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Generation and maturation of carboxylic acids in ancient sediments from the Maracaibo Basin, Venezuela
Advances in Organic Geochemistry 1989
Org. Geochem. Vol. 16, Nos I-3, pp. 211-218, 1990 Printed in Great Britain
0146-6380/90 $3.00+ 0.00 Pergamon Press plc
Generation and maturation of carboxyfic acids in ancient sediments from the Maracaibo Basin, Venezuela RUDOLF JAFFEand PIERO R. GARDINALI Universidad Sim6n Bolivar, Department of Chemistry, Geochemistry and Environmental Chemistry Lab., Apartado 89000, Caracas 1080=A, Venezuela (Received 10 January 1990; accepted 24 March 1990) Abstract---Cyclic and acyclic carboxylic acids in various rock samples from the Misoa and La Luna formations, Maracaibo Basin, Western Venezuela, were analyzed in order to study their generation and maturation processes. For this purpose, rocks from various degrees of maturity (Misoa: %Ro 0.35--0.77; La Luna: %Ro 0.5-1.8), and different origins (Misoa: type III OM; La Luna: type II OM) were studied. Linear acids and tricyclic as well as pentacyclic (hopanoic) triterpenoic acids were analyzed quantitatively by gas chromatography-mass spectrometry. Qualitative data from the isomeric distribution of the hopanoic acids (HAs) showed the expected trend of increasing abundance of the thermodynamically more stable (17~t,21fl) and (17fl,2P,) isomers, with the simultaneous disappearance of the thermodynamically less stable (17fl,21fl) isomers. However, at high maturity levels, the (17fl,21fl) isomers reappeared, to become the only HAs to be present. A similar trend of apparent maturity change was observed for the linear acids by studying the CPI of these compounds. A double generation process of carboxylic acids is suggested and confirmed by quantitative data. The latter showed that acids of different degrees of maturity are generated at various stages prior to the oil generation maximum. It is suggested that a major portion of the acids biosynthesized in recent environments are incorporated into the geopolymeric fraction during early diagenesis and are as such protected from further diagenetic degradation. It is also suggested that their maturation and generation is strongly dependent on their "speciation" within the geopolymer, and the structural changes in the geopolymeric matter during thermal maturation. The importance of these observations with respect to the application of carboxylic acids as biomarkers of oil migration is discussed. Key words--carboxylic acids, generation and maturation (of carboxylic acids), hopanoic acid stereochemistry, Maracaibo Basin
INTRODUCTION Carboxylic acids, and particularly hopanoic acids (HAs), have recently been reported to be useful biological markers of oil migration (Jaff6 et al., 1988a, b; Jaff6 and Gallardo, 1990). The isomeric distribution of the HAs in an important parameter in the determination of the migration histories of crude oils. Apparently, a significant proportion of the HAs are incorporated into the oil during its migration path, presumably via the "leaching" of carboxylic acids from sediments of relatively low maturity. Knowledge of the generation and maturation processes of carboxylic acids, particularly HAs, in ancient sediments is therefore of extreme importance. Until now there have been few studies on the maturation and generation of carboxylic acids and of HAs. The only reports on the maturation of HAs showed a behavior similar to their alkane counterparts, in that the thermodynamically less stable, biogenic isomers with the (17fl,21fl,22R) configuration decrease in favor of the more stable (17fl,21~) and (17~t,21fl) pair of disastereoisomers (22R and 22S) (Ensminger, 1977; Jaff6 et al., 1988b). In contrast to the hopanes however, where the (17~t,21fl)
configuration is thermodynamically the most stable isomer, in the case of the HAs, the (17fl,21~t) isomers have a degree of thermal stability similar to the (l 7~t,21r ) isomers. There are no reports to our knowledge concerning the generation of the HAs. Similarly, little is known about the generation and maturation of acyclic acids in ancient sediments, particularly in the C15 + range. Whilst Mackenzie et al. (1982) have reported on the molecular parameters of maturation of acyclic isoprenoid acids, nothing is known about their generation in ancient sediments. In the case of the n-acids, it is generally assumed that these compounds mimic their alkane counterparts with increasing maturation. Hence, with increasing thermal stress, the molecular distribution of immature acids with a high carbon preference index (CPI, even over odd predominance) is presumed to be transformed into a distribution with a CPI value of approximately one (Kvenvolden, 1966, 1967). The generation of carboxylic acids has been investigated by various authors (Surdam et al., 1984; Kawamura et al., 1986a, b; Kawamura and Kaplan, 1987). These studies suggested that the generation of acids could enhance the migration of oils by way of increased porosity and permeability of source rocks. This effect could therefore, also contribute to an 211
212
RUVOLVJ^vt~ and PIEROR. GARDINALI
enhancement in the incorporation of the less polar, more soluble, longer chain acids during the migration process as suggested by Jaff~ and co-workers (Jaff~ et al., 1988a, b; Jaff6 and Gallardo, 1990). However, little information exists on the generation of the long chain acids in ancient sediments. Laboratory heating experiments demonstrated the generation of long chain n-acids from kerogen (Kawamura et al., 1986a) and from torbanites (Derenne et al., 1988). Both studies showed that bound acids are stabilized or protected from biological and chemical alterations, thus preserving their immature characteristics after incorporation into the geopolymer. With respect to the maximum in the oil generation window, Surdam et al. (1984) reported the maximum generation of short chain acids to occur at a vitrinite reflectance (%Ro) between 0.4 and 0.5, while Kawamura et al. (1986a) suggested the generation of long chain acids occurs at values of %Ro between 0.5 and 0.6. The position of the generation of the acids relative to the maximum oil generation is, as mentioned previously, not only important for porosity enhancements in the source rocks, but also with respect to the incorporation of the acids by the oil during its migration path. Here, the acid "speciation" within the organic matrix, and the possible sequential generation of acids of different molecular and maturity distributions, could become important in the interpretation of the mono-carboxylic acid fingerprints used in oil migration studies. The objectives of this study were, therefore, to investigate in more detail the generation and maturation processes of long chain n-acids and triterpenoic acids in ancient sediments. EXPERIMENTALSECTION Monocarboxylic acids from ten rock samples from the Icotea borehole from the Misoa formation (Eocene, type III OM) and seven rock samples from different boreholes from La Luna formation (Cretaceous, type II OM) where analyzed. These rocks were selected in order to cover a wide maturity range. The samples from the Misoa formation, with a depth between 280 and 2668 m, cover a maturity range between a vitrinite reflectance (%Ro) of 0.35 and 0.77, while the more mature La Luna samples range from 0.5 to 1.8%R o. Between 80-100g of these rock samples were ground into a fine powder and extracted with a mixture of 3:1 v/v of chloroform/methanol. The concentrate of the extract was fractionated into its neutral and acidic fractions using the method described by McCarthy and Duthie (1962), modified by Arpino et al. (1977). The acidic fraction was then methylated with freshly distilled diazomethane. The mono-methyl ester fraction was then obtained after column chromatography and thin layer chromatography and spiked with a synthetic tetracyclic triterpenoic methyl ester, as internal standard for
quantitative analysis. This fraction was then analyzed by high resolution gas chromatography (GC) and by gas chromatography/mass spectrometry (GC/MS). The GC analyses were performed on a Hewlett Packard, model 5840-A gas chromatograph, equipped with a split-splitless injector, a high performance methyl silicone gum capillary column (25m, 0.2mm i.d., HP1), and a flame ionization detector. GC/MS analyses were carried out on a Helwett Packard 5970-A Mass Selective Detector, fitted with a HP 5880-A gas chromatograph, and a HP 5895-A Chemstation data system. The instrument was operated in the selected ion monitoring (SIM) mode at an ionization energy of 70 eV. Selected Ions such as molecular ions and characteristic fragment ions for the HAs ( m / z = 191,207 + n 14, M+), for the tricyclic acids ( m / z = 191, M +) and for the linear acids ( m / z = 74) were monitored for identification and quantification purposes. Mass spectrometric data and gas chromatographic retention behavior previously reported in the literature were used as an aid in the identification of individual components and isomeric distributions (Van Dorsselaer, 1975; Ensminger, 1977; Schmitter et al., 1978; Jaff~ et al., 1988a, b.) Quantitative data were obtained after experimental determination of response factors for the internal standard with respect to a standard solution of eicosanoic acid for the acyclic acids, and the C-30 (17]~,21/~,22R)-hopanoic acid for the cyclic compounds, both as their methyl esters. Quantification of the cyclic components was performed using m / z = 191 ions of these compounds with respect to the same fragment from the internal standard. Since chromatographic coelution of the different HAs does not permit the use of exclusively the m / z = 191 ion (Jaff6 et al., 1988a), these compounds were quantified based on their characteristics m / z = 207 + n 14 ions. The abundance of the m / z = 191 ion was then calculated based on the relative abundances of these characteristic ions with respect to the m / z = 191 ion reported in the literature for each of the HA isomers (van Dorsselaer, 1975). Rock Eval Pyrolysis and vitrinite reflectance data used in this study were kindly supplied by INTEVEP and were determined using standard geochemical methods. RESULTS AND DISCUSSION 1. Geochemical characterization o f the rock samples
