Geochemical characteristics of crude oils from the Cuyo Basin, Argentina

Advmcmis Orp~e Geoc~mistry1989 Org.Geochem.Vol. 16, Nos 1-3, pp. 511-519, 1990 Printed in Great Britain

0146-6380/90 $3.00 + 0.00

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Geochemical characteristics of crude otis from the Cuyo Basin, Argentina H~CTOR J. VILLARl and WILHELMPUTTMANN2 tCentro de Investigaciones en Recursos Geol6gicos, Consejo National de Investigaciones Cientificas y T6cnicas. J. R. Velasco 847, 1414 Buenos Aires, Argentina 2Lehrstuhl fiir Geologie, Geochemie und Lagerstatten des Erd61s und der Kohle, RWTH Aachen, Lochnerstr. 4-20, 5100 Aachen, B.R.D.

(Received 19 September 1989; accepted 24 April 1990) Abstract---Crude oils from the Triassic Cuyo Basin are thought to be derived from a fresh-water lacustrine source rock, mainly the Upper Triassic organic-rich shales of Cacheuta Fro. By means of GC and GC-MS methods, the oils were analysed for characteristic compounds which may reflect biological input to the source sediments and maturity variations. Pristane/phytane ratios, distributions of n-alkanes, and the presence of significant amounts of bacteriohopanoid-derived compounds (hopanes, 8,14-secohopanes, 8,14-monoaromatic secohopanoids, benzohopanes) together with bicyclic sesquiterpanes in all the samples studied, are taken as an indication of possible reworking of algal organic matter by bacteria. Sterane distributions were consistent with a lacustrine origin of the oils. The occurrence of selected alkylnaphthalene derivatives as dominant compounds in the aromatic hydrocarbon fractions was a matter of special attention in the study. The variation of their concentrations with the maturity of the oils is compared to that of monoaromatic 8,14-secohopanoids, The results suggest that 1,2,5-trimethylnaphthalene in these oils is a product of hopanoid degradation under mild thermal conditions.

Key words--alkylnaphthalenes, secohopanoids, drimanes, crude oils, maturity parameters

INTRODUCTION Triassic and Tertiary strata of the Cuyo Basin produce oils which constitute one of the oldest and most important petroleum resources of Argentina. Hydrocarbon accumulations in the basin occur mainly in the form of oils that, in view of their C22-C36n-alkane content, have been classified as high-wax oils (Hedberg, 1968). They are considered to have been generated from a source rock bearing kerogen type I, principally the shale sequences of the Cacheuta Formation and the upper layers of Potrerillos Formation (Chebli etal., 1984). A stratigraphic analysis of the basin evolution made by Strelkov and Alvarez (1984) has shown that the organic-rich shales of the Cacheuta Fm. were deposited in fresh-water lacustrine and fluvial-lacustrine environments which succeeded and culminated the sandy fluvial systems of Potrerillos Fro. during Upper Triassic. However, significant burial of the sediments, and therefore hydrocarbon generation, occurred only during last 10 Myr, when the Triassic rocks where covered by Tertiary deposits in a rapid subsidence episode which caused maturation of the organic matter (Jordan and Ortiz, 1987). Recently, Rosso etal. (1987) have proposed an East-West maturity trend for the basin, based mainly on bulk parameters, alkane distributions and conventional sterane and hopane ratios of oils and

rock-bitumens. Moreover, organic petrographical investigations on the presumed lacustrine source-rock have revealed an important algal input to the sediments, and an overall "immaturity" when compared to verified substantial oil generation (Villar and Laffitte, 1988). The phenomenon was explained as being a consequence of the particular burial history and its effect on the thermal degradation of the lipid-rich kerogen. Within the geological and geochemical frame described above, the present work attempts to characterise the oils by studying the most prominent hydrocarbon biomarkers detected, which may serve as a clue to assess the source-rock biological input and maturity variations. Special emphasis is given to the abundance of secotriterpenoids and bicyclic compounds in both the saturated and aromatic hydrocarbon fractions, as their simultaneous presence in all the samples analysed is thought to be an indication of their genetic relationship. EXPERIMENTAL

Samples A representative set of 10 oils from different locations of the basin (Fig. 1) was subjected to analysis. The oils, listed in Table 1, are from Triassic and Tertiary reservoirs. 511

