (Fig. 5). In contrast, crude oils from the Lower Jurassic Posidonia source rocks of The Netherlands exhibit sterane isomerisation which is somewhat retarded (see Fig. 5). This infers that these source rocks generated crude oil at a relatively lower level of maturity than, for example, the Upper Jurassic Kimmeridge source rocks of the North Sea where sterane isomerisation in derived crude oils is largely complete (see Fig. 5). The lower maturity level of the Posidonia compared to the Kimmeridge source rock appears to be due to a period of uplift and erosion of the overburden of the former which occurred during the Upper Cretaceous with the main phase of hydrocarbon generation occurring after subsequent deposition of a thick overburden of Cenozoic sediments (Bodenhausen and Ott, 1981; Heybroek, 1975). The retarded degree of sterane isomerisation of Posidonia crude oils is in agreement with the API gravities of these crudes
0.1
0.02 0.02 0.04 0.2 0.1 0.1
45.1
47.1" 45.2 40.8 22.1 * 43.9 35.1
Krakama-I 3 l m o River-2 l m o River-2 I m o River-2 Utorugu-7 Utorugu-7 Utorugu-7 Kanbo-3 Kanbo-3 Kanbo-3
0.5* 2.6 12.5 7.8 0.04* 0.2 9 6.4 0* 4.2 14 3.1 5.4 7.1 8.0 12.5 5.7* 23.9 19.5 0
boiling < 120°C
% light ft.
25.4* 26.7 19.3 0* 17.2 3.3
N o t determined
21.7
Not determined 0.03 20 0.07 5
38.0 42.5 37 1
Irian Jaya Kenali A s s a m Ledok-227
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.05 0.13 0.1 0. I 0. I 0.1 0. I 0.01 0.03 0.09
20.9* 34.8 40.9 34.9 20.1 * 21.5* 30.9 44.5 32.8* 42.1 39.5 23.3 37.8 30.3 37.1 35.8 26.9* 48.5 46.5 ND
%S
Champion-1 Fairley-2 Fairley-4X Iron Duke-7X Iron Duke-6X Iron Duke-6X Iron Duke-6X Iron D u k e S - I X Kedidi N o r t h - I X Oaprey-1X Pelican-2X Pelican-2X Scout Rock-2X Seria-248 Seria-270 Seria-676 Sofia-658 ZI-IX Zl-lX Zl-lX
Well
API gravity (60°F)
14 23 36 57 50 53
36
46 30
37 50 41 54 46 40 44 52 80 20 71 42 45 42 50 42 38 13 39 70
% sats
Bulk m a t u r i t y p a r a m e t e r s
1 2 5 26 9 14
6
12 33
22 11 10 12 29 32 12 2 12 2 25 54 8 21 12 12 19 l 4 12
Nigeria
Indonesia
Brunei
1 I I 6 2 3
I
5 8
4 3 1 1 2 3 2 1 3 2 4 4 2 2 3 2 3 2 2 7
% arom % her (NSO)
G r o s s composition
84 ~ 74 ~ 58 ° 11 a 39 ~ 30 ~
57 °
37 ~ 29 ~
37 ~ 36" 48 ~ 33 ~ 23 ° 25 ° 42 ° 45 ~ 5a 764 ~ b 45 ° 35 ° 35" 44 ~ 40 ~ 84" 55 a I 1~
% rest
0.69* 0.84 1.00 0.59* 0.80 0.69
0.73
ND 0.76 0.76
0.55* 0.69 0.76 ND 0.47* 0.59* 0.65 0.76 0.65* 0.76 0.59 0.59 0.8 0.8 0.76 0.85 0.73* 0.85 0.73 0.73
DOM' o f oil.
Table 1. Geochemical d a t a o f crude oils attributed to Tertiary source rocks
iso d
C30 moretane"
0.54 0.35 0.39 0.39 0.35 0.39 0.38 0.29 0.33 0.21
0.38 0.36 0.32
0.47 0.26 0.42 0.30 0.35 0.36 0.36 0.21 0.18 0.36 0.19 0.42 0.40 0.42 0.46 0.48 0.39 0.39 0.24 0.22
0,57 0.45 0.41 0.41 0.53 0.55 0.55 ND 0.26 ND
0,37 0.39 0.39
0.46 0.37 0.44 ND 0.40 0.43 0.42 0.27 0.33 0.49 ND 0.31 0.43 0.53 0.50 0.56 ND 0.53 0.40 0.37
0.20 0.21 0.19 0.19 0.07 0.09 0.15 0.33 0.26 0.30
0.20 0.15 0. I 0
0. t 3 0.19 0.17 0.29 0.17 0.17 0.17 0.17 0.19 0.17 0.30 0.13 0.18 0.18 0.19 0.18 0.14 0.14 0.14 0.20
(20S + 20R) (iso + nomal) C30 17ct(H)-hopane
20S '~
C29 steranes
B i o m a r k e r maturity parameters
>
> Z
34.7 41.3 36.3 37.8 31.8 35.6* 39.6 33.1 41 43 40.8 33.2 41.5 33.9 41.7 40.3
Baram-8 Baronia-29 Baronia-29 W. Bayan-2 Engkabang-2 C2-1 C2-1X C2-1X C2-1X C2-1X C5-1X D30.2 El 1.2 F13.2 M3-1X Miri
0.2 0.2 0.1 0.04 0.1 0.1 0.0 0.2 0.2 0.02 0.12 0.1 0.1 0.1 0.4 0.0
0.3 0.1 0.2 0. l 0.1 0.3 0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.2 0.1 0.2 10.4 31.4 13.7 16.2 1,2 0.4* 4.9 9.3 0 10 14.9 11.5 22.2 0.5 17.6 23.4
12.3 7.8 0.3* 6.5 5.5 5.5 4.9 9.5 8.3 4.0 2.2 7.2 12.6 0.3* 13.9" 7.6* 15.4
51 47 41 71 46 76 73 78 73 35 15 48 39 44
44
51 49 56 44 50 51 52 57 56 53 64 55 61 34 62 42 62 41 ND 11 25 17 13 2 16 22 20 25 15 1 1 29 14
45 15 40 18 45 44 46 39 42 42 22 41 37 61 35 53 36
4 3 3 3 5 6 1 4 2 6 13 4 2 5 3 5 2
3 3 3 1 l 8 5 2 2 3 1 2 4 I
15
Sarawak
Sabah
35~ 25a 4(P 15° 52a b b ~ b 47a 84° 50~ b 434
b
--~ 33° b 36° b ~ b ~ ~ ~ ._._b b b ~ b ~ ~ 0.69 0.76 0.65 0.73 0.55 0.92* 0.84 0.96 0.85 1.00 ND 0.84 0.69 ND 0.80 0.65
0.59 0.69 0.65* 0.69 0.69 0.76 0.85 0.76 0.88 0.80 0.69 0.73 0.76 0.53* 0.62* 0.50* 0.69
*Parameters which may be affected by bacterial degradation of the crude oil. °Gross composition determined by column chromatography, the " % rest" is the material lost through evaporation. ~Gross composition determined by thin layer chromatography-FID. 'DOM = degree of organic metamorphism of oil, vitrinite reflectance equivalent (Lijmbach et al., 1983). q)etermined by use of the fragmentogram m/z 2 1 7 + 2 1 8 + 2 5 9 . eDetermined by use of the fragmentogram m/z 191. ND = not determined.