The Misoa sequence (Eocene, Type III OM) covering a depth range between 280 and 2668 m showed a total organic carbon (TOC) content of 0.6-2.0%, and is therefore considered as a potential source rock (Talukdar et al., 1984). The samples from La Luna formation (Cretaceous, Type II OM), considered to be good source rocks (Talukdar et al., 1986), have TOC values from 1.3 to 6.5%. Both sample sets cover a relatively wide range of maturity as shown in Table I. An excellent correlation was observed
Generation and maturation of carboxylic acids in ancient sediments Table 1. Characteristics of the rock samples from the Misoa (Icotea borehole) and La Luna formations Sample
Depth (m)
Ro (%)
Tmx (°C)
Misoa Formation
I-1 I-2 I-3 I-4 I-5 I-6 I-7 I-8 I-9 1-10
282 532 961 983 1355 1832 1970 2433 2580 2668
0.35 0.40 0.46 0.46 0.57 0.47 0.69 0.74 0.70 0.77
425 436 439 433 441 447 439 450 462 459
La Luna Formation
QM-3 QM-5 Sol-6 UD-102 UD- 149 VLB-704 UD-120
surficial surficial 5242 4757 4984 4178 4733
0.50 0.50 0.72 nd 1.27 1.28 1.80
437 441 427 437 470 550 550+
between the %Ro and Tmax vs the depth for the Icotea samples, indicating the continuity of this rock sequence (Gardinali, 1989). Hence, present day burial depths will be used to indicate the level of maturity. This is not possible for the La Luna samples since these originate from different boreholes across the formation. 2. Characterization o f the acid fractions
Linear as well as cyclic acids were identified in both rock sequences. The distribution of the n-acids in the Icotea samples showed a range between C-8 and C-37, with a bimodal distribution maximizing at C-16, C-18 and C-28. This was observed particularly for the samples of intermediate maturity (530-1970 m) for which the higher molecular weight fraction was most abundant. In all cases, however, the C-16 and C-18 homologues were the more abundant components. No unsaturated acids were observed. Similar distributions have previously been observed in crude oils from various locations (JaffE and Gallardo, 1990; Jaff6 et al., 1988a, b; Behar and Albrecht, 1984; Mackenzie et al., 1983). A strong variation in the CPI was also observed for the acids, the CPI of the short chain acids (up to C-19) being consistently larger than that of the long chain acids (C-20+). The variation in the CPI for the long chain acids with changes in maturity of the rocks is discussed in detail later in this section. The n-acid fraction in the La Luna samples showed a distribution similar to that of the Icotea samples, except for the most mature sample (UD-120) which maximized at C-28. Again, the low molecular weight acids predominated over the C-20+ fraction, with this effect being more prominent for the La Luna type II OM, than for the Icotea type III OM. However, on comparison with the distributions of the n-alkanes there was no correlation in either case, indicating that there was no close correlation between the origin and geochemical behavior of the n-acids and the nalkanes in these samples. Similar findings were re-
213
ported by Kawamura et al. (1986a) when comparing acid and alkane fractions from kerogen pyrolysates. The CPI values for the short chain acids in the La Luna samples were high in all cases. The variations in the CPI values between the La Luna and Icotea samples are discussed later in this section. With respect to the cyclic acids, both sequences showed the presence of hopanoic acids (HAs). These compounds ranged from C-28 to C-33 (with the exception of the C-29) in the Icotea samples, whilst the C-30 to C-32 compounds were detected only in the most immature samples of the La Luna formation. The isomeric distribution of these acids is discussed below. Tricyclic terpenoic acids were only detected in the type II OM of some of the La Luna samples (QM-3, UD-149, VLB-704) with varying degrees of maturity. Only the C-20, C-21, C-24 and C-26 homologues, the latter two as pairs of diastereoisomers, were observed. Similar distributions have previously been reported by Cyr and Strausz (1983) in oil sands, and by Behar and Albrecht (1984) and Jaff6 and Gallardo (1990) in crude oils. As in the case of the linear components, no correlation was found between these acids and their alkane counterparts. Since these compounds showed the same distribution in the three samples, even though these were quite different in their thermal maturity, and as they were not detected in the Icotea sequence, they are not discussed further. 3. Maturation and generation o f HAs
The isomeric distribution of the C-30 to C-32 homologues of the HAs detected in the Icotea rock sequence are shown in Fig. 1. The distribution follows the expected trend of maturation, by which the thermodynamically less stable (17fl,21/~,22R) homologues, abundant in the most immature samples, decrease in abundance relative to the thermodynamically more stable (17//,21~,22R and 22S) and (17~,21/~,22R and 22S) isomers in the more mature samples. The disappearance of the (17I/,21/~) isomers in favor of the (17p,21~) and (17~,21/~) diastereoisomeric pair, seems to be essentially complete at ca 1360 m of depth. However, after this point, and at more elevated maturities, the relative abundance of the thermodynamically less stable isomer begins to increase with respect to the more stable ones, to a point (at a depth of 2440 m) where the (17/~,21//,22R) isomer is the only one present. This unexpected behavior, seems to indicate a secondary generation of HAs with a relatively immature signal at elevated maturities. These results are in agreement with the pyrolysis data from Kawamura et al. (1986a), who suggested that the acids tightly bound to the kerogen matrix preserve their immature character. It is interesting to notice that only one isomer, the (17/~,21/~,22R) is present at high maturities for the C-30 homologue. Despite the observation that the abundance change in the 22R configuration to the 22S configuration of this isomer is relatively fast
214
RUDOLFJAFFEand I~EROR. GARDINALI Depth ( m ) (%Ro)
C-3O ( m/z =235)
282 (~35) (0.35) 961 (0.46)
532 (0.40) 961
(0.46)
o= c o
._
lti
_ _
._aL_
983 (0.46)
1355 (0,57) 1632
J~ o
ll|
C-31 ( m/z = 249)
Depth(m) (%Ro)
I
~
983 (0.46)
_L
(0.47)
282 (0.35) 961 (0.46 ) • 983
(0.46)
' 1355 (0.57)
C-32 (m/z=263)
xla___ •
ALl
1832 (0.47) 1970
(0.69} 1355 (0.57)
1970 (0.69)
2435 (0.74) 1832 (0.47)
2435 (0.74)
1970 (0.89)
2580 (0.70)
2668 (0.77)
Depth ( m ) (% Ro)
~
,
~
.L
2435 (0.74) 2580 (0.70) 2668 (0.77)
Rt
2580 (QTO)
2668 (0.77) Rt
Rt
Fig. 1. Mass chromatograms of the C-30 (m/z =235), C-31 (m/z =249) and C-32 (m/z =263) HAs in the rock sequence of the Misoa formation (depth in meters and % vitrinite reflectance are indicated). (Jaff6 et al. 1988a), it is not observed at elevated maturity levels. Neither is any isomerization to the more stable isomers at positions 17 and 21 apparent. This may be a result of a faster rate of decarboxylation or degradation of the generated acids compared to the rate of isomerization. It may also indicate that this isomerization does not occur in ancient sediments. Quantitative data on these profiles give a much better insight into the processes involved in these observations. Figure 2(a) shows the concentration vs depth profile for the C-32 homologue of the HAs in the Icotea sequence. Similar profiles are observed for all the homologues, with a clear maximum at ca 1100 m. It is interesting to observe that the most stable as well as the least stable isomers are generated simultaneously, the (17~,21#) isomer being the most abundant in all cases. At ca 1800 m, all the HAs reach concentration levels similar to those of the immature rocks. A very careful look at the data also shows a small second maximum of only the (17/~,21~) isomers at ca 2400 m. This secondary generation has already been pointed out previously, and could, as suggested, be caused by secondary cracking of tightly bound "immature" acids from the geopolymers at elevated maturities. The quantitative data also indicate that the original biogenic free HAs seem to be bound in a major proportion to the geopolymers at a very early stage of diagenesis, or at least this bound fraction is preseved from degradation during early diagenesis.
