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Biological marker maturity parameters of marine crude oils and rock extracts from the Maracaibo Basin, Venezuela
Org. Geochem. Vol. 18, No. 2, pp. 215-224, 1992 Printed in Great Britain.All rights reserved
0146-6380192$5.00+ 0.00 Copyright © 1992PergamonPress Ltd
Biological marker maturity parameters of marine crude oils and rock extracts from the Maracaibo Basin, Venezuela OSWALDOGALLANGO and FERNANDOCASSANI Intevep, S. A., Apartado 76343, Caracas 1070-A, Venezuela (Received 19 October 1988; accepted in revised form 14 August 1991)
Abstract--The maturity parameters obtained from the analysis of saturated and aromatic hydrocarbons and petroporphyrins in marine rock extracts (La Luna source rock) and crude oils from the Maracaibo basin were examined by gas chromatography-mass spectrometry (GC-MS) and high performance liquid chromatography (HPLC). The results obtained indicate that in very mature samples there is a preferential destruction of C29 steranes over C27 and C2s as well as an increase in the C2t and C22 regular steranes. These changes seem to indicate that thermal maturity affects the correlation parameter normally used in the characterisation of oils and rock extracts. The demetallated petroporphyrins show a decrease of DPEP series and increase in ETIO components with increasing maturity. In very mature samples the ETIO porphyrin ratio, C2sE'/C27E' and C29E/C2sE, together with the methylphenanthrenes parameter (PP-1), are very useful for classifyingboth rock extracts and crude oils in terms of maturity. Of the parameters presented here, the methylphenanthrene parameter (PP-I) and the ETIO petroporphyrin series are the most promising source of geochemical information for better assessment of maturity of oil and source rock extracts beyond the main oil generation zone (Ro/> 0.9%), after which other molecular maturity parameters have reached equilibrium. Key words--Maracaibo Basin, biological marker, maturity parameter, petroporphyrins, phenanthrene parameter, La Luna Formation, crude oils
INTRODUCTION The use of a molecular parameter in the assessment of thermal maturity of the organic matter in sedimentary rock and crude oils has increased in the last decade. Reaction such as isomerization of hopanes and steranes, aromatization of monoaromatic steroids, the methylphenanthrene index and changes in the distribution of the alkyl porphyrins have been widely used (Seifert and Moldowan, 1978; Mackenzie, 1984, Radke et al., 1982: Barwise and Robert, 1984; Quirke and Maxwell, 1980; Alexander et al., 1986). The isomerization reactions of the hopanes and steranes are of limited use in petroleum exploration since they often reach a maximum before the major generation of hydrocarbons occurs (Mackenzie and Maxwell, 1981), especially in marine crude oils and source rocks. The aromatization ratio of monoaromatic steroids, which involves the loss of Ci9 methyl group leading to the formation of triaromatic steroidal hydrocarbons (Mackenzie et al., 1981), is apparently affected by migration. This effect seems to lead to a preferential depletion of triaromatic components relative to monoaromatic steroidal hydrocarbons in oils (Hoffman et al., 1984). In addition, the use of this parameter is limited to a part of the main oil generation phase. On the other hand, the methylphenanthrene index has been used in recent years in the
assessment of maturity of rock extracts from type III organic matter and especially coals (Radke and Welte, 1983; Radke et al., 1982; Garrigues et al., 1984). However, Cassani (1985) and Radke et al. (1986) found that phenanthrenes are of limited values for the maturity assessment of type II kerogen and marine crude oils, although later result (Cassani et al., 1988) showed improved applicability at high maturity. Porphyrins in sediments are thought to be derived from chlorophyll precursors in plants and photosynthetic bacteria. Immature sediment and crude oils contain predominantly DPEP type porphyrins. With increasing thermal maturity, the DPEP components are thought to cleave at the isocyclic ring to produce etioporphyrins (Didyk et al., 1975) or DPEP decreases relative to ETIO as a consequence of the preferential thermal destruction of DPEP over ETIO petroporphyrins, as proposed by Barwise and Robert (1984). Thus, a measure of the relative concentration of DPEP and ETIO types reflects the thermal maturity of both sediments and oils, The ratio (%DPEP) varies normally from 100 to 0 with increasing maturity. However, the use of maturity parameters is limited due to the fact that it changes in a relative narrow maturity range. Notwithstanding, Cassani (1985) proposed a couple of maturity ratios in the ETIO series as potential maturity parameters for mature samples which can be extended in to the very
215 OG 18/2--F
216
OSWALDO GALLANGO and FERNANDO CASSANI
are shown in Fig. 1. The La Luna formation (Upper Cretaceous) rock samples examined were from essentially the same stratigraphic interval buried to different depths. Samples were obtained of the type section (sample 3D) and from different wells in order to obtain the whole range of thermal maturity. Sampling of the same stratigraphic interval at various depths should minimize effects caused by lithological or source variation. The La Luna Formation consists mainly of limestone and calcareous shales which contain type II organic matter and is the most important source rock in the Maracaibo basin. General geological and geochemical information can be found in Talukdar et al. (1986) and Cassani et al. (1986). The crude oils were obtained from different wells distributed through the basin. In order to avoid differences due to biodegradation, migration and oil mixing, only crude oils that were apparently unaffected by such processes were selected.
mature region. According to him, the variation in these ratios with increasing maturity are probably due to dealkylation reactions. Ekweozor and Strausz (1983) suggested the application of the tricyclic terpane ratio (Cz3/C21 as a potential oil and source rock maturity indicator. More recently, Cassani (1985) and Cassani et al. (1988) reported data in support of this earlier study and added a similar ratio (C23/C24). In this work special attention is paid to maturity ratios of certain alkanes and aromatic biological marker and in particular, to the porphyrin compounds. We report the uses and limitations of steranes, methylphenanthrenes and petroporphyrins in the assessment of maturity of extracts of the La Luna Formation and La Luna derived crude oil of the Maracaibo basin (Venezuela). The destructive effects of excessive thermal maturity on the distribution of biological markers such as hopane and steranes are discussed. EXPERIMENTAL
G C - M S and H P L C
Samples
The GC-MS procedures for the analysis and the calculation of component ratios from mass fragmentograms have been decribed elsewhere (Mackenzie
The locations of the oils and rock samples collected from outcrop and drill holes from Maracaibo basin
GULF OF VENEZUELA
\ l'~
o
0~
)/i
~
°/
_
DM'116"~"--~"'°
""" •
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-.
J \
N
II Maracaibo
.+o"I
"\
J<-,
o~ t,, A ,@ /~_ q, / • Uachiquo,
J
i
VLA-710 .,
~
UD-170 W ~
',,
\ \ • Bachaquero
\
\ •
SOL 6 •
CLA-111A
¢1
•
VLD-273
CL-87
~.
\
I'
VLE.686
\
A,ACAIBO, t - "~
LAKE
Zuha
4" ~e
\
q~i'/i ~'/~-" 1
lkx~.
A
• 25
ROCK SAMPLE
50
Km
F /
Fig. 1. Location of the La Luna marine source rocks and derived crude oils examined from the Maracaibo basin.
Maturity parameters of marine rock and oil
217
Table I. Bulk and biological marker maturity parameters of the La Luna derived crude oils* Aromatic hydrocarbons
Saturate hydrocarbons
Sample DM-116t VLD-273 UD-170 VLA-710 UD-209 ALT-15 CL-87 VLE-686 VLC-612 1I-Y-20X
Reservoir depth (ft) Age 5531 10,023 16,475 13,038 15,600 16,660 15,745 15,270 12,740 16,466
C M C C C C C C E C
S~ (%) 4.8 1.4 1.8 1.3 1.4 0.6 0.4 0.4 0.2 0.2
H/C:~ % 20S % ill/ 1.56 1.69 1.69 1.74 1.76 1.77 1.86 1.85 1.88 1.89
43 48 52 46 58 62 53 58 65 i.d.
50 53 53 56 58 60 57 60 64 i.d.
C23"3 C27 COs C~ CO4-3 (%) (%) (%) 3.4 1.9 2.4 2.2 2.3 1.5 1.5 1.3 i.d. i.d.