32.8 34.4 29.6* 29.6 29.0 29.5 28.7 34.4 33.1 28.1 29.7 24.8 37.0 22.1" 34.7* 25.9* 41.1
Barton-3 Betty-5 Erb West Erb West-3 Erb West-3 Erb West- I S. Furious-2X S. Furious-2X S. Furious-2X S. Furious-2X S. E. Collins St Joselah- 1 Ketam-I Ketam-3 Semarang Semarang- i Semarang-lX 0.45 0.44 0.35 0.48 0.59 0.50 0.53 0.53 0.53 0.44 0.48 0.51 0.53 0.54 0.37 0.54
0.49 0.43 0.37 0.32 0.36 0.48 0.54 0.46 0.46 0.49 0.59 0.48 0.54 0.53 0.40 0.37 0.38 0.44 0.49 0.43 0.41 0.58 0.71 0.66 0.69 0.69 0.67 0.47 0.48 0.71 0.72 0.56 0.59
0.52 0.50 ND 0.42 0.44 0.64 0.56 0.54 0.57 0.53 0,53 0.57 0.55 0.62 0.50 0.48 0.52 0.19 0.19 0.16 0.20 0.15 0.08 0.10 ND 0.15 0.18 0.20 0.19 0.11 0.09 0.09 0.15
0.18 0.20 0.17 0.20 0.17 0.06 0.23 0.25 0.18 0.20 0.14 0.20 0. I I 0.12 0.21 0.19 0.19
I,O
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STERANE EQUILIBRIUM 20S/20S+20R ,x, 0.55
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TERTIARY CRUDES- WORLD WIDE
T L - LOWER TERTIARY/UPPER KU CRETACEOUS CRUDES - GIPPSLANO BASIN OF AUSTRALIA
~
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c/~
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C~ETACEOUS CRU0eSW. VENEZUELA AND NORTHERN OMAN UPPER JURASSIC CRUDES NORTH SEA ; U.K. ; NORWAY; DENMARK ; QATAR. LOWER JURASSIC CRUDESNETHERLAND~$ CAM6RIAN CRUDES - OMAN
180 my
I00
200
500 my
300
~0
500
ATTRIBUTED AGE OF SOURCE ROCK (MILLION YEARS)
Fig. 5. Plot of C29 20S/(20S + 20R) normal [5e(H), 14c~(H), 17e(H)] sterane ratios in crude oils against attributed source rock age.
( ~ 32~), which are not high, and, in some cases, gas chromatograms of saturated fractions which reveal immature hydrocarbon distributions. Of significance however is the incomplete sterane isomerisation of a large number of the crude oils derived from Tertiary source rocks. These tend to have 20S/(20S + 20R) values of less than 0.5, the lowest value encountered being 0.18. Only relatively few of these crudes contain completely isomerised steranes. These Tertiary source rock derived crude oils, judged by bulk geochemical maturity parameters, are normally viewed as being mature. Table I reports a number of these parameters for the Tertiary crude oils. A completely consistent picture is not obtained
08 t
and is also not to be expected since the crudes examined are from a wide variety of Tertiary source rocks. API gravities range from values below 30 (frequently for biodegraded crude oils) up to those above 40 ° (condensates--27% of all samplesL Organic sulphur contents are consistently very low but, in addition to indicating high maturity, this may also be a characteristic of the depositional environment of deltaic, land-plant-containing source rocks (Gransch and Posthuma, 1975). The amount of the low boiling fraction is also variable. It is generally very low in the bacterially-degraded crudes but reaches high values above 20% in non-degraded samples. Finally, the gross compositions tend to indicate very low contents of the hetero-component
o
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Fig. 6. Plot of C29 iso/(iso + normal) sterane ratios in crude oils against attributed source rock age [iso = 50t(H), 14fl(H), 17fl(H)].
Sterane isomerisation and moretane/hopane ratios in crude oils
301
C 5 0 MORETANE C30 HOPANE
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Fig. 7. Plot of C~ moretane/17~(H)-hopane ratios in crude oils against attributed source rock age.
(NSO) fraction and generally high contents of either the saturates or the "rest" (the amount of volatile material lost during determination of gross compositions using column chromatography). Taken as a whole these bulk parameters imply that most of the Tertiary crude oils examined are mature and yet, most consistently show incomplete isomerisation of the steranes. As examples, Fig. 3 shows the results of linked scan (C-C-MS) of three of the Tertiary crude oils. (ii) A similar trend is also found for the C29 iso/(iso + normal) sterane ratios of the same suite of crude oils (Fig. 6). Those from the Tertiary source rocks have a tendency for low ratios (<0.6) whilst crude oils from older, mature source rocks approach equilibrium in the range 0.65-0.75. The latter values are similar to those noted by Mackenzie et al. (1980) in the most mature samples of the Paris Basin. This gives us confidence that the use of data based on the 217 + 2 1 8 + 2 5 9 sterane fragmentogram is reasonable (see "Experimental"). The Lower Jurassic Netherlands crudes contain iso-steranes which are still somewhat retarded compared to the normal steranes but, as indicated above, this can be rationalised as being the result of the relatively low maturity of the source rock at the time of crude oil generation. Note that for Jurassic crude oils, many of which are from the Upper Kimmeridge in the northern North Sea area, the iso/(iso + normal) ratio shows, for the same samples, more scatter than the 20S/(20S + 20R) ratio (compare Figs 5 and 6). The greater scatter in the iso/(iso + normal) ratio has also been noted for sediments of the Wyoming Overthrust Belt (Mackenzie et al., 1983) and in sediments of the North Sea (Mackenzie et al., 1984). This tends to suggest that the formation of iso-steranes proceeds at
a slower rate than the formation of 20S normal steranes. The data presented here for crude oils can be used to support the Mackenzie hypothesis (see "Introduction") that time is an important factor in the time/temperature constraints of sterane isomerisation, i.e. that in the extracts of Tertiary source rocks, which rapidly reach maturity through fast burial, steran¢ isomerisation is incomplete due to the lack of time for the reaction to take place. A second possibility which may explain the retarded sterane isomerisation of these Tertiary crude oils is related to their type of organic matter. The source rocks of the Tertiary crude oils under investigation are predominantly deltaic and contain terrestrially derived organic matter. If these source rocks have generated their crude oil under relatively low heating conditions, then retarded sterane isomerisation may be expected. To determine which of the above possibilities is correct, additional data using a molecular parameter which is more temperature dependent and less time dependent than sterane isomerisation would be helpful in evaluating the maturity of the Tertiary oils. One such parameter is the steroid aromatisation ratio of Mackenzie and McKenzie (1983) although this data for the Tertiary crudes of the present data set is not currently available. Further work will be directed at providing additional and independent means of assessing the maturity of Tertiary sources. However, the currently favoured hypothesis remains that proposed by Mackenzie and McKenzie (1983) since a number of the examined Tertiary crude oils are found in very deep reservoirs (below 10,000 it) and generation of such crude oils under low heating conditions seems unlikely.