However, a large proportion of these bound acids appear to be able to undergo isomerization during thermal maturation in order to produce a generation curve as shown in Fig. 2(a). With respect to the HAs in the samples from the La Luna formation, these were only observed in the most immature samples (QM-3, QM-5, SOL-6). The isomeric distributions of the HAs in these samples showed a very immature character, with the (17fl,21 fl) isomers being the predominant components. Only the SOL-6 sample showed the presence of small amounts of the thermodynamically more stable isomers, while QM-3 consisted exclusively of the (17/~,21fl) isomers of the C-30 to C-32 homologues, and QM-5 contained only the (17fl,21fl) C-32 compound. In all cases the (17fl,21fl) C-32 isomer was the most abundant component found. These distributions are consistent with those observed for the Icotea sequence, considering the maturity range of the La Luna samples (0.50-0.72% Ro). At this maturity range the secondary generation starts in the Icotea sequence. The rest of the La Luna samples are already too mature to contain any HAs. Surprisingly, however, SOL-6 with a %Ro of 0.72 showed the presence of (17/~,21~t) and (17~t,21fl) components. Perhaps the %Ro is not an adequate maturity parameter for these samples which are type II OM. Indeed, the Tm,x values for SOLo6 agree with our observations for the HAs, since it has the lowest Tm,xvalue of the La Luna samples. The data of the La Luna samples are therefore consistent with that for the Icotea sequence.
Generation and maturation of carboxylic acids in ancient sediments C - 3 2 HAs (ng/g) 0
20 40 ,,i.,i.-
60 80 100 t20 ..........
cPI 2 i
n-acids (ng/gl 3
0
i
,
100 i
L
~
I1 / IL /
.--o-
•
200 i
.
215 tot. n-aclds (nglg]
300 .
0 ,
2500 5000 . . . . . . . .
7 5 0 0 10 0 0 0 , • . •
n C-15
•
nc-z~
/I
o c-3o
•
~oto-oo,d,
Q 2000
~
I
•
•
aBS
OaR
t & 3000 J-
(el
J
1¢1
(d) 2
3
cPl
Fig. 2. (a) Generation profiles for the different isomers of the C-32 HAs from the Misoa rock sequence. (b) Depth profile of the CPI of the n-acids from the Misoa rock sequence. (c) Generation profiles of various n-acids from the Misoa rock sequence. (d) Generation profile of the total n-acids with respect to the variation in their CPI from the Misoa rock sequence. 4. Generation and maturation o f high molecular weight n-acids
In order to get a better insight into the proposed hypothesis on the generation of the HAs, the n-acid fraction was also analyzed qualitatively and quantitatively. Qualitatively the CPI ((2-24 + C-26 + C-28/ C-25 + C-27 + C-29) was used as an indicator of the maturity of the generated acids. A depth profile of CPI in the Icotea sequence is shown in Fig. 2(b). Initially, the expected decrease in the CPI with increasing maturity is observed. Surprisingly however, at a depth of ca 1000 m, a strong increase in the CPI is observed, which maximizes at 2400 m, and then decreases. This unexpected observation apparently follows the behavior of the HAs. Hence, n-acids follow a maturation trend down to ca. 1200 m where their CPI value becomes approximately unity. Past this point, a decrease in maturity is observed in the CPI, consistent with the secondary generation of HAs. However, contrary to the behavior of the HAs, a second increase in maturity is observed at the end of the sequence. These observations suggest a multiple step process for the generation of the n-acids. We hypothesize that initially, at low maturities, the loosely bound acids of biogenic origin are detached from the more "exposed" or labile sites of the kerogen matrix to produce, in combination with the free acids, a high CPI. As thermal maturation proceeds, thermal cracking of these acids, and aleatory cracking of various hydrocarbon moieties from the kerogen, generate n-acids with no even-over-odd predominance, giving rise to a low CPI. However, at a certain point during the thermal maturation of the OM, structural changes or degradation of the more refractory part of the geopolymeric material exposes the previously "protected" part of the macromolecule. Further generation of n-acids having a high CPI then occurs
repeating the previous generation sequence. This hypothesis is also consistent with the absence of a second generation of the more mature isomers of the HAs, since presumably these will be absent from the "protected" part of the geopolymer, where steric hindrance is likely to prevent their formation. The distributions of n-acids in the La Luna samples are consistent with the data for the Icotea sequence. Although, there was no correlation between the CPI and the %Ro, in all cases the value of the CPI was, as expected, relatively high (range 2.08-4.64). These samples are therefore located in the range of the secondary generation of the n-acids. Additional information concerning the generation of the n-acids was obtained by analyzing the quantitative data. Fig. 2(c) shows the depth profiles for some of the n-acids determined in this study for the Icotea sequence. A clear maximum of generation can be observed at ca 1350m with a small secondary maximum at ca 2000m. This second maximum is particularly strong for the shorter chain acids, and is almost absent for the long chain acids (C-20+). Apparently, two phases of generation occur, one in which the long chain acids predominate, and a second one, at higher maturity, where the short chain acids are more important. Figure 2(d) shows a combined plot of the CPI and of the total n-acids. Apparently, the first maximum of generation is in the region where the CPI is low (1-1.5), whilst the second maximum coincides with the secondary increase in the CPI. This implies that during the first phase of aleatory cracking of acids from the kerogen both long and short chain acids are generated, while the second generation of acids from the previously "protected" part of the geopolymeric matter generates almost exclusively short chain acids. With respect to the acid generation sequence, our data show that the cyclic HAs are generated first, followed by a general n-acid generation, particularly
216
RUDOLFJAFF£and Pmao R. GAgOxNAU
of long chain acids, and finally the generation of the shorter chain acids. This sequence could be expected due to steric effects, however, with respect to the generation of n-acids another more complex process must be involved in order to produce generation curves which vary in the chain length and in CPI. Possible explanations may be found in the incorporation of the acids into the geopolymers during early diagenesis and in the cracking mechanisms during catagenesis. From the quantitative point of view, the La Luna samples, even though at a much higher maturity level, showed much higher concentrations of the n-acids than the Icotea samples. Very likely, this is due to the elevated %TOC of these samples and to the less complex nature of the OM of type II.