36 35 35 35 36 36 36 36 36 35
30 32 33 33 34 36 34 34 36 39
34 33 32 32 30 28 30 30 29 26
Demetallopetroporphyrins
T/T + M (%)
PP-I
DPEP (°,6)
47 87 82 100 100 100 100 100 i.d. i.d.
1.50 1.10 1.50 1.19 1.14 -0.60 0.57 0.30 0.21
30 10 6 4 0 0 0 0 0
71
CaE' CovE' - -
CO9E c_mE w
-2.04 1.65 1.10 0.62 0.42 0.38 i.d. i.d.
-2.31 1.86 1.14 0.50 0.33 0.33 i.d. i.d.
*For details of measurements see Table 3 and Text. = In whole oil. t L o w maturity oil. Ages: C = Cretaceous; E = Edcene; M = Miocene. i.d. = Indeterminate because of excessive maturity.
et al., 1982, Mackenzie, 1984; Cassani and Eglinton, 1986; Cassani et al., 1988).
The petroporphyrins of crude oils and bitumens were extracted as their free-bases by demetallation with methane sulphonic acid, according to the procedure of Erdman 0965). Typically, crude oil (lg) was heated (100°C, 4h) with at least a five-fold excess of methane sulphonic acid (98%, Aldrich, 5-10 ml). The reaction was quenched by pouring the acid/oil mixture into distillated water (20 ml). After allowing to cool, the coagulated material was removed by filtration. The aqueous filtrate, containing the porphyrins as dication, was extracted three times with dichloromethane, neutralized with NaHCO 3 and dried over Na2SO4. The free-bases obtained in this way were purified through a short alumina column. The free-based petroporphyrins were then analyzed by HPLC using three 25 c m x 4.6 mm i.d. columns coupled in series (Barwise and Park, 1983). A gradient HPLC solvent delivery system (l.00 ml/min) was controlled by a varian 5000 solvent programmer, and the eluting porphyrins were detected using a variable wavelength u.v./visible spectrophotometer (Altex model 153) with the detector monitoring 400 nm. The spectra were recorded using a CIS-HP 1000 data aquisition system. Demetallated petroporphyrins were resolved using the solvent mixture gradient reported by Barwise et al. (1986).
Petroporphyrins peaks were assigned (Cassani, 1985) by separation of the demetallated fraction in individual peaks by HPLC. Assignments were made on the basis of probe mass spectrometric analysis of the collected fraction. In addition, the assignments of C31 and C32 DPEP-porphyrins were given by coinjection with a standard. The assignments of the C27E' and C2sE' etioporphyrins (E' = fl unsubstituted) were tentatively identified by comparison of their retention patterns with Boscan oil (R. Eversbed, unpublished results). The La Luna marine source rock extracts and derived crude oils have very high vanadyl/nickel porphyrin ratios (eg. 100 to 1), thus differences in DPEP and ETIO content or distribution are primarily from vanadyl porphyrins. Elemental analysis determination
Elemental analysis of crude oils and kerogen was performed on a Perkin Elmer model 240B elemental analyzer. The kerogen samples were preextracted with dichloromethane and dried at 90°C under vacuum for 24 h prior to the C and H determination. The rock samples were extracted in a soxhlet extraction apparatus with dichloromethane prior to Rock-Eval pyrolysis. The samples (50-100mg) were analyzed in a Rock-Eval (series I) device, with temperature programming from 250 to 500°C. The
Table 2. Bulk and biological marker maturity parameters of the La Luna source rock samples with increasing degree of thermal maturation* Sample 3D P-114 VLE-738 SOL-6 30F-IX CLA-11 UD-149 VLB-704
Depth (it)
T~axt °C
Outcrop 431 8933 435 16,076 439 17,143 441 15,232 443 15,181 450 16,392 > 4 6 0 13,743 > 460
H/C °
Ro~ %20S
1.30 1.20 1.10 1.00 0.90 0.80 0.58 0.56
0.43 --0.77 -1.09 1.27 1.30
52 47 50 50 41 54 52 47
%[3fl %T/T + M 59 57 58 58 63 56 59 52
55 43 85 87 100 100 i.d. i.d.
PP-I
%DPEP
C a E' COTE,
C~ E Ca E
1.63 1.40 1.60 1.60 1.70 1.50 0.70 0.30
81 60 -------
--1.49 1.41 1.09 0.18 i.d. i.d.
--2.99 1.51 1.34 0.12 i.d. i.d.
t = Tma~ (Rock-Eval) of extracted rock. i.d. = Indeterminate because of excessive maturity. ~Measured in isolated kerogen. Re values are approximate due to the low amounts of vitrinite in these samples. *For details see Table 3 and text.
OSWALDOGALLANGOand FERNANDOCASSANI
218
Parameter %20S
%8,8
%T(T + M)
%DPEP
°/oC27 %C28
Ratio C29 ~t, a, a,-sterane 20S
Table 3. Molecular parameters employed (see Tables 1 and 2) Significance lsomerization at sterane C-20 (S); ratio rises from 0 to ~ 50% with increasing maturity; oil generation could start at ~ 40% (Mackenzie et aL, 1982)
20S + 20R C29 regular steranes: ~,#,# (R+S) Total Steranes Steroid hydrocarbon: C2s- triaromatic C29Mono + C28-triaromatic Porphyrins DPEP DPEP + AETIO
Aromatization of C ring aromatic hydrocarbon; ratio rises from 0 to 100% with increasing maturity; this ratio is more temperature dependent than % 20S sterane ratio (Mackenzie et al., 1982); oil generation could start at ~ 40-60%
Relative abundances of C27, C2s and C29 :t, 8, # steranes
These distributions depend on the nature of the organisms which contribute to the organic matter of the forming sediments (Huang and Meinsehein, 1979)
Tricyclic terpane ratio
Change with increasing maturity probably due to preferential generation of Cu components from precursors in the polar fractions and kerogen (Cassani et al., 1988); useful in the assessment of maturity of Cretaceous marine source rocks and crude oils; varies from ~ 3.5 to ~ 1.0
Maturity parameter:
Rearrangement of,, - 1 and 9 to the more stable 8-2 and 3 components (Radke et aL, 1982); used mainly for very mature marine samples (Cassani et aL, 1988)
lsomerization at sterane C-14 and C-17, ratio rises from 0-50% to ~ 70-75% with increasing maturity: isomerization normally complete before major generation of hydrocarbons (Mackenzie and Maxwell, 1981)
Apparent C42 bond cleavageof isocyclicring in the DPEP porphyrins (e.g. Didyk et al., 1975) or thermal destruction of DPEP porphyrins (Barwise and Park, 1983); ratio falls from ~ 100% to 0; oil generation could start at ~ 80--60% (Barwise and Park, 1983)
%C29 C23 - 3 C24 - 3
PPI
(1 - M P + 9 - MP) (2- MP + 3 - MP) MP = methylphenanthrene
operating parameters are the same as those published earlier by Clementz et al. (1979). Vitrinite reflectance (%Ro) measurements were made on isolated ( H C I / H F procedure) kerogens using a Zeiss Universal microscope. RESULTS AND DISCUSSION Saturated hydrocarbons
Tables 1 and 2 show the maturity parameters o f the La Luna F o r m a t i o n and derived crude oils. Both crude oils and source rocks are listed in order of maturity from low mature to very mature as proposed by Talukdar et al. (1986) and Cassani et al. (1988) based on bulk chemical and some biomarker parameters. It is well known that all these oils are generated from the same source rock (La Luna Formation; Talukdar et al., 1986). As can be observed in Tables 1 and 2, the sterane maturity parameters normally used for sediments and crude oils (% 20S; % tiff) do not show any trend with increasing sample maturity, because the end point of these isomerization reactions have been reached. The proposed % tiff equilibrium values of ca 65-75% (Mackenzie et al., 1982) are not reached even in the extremely mature samples, which present values for these parameters between 50 and 64%. Early reports suggest that these parameters are affected by migration which causes an enrichment of 14fl(H), 17fl(H) components (Seifert et al., 1980). However, this enrichment was not observed in the oils examined suggesting that such changes are probably due to
maturity effects. The similar values obtained in this parameter for the different samples analyzed agree with Mackenzie and Maxwell (1981) who found that this isomerization reaction reaches the end point in most crude oils. The % 20S parameter is more reliable than the previous ratio due to the fact, as we mentioned above, that the % of tiff shows more variations as a result of migration and source effects. The % 20S can be extended into the zone of petroleum generation, according to Mackenzie (1981). Again, in the mature oil samples (Table l), these values reach the end point (range 43-55%). The very mature samples present values in the range of 53-65% which limit the use of this parameter. On the other hand an increase in the relative concentration of C2t and C22 steranes as well as decrease in the regular steranes from C29 to C28 , is observed with increasing maturity (Figs 2 and 3). Thermal destruction of hopanes has also been observed (Fig. 4). As listed in Tables l and 2 and shown in Figs 2 and 3, there is a predominance of C27 steranes over C29 steranes in the low mature samples (i.e. DM-116 crude), while in the mature and very mature samples, the proportion of C29 to other regular steranes decreases. Similar results have been noticed by Cassani and Eglinton (1986) and by Wingert and Pomerantz (1986) during asphaltene and crude oil thermal degradation laboratory experiments, respectively. We believe that a preferential degradation of the steranes seems to be the explanation. Recently, G r a n t h a m and Wakefield (1988) observed similar behavior in the sterane distribution in crude oils derived from marine