302
P.J. GRANTHAM
A third hypothesis for the incomplete sterane isomerisation of the Tertiary crude oils could be that these oils have picked up incompletely isomerised steranes from immature sediments during migration. Since immature sediments contain relatively higher biomarker concentrations than their mature counterparts, the resulting mixture of the original and "immature" biomarkers may exaggerate the latter. This mechanism has been propsed to explain the biomarker distributions and concentrations of crude oils from the North Slope of Alaska and from North-West Germany (Rullk6tter et al., 1984) and, as a possible explanation for immature biomarker distributions, in an Australian crude oil (Philp and Gilbert, 1982). Whilst this effect may occur in particular instances it is difficult to propose this mechanism for all the Tertiary crude oils described in this study since these originate from a wide variety of countries and locations. Moreover if this phenomenon were widespread there should be no reason for it to be solely restricted to Tertiary crude oils. (iii) Turning to the moretane/17~t (H)-hopane ratios of crude oils, it is found that, again, the Tertiary derived crude oils behave exceptionally. Figure 7 shows the ratios in crude oils plotted against the (attributed) geological age of their source rocks. Crudes from Tertiary source rocks have generally higher moretane/hopane ratios (0.1-0.3 with many values between 0.15 and 0.20) whilst the ratios in crudes from progressively older source rocks fall to values of generally 0.1 or less. Only the Lower Jurassic West Netherlands crude oils, which were generated from the relatively low mature Posidonia source rock (see above), have moretane/hopane ratios which are somewhat higher than the 0.1 level. These findings are in agreement with values reported by Seifert and Moldowan (1980) for petroleums (0.03-0.06). These workers did however note 2
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moretane/hopane ratios of 0.15 in crude oils from a Tertiary basin. (As an example Fig. 4 shows the m/z 191 fragmentogram of a Tertiary crude oil compared to that of a mature Cretaceous crude oil--see also the "Experimental" section for a discussion of the method of determination. The data presented here for crude oils suggests that in analogy with the steranes, the conversion of moretane to 17~ (H)-hopane may also be sensitive to time, the amount of time available in the Tertiary being insufficient for extensive conversion. An alternative hypothesis for higher moretane/ hopane ratios in the Tertiary crudes could be that moretane is co-eluting with 17:t (H)-moretane (see the discussion in the "Experimental" section). However, this appears unlikely since there is no known biogcnic source of 17~t(H)-moretane and the formation of this compound is energetically unfavourable under geological conditions (see Seifert and Moldowan, 1980 and references therein). Finally, from the foregoing, it is clear that the majority of Tertiary crude oils, in this data set, are distinguished from oils from older, mature source rocks by their generally incomplete sterane isomerisation and higher moretane/hopane ratios. Therefore a cross plot of a sterane isomerisation parameter against the moretane/hopane ratios should distinguish Tertiary crudes from older mature crudes. Such a plot is shown in Fig. 8. Crudes derived from Cretaceous and older mature source rocks generally have 20S/(20S+20R) ratios of >0.5 and C30 moretane/hopane <0.1 whilst crudes derived from Tertiary source rocks generally have 20S/(20S + 20R) sterane ratios of <0.5 and moretane/hopane ratios of >0.1. The West Netherlands crude oils, generated at a lower maturity level, plot--as expected--outside the area of mature non-Tertiary source rocks and overlap
x x
o Wesl Netherlands crude o~]s from a relafivsly low moture Lower Jurassic source rock
0.05
0.~0
0:15
o.o
o.'2
o.o
C3oMORETANE C3oHOPANE
Fig. 8. Cross plot of the C29 sterane isomerisation parameter 20S/(20S + 20R) against C30 moretane/ 17~(H)-hopane ratios in crude oils.
Sterane isomerisation and moretane/hopane ratios in crude oils with a number o f the Tertiary crude oils (see Fig. 8). Here, geological evidence (see above) has enabled us to rationalise the results, i.e. sterane isomerisation is incomplete in the Tertiary crudes probably because of rapid burial of the source rocks whilst sterane isomerisation is incomplete in the West Netherlands crudes owing to the relatively low maturity of the source rock. CONCLUSIONS (1) Crude oils generated from Tertiary source rocks frequently (but not always) contain incompletely isomerised steranes. A possible explanation is that, on the Mackenzie model of sterane isomerisation vs steroid aromatisation, steranes in Tertiary source rocks are incompletely isomerised since the time available for the reactions to proceed to completion has been too short, despite the generation of crude oil at high temperatures. An alternative hypothesis for the incomplete sterane isomerisation of Tertiary crude oils could be that their deltaic land-plant-containing source rocks have generated crude oil under low heating conditions. However, the Mackenzie hypothesis is currently favoured since a number of the Tertiary crude oils are found in very deep reservoirs and this argues against generation under low heating conditions. A third hypothesis, that the " i m m a t u r e " biomarkers of the Tertiary crude oils may reflect nonindigenous contributions of organic matter picked up during migration o f the crude oils and condensates through or along immature source rocks, although possibly applicable in particular cases, does not appear to be a general, widespread phenomenon. If such were a c o m m o n process then it is difficult to explain why it is generally restricted to Tertiary crude oils. (2) M o r e t a n e / h o p a n e ratios are generally higher in the same Tertiary crude oils which contain incompletely isomerised steranes. This could suggest that constraints similar to those operating on the steranes may exist on moretane to hopane conversions. Acknowledgements--The author wishes to thank Ir G. J. Heiszwolf and Dr S. Hoff for their review and comments on the manuscript, Dr Ulfert Klomp for many stimulating discussions and the management of the Shell International Petroleum Co. Ltd for their permission to publish this article. Messrs A. Baak and P. B. L. Lohbeck are thanked for their mass spectrometric analyses which were carried out with enthusiasm and dedication. REFERENCES
Bodenhausen J. W. A. and Ott W. F. (1981) Habitat of the Rijswijk Oil Province, Onshore, The Netherlands. In Petroleum Geology of the Con tinental Shelf of North - West Europe (Edited by Illing L. V. and Hobson G. D.), pp. 301-309. Institute of Petroleum. Cox H. C., de Lceuw J. W., Schenck P. A., van Koningsveld H., Jansen J. C., van de Graaf B., van Geerestein V. J.,
303
Kanters J. A., Kruk C. and Jans A. W. H. (1986) Bicadinane, a C30 pentacyclic isoprenoid hydrocarbon found in crude oil. Nature (London) 319, 316-319. Djerassi C. (1978) Recent advances in the mass spectrometry of steroids. Pure Appl. Chem. 50, 171-184. van Graas G., Baas J. M. A., van der Graaf B. and de Leeuw J. W. (1982) Theoretical organic geochemistry. I. The thermodynamic stability of several cholestane isomers calculated by molecular mechanics. Geochim. Cosmochim. Acta 46, 2399-2402. Gransch J. A. and Posthuma J. (1975) On the origin of sulphur in crudes. In Advances in Organic Geochemistry 1973 (Edited by Tissot B. and Bienner F.), pp. 727 739. Editions Technip, Paris. Grantham P. J., Posthuma J. and Baak, A. (1983) Triterpanes in a number of Far-Eastern crude oils. In Advances in Organic Geochemistry 1981 (Edited by Bjoroy M. et al.), pp. 675-683. Wiley, Chichester. Heybroek P. (1975) On the structure of the Dutch Part of the Central North Sea Graben. In Petroleum and the Continental Shelf of North West Europe (Edited by Woodland A. W.), Vol. 1, pp. 339-349. Applied Science Publishers. Hughes W. B., Holba A. G., Mullar D. E. and Richardson J. S. (1985) Geochemistry of Greater Ekofisk crude oils. In Organic Geochemistry in Exploration of the Norwegian Shelf(Edited by Thomas B. M. et al.), pp. 75-92. Graham & Trotman, London. Kimble B. J. (1972) The geochemistry of triterpenoid hydrocarbons. Ph.D. thesis, University of Bristol. Lijmbach G. W. M., van der Veen F. M. and Engelhardt E. D. (1983) Geochemical characteristation of crude oils and source rocks using Field Ionisation Mass Spectrometry. In Advances in Organic Geochemistry 1981 (Edited by Bjoro3~ M. et al.), pp. 788-798. Wiley, Chichester. Mackenzie A. S. and McKenzie D. (1983) Isomerisation and aromatization of hydrocarbons in sedimentary basins formed by extension. Geol. Mag. 120, 417~,65. Mackenzie A. S., Patience R. L., Maxwell J. R., Vandenbroucke M. and Durand B. (1980) Molecular parameters of maturation in the Toarcian shales, Paris Basin, France --I. Changes in the configurations of acyclic isoprenoids alkanes, steranes and triterpanes. Geochim. Cosmochim, Acta 44, 1709-1721. Mackenzie A. S., Patience R. L. and Maxwell J. R. (1981) Molecular changes and the maturation of sedimentary organic matter In Origin and Chemistry of Petroleum (Edited by Atkinson G. and Zuckerman J. J.), Chap. 1, pp. 1-31. Pergamon Press, Oxford. Mackenzie A. S., Brassell S. C., Eglinton G. and Maxwell J. R. (1982a) Steroid hydrocarbons and the thermal history of sediments. Nature (London) 295, 223~26. Mackenzie A. S., Brassell S. C., Eglinton G. and Maxwell J. R. (1982b) Chemical fossils: the geological fate of steroids. Science 217, 491-504. Mackenzie A. S., Li R.-W., Maxwell J. R., Moldowan J. M. and Seifert W. K. (1983) Molecular measurements of thermal maturation of Cretaceous Shales from the Overthrust Belt, Wyoming, USA. In Advances in Organic Geochemistry 1981 (Edited by Bjor~y M. et al.), pp. 496-503. Wiley, Chichester. Mackenzie A. S., Maxwell J. R., Coleman M. L. and Deegan C. E. (1984) Biological marker and isotope studies of North Sea crude oils and sediments. In Proc. llth Worm Pet. Congr., Vol. 2, pp. 45--56. Wiley, Chichester. McKenzie D., Mackenzie A. S., Maxwell J. R. and Sajgo Cs. (1983) Isomerisation and aromatization of hydrocarbons in stretched sedimentary basins. Nature (London) 301, 504-506. Philp R. P. and Gilbert T. D. (1982) Unusual distribution of biological markers in an Australian crude oil. Nature (London) 299, 245-247.