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Methods Whole oil samples were separated into saturated and aromatic hydrocarbon fractions and subsequently analysed by GC and GC-MS. Procedures, conditions of analysis and criteria for compound peak identification can be found in P/ittmann and Villar (1987) and Villar et al. (1988). Quantitation of selected compounds was carried out as follows: (a) trimethylnaphthalenes and tetramethylnaphthalenes: peak areas in respective m/z 170 and m/z 184 chromatograms, and normalised to 2,6-dimethylnaphthalene standard (m/z 156 chromatogram); (b) tetramethyltetrahydronaphthalenes: peak areas in RIC trace and normalised to d8-anthracene standard; (c) monoaromatic secohopanoids (peaks S1 and $2 in Fig. 3): peak areas in RIC trace and normalised to dS-anthracene standard. RESULTS AND DISCUSSION

Some selected source and maturity parameters of the oils are shown in Table 1; Figs 2 and 3 display typical chromatograms of the saturated and aromatic hydrocarbon fractions for three samples of different maturity. From both tabulated data and GC-traces, a unique source for the oils can be envisaged. However, significant maturity variations, as determined by molecular measurements, occur. Taking into account the global evaluation of triterpenoid and steroid

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The saturate fraction gas chromatograms (Fig. 2) are all strongly dominated by n-alkanes ranging from i C~0 to C35 with slight odd-over-even predominance, Zb maxima at n-C15_21, and ratios Y.n-C21_31/Y.n-C15_2o indicating a moderate wax content (Table 1). The major components < C23 are interpreted to be derived from bacterial and/or algal debris, according to Hart and Calvin (1969), Gclpi et al. (1970) and Youngblood et al. (1971), whereas the biological sources for n-alkanes > C23 can be attributed to terrestrial plant waxes and/or algal lipids (Gelpi et al., ~)IL B-306[ 1970; Moldowan et al., 1985; McKirdy et al., 1986). 23 TslTm : 0,89 Changes in the profile of paraffins, mainly a decrease % 20S st,.50 in the degree of waxiness (Y.n-C21_3J~,n-Cl5_2o in Table 1; Fig. 2) are considered to be maturity- and not source-related, implying a cracking effect, albeit minor, on chain lengths. Additional source-type information obtained from the sterane distribution, sterane/hopane ratios (Table 1), pristane/phytane ratios (range: 2.0-3.0) and absence of C30 steranes, is consistent with a nonmarine, lacustrine origin for the oils, according to Huang and Meinschein (1979) and lOlL LP-11 ] Moldowan et al. (1985). However, it remains unclear 15 20 Ts/Trn ; 1.22 whether the relative abundance of C29 steranes (range: % 20S s!: 57 32-43%) reflects some higher plant input. Similar I abundances have been reported in nonmarine crude oils in which the presence of waxy hydrocarbons was attributed to an algal origin (McKirdy et al., 1986). Moreover, a recent paper has questioned the utility of the predominance of C29 steranes to denote a vascular plant contribution (Mello et al., 1988). Although such Fig. 2. Capillary gas chromatograms of the saturate hydro- input to these Triassic oils cannot be conclusively carbon fractions of three crude oils of different maturity evaluated, it is noteworthy to mention that microfrom the Cuyo Basin. Labelled peaks indicate carbon number of n-alkanes; Pr, pristane; Ph, phytane. For TdTm scopical studies on the assigned source rock (Villar and Laffitte, 1988) have shown a major algal contriand % 20Sst, see Table 1. bution, whilst land-plant components appeared largely subordinated. Among cyclic terpenoids, hopanes and, seconratios (TJTm, 20Sst, ~flst, C20 Ar.st) as a maturity base, the samples are ranked in Table 1 in an darily, 8,14-secohopanes and C~-C~6 bicyclanes, approximate sequence of increasing maturity. strongly dominate. Saturated secohopanes (assigned Samples L-19 and EQ-5, located respectively in the using m / z = 123 chromatograms) showed similar northern and southern extremes of the studied area distributions in all the oils. Retention behaviour and (Fig. 1), show to be the most immature oils, whereas fragmentation patterns published by Schmitter et al. the western and deepest pooled oils LP-I1 and (1982) provided the clues to identify two C27-, three PPC X-I exhibit the highest maturity. The relative C29- and one C30-compounds, although higher carmaturity order of the intermediate group of six ben-numbered homologues of the series are apparent samples (PB-122 to PC-21; Table 1) is rather (Fig. 4). Their relative abundance increases with arbitrary, since insufficient variation in the values maturity of the oils, reaching a maximum value of does not allow a confident ranking. 0.53 for the ratio secohopanes/hopanes (mass chroThe maturity assessments based on biomarker matograms m / z 123 and 191, respectively) in sample parameters correlate roughly with variations in the LP-II, and a minimum of 0.13 in sample EQ-5, chemical class compositional data (saturate, aromatic pointing to the influence of thermal stress in the and polar fractions) and with slight variations in the cleavage of the 8(14)-bond in hopanes, as has been CPI values (Table 1) of the oils. On the other hand, suggested by Schmitter et al. (1982) and Robinson parameters based on di- and triaromatic hydro- et al. (1986). carbons such as Trimethylnaphthalene Ratio~TNR A quantitative relationship between saturated seco(Alexander et al., 1985) and Methylphenanthrene hopanes and bicyclic sesquiterpanes of the drimane Index--MPI (Radke et al., 1982) appear to show a type in the oils studied becomes evident from Fig. 5, poor dependence with the maturity indicators derived since an increment in maturity (indicatively denoted from biological marker ratios. by T i T m ratios) reflects an increase in the relative I$