Maturity parameters of marine rock and oil 27
A)
219
29
21ST
MIZ 218
28
B) ¢II CO
UD.170
21ST VLC-612
c)
27
i
- -
Retention time
Fig. 2. Mass fragmentograms of regular sterane (m/z 218) distributions for: (A) low mature; (B) mature; and (C) very mature marine La Luna derived crude oils. carbonate and siliciclastic source rocks through time. According to the authors these changes are due to a diversification of marine sources instead of thermal alteration. However, Grantham and Wakefield (1988) did not take into consideration the maturity effect in the steranes distribution. Our results show that there are variations in the sterane carbon number distributions during maturation of both source rock and crude oils. Aromatic hydrocarbons A further measurement of maturity which has been shown to be reliable in a number of basins is the extent to which C-ring monoaromatic steroid hydrocarbons (M) have been aromatized to triaromatic steroid hydrocarbons (T) (Mackenzie et al., 1984). These changes are measured by the aromatization ratio (% T/T + M) and are commonly applied in the maturity assessment of source rocks and crude oils.
The ratio increases with maturity from 0 to 100% and is more temperature dependent than the sterane ratio % 20S (Mackenzie et al., 1982). In the case of the sample examined in this study (Tables 1 and 2), this parameter tends to increase from the less mature crude oil (DM-116) and rock extract (3D) towards the most mature samples (VLE-686 and CLA-111), respectively. On the other hand, there is a good correlation between the atomic H/C ratio measured in the isolated kerogen and in the whole oil with the aromatization values. However, the use of the aromatization parameter % T/(T + M) for the maturity assessment of very mature crude oil and rock extracts is limited because the aromatization values of these samples reach their maximum in the early stage of maturation. This is probably due to the temperature dependence of this parameter (Mackenzie et al., 1982). Moreover, among the very mature samples (Tables 1 and 2) it is difficult to establish any further
220
OSWALDOGALLANGOand FEP.NANDOCASSA~
27 21ST A]
28 29
P-114 =
.
/
22St J •
[
k
•
..
J~
MIZ
218
.l iiJ.. 28
B)
.E ol
SOL-6
0
....
£
~ ......
L~
1
t,
C) CLA-111
29
--- TI --
Retention time
Fig. 3. Mass fragmentograms of regular sterane (m/z 218) distributions for: (A) low mature; (B) mature; and (C) very mature La Luna rock extracts.
VLC-612
M/Z 191 lr"-Hopanes - 1
Tricyclic terpanes
,
A)
23
;1
26
21St
M/Z 218
B) 27
29
Retention time "~ Fig. 4. Mass fragmentograms of (A) triterpanes (m/z 191) and (B) regular steranes (m/z 218) for a very mature marine La Luna Formation derived crude oil (well VLC-612). Notice the thermal destruction of the hopanes and the decreasing in the relative concentration of the regular steranes compared to C2mand C22 steranes.
Maturity parameters of marine rock and oil maturity trend based on this parameter. The same limitation applies for other molecular parameters of alkanes such as %20S and the tricyclic terpane ratio, which can only be used in the main oil generation zone (Ro ~ 0.5-0.9%). In an attempt to find another maturity parameter which could be applied to the La Luna source rock and derived crude oils beyond the main oil generation zone, (Ro > 0.9%), we examined the distribution of methyiphenanthrenes by G C - M S and calculated the methylphenanthrene parameter PP-1 (Cessani et al., 1988). As shown in Tables 1 and 2, no major changes in PP-1 values for the low mature to mature samples are observed. In addition, there is no correlation between the PP-1 values and the other maturity parameters. However, for very mature samples a
C32D C31o
A)
DM-116
C28E' C29 E
B)
L
221
C32D
A)
I
I C31D C32D
"p..
C29E C28E" I
~0 13) u~ t~
SOL
(J
l
C27E" C28E" I C29E 3
0
F
-
I
~
C27E"
'P._.
-C27E' C28E' I C29E
l
ilL' C27E'
Retenliontime
b
Fig. 6. HPLC chromatograms of the La Luna marine rock extracts showing the changes in the demetallopetroporphyrin distributions with increasing maturity. decrease of PP-1 values occurs with increasing maturity of crudes and rock extracts. This agrees with Cassani et al. (1988) and differs from previous published results (Radke et al., 1986). As Cassani et al. (1988) pointed out, the methylphenanthrene parameter (PP-I) seems to be very useful in the assessment of maturity of very mature marine source rocks and crude oils.
C28E Petroporphyrins
D) C28E'
za - -
Retention time
Fig. 5. HPLC chromatograms of the La Luna derived crude oils showing the changes in the demetallopetroporphyrin distributions with increasing maturity.
Figures 5 and 6 show the HPLC chromatograms of the demetallated petroporphyrins for the oils and rock extracts examined. As can be seen in these figures, there are significant changes in the porphyrin distributions with increasing maturity. The low maturity samples of crude oils and rock extracts contain mostly DPEP type (Tables 1 and 2) with ETIO porphyrins present in very low concentration. With increasing maturity, the DPEP content decrease and the ETIO porphyrins increase. In very mature
222
OSWALDO GALLANGO a n d FERNANDO CASSANI
samples the DPEP components are absent and the ETIO type shows a simple distribution with a predominance of the C28E' and C29E components (Figs 5 and 6). In the most mature samples a progressive increment of C27E' and C28E over C27E' and C29E is observed. Similar results were obtained by Cassani (1985) during laboratory thermal experiments performed on the Boscan oil. Mechanisms proposed for the thermal DPEP to ETIO conversion involves cleavage of the isocyclic ring which should give rise to etioporphyrins (Quirke and Maxwell, 1980), or as a consequence of preferential thermal destruction of DPEP over ETIO petroporphyrins as proposed by Barwise and Robert (1984). However, the distribution of ETIO type cannot be visualized as a simple concentration of these compounds by thermal destruction of the DPEP types as suggested by Barwise and Park (1983). Dealkylation reactions apparently also control the distribution of these ETIO porphyrin compounds. This type of reaction has been reported
Structure
previously in laboratory thermal alteration experiments of ETIO porphyrins (Yen et al., 1969; Casagrande and Hodgson, 1971, 1974; Cassani, 1985). These dealkylation reactions tend toward the lower molecular weight components with increasing thermal maturation. According to the results obtained by Cassani (1985) and the observation in natural samples examined in the present study, we propose the use of the ratios C2s E'/C27E' and C29 E/C28E in the ETIO series for very mature samples in order to extend the use of petroporphyrins in the assessment of thermal maturity of both crude oils and rock extracts beyond the maximum zone of oil generation (Re >/0.9%, Fig. 7). Our results (Tables 1 and 2 and Figs 6-8) show that there is a good correlation between PP-I values and the ETIO petroporphyrin maturity ratio, and that both maturity parameters lead us to distinguish maturity differences among crude oils or rock extracts. Furthermore, in the extremely mature samples
Biological marker
Low mature (Ro<0.6%)*
Steranes
% 20S, % ~l~ (0-55)*
Triterpanes
Mature (Re 0.6-0.9%)*
C23/C 24 (4.0-1.0)*
Phenanthrenes
R
Very mature (Ro>0.9%)*
PP-I (1-0.2)*
R
Aromatic steroids
% T1T+M (0-60)*
% TIT+M (60-100)*
Porphyrins
% DPEP (100-50) ~
% DPEP (50-0) ~
H R
R
R R
R
~
BR ETIO C28E' ' C29E C27E' ' ~
R R
R
* = values are approximated
Fig. 7. Biological markers and the molecular maturity parameters applied to the La Luna source rocks and derived crude oils. According to the level of maturity, different parameters are used for the assessment of the level of maturity.