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Rullk6tter J., Mackenzie A. S., Welte D. H., Leythaeuser D. and Radke M. (1984) Quantitative gas chromatographymass spectrometry analysis of geological samples. In Advances in Organic Geochemistry 1983 (Edited by Schenck P. A., de Leeuw J. W. and Lijmbach G. W. M.). Org. Geochem. 6, 817-827. Pergamon Press, Oxford. Seifert W. K. and Moldowan J. M. (1979) The effects of biodegradation on steranes and terpanes in crude oils. Geochim. Cosmochim. Acta 43, 111-126.
Seifert W. K. and Moldowan J. M. (1980) The effect of thermal stress on source rock quality as measured by hopane stereochemistry. In Advances in Organic Geochemistry 1979 (Edited by Douglas A. G. and Maxwell J. R.), pp. 229-237. Pergamon Press, Oxford. Warburton G. A. and Zumberge J. E. (1983) Determination of petroleum sterane distributions by mass spectrometry with selective metastable ion monitoring. Anal. Chem. 55, 123- 126.
0146-6380/86 $3.00+ 0.00 Pergamon Journals Ltd
Sterane isomerisation and moretanelhopane ratios in crude oils derived from Tertiary source rocks P. J. GRANTHAM Koninklijke/Shell Exploratie en Produktie Laboratorium, Postbus 60, 2280 AB Rijswijk (Z.H.), The Netherlands (Received 22 October 1985; accepted 26 March 1986) Abstract--The extent of sterane isomerisation reactions and the moretane/hopane ratios of 234 crude oils, taken world wide, from a wide variety of source rocks of differing geological ages, have been measured. This data indicates that in 78 crude oils derived from Tertiary source rocks, sterane isomerisation reactions as determined by the 20S/(20S + 20R) ratio of the C~ 5~(H), 14~(H), 17ct(H) normal-steranes and the C29 iso/(iso + normal) ratio [iso = 5~(H), 14fl(H), 17fl(H)] are mainly incomplete and sometimes considerably so. In addition, the same crude oils have 17fl(H), 21~t(H)-moretane/17~(H),21fl(H)-hopane ratios which are significantly greater (predominantly in the range 0.10-0.30) than those of crude oils derived from older, mature source rocks (mainly less than 0.1). This data, for crude oils, lends support to the hypothesis, proposed by Mackenzie and McKenzie (1983) for source rock extracts, that the time/temperature constraints of sterane isomerisation reactions are such that the time available for isomerisation in Tertiary sediments is generally insufficient,despite generation of crude oil at relatively high temperatures. An alternative hypothesis is that the incomplete sterane isomerisation of Tertiary crude oils may be due to generation of these crude oils from their deltaic, land plant-containing source rocks under low heating conditions. A third hypothesis proposes that the Tertiary crude oils may have picked up the incompletelyisomerised steranes from immature sediments during migration. Although possible in particular instances, such a mechanism does not appear to be generally applicable since, in that case, the phenomenon would then appear to be restricted to the Tertiary. The higher moretane/hopane ratios of the Tertiary crude oils could suggest that constraints, similar to those applying in sterane isomerisation, also operate in the conversion of moretane to 17ct(H)-hopane. Key words: steranes, moretane, hopane, Tertiary, Cretaceous, time of oil generation, isomerisation, crude oil, source rocks
INTRODUCTION The search for reliable geochemical tools for the determination of the maturity of a source rock or crude oil is a major concern of geochemistry. In recent years an increasing number of studies on the effects of maturity on steranes and triterpanes have been described. In these publications the isomerisation of steranes and the reactions determining the relative concentrations of moretanes and hopanes have received much attention. Two main sterane isomerisation reactions have been described (Mackenzie et al., 1980, 1981, 1982a, b, 1983, 1984; McKenzie et al., 1983; Mackenzie and McKenzie, 1983). The first is the isomerisation of "normal" 5ct(H), 14~(H), 17~t(H)-20R steranes which occur as the most prominent steranes of very immature sediments (see Fig. 1 for structures). As the maturity increases, isomerisation at the chiral carbon-20 takes place and, compared to the 20R isomer, the 20S isomer increases in concentration. This increase continues until an equilibrium condition is reached [C~ 20S/(20S + 2 0 R ) ~ 0.55] after which the relative isomer concentrations remain the same even though source rock maturity may continue to increase. O . G 9/6-- B
The second sterane isomerisation reaction concerns the increase in 5~t(H), 14fl(H), 17fl(H) "iso" steranes relative to the "normal" steranes as source rock maturity increases (Fig. 1). Again, isomerisation proceeds until an equilibrium condition is reached [C29 iso/(iso + normal) ~ 0.75] whereby the iso-steranes are more stable than the normal steranes. Beyond equilibrium no further change is reported to take place even though, again, the maturity of the source rock may continue to increase. Moreover, the relative stabilities of the isomers of the normal steranes, and the stabilities of the iso- with respect to the normal steranes, observed in nature, are in agreement with their calculated thermodynamic stabilities using molecular mechanics (van Graas et al., 1982). Time appears to play an important role in sterane isomerisation reactions. Mackenzie and McKenzie (1983) and McKenzie et al. (1983) have published data which indicates that: (a) Sterane aromatisation is strongly dependent on source rock temperature and has a higher activation energy compared to sterane isomerisation. (b) sterane isomerisation requires time to proceed to equilibrium (there is a slow rate determining step), despite high temperatures.
293
294
P.J. GRANTHAM
S H
H
,5=(H), 14= (H), t7==(HJ - 20 _R 24-ETHYL ' N O R M A l : CHOLESTANE
~)a( H),t4(z ( H ), t7a( H ) - L:'O~ 24-ETHYL ' N O R M A l ' CHOLESTANE
F O R M A T I O N OF 2OS ' N O R M A L ~S T E R A N E S
S
i
H
5(=(H),~I~(H),I7~(I-I)-201~_ AND20~} 24-ETHYL'1SO' CHOLESTANES
5a(H),I4a(H),ITa(H)-20R AND 2OS 24.ETHYL"NOBMA[ CHOLESTANES FORMATION OF'ISO'
STERANES
,,H
i7¢=(H),2t/~(H)- HOPANE
i?/3(H),21(=(H)-MORETANE
Fig. 1. Structures of the biomarkers discussed in this report.
Thus, in young Tertiary basins, which have been rapidly buried and have rapidly reached a high temperature, sterane aromatisation has proceeded faster than sterane isomerisation, which remains incomplete. In older sediments which have never reached a high temperature, sterane isomerisation is more advanced than sterane aromatisation. The differences in the rates of steroid isomerisation and aromatisation reactions have been demonstrated by the use of aromatisation/isomerisation (AI) diagrams which plot the ratios of triaromatic/ (triaromatic + monoaromatic) steroid hydrocarbons against the ratio of 20S/(20S + 20R) C29 normal steranes (Mackenzie and McKenzie, 1983). Another reaction which has received attention as a possible maturity tool is the relative concentrations of 17#(H),21a(H) moretanes and 17ct(H),21#(H) hopanes (Seifert and Moldowan, 1980~see Fig. 1 for structures). These workers found that in extracts of low mature source rocks (apparently below % Ro levels of 0.62-0.69) there was no correlation of C30 moretane/hopane ratios (generally high, 0.25-0.60) with increasing source rock maturity. However, the ratio decreased in pyrolyzates of the same source rocks as the maturity became higher, and, in extracts of mature source rocks, and in crude oils, the ratio was low (<0.1).