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relative concentration with resp~t to hopanes with maturity, but maintaining a rather constant proportion between them. This interpretation seam to explain plausibly the direct relationship shown in Fig. 5. Additionally, it must be mentioned that other bicyclic sesquiterpanes follow a parallel trend to that of drimanes, that is a relative increase with maturity and, hence, with the relative concentration of secohopanes. The relative abundance of two C]5 cyclonaphthcnes (m.w.=208) with base peak m/z = 193, possibly identical to those suggested by Alexander et al. (1984) to be drimane rearrangement products, arc plotted as reference in Fig. 5, as they are the most prominent bicyclanes after the mentioned drimanes. The aromatic hydrocarbon fractions of the oils (Fig. 3) show a marked predominance of naphthalene derivatives. A characteristic feature is the increase in their concentration with maturity. In the high-boiling range of the chromatograms, the most prominent peaks identified belong to C29-and C30-monoaromatic 8,14-secohopanoids (S1 and $2 in Fig. 3), aromatic (mainly triaromatic) steroids and benzohopanes. The general picture of the chromatograms shows a bimodal distribution pattern, with alkynaphthalenes comprising a first "mode". A second "mode" is constituted by a hump in which S1 and $2 dominate. The intermediate region of the GC-trace shows a poor representation of phenanthrene derived compounds when compared to naphthalcnes. This bimodal distribution tends to disappear with increasing maturity, as the abundance of the secohopanoids Sl and $2 diminishes sharply with respect to total alkylnaphthalenes. Moreover, the total distribution of alkylnaphthalenes shifts to mono- and di-substituted homologues. In the specific case of trimethylnaphthalenes, a remarkable characteristic is the maturity-dependent decrease of the proportions of 1,2,5-trimethylnaphthalene (TMN 6) with respect to total trimethylnaphthalenes, as envisaged by variations in biomarker ratios. Its percentage ratio varies from 43.4% in sample EQ-5 to 12.6% in sample LP-11, while the other isomers individually resolved in the chromatograms display in general an increase in their relative proportions. In the lower part of Fig. 6, absolute concentrations of selected compounds are plotted against a maturity parameter (TdTm; Table 1). C4THN and C4N, structurally related to TMN 6, are also included. Rough trends are apparent: an expected decrease in C4THN concentrations, an increment of TMN 1, TMN 2 and TMN 4, and an irregular variation for TMN 6 and C4N. Similar variations can be observed for all the isomers when other molecular measurements (20Sst, //// st, C20Ar.st; Table 1) are used. Regarding the abundance of the secohopanoids S 1 and $2, changes depending on the maturity level of the oils can be envisaged as well. The plots in Fig. 7 show the variation of their concentration with different parameters. The overall picture indicates to some extent a diminution of (S1 + $2)