Maturity parameters of marine rock and oil
H/C (m g/g.C) 0 ~
%Ro
Alkanes
0 ,"-' ,c" ',"
./, ::o~,',
Wo)
Aromatics 0
lO0
0.5
0
223
, FPP
I00
0
60 50 65
'°°l
% T/(T-I-M) r~ N 2 ~
°
T
DPEP/ETIO ETIO
~' a S
RATIOS
PP-I
?I 2.0
(Modified from Mackenzie ef al.,1982)
FPP = Free bose petroporphyrins PP-1 = Methylphenonthrens index S and H.T.D = Beginning of sterone and hopone thermal destruction TTR = Tricyclic terpone ratios
Fig. 8. Range of individual molecular measurements of thermal maturation including the vitrinite reflectance values for the marine La Luna source rocks and derived crude oils (modified from Mackenzie et al., 1982). where the ETIO petroporphyrins have been thermally destroyed, the PP-I can still be used for the maturity assessment of both crude oils and rock extracts. Taking into consideration the results obtained in the present study, we propose three main maturity zones (low mature, mature and very mature) as shown in Fig. 7 for the classification of the Venezuelan crude oils and source rock extracts. According to the maturity range of the samples, we used different parameters for the assessment of the level of maturity. The assessment of the level of maturity of crude oils is very important to relate them with their time of generation, and to study the migration path that oil follows in some basin. The extent of thermal maturation of crude oils can be accurately determined by selected molecular parameters. For instance, when reactions such as isomerization of steranes (% 20S and %tiff) and triterpanes and the aromatization of the monoaromatic steroid hydrocarbons have reached their end point (R o < 0.9), the ETIO porphyrin ratio C2sE'/C27 E' and C29/C28E and the methylphenanthrene parameter (PP-1) can be applied in order to rank crude oils according to maturity differences. Furthermore, bitumen derived from kerogen with type II organic matter contain abundant petroporphyrins. This type of kerogen is usually lacking in vitrinite particles making reflectance measurements difficult. However, it is under these circumstances that the petroporphyrins can provide a useful alternative for maturity assessment in a very wide range of R o equivalents (0.3 to ~ 1.1%).
Individual molecular measurements for thermal maturation have been modified for the La Luna rock extracts and derived crude oils (Mackenzie et al., 1982; Fig. 8). Our results show, in agreement with Mackenzie et al. (1984), that different reaction types, such as configuration isomerization and aromatization might be expected to have significantly different temperature dependence. The relationships shown in Fig. 8 for the alkanes and aromatics cannot be considered general and have to be calibrated in each basin. For the La Luna extracts and derived crude oils, the isomerization reactions of the hopanes and steranes reach equilibrium at about Ro values of 0.6%, while the aromatization of the monoaromatic steroid hydrocarbons, as well as the thermal destruction of the DPEP porphyrins series, is reached at Ro values of 0.9%. As shown in Fig. 8, the methylphenanthrene parameter (PP-1) and the ETIO porphyrin ratios can be used to extend the thermal maturity assessment for the La Luna source rock extracts and derived crude oils to higher maturity levels. CONCLUSIONS
The combination of various molecular maturity parameters according to the maturity range of the rock extracts and crude oils have been successfully applied in order to assess the level of maturity of the La Luna marine rock extracts and derived crude oils. In addition, the uses and limitations of different
O S W A ~ GALLANGOand EERNANDOCASSANI
224
parameters have been established. F o r very mature samples, the relative maturity level can be assessed by the methylphenanthrene index (PP-1) and by the E T I O porphyrin ratios C2s E'/C27 E' and Ce9 E/C2s E. These last ratios change with increasing maturity, probably due to dealkylation reactions which leads towards the lower molecular weight E T I O components. Variation in the steranes carbon number distribution in both rock extracts and crude oils with increasing maturity have been observed. Such variation can affect the correlation parameters which are normally used in the characterization of source rocks and crude oils. On the other hand, the relative increase in the C2, and C22 steranes with increasing maturity can be useful in the maturity assessment of very mature rock extracts and crude oils. The increase in C21 and C22 steranes could be due to a preferential thermal destruction of regular steranes from C27 to C29 or by converting these to C2, and C22. Heating experiments are in progress to elucidate this.
REFERENCES
Alexander R., Strachan M. G., Kagi R. I. and Van Bronswijk W. (1986) Heating rate effects on aromatic maturity indicators. In Advances in Organic Geochemistry 1985 (Edited by Leythaeuser D. and Rulkotter J.), pp. 997-1003. Pergamon Press, Oxford. Barwise A. J. G. and Park P. J. D. (1983) Petroporphyrin fingerprinting as a geochemical marker. In Advances in Organic Geochemistry 1981 (Edited by Bjgroy et aL), pp. 668~574. Wiley, Chichester. Barwise A. J. G. and Robert I. (1984) Diagenetic and catagenetic pathways for porphyrins in sediment. Org. Geochem. 6, 587-595. Barwise A. J. G., Evershed R. P., Wolff G. A., Eglinton G. and Maxwell J. R. (1986) High-performance liquid chromatography analysis of free-base porphyrins. J. Chromatogr. 368, 1-9. Casagrande D. J. and Hodgson G. W. (1971) Geochemical simulation evidence for the generation of homologous decarboxylated porphyrins. Nat. Phys. Sci. 233, 123-124. Casagrande D. J. and Hodgson G. W. (1974) Generation of homologous porphyrins under simulated geochemical conditions. Geochim. Cosmochim. Acta 38, 1745--1758. Cassani F. (1985) Organic geochemistry of extra-heavy crude oils from the Eastern Venezuelan basin. Ph. D. dissertation, University of Bristol, England. Cassani F. and Eglinton G. (1986) Organic geochemistry of Venezuelan extra-heavy oils. 1. Pyrolysis of asphaltenes: a technique for the correlation and maturity evaluation of crude oils. Chem. Geol. 56, 167-183. Cassani F., Gallango O., Talukdar S., Vallejos C. and Ehrmann U. (1988) Methylphenanthrene maturity index of marine source rock extracts and crude oils from the Maracaibo basin. In Advances in Organic Geochemistry 1987(Edited by Mattavelli L. and Novelli L.), pp. 73-80. Pergamon Press, Oxford. Clementz D. M., Demaison G. J. and Daly R. (1979) Well site geochemistry by programmed pyrolysis. Proc. llth Annual Offshore Technol. Conf., pp. 465-470. Didyk B. M., Alturky Y. I. A., Pillinger C. T. and Eglinton G (1975) Petroporphyrins as indicators of geothermal maturation. Nature (London) 256, 563-575. Ekweozor C. M. and Strausz O. P. (1983) Tricyclic terpanes in the Athabasca oil sands: their geochemistry. In