Seifert and Moldowan (1980) account for the above phenomena by a reaction scheme which allows for moretane to hopane conversion at high (oil generation) temperatures. At low temperatures this conversion is not possible and, in extracts, moretane/ hopane ratios do not reflect maturity. It has been noted however by Mackenzie et al. (1980) that in extracts of sediments in the Paris Basin there does appear to be loss of moretane with respect to hopane as maturity increases. As part of our search for reliable maturity parameters amongst biomarkers, the preceding concepts of sterane isomerisation and moretane/hopane ratios have been tested on a large number of crude oils. This paper describes the results of this research with particular emphasis being placed on those crudes derived from Tertiary source rocks. EXPERIMENTAL
Sterane isomerisation reactions
The progress of sterane isomerisation reactions has been monitored by combined gas chromatographicmass spectrometric analysis (GC-MS) of saturated (branched/cyclic) fractions isolated from a crude oil or source rock extract; Use was made of multiple ion detection (MID) of selected fragments. The presence
Sterane isomerisation and moretane/hopane ratios in crude oils of steranes in a geological sample can be monitored using a variety of fragment ions (m/z 217, 218, 259, 232, 149, 151 plus the parent ions m/z 372, 386 and 400--see Seifert and Moldowan, 1979 and references therein). Published fragmentograms of sterane distributions are usually based on the base peak of the Set(H), 14~t(H), 17~t(H) normal steranes i.e. m/z 217 (see for example Mackenzie et al., 1981) whilst the reported ratio of the C:9 20S/(20S + 20R) steranes has been calculated from peak areas using this fragment ion (Mackenzie et al., 1981). However, the ratio of C29 iso/(iso + normal) steranes based on peak area measurement using m/z 217 would discriminate against the actual amount of iso steranes present since the latter have m/z 218 as base peak, not m/z 217 (Djerassi, 1978; Seifert and Moldowan, 1979). Therefore, we have calculated C29 20S/(20S + 20R) and C29 iso/(iso + normal) sterane ratios from peak area measurements based on a fragmentogram which sums the intensities of the fragment ions 217, 218 and 259 (the latter being a fragment ion characteristic of 13fl(H), 17ct(H)-rearranged or -diasterancs--see
100.0
Fig. 2 for examples). The aim has been to calculate relative sterane intensities, from one fragmentogram, which reflect as accurately as possible those in nature and those calculated by others (see results below). As a check on possible co-elution of the C29 steranes with other components in the Tertiary crude oils, three samples have also been analysed using linked scan combined gas chromatography-mass spectrometry (Warburton and Zumberge, 1983). This technique enables only those fragment ions (e.g. m/z 217) which result from the fragmentation of a given stcrane parent ion to be identified. These analyses confirmed the incomplete sterane isomerisation of the analysed samples (see Fig. 3) and indicated that co-elution of the C29 steranes with other compounds is not a serious problem.
Determination of moretane /hopane ratios The relative concentrations of moretane and hopane are monitored using the m/z 191 fragment ion, an approach used by other workers (see for
14
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Fig. 2. Sterane fragmentogram (m/z 217 + 218 + 259) of a Tertiary source rock derived crude oil compared to that of a crude oil derived from a mature Cretaceous source rock. Gas chromatographic conditions: 22 m (x 0.32 mm i.d.) fused silica capillary column coated with CP Sil 5; helium flow rate 35 cm/sec; temperature programmed from 50-160°C at 20°C/rain, then from 160-290°C at 2°C/min. Isothermal at 290°C for 10 min.
296
P.J. GRANTHAM BARAM-8. SARAWAK
J ~.
20000
~.~, I
KEDIDI NORTH-IX, BRUNEI
Ij2~ ~_
15C00 '
! 291
N = normal ~QQO I = i,o, */~B R = te0rr0nged ,~G
,:,~.,-.';k.,.,.,.,.,.,.w.,.,.,.,.,.,.,.,.,I 29N
30000.
IRON DUKE. BRUNEI
291
20a_ 20a_ GaS chron~tOgtaphic
29R 29R >" I-
conditions: 25 m {x 0.22 mm i.d3 fused =lica capi,aW column coated with CP Si~ S CB. Temp. p, Ov,~,,,,med from
29N1
20000-
Z m/z 217 z 10000.
le0 to 300°C at
2°C/min. /sec.
i
, i ,
i - i .
i - I ,
i,
i,
i-
i,
i-
i - i - i
,i
.~,.,
.--, :,~,,
INCREASING RETENTION TIME-~-.-'Im'-
Fig. 3. Linked scan fragmentograms of metastable m/z 217 ions originating from parent ions m/z 400 (C29 steranes) in three crude oils from Tertiary source rocks in the Far East.
example Seifert and Moldowan, 1980; Hughes et al., 1985). This paper presents data which demonstrates that crude oils derived from Tertiary source rocks contain, relative to 17~t(H),21fl(H)-hopane, large concentrations of 17fl(H),21~t(H)-moretane. Since the large majority of the 78 Tertiary crude oils are from land-plant-containing deltaic environments it is theoretically possible for the large moretane peak to result from co-elution of moretane with other landplant-derived triterpanes. Kimble (1972) has determined the mass spectra and Kovats retention indices of a large number of triterpanes and her data indicate that the triterpanes most likely to co-elute with moretane are glutane and 17ct(H) moretane (Kovats retention indices: Dexsil 300, 280°C, glutane, 3321, 17~(H)-moretane, 3324, moretane, 3327; OV-101, 250°C, glutane, 3118, moretane, 3112; OV-101, 280°C, 17ct(H)-moretane, 3174, moretane, 3178). The phase used in our analyses, CP Sil-5, is a Silicone gum, similar to Dexsil 300 and OV-101 and similar retention behaviour is expected. However, there are many other cyclic hydrocarbons present in these Far Eastern crude oils, the retention data of which are not known, and these may also co-elute with moretane.
As a check on possible co-elution of any components with moretane, four of the Tertiary crude oils were analysed using cyclic scanning of all fragments (rn/z 60-600). In all cases the mass spectrum of the moretane component was in agreement with the published spectrum of moretane and indistinguishable from that of 17~t(H)-moretane (Kimble, 1972). It is therefore possible that 17~t(H)-moretane may also be present in these samples (but see the Results and Discussion below). "[he mass spectrum of glutane contains only a minor m/z 191 fragment ion and hence moretane/hopane determinations based on this ion will not be affected by possible glutane co-elution. Figure 4 shows the m/z 191 fragmentogram of a Tertiary crude oil compared to that of a mature Cretaceous crude oil. The 17~(H), 21fl(H) hopane peak is, however, not pure, being made up of (predominantly) hopane with contributions from the closely eluting 18~t(H)-oleanane, component "J" and the small peak which elutes immediately after the hopane peak. The moretane/hopane ratios have therefore been determined on computer enhanced m/z 191 fragmentograms and in the example shown (Fig. 4) the ratio was determined to be 0.25.
Sterane isomerisation and moretane/hopane ratios in crude oils 30 ,,
J
il I
m/z 19l
TERTIARY CRUDE OIL LEGEND TI 18all TRISNORHOPANE Tm 17all TRISNORHOPANE 2g-¢ ITaH ,2IBH NORHOPANE 30-a 17(IHt21~H HOP&NE 31 -e I~oH ,21~H HOMOHOPANE 32-¢1 17¢IH,2~H IBISHOMOHOPANE 33-a 17aH,21~H TRISHOMOHOPAN[ 34-(I I?aH,Z$~H TETRAHOMOHOP&NE 55-a 170H,21~H PENTAHOMOH0PANE O,..~n) 18GHOLEANANE J COMPOUNDJ
,. .... o.oc...o.,3,, l l
M
M
| M
if 16oo
COXET AL ,19~)
I'?J~H,ZlGH MORETANE 17~H,21{IH NORMORETANIE
31-o
L/I
2200
30-o
297
32°
2400
2600 SCANS
MATURE, CRETACEOUS CRUDE OIL
29-o z 31-0
2o00
S
NS
Fig. 4. Triterpan¢ fragmentogram (m/z 191) of a Tertiary source rock derived crude oil compared to that
of a crude oil derived from a mature Cretaceous source rock. Gas chromatographicconditions: see Fig. 2.