These observations may be of considerable importance in understanding the effect of acid generation on oil migration and the molecular distributions of acids in crude oils. First, as previously reported by other authors (Surdam et al., 1984; MacGowan and Surdam, 1988; Kawamura et al., 1986a, b; Kawamura and Kaplan, 1987), the acid generation prior to the oil generation maximum could enhance rock porosity as a result of carbonate and/or aluminosilicate dissolution. Second, the generation of acids, particularly of HAs, prior to the oil generation maximum will influence the incorporation of acids into petroleum during migration, and subsequently, the interpretation of HA fingerprints in migrated oils (Jaff6 and Gallardo, 1990).
5. Generation o f carboxylic acids with respect to the oil generation window
CONCLUSIONS
The generation of the HAs and n-acids with respect to the Petroleum Potential (SI + $2), obtained from Rock-Eval data, are shown in Figs 3(a) and (b) respectively. The depth of maximum Petroleum Potential for the Icotea borehole is observed at ca 2000 m (%Ro 0.7 approx.). For both the HAs and the n-acids, a clear maximum of generation is observed prior to the maximum Petroleum Potential. These observations are in agreement with data from Surdam et al. (1984) for short chain acids. It is interesting to observe that the second maximum of the n-acid generation coincides with the maximum Petroleum Potential. The exact position of these maxima is however, difficult to discern due to the limited amounts of samples analyzed. Little information was obtained on the samples from the La Luna formation, mainly due to the elevated maturity of these rocks.
1. The generation of HAs in ancient sediments, was found to be controlled by a double step process during thermal maturation. It is proposed that, besides the free HAs, those bound to the organic geopolymers also undergo isomerization during thermal maturation. Those acids bound to the less "protected" fraction of the geopolymers undergo isomerization until they reach their thermodynamically most stable configurations. However, those bound to the "protected" fraction of the geopolymer, will be hindered by steric effects from this process, and remain principally in the immature configuration. Due to this "speciation", the HAs from both fractions will be generated with different degrees of maturity at different maturity levels, and in different amounts. The n-acids also show a double generation process. Here, the maturity signal of the acids (CPI) decreases first as the thermal aleatory cracking of chains
$1 + S2 (a)
0
0.5 i .....
1.0
, .....
$ I + S2 1.5
= ..... •
A
2.0
i .....
(b)
o
0.5
1.o
1.5
J-
$1+S2
2.o
i
$1+S2
•
-
.
$
$
1000
1000
2000
2000
3000
3000 0
1000
t o t , HAs (ng/g)
2000
.
.
. . . 2500
.
. . 5000
.
. . 7500
10,000
tot,. n-aclds (ng/g)
Fig. 3. (a) Generation profile of the total HAs with respect to the Petroleum Potential (S~ + $2) from the Misoa rock sequence• (b) Generation profile of the total n-acids with respect to the Petroleum Potential (Sl + $2) from the Misoa rock sequence.
Generation and maturation of carboxylic acids in ancient sediments becomes more important than the generation of the original biogenic acids bound to the organic matrix. Once this geopolymeric material suffers from structural rearrangement or degradation due to thermal stress, exposing its "protected" fractions, this generation process is repeated, producing first an increase followed by a decrease in the CPI of the acids being generated. 2. The simultaneous generation of HAs of different maturities, as well as the generation of the thermodynamically most immature isomers of the HAs at elevated maturities, is a clear indication that the thermodynamics and kinetics of stereochemical equilibria of biomarkers in ancient sediments may be much more complex than presently believed. 3. The acids generated during thermal maturation, are produced by thermal cracking in a sequential manner. The largest, sterically most favored pentacyclic acids are generated first, followed by the nacids (particularly long chain acids) and finally by the short chain acids. 4. Comparing the large amounts of acids generated by thermal cracking from geopolymers, with the much smaller amounts initially present as free acids in the most immature samples, it is suggested that a major portion of the acid fraction produced during primary production is incorporated into the geopolymers in recent depositional environments. As such, these compounds are protected from further diagenetic degradation and preserved in the sedimentary record. This incorporation however, is preceded by a partial alteration of the original acid fraction during early diagenesis. 5. In agreement with previous reports (Surdam et al., 1984; Kawamura et al., 1986a, b; Kawamura and Kaplan, 1987), our data imply that the generation maxima of the carboxylic acids with respect to the oil generation maximum is an important factor in the development of secondary porosity as a result of carbonate and/or aluminosilicate dissolution. 6. The knowledge on the generation and maturation of carboxylic acids in ancient sediments is of importance for the understanding of the processes involved in the incorporation of these acids into the oils during their migration. It also provides an important tool in the interpretation of HA fingerprints with respect to their use as indicators of oil migration. Acknowledgements--The authors wish to acknowledge the Organic Geochemistry Group from INTEVEP for supplying the samples, %R o and Rock-Eval data used in this study. Special thanks to the laboratory of POLAR for allowing access to their GC/MS system, to Drs P. Albrecht and J. M. Trendel for supplying the standard used for quantitation, to Drs F. Cassani and G. Wolff for helpful discussions, to Dr G. Wolff for valuable comments on the manuscript, and to INTEVEP, CONICIT, and the Universidad Simrn Bolivar for financial assistance. P. Gardinali thanks the Venezuelan government for a fellowship. 0(3 16/1-3--0
217
REFERENCES
Arpino P., Schmitter J. M., Guiochon G., Ramljak Z. and Solc A. (1977) Separation of acids from asphalts. Anal. Chem. 49, 1222-1225. Behar F. and Albrecht P. (1984), Correlations between carboxylic acids and hydrocarbons in several crude oils. Alteration by biodegradation. Org. Geochem. 6, 597-604. Cyr T. D. and Strausz O. P. (1983) The structures of tricyclic terpenoid carboxylic acids and their parent alkanes in the Alberta oil sands. J. Chem. Soc. Chem. Commun. 1028-1030. Derenne S., Largeau C., Casadevall E. and Connan J. (1988) Comparison of torbanites of various origins and evolutionary stages. Bacterial contribution to their formation. Cause of lack of botryococcane in bitumens. Org. Geochem. 12, 43-59. Ensminger A. (1977) Evolution de composrs polycycliques srdimentaires. Thrse, Universit6 Louis Pasteur, Strasbourg, France. Gardinali P. R. (1989) Generacirn y maduraci6n de ,'icidos carboxilicos en rocas de la Cuenca de Maracaibo. Tesis de Maestria, Universidad Sim6n Bolivar, Venezuela. Jaff6 R. and Gallardo M. T. (1990) Application of carboxylic acids as biomarkers of biodegradation and oil migration in the Maracaibo Basin, Venezuela. (submitted). Jaff6 R., Albrecht P. and Oudin J. L. (1988a) Carboxylic acids as indicators of oil migration--I. Occurrence and geochemical significance of C-22 diastereoisomers of the (17/~,21/1) C-30 hopanoic acid in geological samples. Advances in Organic Geochemistry 1987 (Edited by Mattavelli L. and Novelli L.). Org. Geochem. 13, 483-488. Pergamon Press, Oxford. Jaff~ R., Albrecht P. and Oudin J. L. (1988b) Carboxylic acids as indicators of oil migration--II. The case of the Mahakam Delta, Indonesia. Geochim. Cosmochim. Acta 52, 2599-2607. Kawamura K. and Kaplan I. R. (1987) Dicarboxylic acids generated by thermal alteration of kerogens and humic acids. Geochim. Cosmochim. Acta 51, 3201-3207. Kawamura K., Tannenbaum E., Huizinga B. J. and Kaplan I. R. (1986a) Long chain acids in pyrolyzates of Green River kerogen. Advances in Organic Geochemistry 1985 (Edited by Leythaeuser D. and Rullk6tter J.). Org. Geochem. I0, 1059-1065. Pergamon Press, Oxford. Kawamura K., Tannenbaum E., Huizinga B. J. and Kaplan I. R. (1986b) Volatile organic acids generated from kerogen during laboratory heating. Geochem. J. 20, 51-59. Kvenvolden K. (1967) Normal fatty acids in sediments. J. Am. Oil Chem. Soc. 44, 628-636. Kvenvolden K. (1966) Molecular distribution of normal fatty acids and paraffins in some cretaceous sediments. Nature 209, 573-577. MacGowan D. B. and Surdam R. C. (1988) Difunctional carboxylic acid anions in oilfield waters. Org. Geochem. 12, 245-259. Mackenzie A. S., Patience D. A., Yon D. A. and Maxwell J. R. (I 982) The effect of maturation on the configuration of acyclic isoprenoid acids in sediments. Geochim. Cosmochim. Acta 46, 783-792. Mackenzie A. S., Wolff G. A. and Maxwell J. R. (1983) Fatty acids in some biodegrated petroleums. Possible origin and significance. Advances in Organic Geochemistry 1981 (Edited by Bjoray M. et al.). pp. 637-649. Wiley, New York. McCarthy R. D. and Duthie A. H. (1962) A rapid method for the separation of free acids from other lipids. J. Lipid Res. 3, 117-119.