Advances in Organic Geochemistry 1981 (Edited by Bj~roy et al.), pp. 746-766. Wiley, Chichester. Erdman J. G. (1965) U.S. Patent 3 190, 829. Garrignes P., de Vazelhes-DeSury R., Angelin M. M., Ewald M., Oudin J. L. and Connan J. (1984) Analysis of series of aromatics isomers by high resolution spectrofluorometry and capillary gas chromatography in HPLC fractions of crude petroleum and sedimentary rock extracts. In Advances in Organic Geochemistry 1983 (Edited by Schenk P. A., de Leeuw J. W. and Lijmbach G. W. M.), pp. 829-837. Pergamon Press, Oxford. Grantham P. J. and Wakefield L. L. (1988) Variations in the sterane carbon number distributions of marine source rock derived crude oils through geological time. Org. Geochem. 12, 61-73. Hoffmann C. F., Mackenzie A. S., Lewis C. A., Maxwell J. R., Oudin J. L., Durand B. and Vanderbroucke M. (1984) A biological marker study of coals, shales and oils from the Mahakam delta, Kalimantanm, Indonesia. Chem. Geol. 42, 1-23. Mackenzie A. S. (1984) Application of biological markers in petroleum geochemistry. In Advances in Organic Geochemistry (Edited by Brooks J. and Welte D.), pp. 115-214, Academic Press, London. Mackenzie A. S. and Maxwell J. R. (1981) Assessment of thermal maturation in sedimentary rocks by molecular measurements. In Organic Maturation Studies and Fossil Fuel Exploration (Edited by Brooks J.), pp. 239-254. Academic Press, London. Mackenzie A. S., Hoffman C. F. and Maxwell J. R. (1981) Molecular parameters of maturation in the Toarcian shales, Paris basin, France--III. Changes in aromatic steroid hydrocarbons. Geochim. Cosmochim. Acta 45, 1345-1355. Mackenzie A. S., Brassell S. C., Eglinton G. and Maxwell J. R. (1982) Chemical fossil: the geological fate of steroids. Science 217, 491-504. Quirke J. M. E. and Maxwell J. R. (1980) Petroporphyrins III. Tetrahedron Lett. 36, 3453-3456. Radke M. and Welte D. H. (1983) The methylphenanthrene index (MPI): a maturity parameter based on aromatic hydrocarbons. In Advances in Organic Geochemistry 1983 (Edited by Bj~roy M. et al.), pp. 504-512. Wiley, Chichester. Radke M., Welte D. H. and Willsch H (1982) Geochemical study on a well in the Western Canada basin: relation of the aromatic distribution pattern to maturity of organic matter. Geochim. Cosmochim. Acta 46, 1-10. Radke M., Welte D. H. and Willsch H. (1986) Maturity parameters based on aromatic hydrocarbons: influence of organic matter type. In Advances in Organic Geochemistry 1985 (Edited by Leythaeuser D. and Rullkrter J.), pp. 51--63, Pergamon Press, Oxford. Seifert W. K. and Moldowan J. M. (1978) Application of steranes, terpanes and monoaromatics to the maturation, migration and source of crude oils. Geochim. Cosmochim. Acta 42, 77-92. Seifert W. K., Moldowan J. M. and Jones R. W. (1980) Application of biological marker chemistry to petroleum exploration. Proc. Tenth World Petroleum Congr., Vol. 2, pp. 425-438. Heyden, London. Talukdar S., Gallango O. and Chin-Alien M. (1986) Generation and migration of hydrocarbons in the Maracaibo basin: an integrated basin study. In Advances in Organic Geochemistry 1985 (Edited by Leythaeuser D. and Rullk6ter J.), pp. 261-279. Pergamon Press, Oxford. Wingert S. W. and Pomerantz A. (1986) Structure and significance of some twenty-one and twenty-two carbon petroleum steranes. Geochim. Cosmochim. Acta 50, 2763-2769. Yen T. F., Boucher L. J., Tynan E. C. and Vaughan G. B. (1969) Vanadium complexes and porphyrins in asphaltenes. Am. Chem. Soc. Prepr. Div. Pet. Chem. 14. E. 32, 55, 87-99.
0146-6380192$5.00+ 0.00 Copyright © 1992PergamonPress Ltd
Biological marker maturity parameters of marine crude oils and rock extracts from the Maracaibo Basin, Venezuela OSWALDOGALLANGO and FERNANDOCASSANI Intevep, S. A., Apartado 76343, Caracas 1070-A, Venezuela (Received 19 October 1988; accepted in revised form 14 August 1991)
Abstract--The maturity parameters obtained from the analysis of saturated and aromatic hydrocarbons and petroporphyrins in marine rock extracts (La Luna source rock) and crude oils from the Maracaibo basin were examined by gas chromatography-mass spectrometry (GC-MS) and high performance liquid chromatography (HPLC). The results obtained indicate that in very mature samples there is a preferential destruction of C29 steranes over C27 and C2s as well as an increase in the C2t and C22 regular steranes. These changes seem to indicate that thermal maturity affects the correlation parameter normally used in the characterisation of oils and rock extracts. The demetallated petroporphyrins show a decrease of DPEP series and increase in ETIO components with increasing maturity. In very mature samples the ETIO porphyrin ratio, C2sE'/C27E' and C29E/C2sE, together with the methylphenanthrenes parameter (PP-1), are very useful for classifyingboth rock extracts and crude oils in terms of maturity. Of the parameters presented here, the methylphenanthrene parameter (PP-I) and the ETIO petroporphyrin series are the most promising source of geochemical information for better assessment of maturity of oil and source rock extracts beyond the main oil generation zone (Ro/> 0.9%), after which other molecular maturity parameters have reached equilibrium. Key words--Maracaibo Basin, biological marker, maturity parameter, petroporphyrins, phenanthrene parameter, La Luna Formation, crude oils
INTRODUCTION The use of a molecular parameter in the assessment of thermal maturity of the organic matter in sedimentary rock and crude oils has increased in the last decade. Reaction such as isomerization of hopanes and steranes, aromatization of monoaromatic steroids, the methylphenanthrene index and changes in the distribution of the alkyl porphyrins have been widely used (Seifert and Moldowan, 1978; Mackenzie, 1984, Radke et al., 1982: Barwise and Robert, 1984; Quirke and Maxwell, 1980; Alexander et al., 1986). The isomerization reactions of the hopanes and steranes are of limited use in petroleum exploration since they often reach a maximum before the major generation of hydrocarbons occurs (Mackenzie and Maxwell, 1981), especially in marine crude oils and source rocks. The aromatization ratio of monoaromatic steroids, which involves the loss of Ci9 methyl group leading to the formation of triaromatic steroidal hydrocarbons (Mackenzie et al., 1981), is apparently affected by migration. This effect seems to lead to a preferential depletion of triaromatic components relative to monoaromatic steroidal hydrocarbons in oils (Hoffman et al., 1984). In addition, the use of this parameter is limited to a part of the main oil generation phase. On the other hand, the methylphenanthrene index has been used in recent years in the
assessment of maturity of rock extracts from type III organic matter and especially coals (Radke and Welte, 1983; Radke et al., 1982; Garrigues et al., 1984). However, Cassani (1985) and Radke et al. (1986) found that phenanthrenes are of limited values for the maturity assessment of type II kerogen and marine crude oils, although later result (Cassani et al., 1988) showed improved applicability at high maturity. Porphyrins in sediments are thought to be derived from chlorophyll precursors in plants and photosynthetic bacteria. Immature sediment and crude oils contain predominantly DPEP type porphyrins. With increasing thermal maturity, the DPEP components are thought to cleave at the isocyclic ring to produce etioporphyrins (Didyk et al., 1975) or DPEP decreases relative to ETIO as a consequence of the preferential thermal destruction of DPEP over ETIO petroporphyrins, as proposed by Barwise and Robert (1984). Thus, a measure of the relative concentration of DPEP and ETIO types reflects the thermal maturity of both sediments and oils, The ratio (%DPEP) varies normally from 100 to 0 with increasing maturity. However, the use of maturity parameters is limited due to the fact that it changes in a relative narrow maturity range. Notwithstanding, Cassani (1985) proposed a couple of maturity ratios in the ETIO series as potential maturity parameters for mature samples which can be extended in to the very
215 OG 18/2--F
216
OSWALDO GALLANGO and FERNANDO CASSANI
are shown in Fig. 1. The La Luna formation (Upper Cretaceous) rock samples examined were from essentially the same stratigraphic interval buried to different depths. Samples were obtained of the type section (sample 3D) and from different wells in order to obtain the whole range of thermal maturity. Sampling of the same stratigraphic interval at various depths should minimize effects caused by lithological or source variation. The La Luna Formation consists mainly of limestone and calcareous shales which contain type II organic matter and is the most important source rock in the Maracaibo basin. General geological and geochemical information can be found in Talukdar et al. (1986) and Cassani et al. (1986). The crude oils were obtained from different wells distributed through the basin. In order to avoid differences due to biodegradation, migration and oil mixing, only crude oils that were apparently unaffected by such processes were selected.