RESULTS AND DISCUSSION
The sterane isomerisation parameters [C~ 20S/(20S + 20R) and iso/(iso + normal)] and the moretane/ hopane ratios of 234 crude oils, generated from a wide variety of source rocks which vary in age from the Tertiary to the Cambrian, have been determined (see "Experimental"). Of these crude oils, 78 have been generated from Tertiary source rocks and are mainly from the Far East (Sarawak, Brunei, Sabah and Indonesia) and Nigeria. The large majority of these crude oils are from deltaic source rocks which contain land-plant derived organic matter. Those from the Far East are frequently characterised by the triterpane 18~(H)-oleanane and the compounds "W", "T" and "R" (Grantham et al., 1981; Cox et al., 1986). Table 1 gives the biomarker maturity parameters of 73 of these Tertiary crude oils plus a number of their bulk maturity parameters. Figures 5, 6 and 7 show the two sterane and the moretane/hopane parameters of all analysed crude oils plotted against the attributed geological age of their source rocks. A number of observations can be made:
(i) Crude oils from mature (% Ro above 0.65) Cretaceous or older source rocks tend to give C29 20S/(20S + 20R) sterane isomer ratios of between 0.5 and 0.6 (mean 0.56) which is in agreement with published values (Mackenzie and McKenzie, 1983) for mature source rocks (Fig. 5). In contrast, crude oils from the Lower Jurassic Posidonia source rocks of The Netherlands exhibit sterane isomerisation which is somewhat retarded (see Fig. 5). This infers that these source rocks generated crude oil at a relatively lower level of maturity than, for example, the Upper Jurassic Kimmeridge source rocks of the North Sea where sterane isomerisation in derived crude oils is largely complete (see Fig. 5). The lower maturity level of the Posidonia compared to the Kimmeridge source rock appears to be due to a period of uplift and erosion of the overburden of the former which occurred during the Upper Cretaceous with the main phase of hydrocarbon generation occurring after subsequent deposition of a thick overburden of Cenozoic sediments (Bodenhausen and Ott, 1981; Heybroek, 1975). The retarded degree of sterane isomerisation of Posidonia crude oils is in agreement with the API gravities of these crudes
0.1
0.02 0.02 0.04 0.2 0.1 0.1
45.1
47.1" 45.2 40.8 22.1 * 43.9 35.1
Krakama-I 3 l m o River-2 l m o River-2 I m o River-2 Utorugu-7 Utorugu-7 Utorugu-7 Kanbo-3 Kanbo-3 Kanbo-3
0.5* 2.6 12.5 7.8 0.04* 0.2 9 6.4 0* 4.2 14 3.1 5.4 7.1 8.0 12.5 5.7* 23.9 19.5 0
boiling < 120°C
% light ft.
25.4* 26.7 19.3 0* 17.2 3.3
N o t determined
21.7
Not determined 0.03 20 0.07 5
38.0 42.5 37 1
Irian Jaya Kenali A s s a m Ledok-227
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.05 0.13 0.1 0. I 0. I 0.1 0. I 0.01 0.03 0.09
20.9* 34.8 40.9 34.9 20.1 * 21.5* 30.9 44.5 32.8* 42.1 39.5 23.3 37.8 30.3 37.1 35.8 26.9* 48.5 46.5 ND
%S
Champion-1 Fairley-2 Fairley-4X Iron Duke-7X Iron Duke-6X Iron Duke-6X Iron Duke-6X Iron D u k e S - I X Kedidi N o r t h - I X Oaprey-1X Pelican-2X Pelican-2X Scout Rock-2X Seria-248 Seria-270 Seria-676 Sofia-658 ZI-IX Zl-lX Zl-lX
Well
API gravity (60°F)
14 23 36 57 50 53
36
46 30
37 50 41 54 46 40 44 52 80 20 71 42 45 42 50 42 38 13 39 70
% sats
Bulk m a t u r i t y p a r a m e t e r s
1 2 5 26 9 14
6
12 33
22 11 10 12 29 32 12 2 12 2 25 54 8 21 12 12 19 l 4 12
Nigeria
Indonesia
Brunei
1 I I 6 2 3
I
5 8
4 3 1 1 2 3 2 1 3 2 4 4 2 2 3 2 3 2 2 7
% arom % her (NSO)
G r o s s composition
84 ~ 74 ~ 58 ° 11 a 39 ~ 30 ~
57 °
37 ~ 29 ~
37 ~ 36" 48 ~ 33 ~ 23 ° 25 ° 42 ° 45 ~ 5a 764 ~ b 45 ° 35 ° 35" 44 ~ 40 ~ 84" 55 a I 1~
% rest
0.69* 0.84 1.00 0.59* 0.80 0.69
0.73
ND 0.76 0.76
0.55* 0.69 0.76 ND 0.47* 0.59* 0.65 0.76 0.65* 0.76 0.59 0.59 0.8 0.8 0.76 0.85 0.73* 0.85 0.73 0.73
DOM' o f oil.
Table 1. Geochemical d a t a o f crude oils attributed to Tertiary source rocks
iso d
C30 moretane"
0.54 0.35 0.39 0.39 0.35 0.39 0.38 0.29 0.33 0.21
0.38 0.36 0.32
0.47 0.26 0.42 0.30 0.35 0.36 0.36 0.21 0.18 0.36 0.19 0.42 0.40 0.42 0.46 0.48 0.39 0.39 0.24 0.22
0,57 0.45 0.41 0.41 0.53 0.55 0.55 ND 0.26 ND
0,37 0.39 0.39
0.46 0.37 0.44 ND 0.40 0.43 0.42 0.27 0.33 0.49 ND 0.31 0.43 0.53 0.50 0.56 ND 0.53 0.40 0.37
0.20 0.21 0.19 0.19 0.07 0.09 0.15 0.33 0.26 0.30
0.20 0.15 0. I 0
0. t 3 0.19 0.17 0.29 0.17 0.17 0.17 0.17 0.19 0.17 0.30 0.13 0.18 0.18 0.19 0.18 0.14 0.14 0.14 0.20
(20S + 20R) (iso + nomal) C30 17ct(H)-hopane
20S '~
C29 steranes
B i o m a r k e r maturity parameters
>
> Z
34.7 41.3 36.3 37.8 31.8 35.6* 39.6 33.1 41 43 40.8 33.2 41.5 33.9 41.7 40.3
Baram-8 Baronia-29 Baronia-29 W. Bayan-2 Engkabang-2 C2-1 C2-1X C2-1X C2-1X C2-1X C5-1X D30.2 El 1.2 F13.2 M3-1X Miri
0.2 0.2 0.1 0.04 0.1 0.1 0.0 0.2 0.2 0.02 0.12 0.1 0.1 0.1 0.4 0.0
0.3 0.1 0.2 0. l 0.1 0.3 0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.2 0.1 0.2 10.4 31.4 13.7 16.2 1,2 0.4* 4.9 9.3 0 10 14.9 11.5 22.2 0.5 17.6 23.4
12.3 7.8 0.3* 6.5 5.5 5.5 4.9 9.5 8.3 4.0 2.2 7.2 12.6 0.3* 13.9" 7.6* 15.4
51 47 41 71 46 76 73 78 73 35 15 48 39 44
44
51 49 56 44 50 51 52 57 56 53 64 55 61 34 62 42 62 41 ND 11 25 17 13 2 16 22 20 25 15 1 1 29 14
45 15 40 18 45 44 46 39 42 42 22 41 37 61 35 53 36
4 3 3 3 5 6 1 4 2 6 13 4 2 5 3 5 2
3 3 3 1 l 8 5 2 2 3 1 2 4 I
15
Sarawak
Sabah
35~ 25a 4(P 15° 52a b b ~ b 47a 84° 50~ b 434
b
--~ 33° b 36° b ~ b ~ ~ ~ ._._b b b ~ b ~ ~ 0.69 0.76 0.65 0.73 0.55 0.92* 0.84 0.96 0.85 1.00 ND 0.84 0.69 ND 0.80 0.65
0.59 0.69 0.65* 0.69 0.69 0.76 0.85 0.76 0.88 0.80 0.69 0.73 0.76 0.53* 0.62* 0.50* 0.69
*Parameters which may be affected by bacterial degradation of the crude oil. °Gross composition determined by column chromatography, the " % rest" is the material lost through evaporation. ~Gross composition determined by thin layer chromatography-FID. 'DOM = degree of organic metamorphism of oil, vitrinite reflectance equivalent (Lijmbach et al., 1983). q)etermined by use of the fragmentogram m/z 2 1 7 + 2 1 8 + 2 5 9 . eDetermined by use of the fragmentogram m/z 191. ND = not determined.