218
RUDOLFJA~'~ and I~RO R. GARDINALI
Schmitter J. M., Arpino P. and Guiochon G. (1978) Investigation of high molecular weight carboxylic acids in petroleum by different combinations of chromatography (gas and liquid) and mass spectrometry (electron impact and chemical ionization). J. Chrom. 167, 149-158. Surdam R. C., Boost S. W. and Crossey L. J. (1984) The chemistry of secondary porosity. In Clastic Diagenesis (Edited by McDonald and Surdam) Am. Assoc. Pet Geol. Mem. 37, 127-150. Talukdar S., Gallango O. and Chin-a-Lien M. (1984) Estudio geoquimico regional de la Cuenca de Maracaibo.
Technical report No. INT-OO90Z84. INTEVEP, Caracas, Venezuela. Talukdar S., Gallango O. and Chin-a-Lien M. (1986) Correlation and migration of hydrocarbons in the Maracaibo Basin, Venezuela: an integrated Basin study. Advances in Organic Geochemistry 1985 (Edited by I.~ythaeuser D. and Rullk6tter J.) Org. Geochem. 10, 261-279. Pergamon Press, Oxford. Van Dorssclaer A. (1975) Tritfirpcnes de S~diments. Th~se de doctorates Sciences, Universit6 Louis Pasteur, Strasbourg, France.
Org. Geochem. Vol. 16, Nos I-3, pp. 211-218, 1990 Printed in Great Britain
0146-6380/90 $3.00+ 0.00 Pergamon Press plc
Generation and maturation of carboxyfic acids in ancient sediments from the Maracaibo Basin, Venezuela RUDOLF JAFFEand PIERO R. GARDINALI Universidad Sim6n Bolivar, Department of Chemistry, Geochemistry and Environmental Chemistry Lab., Apartado 89000, Caracas 1080=A, Venezuela (Received 10 January 1990; accepted 24 March 1990) Abstract---Cyclic and acyclic carboxylic acids in various rock samples from the Misoa and La Luna formations, Maracaibo Basin, Western Venezuela, were analyzed in order to study their generation and maturation processes. For this purpose, rocks from various degrees of maturity (Misoa: %Ro 0.35--0.77; La Luna: %Ro 0.5-1.8), and different origins (Misoa: type III OM; La Luna: type II OM) were studied. Linear acids and tricyclic as well as pentacyclic (hopanoic) triterpenoic acids were analyzed quantitatively by gas chromatography-mass spectrometry. Qualitative data from the isomeric distribution of the hopanoic acids (HAs) showed the expected trend of increasing abundance of the thermodynamically more stable (17~t,21fl) and (17fl,2P,) isomers, with the simultaneous disappearance of the thermodynamically less stable (17fl,21fl) isomers. However, at high maturity levels, the (17fl,21fl) isomers reappeared, to become the only HAs to be present. A similar trend of apparent maturity change was observed for the linear acids by studying the CPI of these compounds. A double generation process of carboxylic acids is suggested and confirmed by quantitative data. The latter showed that acids of different degrees of maturity are generated at various stages prior to the oil generation maximum. It is suggested that a major portion of the acids biosynthesized in recent environments are incorporated into the geopolymeric fraction during early diagenesis and are as such protected from further diagenetic degradation. It is also suggested that their maturation and generation is strongly dependent on their "speciation" within the geopolymer, and the structural changes in the geopolymeric matter during thermal maturation. The importance of these observations with respect to the application of carboxylic acids as biomarkers of oil migration is discussed. Key words--carboxylic acids, generation and maturation (of carboxylic acids), hopanoic acid stereochemistry, Maracaibo Basin
INTRODUCTION Carboxylic acids, and particularly hopanoic acids (HAs), have recently been reported to be useful biological markers of oil migration (Jaff6 et al., 1988a, b; Jaff6 and Gallardo, 1990). The isomeric distribution of the HAs in an important parameter in the determination of the migration histories of crude oils. Apparently, a significant proportion of the HAs are incorporated into the oil during its migration path, presumably via the "leaching" of carboxylic acids from sediments of relatively low maturity. Knowledge of the generation and maturation processes of carboxylic acids, particularly HAs, in ancient sediments is therefore of extreme importance. Until now there have been few studies on the maturation and generation of carboxylic acids and of HAs. The only reports on the maturation of HAs showed a behavior similar to their alkane counterparts, in that the thermodynamically less stable, biogenic isomers with the (17fl,21fl,22R) configuration decrease in favor of the more stable (17fl,21~) and (17~t,21fl) pair of disastereoisomers (22R and 22S) (Ensminger, 1977; Jaff6 et al., 1988b). In contrast to the hopanes however, where the (17~t,21fl)
configuration is thermodynamically the most stable isomer, in the case of the HAs, the (17fl,21~t) isomers have a degree of thermal stability similar to the (l 7~t,21r ) isomers. There are no reports to our knowledge concerning the generation of the HAs. Similarly, little is known about the generation and maturation of acyclic acids in ancient sediments, particularly in the C15 + range. Whilst Mackenzie et al. (1982) have reported on the molecular parameters of maturation of acyclic isoprenoid acids, nothing is known about their generation in ancient sediments. In the case of the n-acids, it is generally assumed that these compounds mimic their alkane counterparts with increasing maturation. Hence, with increasing thermal stress, the molecular distribution of immature acids with a high carbon preference index (CPI, even over odd predominance) is presumed to be transformed into a distribution with a CPI value of approximately one (Kvenvolden, 1966, 1967). The generation of carboxylic acids has been investigated by various authors (Surdam et al., 1984; Kawamura et al., 1986a, b; Kawamura and Kaplan, 1987). These studies suggested that the generation of acids could enhance the migration of oils by way of increased porosity and permeability of source rocks. This effect could therefore, also contribute to an 211
212
RUVOLVJ^vt~ and PIEROR. GARDINALI