mature region. According to him, the variation in these ratios with increasing maturity are probably due to dealkylation reactions. Ekweozor and Strausz (1983) suggested the application of the tricyclic terpane ratio (Cz3/C21 as a potential oil and source rock maturity indicator. More recently, Cassani (1985) and Cassani et al. (1988) reported data in support of this earlier study and added a similar ratio (C23/C24). In this work special attention is paid to maturity ratios of certain alkanes and aromatic biological marker and in particular, to the porphyrin compounds. We report the uses and limitations of steranes, methylphenanthrenes and petroporphyrins in the assessment of maturity of extracts of the La Luna Formation and La Luna derived crude oil of the Maracaibo basin (Venezuela). The destructive effects of excessive thermal maturity on the distribution of biological markers such as hopane and steranes are discussed. EXPERIMENTAL
G C - M S and H P L C
Samples
The GC-MS procedures for the analysis and the calculation of component ratios from mass fragmentograms have been decribed elsewhere (Mackenzie
The locations of the oils and rock samples collected from outcrop and drill holes from Maracaibo basin
GULF OF VENEZUELA
\ l'~
o
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~
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\ •
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¢1
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~.
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\
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lkx~.
A
• 25
ROCK SAMPLE
50
Km
F /
Fig. 1. Location of the La Luna marine source rocks and derived crude oils examined from the Maracaibo basin.
Maturity parameters of marine rock and oil
217
Table I. Bulk and biological marker maturity parameters of the La Luna derived crude oils* Aromatic hydrocarbons
Saturate hydrocarbons
Sample DM-116t VLD-273 UD-170 VLA-710 UD-209 ALT-15 CL-87 VLE-686 VLC-612 1I-Y-20X
Reservoir depth (ft) Age 5531 10,023 16,475 13,038 15,600 16,660 15,745 15,270 12,740 16,466
C M C C C C C C E C
S~ (%) 4.8 1.4 1.8 1.3 1.4 0.6 0.4 0.4 0.2 0.2
H/C:~ % 20S % ill/ 1.56 1.69 1.69 1.74 1.76 1.77 1.86 1.85 1.88 1.89
43 48 52 46 58 62 53 58 65 i.d.
50 53 53 56 58 60 57 60 64 i.d.
C23"3 C27 COs C~ CO4-3 (%) (%) (%) 3.4 1.9 2.4 2.2 2.3 1.5 1.5 1.3 i.d. i.d.
36 35 35 35 36 36 36 36 36 35
30 32 33 33 34 36 34 34 36 39
34 33 32 32 30 28 30 30 29 26
Demetallopetroporphyrins
T/T + M (%)
PP-I
DPEP (°,6)
47 87 82 100 100 100 100 100 i.d. i.d.
1.50 1.10 1.50 1.19 1.14 -0.60 0.57 0.30 0.21
30 10 6 4 0 0 0 0 0
71
CaE' CovE' - -
CO9E c_mE w
-2.04 1.65 1.10 0.62 0.42 0.38 i.d. i.d.
-2.31 1.86 1.14 0.50 0.33 0.33 i.d. i.d.
*For details of measurements see Table 3 and Text. = In whole oil. t L o w maturity oil. Ages: C = Cretaceous; E = Edcene; M = Miocene. i.d. = Indeterminate because of excessive maturity.
et al., 1982, Mackenzie, 1984; Cassani and Eglinton, 1986; Cassani et al., 1988).
The petroporphyrins of crude oils and bitumens were extracted as their free-bases by demetallation with methane sulphonic acid, according to the procedure of Erdman 0965). Typically, crude oil (lg) was heated (100°C, 4h) with at least a five-fold excess of methane sulphonic acid (98%, Aldrich, 5-10 ml). The reaction was quenched by pouring the acid/oil mixture into distillated water (20 ml). After allowing to cool, the coagulated material was removed by filtration. The aqueous filtrate, containing the porphyrins as dication, was extracted three times with dichloromethane, neutralized with NaHCO 3 and dried over Na2SO4. The free-bases obtained in this way were purified through a short alumina column. The free-based petroporphyrins were then analyzed by HPLC using three 25 c m x 4.6 mm i.d. columns coupled in series (Barwise and Park, 1983). A gradient HPLC solvent delivery system (l.00 ml/min) was controlled by a varian 5000 solvent programmer, and the eluting porphyrins were detected using a variable wavelength u.v./visible spectrophotometer (Altex model 153) with the detector monitoring 400 nm. The spectra were recorded using a CIS-HP 1000 data aquisition system. Demetallated petroporphyrins were resolved using the solvent mixture gradient reported by Barwise et al. (1986).
Petroporphyrins peaks were assigned (Cassani, 1985) by separation of the demetallated fraction in individual peaks by HPLC. Assignments were made on the basis of probe mass spectrometric analysis of the collected fraction. In addition, the assignments of C31 and C32 DPEP-porphyrins were given by coinjection with a standard. The assignments of the C27E' and C2sE' etioporphyrins (E' = fl unsubstituted) were tentatively identified by comparison of their retention patterns with Boscan oil (R. Eversbed, unpublished results). The La Luna marine source rock extracts and derived crude oils have very high vanadyl/nickel porphyrin ratios (eg. 100 to 1), thus differences in DPEP and ETIO content or distribution are primarily from vanadyl porphyrins. Elemental analysis determination
Elemental analysis of crude oils and kerogen was performed on a Perkin Elmer model 240B elemental analyzer. The kerogen samples were preextracted with dichloromethane and dried at 90°C under vacuum for 24 h prior to the C and H determination. The rock samples were extracted in a soxhlet extraction apparatus with dichloromethane prior to Rock-Eval pyrolysis. The samples (50-100mg) were analyzed in a Rock-Eval (series I) device, with temperature programming from 250 to 500°C. The
Table 2. Bulk and biological marker maturity parameters of the La Luna source rock samples with increasing degree of thermal maturation* Sample 3D P-114 VLE-738 SOL-6 30F-IX CLA-11 UD-149 VLB-704
Depth (it)
T~axt °C
Outcrop 431 8933 435 16,076 439 17,143 441 15,232 443 15,181 450 16,392 > 4 6 0 13,743 > 460
H/C °
Ro~ %20S
1.30 1.20 1.10 1.00 0.90 0.80 0.58 0.56
0.43 --0.77 -1.09 1.27 1.30
52 47 50 50 41 54 52 47
%[3fl %T/T + M 59 57 58 58 63 56 59 52
55 43 85 87 100 100 i.d. i.d.
PP-I
%DPEP
C a E' COTE,
C~ E Ca E
1.63 1.40 1.60 1.60 1.70 1.50 0.70 0.30
81 60 -------
--1.49 1.41 1.09 0.18 i.d. i.d.
--2.99 1.51 1.34 0.12 i.d. i.d.
t = Tma~ (Rock-Eval) of extracted rock. i.d. = Indeterminate because of excessive maturity. ~Measured in isolated kerogen. Re values are approximate due to the low amounts of vitrinite in these samples. *For details see Table 3 and text.