32.8 34.4 29.6* 29.6 29.0 29.5 28.7 34.4 33.1 28.1 29.7 24.8 37.0 22.1" 34.7* 25.9* 41.1
Barton-3 Betty-5 Erb West Erb West-3 Erb West-3 Erb West- I S. Furious-2X S. Furious-2X S. Furious-2X S. Furious-2X S. E. Collins St Joselah- 1 Ketam-I Ketam-3 Semarang Semarang- i Semarang-lX 0.45 0.44 0.35 0.48 0.59 0.50 0.53 0.53 0.53 0.44 0.48 0.51 0.53 0.54 0.37 0.54
0.49 0.43 0.37 0.32 0.36 0.48 0.54 0.46 0.46 0.49 0.59 0.48 0.54 0.53 0.40 0.37 0.38 0.44 0.49 0.43 0.41 0.58 0.71 0.66 0.69 0.69 0.67 0.47 0.48 0.71 0.72 0.56 0.59
0.52 0.50 ND 0.42 0.44 0.64 0.56 0.54 0.57 0.53 0,53 0.57 0.55 0.62 0.50 0.48 0.52 0.19 0.19 0.16 0.20 0.15 0.08 0.10 ND 0.15 0.18 0.20 0.19 0.11 0.09 0.09 0.15
0.18 0.20 0.17 0.20 0.17 0.06 0.23 0.25 0.18 0.20 0.14 0.20 0. I I 0.12 0.21 0.19 0.19
I,O
o.
Po a
o
o
o-
o ,-4
o
300
P.J.
f
_~..o
I
STERANE EQUILIBRIUM 20S/20S+20R ,x, 0.55
+
o~
O Ca U3
0.4 •
GRANTHAM
C°JL
~ T ~ o
~ o
o
03
T
TERTIARY CRUDES- WORLD WIDE
T L - LOWER TERTIARY/UPPER KU CRETACEOUS CRUDES - GIPPSLANO BASIN OF AUSTRALIA
~
K
°° o
JU
c/~
LLJ Z <~
o 8
02 ~-
JL
O~ OJ
-(:
0.1 30 rrly
65 my
t00 nlv
145 r~y
C~ETACEOUS CRU0eSW. VENEZUELA AND NORTHERN OMAN UPPER JURASSIC CRUDES NORTH SEA ; U.K. ; NORWAY; DENMARK ; QATAR. LOWER JURASSIC CRUDESNETHERLAND~$ CAM6RIAN CRUDES - OMAN
180 my
I00
200
500 my
300
~0
500
ATTRIBUTED AGE OF SOURCE ROCK (MILLION YEARS)
Fig. 5. Plot of C29 20S/(20S + 20R) normal [5e(H), 14c~(H), 17e(H)] sterane ratios in crude oils against attributed source rock age.
( ~ 32~), which are not high, and, in some cases, gas chromatograms of saturated fractions which reveal immature hydrocarbon distributions. Of significance however is the incomplete sterane isomerisation of a large number of the crude oils derived from Tertiary source rocks. These tend to have 20S/(20S + 20R) values of less than 0.5, the lowest value encountered being 0.18. Only relatively few of these crudes contain completely isomerised steranes. These Tertiary source rock derived crude oils, judged by bulk geochemical maturity parameters, are normally viewed as being mature. Table I reports a number of these parameters for the Tertiary crude oils. A completely consistent picture is not obtained
08 t
and is also not to be expected since the crudes examined are from a wide variety of Tertiary source rocks. API gravities range from values below 30 (frequently for biodegraded crude oils) up to those above 40 ° (condensates--27% of all samplesL Organic sulphur contents are consistently very low but, in addition to indicating high maturity, this may also be a characteristic of the depositional environment of deltaic, land-plant-containing source rocks (Gransch and Posthuma, 1975). The amount of the low boiling fraction is also variable. It is generally very low in the bacterially-degraded crudes but reaches high values above 20% in non-degraded samples. Finally, the gross compositions tend to indicate very low contents of the hetero-component
o
0.7~
o
o o
0.6
STERANE EQUILIBRIUM ISO/ISO+NORMAL ~ 0.7
o
o o
TL-KU
o
o
It~ o JU 8
0.5,
o JL
o
m
0.4,
T
o L E G E N D : S E E FIG.S
© -
0.3,
Z < 0.2,
1,-(/3 O) C,.)
0.1 30 my
65 my
100 my
145 my 180 my
c ~ my
100 . r r R , B U T E O . a E OF SOURCE ROCK ,M,LL,ON YEARS
---
Fig. 6. Plot of C29 iso/(iso + normal) sterane ratios in crude oils against attributed source rock age [iso = 50t(H), 14fl(H), 17fl(H)].
Sterane isomerisation and moretane/hopane ratios in crude oils
301
C 5 0 MORETANE C30 HOPANE
o.3o. 0.25-
o
0.20.
T F
o o
~o
0.15
co
~0~--. o TL-KU
00 r~,~,___u.,. -
LEGEND: SEE FIG.5
-
o --o--
o
--
-K
,~ = ~=%,
ooo
O, 05
ooo
30 my
_
65 , ~
100 my
o
JU
~
_
oom
JL
ooo~o
o:lDo
~-~ . ~ . . - .~ oo
145 my
~- - - : - ..nH;
180 my
ATTRIBUTED
oo
AGE OF SOURCE
ROCK
(MILLION
YEARS)
-~
Fig. 7. Plot of C~ moretane/17~(H)-hopane ratios in crude oils against attributed source rock age.