enhancement in the incorporation of the less polar, more soluble, longer chain acids during the migration process as suggested by Jaff~ and co-workers (Jaff~ et al., 1988a, b; Jaff6 and Gallardo, 1990). However, little information exists on the generation of the long chain acids in ancient sediments. Laboratory heating experiments demonstrated the generation of long chain n-acids from kerogen (Kawamura et al., 1986a) and from torbanites (Derenne et al., 1988). Both studies showed that bound acids are stabilized or protected from biological and chemical alterations, thus preserving their immature characteristics after incorporation into the geopolymer. With respect to the maximum in the oil generation window, Surdam et al. (1984) reported the maximum generation of short chain acids to occur at a vitrinite reflectance (%Ro) between 0.4 and 0.5, while Kawamura et al. (1986a) suggested the generation of long chain acids occurs at values of %Ro between 0.5 and 0.6. The position of the generation of the acids relative to the maximum oil generation is, as mentioned previously, not only important for porosity enhancements in the source rocks, but also with respect to the incorporation of the acids by the oil during its migration path. Here, the acid "speciation" within the organic matrix, and the possible sequential generation of acids of different molecular and maturity distributions, could become important in the interpretation of the mono-carboxylic acid fingerprints used in oil migration studies. The objectives of this study were, therefore, to investigate in more detail the generation and maturation processes of long chain n-acids and triterpenoic acids in ancient sediments. EXPERIMENTALSECTION Monocarboxylic acids from ten rock samples from the Icotea borehole from the Misoa formation (Eocene, type III OM) and seven rock samples from different boreholes from La Luna formation (Cretaceous, type II OM) where analyzed. These rocks were selected in order to cover a wide maturity range. The samples from the Misoa formation, with a depth between 280 and 2668 m, cover a maturity range between a vitrinite reflectance (%Ro) of 0.35 and 0.77, while the more mature La Luna samples range from 0.5 to 1.8%R o. Between 80-100g of these rock samples were ground into a fine powder and extracted with a mixture of 3:1 v/v of chloroform/methanol. The concentrate of the extract was fractionated into its neutral and acidic fractions using the method described by McCarthy and Duthie (1962), modified by Arpino et al. (1977). The acidic fraction was then methylated with freshly distilled diazomethane. The mono-methyl ester fraction was then obtained after column chromatography and thin layer chromatography and spiked with a synthetic tetracyclic triterpenoic methyl ester, as internal standard for
quantitative analysis. This fraction was then analyzed by high resolution gas chromatography (GC) and by gas chromatography/mass spectrometry (GC/MS). The GC analyses were performed on a Hewlett Packard, model 5840-A gas chromatograph, equipped with a split-splitless injector, a high performance methyl silicone gum capillary column (25m, 0.2mm i.d., HP1), and a flame ionization detector. GC/MS analyses were carried out on a Helwett Packard 5970-A Mass Selective Detector, fitted with a HP 5880-A gas chromatograph, and a HP 5895-A Chemstation data system. The instrument was operated in the selected ion monitoring (SIM) mode at an ionization energy of 70 eV. Selected Ions such as molecular ions and characteristic fragment ions for the HAs ( m / z = 191,207 + n 14, M+), for the tricyclic acids ( m / z = 191, M +) and for the linear acids ( m / z = 74) were monitored for identification and quantification purposes. Mass spectrometric data and gas chromatographic retention behavior previously reported in the literature were used as an aid in the identification of individual components and isomeric distributions (Van Dorsselaer, 1975; Ensminger, 1977; Schmitter et al., 1978; Jaff~ et al., 1988a, b.) Quantitative data were obtained after experimental determination of response factors for the internal standard with respect to a standard solution of eicosanoic acid for the acyclic acids, and the C-30 (17]~,21/~,22R)-hopanoic acid for the cyclic compounds, both as their methyl esters. Quantification of the cyclic components was performed using m / z = 191 ions of these compounds with respect to the same fragment from the internal standard. Since chromatographic coelution of the different HAs does not permit the use of exclusively the m / z = 191 ion (Jaff6 et al., 1988a), these compounds were quantified based on their characteristics m / z = 207 + n 14 ions. The abundance of the m / z = 191 ion was then calculated based on the relative abundances of these characteristic ions with respect to the m / z = 191 ion reported in the literature for each of the HA isomers (van Dorsselaer, 1975). Rock Eval Pyrolysis and vitrinite reflectance data used in this study were kindly supplied by INTEVEP and were determined using standard geochemical methods. RESULTS AND DISCUSSION 1. Geochemical characterization o f the rock samples
The Misoa sequence (Eocene, Type III OM) covering a depth range between 280 and 2668 m showed a total organic carbon (TOC) content of 0.6-2.0%, and is therefore considered as a potential source rock (Talukdar et al., 1984). The samples from La Luna formation (Cretaceous, Type II OM), considered to be good source rocks (Talukdar et al., 1986), have TOC values from 1.3 to 6.5%. Both sample sets cover a relatively wide range of maturity as shown in Table I. An excellent correlation was observed
Generation and maturation of carboxylic acids in ancient sediments Table 1. Characteristics of the rock samples from the Misoa (Icotea borehole) and La Luna formations Sample
Depth (m)
Ro (%)
Tmx (°C)
Misoa Formation
I-1 I-2 I-3 I-4 I-5 I-6 I-7 I-8 I-9 1-10
282 532 961 983 1355 1832 1970 2433 2580 2668
0.35 0.40 0.46 0.46 0.57 0.47 0.69 0.74 0.70 0.77
425 436 439 433 441 447 439 450 462 459
La Luna Formation
QM-3 QM-5 Sol-6 UD-102 UD- 149 VLB-704 UD-120
surficial surficial 5242 4757 4984 4178 4733
0.50 0.50 0.72 nd 1.27 1.28 1.80
437 441 427 437 470 550 550+
between the %Ro and Tmax vs the depth for the Icotea samples, indicating the continuity of this rock sequence (Gardinali, 1989). Hence, present day burial depths will be used to indicate the level of maturity. This is not possible for the La Luna samples since these originate from different boreholes across the formation. 2. Characterization o f the acid fractions
Linear as well as cyclic acids were identified in both rock sequences. The distribution of the n-acids in the Icotea samples showed a range between C-8 and C-37, with a bimodal distribution maximizing at C-16, C-18 and C-28. This was observed particularly for the samples of intermediate maturity (530-1970 m) for which the higher molecular weight fraction was most abundant. In all cases, however, the C-16 and C-18 homologues were the more abundant components. No unsaturated acids were observed. Similar distributions have previously been observed in crude oils from various locations (JaffE and Gallardo, 1990; Jaff6 et al., 1988a, b; Behar and Albrecht, 1984; Mackenzie et al., 1983). A strong variation in the CPI was also observed for the acids, the CPI of the short chain acids (up to C-19) being consistently larger than that of the long chain acids (C-20+). The variation in the CPI for the long chain acids with changes in maturity of the rocks is discussed in detail later in this section. The n-acid fraction in the La Luna samples showed a distribution similar to that of the Icotea samples, except for the most mature sample (UD-120) which maximized at C-28. Again, the low molecular weight acids predominated over the C-20+ fraction, with this effect being more prominent for the La Luna type II OM, than for the Icotea type III OM. However, on comparison with the distributions of the n-alkanes there was no correlation in either case, indicating that there was no close correlation between the origin and geochemical behavior of the n-acids and the nalkanes in these samples. Similar findings were re-