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Fig. 6. Top, variation of the concentration of the monoaromatic secohopanoids SI and $2 (Fig. 3), with the concentration of selected alkylnaphthalene derivatives (Fig. 3). Bottom, dependence of the concentration of alkylnaphthalene derivatives with maturity (Ts/Tm; see Table 1). Measurements done with respect to the aromatic hydrocarbon fractions. with increasing maturity. C:oAr.st is the maturity parameter displaying the best-fitting correlation curve. On the other hand, the TNR-plot shows a large dispersion of the values. Comparison of the abundance of total monoaromatic secohopanoids (S1 + $2) vs alkylnaphthalene derivatives is shown in Fig. 6 (upper part). These pairs of variables seem to be linked to the maturation of the oils (see Fig. 7 and lower part of Fig. 6). The most conspicuous feature results from the correlation of T M N 6 and (S1 + S2)-concentrations, since high amounts of TMN 6 appear to be associated with the abundance of (SI + $2) much more markedly than they are with the maturity of the samples. Aromatisation of bicyclic diterpenoids present in higher-plants, such as agathic acid, has been considered a likely source for 1,2,5-trimethylnaphthalene (Thomas, 1969). In fact, many of the bicyclics of the labdane and sclarane type identified in ambers and resins by Grimalt et al. (1988) may act as primary precursors. Further publications have afforded

evidence of the probable origin of 1,2,5-trimethylnaphthalene from degradation of higher-plant triterpenoids in geological samples (Hayatsu et aL, 1987; Piittmann and Villar, 1987; Villar et al., 1988; Strachan et al., 1988), and amyrin related compounds have been proposed as precursors in post-Middle Cretaceous materials. Alternatively, ubiquitous compounds such as bacterial hopanes or C~5 sesquiterpenoid bicyclanes have been respectively suggested by Piittmann and Villar (1987) and Strachan et al. (1988) to serve as possible precursors in samples where contribution of higher-plant terpenoid materials could be discarded. In the Triassic oils investigated here, an amyrin origin is obviously excluded, whereas a resin diterpenoid source seems unlikely. Although some conifer input to the PotreriUos Fm. can be inferred from palynological studies (Zavattieri, 1987), its contribution to the lake biomass is thought to be of secondary importance, as no typical resin-derived bicyclic diterpenoids could be detected in the alkane fractions. Additionally, aromatic compounds of the

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Geochemical characteristics of crude oils from the Cuyo Basin cadalene, pimanthrcne or rctcne structures, normally associated with a resin source as documented by Grimalt et al. (1988), if present, were minor components of the oils. A recent report on the presence of trimethylnaphthalencs in sediments and crude oils (Strachan et al., 1988) has shown that the relative distributions of these compounds vary strongly with source and maturity. Particularly, in sets of samples in which 1,2,5-trimcthylnaphthalcne ( = T M N 6) predominates, the authors note that the relative abundance of this isomer diminishes markedly with maturity and they propose a 1,2,5-/1,3,6-trimethylnaphthalene ratio in order to follow this variation. When the ratio is applied to the Triassic oils analysed in the present study, an analogous general decrease with maturity (from 2.70 in sample EQ-5 to 0.68 in sample LP-1 l; Fig. 3) is observed. This drastic variation contrasts with the apparently less significant TNR changes (Table 1), suggesting that the diminution of the relative concentration of 1,2,5-trimethylnaphthalene reflects maturity differences of this set of oils better than the TNR does, possibly due to a primary source effect common to all samples. Following the concepts of a previous paper (Piittmann and Villar, 1987) and in vicw of the graphic relations discussed above, our proposal is that TMN 6 may originate in these crudes from the A/B ring moiety of bacteriohopanoids. Tetracyclic monoaromatic 8,14-sccohopanoids, which are enhanced cyclic components of the oils, arc considered plausible intermediates of this degradation. A likely pathway may occur through full aromatisation of ring B and further cleavage of the l l(12)-bond, to yield C4THN. Subsequently, TMN 6 or C4N (if a 1,2-methyl shift of the gcminal group is involved) would be formed via aromatisation of ring A. The relation between the proposed precursors and end primary products is apparent from Fig. 8. Whether saturated secohopanoids and drimanes (which arc enhanced cyclic triterpenoids of these oils) can bc linked to this degradation scheme is a matter of discussion. In fact, an integrated pathway comprising precursors and both aliphatic and aromatic products seems difficult to be postulated due to the complexity of the hypothetical reactions involved. However, some hinting remarks related to the maturity of the oils must be stressed. First, monoaromatic secohopanoids are more abundant in the less mature samples, in which TMN 6 (or TMN 6 + C4THN) tends to dominate the trimethylnaphthalene pattern, suggesting a diagenetic or low-temperature process in their formation. On the contrary, relative abundances of saturated secohopanes and drimancs tend to augment with maturity. Furthermore, the relative concentrations of TMN 6 with respect to ($1 + $2) vary within a very narrow range showing no maturity trend, at the same time that the absolute concentrations of TMN 6 and (S~ +$2) decrease with increasing maturity. Consequently, TMN 6 can be