(NSO) fraction and generally high contents of either the saturates or the "rest" (the amount of volatile material lost during determination of gross compositions using column chromatography). Taken as a whole these bulk parameters imply that most of the Tertiary crude oils examined are mature and yet, most consistently show incomplete isomerisation of the steranes. As examples, Fig. 3 shows the results of linked scan (C-C-MS) of three of the Tertiary crude oils. (ii) A similar trend is also found for the C29 iso/(iso + normal) sterane ratios of the same suite of crude oils (Fig. 6). Those from the Tertiary source rocks have a tendency for low ratios (<0.6) whilst crude oils from older, mature source rocks approach equilibrium in the range 0.65-0.75. The latter values are similar to those noted by Mackenzie et al. (1980) in the most mature samples of the Paris Basin. This gives us confidence that the use of data based on the 217 + 2 1 8 + 2 5 9 sterane fragmentogram is reasonable (see "Experimental"). The Lower Jurassic Netherlands crudes contain iso-steranes which are still somewhat retarded compared to the normal steranes but, as indicated above, this can be rationalised as being the result of the relatively low maturity of the source rock at the time of crude oil generation. Note that for Jurassic crude oils, many of which are from the Upper Kimmeridge in the northern North Sea area, the iso/(iso + normal) ratio shows, for the same samples, more scatter than the 20S/(20S + 20R) ratio (compare Figs 5 and 6). The greater scatter in the iso/(iso + normal) ratio has also been noted for sediments of the Wyoming Overthrust Belt (Mackenzie et al., 1983) and in sediments of the North Sea (Mackenzie et al., 1984). This tends to suggest that the formation of iso-steranes proceeds at
a slower rate than the formation of 20S normal steranes. The data presented here for crude oils can be used to support the Mackenzie hypothesis (see "Introduction") that time is an important factor in the time/temperature constraints of sterane isomerisation, i.e. that in the extracts of Tertiary source rocks, which rapidly reach maturity through fast burial, steran¢ isomerisation is incomplete due to the lack of time for the reaction to take place. A second possibility which may explain the retarded sterane isomerisation of these Tertiary crude oils is related to their type of organic matter. The source rocks of the Tertiary crude oils under investigation are predominantly deltaic and contain terrestrially derived organic matter. If these source rocks have generated their crude oil under relatively low heating conditions, then retarded sterane isomerisation may be expected. To determine which of the above possibilities is correct, additional data using a molecular parameter which is more temperature dependent and less time dependent than sterane isomerisation would be helpful in evaluating the maturity of the Tertiary oils. One such parameter is the steroid aromatisation ratio of Mackenzie and McKenzie (1983) although this data for the Tertiary crudes of the present data set is not currently available. Further work will be directed at providing additional and independent means of assessing the maturity of Tertiary sources. However, the currently favoured hypothesis remains that proposed by Mackenzie and McKenzie (1983) since a number of the examined Tertiary crude oils are found in very deep reservoirs (below 10,000 it) and generation of such crude oils under low heating conditions seems unlikely.
302
P.J. GRANTHAM
A third hypothesis for the incomplete sterane isomerisation of the Tertiary crude oils could be that these oils have picked up incompletely isomerised steranes from immature sediments during migration. Since immature sediments contain relatively higher biomarker concentrations than their mature counterparts, the resulting mixture of the original and "immature" biomarkers may exaggerate the latter. This mechanism has been propsed to explain the biomarker distributions and concentrations of crude oils from the North Slope of Alaska and from North-West Germany (Rullk6tter et al., 1984) and, as a possible explanation for immature biomarker distributions, in an Australian crude oil (Philp and Gilbert, 1982). Whilst this effect may occur in particular instances it is difficult to propose this mechanism for all the Tertiary crude oils described in this study since these originate from a wide variety of countries and locations. Moreover if this phenomenon were widespread there should be no reason for it to be solely restricted to Tertiary crude oils. (iii) Turning to the moretane/17~t (H)-hopane ratios of crude oils, it is found that, again, the Tertiary derived crude oils behave exceptionally. Figure 7 shows the ratios in crude oils plotted against the (attributed) geological age of their source rocks. Crudes from Tertiary source rocks have generally higher moretane/hopane ratios (0.1-0.3 with many values between 0.15 and 0.20) whilst the ratios in crudes from progressively older source rocks fall to values of generally 0.1 or less. Only the Lower Jurassic West Netherlands crude oils, which were generated from the relatively low mature Posidonia source rock (see above), have moretane/hopane ratios which are somewhat higher than the 0.1 level. These findings are in agreement with values reported by Seifert and Moldowan (1980) for petroleums (0.03-0.06). These workers did however note 2
I
°°I
, J. L--,
r
,.._I
x
I /e
Area enclosed by
0.3'
matow,,
Netherlands crude oils
x
a- , , x .
x
x
x
;
,o
~1~ m
2#x
x
0.2.
0
x
~x
x x
x •
x
Legend
x
x x
x Terliary crude oils world w~de
x
• Crude oils from mature Cretaceous. upper Jurassic and Cambrlon source rocks
0,1
x x
x
CO
(D
x
-.--.--------m~¢l~
,he ,..°,,v.., ,o,,
0.4.
° x
4".x
0
0 £M + CO 0 ¢J
moretane/hopane ratios of 0.15 in crude oils from a Tertiary basin. (As an example Fig. 4 shows the m/z 191 fragmentogram of a Tertiary crude oil compared to that of a mature Cretaceous crude oil--see also the "Experimental" section for a discussion of the method of determination. The data presented here for crude oils suggests that in analogy with the steranes, the conversion of moretane to 17~ (H)-hopane may also be sensitive to time, the amount of time available in the Tertiary being insufficient for extensive conversion. An alternative hypothesis for higher moretane/ hopane ratios in the Tertiary crudes could be that moretane is co-eluting with 17:t (H)-moretane (see the discussion in the "Experimental" section). However, this appears unlikely since there is no known biogcnic source of 17~t(H)-moretane and the formation of this compound is energetically unfavourable under geological conditions (see Seifert and Moldowan, 1980 and references therein). Finally, from the foregoing, it is clear that the majority of Tertiary crude oils, in this data set, are distinguished from oils from older, mature source rocks by their generally incomplete sterane isomerisation and higher moretane/hopane ratios. Therefore a cross plot of a sterane isomerisation parameter against the moretane/hopane ratios should distinguish Tertiary crudes from older mature crudes. Such a plot is shown in Fig. 8. Crudes derived from Cretaceous and older mature source rocks generally have 20S/(20S+20R) ratios of >0.5 and C30 moretane/hopane <0.1 whilst crudes derived from Tertiary source rocks generally have 20S/(20S + 20R) sterane ratios of <0.5 and moretane/hopane ratios of >0.1. The West Netherlands crude oils, generated at a lower maturity level, plot--as expected--outside the area of mature non-Tertiary source rocks and overlap
x x
o Wesl Netherlands crude o~]s from a relafivsly low moture Lower Jurassic source rock
0.05
0.~0
0:15
o.o
o.'2
o.o
C3oMORETANE C3oHOPANE
Fig. 8. Cross plot of the C29 sterane isomerisation parameter 20S/(20S + 20R) against C30 moretane/ 17~(H)-hopane ratios in crude oils.
Sterane isomerisation and moretane/hopane ratios in crude oils with a number o f the Tertiary crude oils (see Fig. 8). Here, geological evidence (see above) has enabled us to rationalise the results, i.e. sterane isomerisation is incomplete in the Tertiary crudes probably because of rapid burial of the source rocks whilst sterane isomerisation is incomplete in the West Netherlands crudes owing to the relatively low maturity of the source rock. CONCLUSIONS (1) Crude oils generated from Tertiary source rocks frequently (but not always) contain incompletely isomerised steranes. A possible explanation is that, on the Mackenzie model of sterane isomerisation vs steroid aromatisation, steranes in Tertiary source rocks are incompletely isomerised since the time available for the reactions to proceed to completion has been too short, despite the generation of crude oil at high temperatures. An alternative hypothesis for the incomplete sterane isomerisation of Tertiary crude oils could be that their deltaic land-plant-containing source rocks have generated crude oil under low heating conditions. However, the Mackenzie hypothesis is currently favoured since a number of the Tertiary crude oils are found in very deep reservoirs and this argues against generation under low heating conditions. A third hypothesis, that the " i m m a t u r e " biomarkers of the Tertiary crude oils may reflect nonindigenous contributions of organic matter picked up during migration o f the crude oils and condensates through or along immature source rocks, although possibly applicable in particular cases, does not appear to be a general, widespread phenomenon. If such were a c o m m o n process then it is difficult to explain why it is generally restricted to Tertiary crude oils. (2) M o r e t a n e / h o p a n e ratios are generally higher in the same Tertiary crude oils which contain incompletely isomerised steranes. This could suggest that constraints similar to those operating on the steranes may exist on moretane to hopane conversions. Acknowledgements--The author wishes to thank Ir G. J. Heiszwolf and Dr S. Hoff for their review and comments on the manuscript, Dr Ulfert Klomp for many stimulating discussions and the management of the Shell International Petroleum Co. Ltd for their permission to publish this article. Messrs A. Baak and P. B. L. Lohbeck are thanked for their mass spectrometric analyses which were carried out with enthusiasm and dedication. REFERENCES
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