213
ported by Kawamura et al. (1986a) when comparing acid and alkane fractions from kerogen pyrolysates. The CPI values for the short chain acids in the La Luna samples were high in all cases. The variations in the CPI values between the La Luna and Icotea samples are discussed later in this section. With respect to the cyclic acids, both sequences showed the presence of hopanoic acids (HAs). These compounds ranged from C-28 to C-33 (with the exception of the C-29) in the Icotea samples, whilst the C-30 to C-32 compounds were detected only in the most immature samples of the La Luna formation. The isomeric distribution of these acids is discussed below. Tricyclic terpenoic acids were only detected in the type II OM of some of the La Luna samples (QM-3, UD-149, VLB-704) with varying degrees of maturity. Only the C-20, C-21, C-24 and C-26 homologues, the latter two as pairs of diastereoisomers, were observed. Similar distributions have previously been reported by Cyr and Strausz (1983) in oil sands, and by Behar and Albrecht (1984) and Jaff6 and Gallardo (1990) in crude oils. As in the case of the linear components, no correlation was found between these acids and their alkane counterparts. Since these compounds showed the same distribution in the three samples, even though these were quite different in their thermal maturity, and as they were not detected in the Icotea sequence, they are not discussed further. 3. Maturation and generation o f HAs
The isomeric distribution of the C-30 to C-32 homologues of the HAs detected in the Icotea rock sequence are shown in Fig. 1. The distribution follows the expected trend of maturation, by which the thermodynamically less stable (17fl,21/~,22R) homologues, abundant in the most immature samples, decrease in abundance relative to the thermodynamically more stable (17//,21~,22R and 22S) and (17~,21/~,22R and 22S) isomers in the more mature samples. The disappearance of the (17I/,21/~) isomers in favor of the (17p,21~) and (17~,21/~) diastereoisomeric pair, seems to be essentially complete at ca 1360 m of depth. However, after this point, and at more elevated maturities, the relative abundance of the thermodynamically less stable isomer begins to increase with respect to the more stable ones, to a point (at a depth of 2440 m) where the (17/~,21//,22R) isomer is the only one present. This unexpected behavior, seems to indicate a secondary generation of HAs with a relatively immature signal at elevated maturities. These results are in agreement with the pyrolysis data from Kawamura et al. (1986a), who suggested that the acids tightly bound to the kerogen matrix preserve their immature character. It is interesting to notice that only one isomer, the (17/~,21/~,22R) is present at high maturities for the C-30 homologue. Despite the observation that the abundance change in the 22R configuration to the 22S configuration of this isomer is relatively fast
214
RUDOLFJAFFEand I~EROR. GARDINALI Depth ( m ) (%Ro)
C-3O ( m/z =235)
282 (~35) (0.35) 961 (0.46)
532 (0.40) 961
(0.46)
o= c o
._
lti
_ _
._aL_
983 (0.46)
1355 (0,57) 1632
J~ o
ll|
C-31 ( m/z = 249)
Depth(m) (%Ro)
I
~
983 (0.46)
_L
(0.47)
282 (0.35) 961 (0.46 ) • 983
(0.46)
' 1355 (0.57)
C-32 (m/z=263)
xla___ •
ALl
1832 (0.47) 1970
(0.69} 1355 (0.57)
1970 (0.69)
2435 (0.74) 1832 (0.47)
2435 (0.74)
1970 (0.89)
2580 (0.70)
2668 (0.77)
Depth ( m ) (% Ro)
~
,
~
.L
2435 (0.74) 2580 (0.70) 2668 (0.77)
Rt
2580 (QTO)
2668 (0.77) Rt
Rt
Fig. 1. Mass chromatograms of the C-30 (m/z =235), C-31 (m/z =249) and C-32 (m/z =263) HAs in the rock sequence of the Misoa formation (depth in meters and % vitrinite reflectance are indicated). (Jaff6 et al. 1988a), it is not observed at elevated maturity levels. Neither is any isomerization to the more stable isomers at positions 17 and 21 apparent. This may be a result of a faster rate of decarboxylation or degradation of the generated acids compared to the rate of isomerization. It may also indicate that this isomerization does not occur in ancient sediments. Quantitative data on these profiles give a much better insight into the processes involved in these observations. Figure 2(a) shows the concentration vs depth profile for the C-32 homologue of the HAs in the Icotea sequence. Similar profiles are observed for all the homologues, with a clear maximum at ca 1100 m. It is interesting to observe that the most stable as well as the least stable isomers are generated simultaneously, the (17~,21#) isomer being the most abundant in all cases. At ca 1800 m, all the HAs reach concentration levels similar to those of the immature rocks. A very careful look at the data also shows a small second maximum of only the (17/~,21~) isomers at ca 2400 m. This secondary generation has already been pointed out previously, and could, as suggested, be caused by secondary cracking of tightly bound "immature" acids from the geopolymers at elevated maturities. The quantitative data also indicate that the original biogenic free HAs seem to be bound in a major proportion to the geopolymers at a very early stage of diagenesis, or at least this bound fraction is preseved from degradation during early diagenesis.
However, a large proportion of these bound acids appear to be able to undergo isomerization during thermal maturation in order to produce a generation curve as shown in Fig. 2(a). With respect to the HAs in the samples from the La Luna formation, these were only observed in the most immature samples (QM-3, QM-5, SOL-6). The isomeric distributions of the HAs in these samples showed a very immature character, with the (17fl,21 fl) isomers being the predominant components. Only the SOL-6 sample showed the presence of small amounts of the thermodynamically more stable isomers, while QM-3 consisted exclusively of the (17/~,21fl) isomers of the C-30 to C-32 homologues, and QM-5 contained only the (17fl,21fl) C-32 compound. In all cases the (17fl,21fl) C-32 isomer was the most abundant component found. These distributions are consistent with those observed for the Icotea sequence, considering the maturity range of the La Luna samples (0.50-0.72% Ro). At this maturity range the secondary generation starts in the Icotea sequence. The rest of the La Luna samples are already too mature to contain any HAs. Surprisingly, however, SOL-6 with a %Ro of 0.72 showed the presence of (17/~,21~t) and (17~t,21fl) components. Perhaps the %Ro is not an adequate maturity parameter for these samples which are type II OM. Indeed, the Tm,x values for SOLo6 agree with our observations for the HAs, since it has the lowest Tm,xvalue of the La Luna samples. The data of the La Luna samples are therefore consistent with that for the Icotea sequence.
Generation and maturation of carboxylic acids in ancient sediments C - 3 2 HAs (ng/g) 0
20 40 ,,i.,i.-
60 80 100 t20 ..........
cPI 2 i
n-acids (ng/gl 3
0
i
,
100 i
L
~
I1 / IL /
.--o-
•
200 i
.
215 tot. n-aclds (nglg]
300 .
0 ,
2500 5000 . . . . . . . .