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Fig. 8. Dependence of the concentration of (C4THN + TMN 6 + C4N) (Fig. 3) and of total trimethylnaphthalenes (TMNIot) with maturity (TJTm; see Table 1), and concentration of the monoaromatic secohopanoids S1 and $2 (Fig. 3). Measurements done with respect to the aromatic hydrocarbon fractions. considered a precursor for other more temperature: stable trimethylnaphthalenes, as has been demonstrated by Strachan et al. (1988). At certain stages of thermal maturation, TMN 6 could actually be generated from the A/B ring moiety of monoaromatic sccohopanoids and, simultaneously, act as a precursor for other trimethylnaphthalene isomers. In fact, this can explain the positive correlation between TMN 6- and ($1 +S2)-concentrations shown in Fig. 6 or, alternatively, between (C4THN + TMN 6 +C4N)- and (S1 +S2)-concentrations shown in Fig. 8. The genetic pathway from monoaromatic secohopanoids to other trimethylnaphthalene derivatives, with C4THN and TMN 6 as intermediates, cannot be considered the unique source for these compounds, since other sources are likely to exist. However, in the oils studied, the postulated reaction scheme explains satisfactory the observed data, that is, the variations in the absolute and relative concentrations of the major aromatic species detected (Fig. 3). Concerning the formation of monoaromatic 8,14sccohopanoids, Hussler et aL (1984) are of the view that the fragmentation of the 8(14) bond of hopanoid precursors could occur due to a thermo-catalytical process and postulate that further cleavage of the rather fragile 11(12) bond could yield bicyclic compounds. In addition, Hayatsu et al. (1987) have demonstrated that montmorillonite clay freshly activated with acid plays an important role in the thermal degradation and/or aromatisation of higher-plant

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derived pentacyclic triterpenoids. Strachan et al. (1988) have proposed that the diagenetic formation of particular trimethylnaphthalenes from pentacyclic triterpenoids should be favoured by conditions conducive to aromatisation and cleavage processes in acidic depositional environments such as coal swamps. Furthermore, strong acidic conditions have also been invoked by Pfittmann and Villar (1987) to explain A-ring rearrangements of terpenoids which lack an oxygenated function at C-3, thus accounting for the occurrence of 1,2,5,6-tetramethylnaphthalet~e (C4N) in sediments with no higher-plant contribution. It is therefore likely that both the formation of monoaromatic secohopanoids and the subsequent generation of alkylnaphthalene derivatives, mainly C4THN, T M N 6 and C4N, have occurred in the source sediments mediated by acid catalysed processes under mild thermal conditions.

Alexander R., Kagi R. I., Rowland S. J., Sheppard P. N. and Chirila T. V. (1985) The effects of thermal maturity on distributions of dimethyinaphthalene, and trimethylnaphthalenes in some ancient sediments and petroleums. Geoehim. Cosmochim. Acta 49, 385-395. Chebli G. A., Labayrn I. L., l.af~tte G. A. and Rosso

M. R. (1984) Materia orgfinica, ambiente deposicional y evaluacirn oleogen~tica de la Cuenca Cuyana. IX Congr. Geol. Argentino, S.C. Bariloche, Actas VII, pp. 68-85. Dimmler A., Cyr T. D. and Stransz O. P. (1984) Identification of bicyclic hydrocarbons in the saturate fraction of Athabasea oil sand bitumen. Org. Geochem. 7, 231-238. Gelpi E., Schneider H., Mann J. and Oro J. (1970) Hydrocarbons of geochemical significance in microscopic algae. Phytochemistry 9, 603-612. Grimalt J. O., Simoneit B. R. T., Hatcher P. G. and Nissenbaum A. (1988) The molecular composition of ambers. In Advances in Organic Geochemistry 1987 (Edited by Mattavelli L. and Novelli L.). Org. Geochem. 13, 677-690. Pergamon Press, Oxford. Han J. and Calvin M. (1969) Hydrocarbon distributions of algae and bacteria and microbial activity in sediments. Proc. Nat. Acad. Sci. U.S.A. 64, 436-443. SUMMARY AND CONCLUSIONS Hayatsu R., Botto R. E., Scott R. G., McBeth R. L. and Winans R. E. 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