OSWALDOGALLANGOand FERNANDOCASSANI
218
Parameter %20S
%8,8
%T(T + M)
%DPEP
°/oC27 %C28
Ratio C29 ~t, a, a,-sterane 20S
Table 3. Molecular parameters employed (see Tables 1 and 2) Significance lsomerization at sterane C-20 (S); ratio rises from 0 to ~ 50% with increasing maturity; oil generation could start at ~ 40% (Mackenzie et aL, 1982)
20S + 20R C29 regular steranes: ~,#,# (R+S) Total Steranes Steroid hydrocarbon: C2s- triaromatic C29Mono + C28-triaromatic Porphyrins DPEP DPEP + AETIO
Aromatization of C ring aromatic hydrocarbon; ratio rises from 0 to 100% with increasing maturity; this ratio is more temperature dependent than % 20S sterane ratio (Mackenzie et al., 1982); oil generation could start at ~ 40-60%
Relative abundances of C27, C2s and C29 :t, 8, # steranes
These distributions depend on the nature of the organisms which contribute to the organic matter of the forming sediments (Huang and Meinsehein, 1979)
Tricyclic terpane ratio
Change with increasing maturity probably due to preferential generation of Cu components from precursors in the polar fractions and kerogen (Cassani et al., 1988); useful in the assessment of maturity of Cretaceous marine source rocks and crude oils; varies from ~ 3.5 to ~ 1.0
Maturity parameter:
Rearrangement of,, - 1 and 9 to the more stable 8-2 and 3 components (Radke et aL, 1982); used mainly for very mature marine samples (Cassani et aL, 1988)
lsomerization at sterane C-14 and C-17, ratio rises from 0-50% to ~ 70-75% with increasing maturity: isomerization normally complete before major generation of hydrocarbons (Mackenzie and Maxwell, 1981)
Apparent C42 bond cleavageof isocyclicring in the DPEP porphyrins (e.g. Didyk et al., 1975) or thermal destruction of DPEP porphyrins (Barwise and Park, 1983); ratio falls from ~ 100% to 0; oil generation could start at ~ 80--60% (Barwise and Park, 1983)
%C29 C23 - 3 C24 - 3
PPI
(1 - M P + 9 - MP) (2- MP + 3 - MP) MP = methylphenanthrene
operating parameters are the same as those published earlier by Clementz et al. (1979). Vitrinite reflectance (%Ro) measurements were made on isolated ( H C I / H F procedure) kerogens using a Zeiss Universal microscope. RESULTS AND DISCUSSION Saturated hydrocarbons
Tables 1 and 2 show the maturity parameters o f the La Luna F o r m a t i o n and derived crude oils. Both crude oils and source rocks are listed in order of maturity from low mature to very mature as proposed by Talukdar et al. (1986) and Cassani et al. (1988) based on bulk chemical and some biomarker parameters. It is well known that all these oils are generated from the same source rock (La Luna Formation; Talukdar et al., 1986). As can be observed in Tables 1 and 2, the sterane maturity parameters normally used for sediments and crude oils (% 20S; % tiff) do not show any trend with increasing sample maturity, because the end point of these isomerization reactions have been reached. The proposed % tiff equilibrium values of ca 65-75% (Mackenzie et al., 1982) are not reached even in the extremely mature samples, which present values for these parameters between 50 and 64%. Early reports suggest that these parameters are affected by migration which causes an enrichment of 14fl(H), 17fl(H) components (Seifert et al., 1980). However, this enrichment was not observed in the oils examined suggesting that such changes are probably due to
maturity effects. The similar values obtained in this parameter for the different samples analyzed agree with Mackenzie and Maxwell (1981) who found that this isomerization reaction reaches the end point in most crude oils. The % 20S parameter is more reliable than the previous ratio due to the fact, as we mentioned above, that the % of tiff shows more variations as a result of migration and source effects. The % 20S can be extended into the zone of petroleum generation, according to Mackenzie (1981). Again, in the mature oil samples (Table l), these values reach the end point (range 43-55%). The very mature samples present values in the range of 53-65% which limit the use of this parameter. On the other hand an increase in the relative concentration of C2t and C22 steranes as well as decrease in the regular steranes from C29 to C28 , is observed with increasing maturity (Figs 2 and 3). Thermal destruction of hopanes has also been observed (Fig. 4). As listed in Tables l and 2 and shown in Figs 2 and 3, there is a predominance of C27 steranes over C29 steranes in the low mature samples (i.e. DM-116 crude), while in the mature and very mature samples, the proportion of C29 to other regular steranes decreases. Similar results have been noticed by Cassani and Eglinton (1986) and by Wingert and Pomerantz (1986) during asphaltene and crude oil thermal degradation laboratory experiments, respectively. We believe that a preferential degradation of the steranes seems to be the explanation. Recently, G r a n t h a m and Wakefield (1988) observed similar behavior in the sterane distribution in crude oils derived from marine
Maturity parameters of marine rock and oil 27
A)
219
29
21ST
MIZ 218
28
B) ¢II CO
UD.170
21ST VLC-612
c)
27
i
- -
Retention time
Fig. 2. Mass fragmentograms of regular sterane (m/z 218) distributions for: (A) low mature; (B) mature; and (C) very mature marine La Luna derived crude oils. carbonate and siliciclastic source rocks through time. According to the authors these changes are due to a diversification of marine sources instead of thermal alteration. However, Grantham and Wakefield (1988) did not take into consideration the maturity effect in the steranes distribution. Our results show that there are variations in the sterane carbon number distributions during maturation of both source rock and crude oils. Aromatic hydrocarbons A further measurement of maturity which has been shown to be reliable in a number of basins is the extent to which C-ring monoaromatic steroid hydrocarbons (M) have been aromatized to triaromatic steroid hydrocarbons (T) (Mackenzie et al., 1984). These changes are measured by the aromatization ratio (% T/T + M) and are commonly applied in the maturity assessment of source rocks and crude oils.
The ratio increases with maturity from 0 to 100% and is more temperature dependent than the sterane ratio % 20S (Mackenzie et al., 1982). In the case of the sample examined in this study (Tables 1 and 2), this parameter tends to increase from the less mature crude oil (DM-116) and rock extract (3D) towards the most mature samples (VLE-686 and CLA-111), respectively. On the other hand, there is a good correlation between the atomic H/C ratio measured in the isolated kerogen and in the whole oil with the aromatization values. However, the use of the aromatization parameter % T/(T + M) for the maturity assessment of very mature crude oil and rock extracts is limited because the aromatization values of these samples reach their maximum in the early stage of maturation. This is probably due to the temperature dependence of this parameter (Mackenzie et al., 1982). Moreover, among the very mature samples (Tables 1 and 2) it is difficult to establish any further
220
OSWALDOGALLANGOand FEP.NANDOCASSA~
27 21ST A]
28 29
P-114 =
.
/
22St J •
[
k
•
..
J~
MIZ
218
.l iiJ.. 28
B)
.E ol
SOL-6
0
....
£
~ ......
L~
1
t,
C) CLA-111
29
--- TI --
Retention time
Fig. 3. Mass fragmentograms of regular sterane (m/z 218) distributions for: (A) low mature; (B) mature; and (C) very mature La Luna rock extracts.
VLC-612
M/Z 191 lr"-Hopanes - 1
Tricyclic terpanes
,
A)
23
;1
26
21St
M/Z 218
B) 27
29
Retention time "~ Fig. 4. Mass fragmentograms of (A) triterpanes (m/z 191) and (B) regular steranes (m/z 218) for a very mature marine La Luna Formation derived crude oil (well VLC-612). Notice the thermal destruction of the hopanes and the decreasing in the relative concentration of the regular steranes compared to C2mand C22 steranes.
Maturity parameters of marine rock and oil maturity trend based on this parameter. The same limitation applies for other molecular parameters of alkanes such as %20S and the tricyclic terpane ratio, which can only be used in the main oil generation zone (Ro ~ 0.5-0.9%). In an attempt to find another maturity parameter which could be applied to the La Luna source rock and derived crude oils beyond the main oil generation zone, (Ro > 0.9%), we examined the distribution of methyiphenanthrenes by G C - M S and calculated the methylphenanthrene parameter PP-1 (Cessani et al., 1988). As shown in Tables 1 and 2, no major changes in PP-1 values for the low mature to mature samples are observed. In addition, there is no correlation between the PP-1 values and the other maturity parameters. However, for very mature samples a
C32D C31o
A)
DM-116
C28E' C29 E
B)
L
221
C32D
A)
I
I C31D C32D
"p..