7 5 0 0 10 0 0 0 , • . •
n C-15
•
nc-z~
/I
o c-3o
•
~oto-oo,d,
Q 2000
~
I
•
•
aBS
OaR
t & 3000 J-
(el
J
1¢1
(d) 2
3
cPl
Fig. 2. (a) Generation profiles for the different isomers of the C-32 HAs from the Misoa rock sequence. (b) Depth profile of the CPI of the n-acids from the Misoa rock sequence. (c) Generation profiles of various n-acids from the Misoa rock sequence. (d) Generation profile of the total n-acids with respect to the variation in their CPI from the Misoa rock sequence. 4. Generation and maturation o f high molecular weight n-acids
In order to get a better insight into the proposed hypothesis on the generation of the HAs, the n-acid fraction was also analyzed qualitatively and quantitatively. Qualitatively the CPI ((2-24 + C-26 + C-28/ C-25 + C-27 + C-29) was used as an indicator of the maturity of the generated acids. A depth profile of CPI in the Icotea sequence is shown in Fig. 2(b). Initially, the expected decrease in the CPI with increasing maturity is observed. Surprisingly however, at a depth of ca 1000 m, a strong increase in the CPI is observed, which maximizes at 2400 m, and then decreases. This unexpected observation apparently follows the behavior of the HAs. Hence, n-acids follow a maturation trend down to ca. 1200 m where their CPI value becomes approximately unity. Past this point, a decrease in maturity is observed in the CPI, consistent with the secondary generation of HAs. However, contrary to the behavior of the HAs, a second increase in maturity is observed at the end of the sequence. These observations suggest a multiple step process for the generation of the n-acids. We hypothesize that initially, at low maturities, the loosely bound acids of biogenic origin are detached from the more "exposed" or labile sites of the kerogen matrix to produce, in combination with the free acids, a high CPI. As thermal maturation proceeds, thermal cracking of these acids, and aleatory cracking of various hydrocarbon moieties from the kerogen, generate n-acids with no even-over-odd predominance, giving rise to a low CPI. However, at a certain point during the thermal maturation of the OM, structural changes or degradation of the more refractory part of the geopolymeric material exposes the previously "protected" part of the macromolecule. Further generation of n-acids having a high CPI then occurs
repeating the previous generation sequence. This hypothesis is also consistent with the absence of a second generation of the more mature isomers of the HAs, since presumably these will be absent from the "protected" part of the geopolymer, where steric hindrance is likely to prevent their formation. The distributions of n-acids in the La Luna samples are consistent with the data for the Icotea sequence. Although, there was no correlation between the CPI and the %Ro, in all cases the value of the CPI was, as expected, relatively high (range 2.08-4.64). These samples are therefore located in the range of the secondary generation of the n-acids. Additional information concerning the generation of the n-acids was obtained by analyzing the quantitative data. Fig. 2(c) shows the depth profiles for some of the n-acids determined in this study for the Icotea sequence. A clear maximum of generation can be observed at ca 1350m with a small secondary maximum at ca 2000m. This second maximum is particularly strong for the shorter chain acids, and is almost absent for the long chain acids (C-20+). Apparently, two phases of generation occur, one in which the long chain acids predominate, and a second one, at higher maturity, where the short chain acids are more important. Figure 2(d) shows a combined plot of the CPI and of the total n-acids. Apparently, the first maximum of generation is in the region where the CPI is low (1-1.5), whilst the second maximum coincides with the secondary increase in the CPI. This implies that during the first phase of aleatory cracking of acids from the kerogen both long and short chain acids are generated, while the second generation of acids from the previously "protected" part of the geopolymeric matter generates almost exclusively short chain acids. With respect to the acid generation sequence, our data show that the cyclic HAs are generated first, followed by a general n-acid generation, particularly
216
RUDOLFJAFF£and Pmao R. GAgOxNAU
of long chain acids, and finally the generation of the shorter chain acids. This sequence could be expected due to steric effects, however, with respect to the generation of n-acids another more complex process must be involved in order to produce generation curves which vary in the chain length and in CPI. Possible explanations may be found in the incorporation of the acids into the geopolymers during early diagenesis and in the cracking mechanisms during catagenesis. From the quantitative point of view, the La Luna samples, even though at a much higher maturity level, showed much higher concentrations of the n-acids than the Icotea samples. Very likely, this is due to the elevated %TOC of these samples and to the less complex nature of the OM of type II.
These observations may be of considerable importance in understanding the effect of acid generation on oil migration and the molecular distributions of acids in crude oils. First, as previously reported by other authors (Surdam et al., 1984; MacGowan and Surdam, 1988; Kawamura et al., 1986a, b; Kawamura and Kaplan, 1987), the acid generation prior to the oil generation maximum could enhance rock porosity as a result of carbonate and/or aluminosilicate dissolution. Second, the generation of acids, particularly of HAs, prior to the oil generation maximum will influence the incorporation of acids into petroleum during migration, and subsequently, the interpretation of HA fingerprints in migrated oils (Jaff6 and Gallardo, 1990).
5. Generation o f carboxylic acids with respect to the oil generation window
CONCLUSIONS
The generation of the HAs and n-acids with respect to the Petroleum Potential (SI + $2), obtained from Rock-Eval data, are shown in Figs 3(a) and (b) respectively. The depth of maximum Petroleum Potential for the Icotea borehole is observed at ca 2000 m (%Ro 0.7 approx.). For both the HAs and the n-acids, a clear maximum of generation is observed prior to the maximum Petroleum Potential. These observations are in agreement with data from Surdam et al. (1984) for short chain acids. It is interesting to observe that the second maximum of the n-acid generation coincides with the maximum Petroleum Potential. The exact position of these maxima is however, difficult to discern due to the limited amounts of samples analyzed. Little information was obtained on the samples from the La Luna formation, mainly due to the elevated maturity of these rocks.
1. The generation of HAs in ancient sediments, was found to be controlled by a double step process during thermal maturation. It is proposed that, besides the free HAs, those bound to the organic geopolymers also undergo isomerization during thermal maturation. Those acids bound to the less "protected" fraction of the geopolymers undergo isomerization until they reach their thermodynamically most stable configurations. However, those bound to the "protected" fraction of the geopolymer, will be hindered by steric effects from this process, and remain principally in the immature configuration. Due to this "speciation", the HAs from both fractions will be generated with different degrees of maturity at different maturity levels, and in different amounts. The n-acids also show a double generation process. Here, the maturity signal of the acids (CPI) decreases first as the thermal aleatory cracking of chains
$1 + S2 (a)
0
0.5 i .....
1.0
, .....
$ I + S2 1.5
= ..... •
A
2.0
i .....
(b)
o
0.5
1.o
1.5
J-
$1+S2
2.o
i
$1+S2
•
-
.
$
$
1000
1000
2000
2000
3000
3000 0
1000
t o t , HAs (ng/g)
2000
.
.
. . . 2500
.
. . 5000
.
. . 7500
10,000
tot,. n-aclds (ng/g)
Fig. 3. (a) Generation profile of the total HAs with respect to the Petroleum Potential (S~ + $2) from the Misoa rock sequence• (b) Generation profile of the total n-acids with respect to the Petroleum Potential (Sl + $2) from the Misoa rock sequence.
Generation and maturation of carboxylic acids in ancient sediments becomes more important than the generation of the original biogenic acids bound to the organic matrix. Once this geopolymeric material suffers from structural rearrangement or degradation due to thermal stress, exposing its "protected" fractions, this generation process is repeated, producing first an increase followed by a decrease in the CPI of the acids being generated. 2. The simultaneous generation of HAs of different maturities, as well as the generation of the thermodynamically most immature isomers of the HAs at elevated maturities, is a clear indication that the thermodynamics and kinetics of stereochemical equilibria of biomarkers in ancient sediments may be much more complex than presently believed. 3. The acids generated during thermal maturation, are produced by thermal cracking in a sequential manner. The largest, sterically most favored pentacyclic acids are generated first, followed by the nacids (particularly long chain acids) and finally by the short chain acids. 4. Comparing the large amounts of acids generated by thermal cracking from geopolymers, with the much smaller amounts initially present as free acids in the most immature samples, it is suggested that a major portion of the acid fraction produced during primary production is incorporated into the geopolymers in recent depositional environments. As such, these compounds are protected from further diagenetic degradation and preserved in the sedimentary record. This incorporation however, is preceded by a partial alteration of the original acid fraction during early diagenesis. 5. In agreement with previous reports (Surdam et al., 1984; Kawamura et al., 1986a, b; Kawamura and Kaplan, 1987), our data imply that the generation maxima of the carboxylic acids with respect to the oil generation maximum is an important factor in the development of secondary porosity as a result of carbonate and/or aluminosilicate dissolution. 6. The knowledge on the generation and maturation of carboxylic acids in ancient sediments is of importance for the understanding of the processes involved in the incorporation of these acids into the oils during their migration. It also provides an important tool in the interpretation of HA fingerprints with respect to their use as indicators of oil migration. Acknowledgements--The authors wish to acknowledge the Organic Geochemistry Group from INTEVEP for supplying the samples, %R o and Rock-Eval data used in this study. Special thanks to the laboratory of POLAR for allowing access to their GC/MS system, to Drs P. Albrecht and J. M. Trendel for supplying the standard used for quantitation, to Drs F. Cassani and G. Wolff for helpful discussions, to Dr G. Wolff for valuable comments on the manuscript, and to INTEVEP, CONICIT, and the Universidad Simrn Bolivar for financial assistance. P. Gardinali thanks the Venezuelan government for a fellowship. 0(3 16/1-3--0
217
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