C29E C28E" I
~0 13) u~ t~
SOL
(J
l
C27E" C28E" I C29E 3
0
F
-
I
~
C27E"
'P._.
-C27E' C28E' I C29E
l
ilL' C27E'
Retenliontime
b
Fig. 6. HPLC chromatograms of the La Luna marine rock extracts showing the changes in the demetallopetroporphyrin distributions with increasing maturity. decrease of PP-1 values occurs with increasing maturity of crudes and rock extracts. This agrees with Cassani et al. (1988) and differs from previous published results (Radke et al., 1986). As Cassani et al. (1988) pointed out, the methylphenanthrene parameter (PP-I) seems to be very useful in the assessment of maturity of very mature marine source rocks and crude oils.
C28E Petroporphyrins
D) C28E'
za - -
Retention time
Fig. 5. HPLC chromatograms of the La Luna derived crude oils showing the changes in the demetallopetroporphyrin distributions with increasing maturity.
Figures 5 and 6 show the HPLC chromatograms of the demetallated petroporphyrins for the oils and rock extracts examined. As can be seen in these figures, there are significant changes in the porphyrin distributions with increasing maturity. The low maturity samples of crude oils and rock extracts contain mostly DPEP type (Tables 1 and 2) with ETIO porphyrins present in very low concentration. With increasing maturity, the DPEP content decrease and the ETIO porphyrins increase. In very mature
222
OSWALDO GALLANGO a n d FERNANDO CASSANI
samples the DPEP components are absent and the ETIO type shows a simple distribution with a predominance of the C28E' and C29E components (Figs 5 and 6). In the most mature samples a progressive increment of C27E' and C28E over C27E' and C29E is observed. Similar results were obtained by Cassani (1985) during laboratory thermal experiments performed on the Boscan oil. Mechanisms proposed for the thermal DPEP to ETIO conversion involves cleavage of the isocyclic ring which should give rise to etioporphyrins (Quirke and Maxwell, 1980), or as a consequence of preferential thermal destruction of DPEP over ETIO petroporphyrins as proposed by Barwise and Robert (1984). However, the distribution of ETIO type cannot be visualized as a simple concentration of these compounds by thermal destruction of the DPEP types as suggested by Barwise and Park (1983). Dealkylation reactions apparently also control the distribution of these ETIO porphyrin compounds. This type of reaction has been reported
Structure
previously in laboratory thermal alteration experiments of ETIO porphyrins (Yen et al., 1969; Casagrande and Hodgson, 1971, 1974; Cassani, 1985). These dealkylation reactions tend toward the lower molecular weight components with increasing thermal maturation. According to the results obtained by Cassani (1985) and the observation in natural samples examined in the present study, we propose the use of the ratios C2s E'/C27E' and C29 E/C28E in the ETIO series for very mature samples in order to extend the use of petroporphyrins in the assessment of thermal maturity of both crude oils and rock extracts beyond the maximum zone of oil generation (Re >/0.9%, Fig. 7). Our results (Tables 1 and 2 and Figs 6-8) show that there is a good correlation between PP-I values and the ETIO petroporphyrin maturity ratio, and that both maturity parameters lead us to distinguish maturity differences among crude oils or rock extracts. Furthermore, in the extremely mature samples
Biological marker
Low mature (Ro<0.6%)*
Steranes
% 20S, % ~l~ (0-55)*
Triterpanes
Mature (Re 0.6-0.9%)*
C23/C 24 (4.0-1.0)*
Phenanthrenes
R
Very mature (Ro>0.9%)*
PP-I (1-0.2)*
R
Aromatic steroids
% T1T+M (0-60)*
% TIT+M (60-100)*
Porphyrins
% DPEP (100-50) ~
% DPEP (50-0) ~
H R
R
R R
R
~
BR ETIO C28E' ' C29E C27E' ' ~
R R
R
* = values are approximated
Fig. 7. Biological markers and the molecular maturity parameters applied to the La Luna source rocks and derived crude oils. According to the level of maturity, different parameters are used for the assessment of the level of maturity.
Maturity parameters of marine rock and oil
H/C (m g/g.C) 0 ~
%Ro
Alkanes
0 ,"-' ,c" ',"
./, ::o~,',
Wo)
Aromatics 0
lO0
0.5
0
223
, FPP
I00
0
60 50 65
'°°l
% T/(T-I-M) r~ N 2 ~
°
T
DPEP/ETIO ETIO
~' a S
RATIOS
PP-I
?I 2.0
(Modified from Mackenzie ef al.,1982)
FPP = Free bose petroporphyrins PP-1 = Methylphenonthrens index S and H.T.D = Beginning of sterone and hopone thermal destruction TTR = Tricyclic terpone ratios
Fig. 8. Range of individual molecular measurements of thermal maturation including the vitrinite reflectance values for the marine La Luna source rocks and derived crude oils (modified from Mackenzie et al., 1982). where the ETIO petroporphyrins have been thermally destroyed, the PP-I can still be used for the maturity assessment of both crude oils and rock extracts. Taking into consideration the results obtained in the present study, we propose three main maturity zones (low mature, mature and very mature) as shown in Fig. 7 for the classification of the Venezuelan crude oils and source rock extracts. According to the maturity range of the samples, we used different parameters for the assessment of the level of maturity. The assessment of the level of maturity of crude oils is very important to relate them with their time of generation, and to study the migration path that oil follows in some basin. The extent of thermal maturation of crude oils can be accurately determined by selected molecular parameters. For instance, when reactions such as isomerization of steranes (% 20S and %tiff) and triterpanes and the aromatization of the monoaromatic steroid hydrocarbons have reached their end point (R o < 0.9), the ETIO porphyrin ratio C2sE'/C27 E' and C29/C28E and the methylphenanthrene parameter (PP-1) can be applied in order to rank crude oils according to maturity differences. Furthermore, bitumen derived from kerogen with type II organic matter contain abundant petroporphyrins. This type of kerogen is usually lacking in vitrinite particles making reflectance measurements difficult. However, it is under these circumstances that the petroporphyrins can provide a useful alternative for maturity assessment in a very wide range of R o equivalents (0.3 to ~ 1.1%).
Individual molecular measurements for thermal maturation have been modified for the La Luna rock extracts and derived crude oils (Mackenzie et al., 1982; Fig. 8). Our results show, in agreement with Mackenzie et al. (1984), that different reaction types, such as configuration isomerization and aromatization might be expected to have significantly different temperature dependence. The relationships shown in Fig. 8 for the alkanes and aromatics cannot be considered general and have to be calibrated in each basin. For the La Luna extracts and derived crude oils, the isomerization reactions of the hopanes and steranes reach equilibrium at about Ro values of 0.6%, while the aromatization of the monoaromatic steroid hydrocarbons, as well as the thermal destruction of the DPEP porphyrins series, is reached at Ro values of 0.9%. As shown in Fig. 8, the methylphenanthrene parameter (PP-1) and the ETIO porphyrin ratios can be used to extend the thermal maturity assessment for the La Luna source rock extracts and derived crude oils to higher maturity levels. CONCLUSIONS
The combination of various molecular maturity parameters according to the maturity range of the rock extracts and crude oils have been successfully applied in order to assess the level of maturity of the La Luna marine rock extracts and derived crude oils. In addition, the uses and limitations of different
O S W A ~ GALLANGOand EERNANDOCASSANI
224
parameters have been established. F o r very mature samples, the relative maturity level can be assessed by the methylphenanthrene index (PP-1) and by the E T I O porphyrin ratios C2s E'/C27 E' and Ce9 E/C2s E. These last ratios change with increasing maturity, probably due to dealkylation reactions which leads towards the lower molecular weight E T I O components. Variation in the steranes carbon number distribution in both rock extracts and crude oils with increasing maturity have been observed. Such variation can affect the correlation parameters which are normally used in the characterization of source rocks and crude oils. On the other hand, the relative increase in the C2, and C22 steranes with increasing maturity can be useful in the maturity assessment of very mature rock extracts and crude oils. The increase in C21 and C22 steranes could be due to a preferential thermal destruction of regular steranes from C27 to C29 or by converting these to C2, and C22. Heating experiments are in progress to elucidate this.
REFERENCES
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