Correlation of source rocks and migrated hydrocarbons by GC-MS in the middle Triassic of Svalbard

Org. Geochem. Vol.6, pp. 513-520,1984 Printedin Great Britain.Allrightsreserved

0146-6380/84 $03.00+00.00 Copyright© 1984 Pergamon Press Ltd

Correlation of source rocks and migrated hydrocarbons by GC-MS in the middle Triassic of Svalbard L. SCHOU, A. M~RK and M. BJORgIY Continental Shelf Institute, P.O. Box 1883, N-7001 Trondheim, Norway Abstract--Samples of Lower and Middle Triassic sediments from nine different locations on Svalbard have been analysed by GC-MS to study migration processes. Short range migration was investigated by analyses of interbedded shales and siltstones from three of the locations. In addition, shale samples from six locations were analysed to study long range migration. The maturity of the samples varied from immature through moderately mature to oil window maturity. The qualitative distribution of biological markers was seen to be fairly similar in all the analysed locations, irrespective of lithology. This implies that no source characteristic parameters from GC-MS analyses could be applied to exclude any of the analysed source rocks. Based on maturity considerations it was concluded that short range migration from shales to siltstones had probably been the most important process. Key words: source rocks, migrated hydrocarbons, correlation, GC-MS, Middle Triassic Svalbard INTRODUCTION

THE ORGANIC MATTER

Geological investigations of the Mesozoic succession of Svalbard have taken place at the Continental Shelf Institute (IKU) during the last 10 years. A brief evaluation of source rock potential within this succession is given by M~rk and Bjor0y (1984), while M0rk et al. (1982) have presented the geological setting, stratigraphy, sedimentology and facies development of the Lower and Middle Triassic sediments investigated here. This study has concentrated on the uniformly developed units in the central and eastern areas (Fig. 1). Contrary to the low to moderate maturity in this sequence (Fig. 2) abnormally high concentrations of C1~+ hydrocarbons were encountered in shales, siltstones and sandstones. Several theories have been put forward as possible explanations for this: (a) Hydrocarbons have been generated at lower maturity levels than normal, and short range migration into coarser lithologies has taken place. (b) Long range lateral migration of hydrocarbons generated in strata of higher maturity levels has occurred along natural migration pathways. (c) Vertical migration from deeper, more mature source rocks is responsible. In order to assess the viability of these hypotheses, sample pairs of interbedded shales and siltstones have been analysed to study the effect of short range migration (a). Mature samples from the central area have been used to investigate possible long range migration (b). Vertical migration from more deeply buried source rocks (pre-Mesozoic) has not b e e n studied, and thus no conclusion as to the feasibility of hypothesis (c) is forwarded. The regional maturity level, as represented by Tma× temperatures from Rock-Eval pyrolysis is shown in Fig. 2. Only G C - M S data are discussed in any detail, other geochemical analyses have been summarised by M0rk and Bjor~y (1984).

The organic content increases up through the Lower and Middle Triassic succession, and Fig. 3 shows a typical example from the eastern area, with values up to 12% (mean 6-7%) in the Botneheia Member. The same trend was seen in western Spitsbergen with organic content increasing up through the sequence with a maximum at 3% TOC in the Middle Triassic shales. The kerogen types vary from type III dominating in the western areas through a mixed II and III to a dominantly type II towards the east (Botneheia Member). This mirrors the palaeogeography with a land area in the west with deltaic sediment input and increasingly open marine conditions towards the east, with the development of anoxic bottom conditions in parts (M~rk et al., 1982; M0rk and Bjor0y, 1984). Based on vitrinite reflectance measurements, Tm~x of Rock-Eval pyrolysis and the thermal alteration index, the maturity was found to be highest in the west, along a narrow north-south trending zone, correlating with the Tertiary deformation zone (M~rk and Bjor0y, 1984). The maturity decreases towards the east and north-east (Fig. 2). In a previous investigation Forsberg and Bjoroy (1983) discussed the effect of weathering on outcrop samples from the Botneheia Member. They concluded that the total abundance of organic carbon was hardly affected at all, while an average loss of 10% was seen in the abundance of extractable organic matter. The aromatic hydrocarbons were more affected than the saturated compounds, and the n-alkanes more than the isoprenoids. The degree of weathering was thought to be too low to have affected the steranes and terpanes in these samples. To verify this, mass chromatograms of weathered and nonweathered samples (as determined from GC profiles) were compared.

513

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L. ScHou et al.

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Rock-Eval production indices and extraction data show that migrated hydrocarbons have been found in sandstones and siltstones in the Botneheia and Sticky Keep Members (Leythauser et al., 1983; M0rk and BjorCy, 1984). In the present study this enrichment of hydrocarbons was investigated by GC-MS analysis of interbedded shales and siltstones in an attempt to study the distribution of steranes and terpanes in the two lithologies in connection with short range migration (a). To ensure that the analysed sequences of interbedded shales and siltstones showed this enrichment/depletion behaviour, Rock-Eval pyrolysis and extraction data were registered for the samples prior to the detailed GC-MS analysis. To study possible long range migration (b) from more mature sections in the central part of the area shale samples from nine different locations were analysed for their steranes and terpanes distributions.

EXPERIMENTAL

Experimental procedures are described by Bjor0y et al. (1983a) and include total organic carbon (TOC)

analysis, Rock-Eval pyrolysis, extraction, chromatographic separation and gas chromatography. The GC-MS analyses were performed by Multiple Ion Detection (MID) on a VG 70-70H MS-DS coupled to a Varian 3700 GC, fitted with a 20 m OV-1 fused silica column. Hydrogen was used as carrier gas (1 ml min -~) and injections were performed in split mode (1 : 10). GC oven temperature was programmed from 120 to 280°C at 4°C min-l.

RESULTS AND DISCUSSION

The GC-MS data are used both as source characteristic and maturity parameters. For more details about the various molecular ratios it is referred to the

GC-MS in the Middle Triassic of Svalbard

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Fig. 2. Regional maturation levels based on Rock-Eval pyrolysis Tmaxvalues for the samples from the Botneheia Member. Maturation is high in the Western Tertiary Fold Belt. On Central and eastern Spitsbergen maturation is controlled by an east- and north-eastward thinning of the sediment in the Tertiary Basin (MCrk and Bjorcy, 1984). The maturation level on Barentscya and Edgecya have been partly influenced by local intrusions in the cross-hatched area.

literature (Seifert and Moldowan, 1978, 1980; Seifert et al., 1980; Mackenzie et al., 1981). The term maturity is here used both to describe the maturity of extractable hydrocarbons as determined from the G C - M S parameters, and to describe the maturity of the kerogen itself as determined from Tmax and vitrinite reflectance measurements. For discussion of short range migration, biomarker values for individual samples are applied, while for the discussion of long range migration values have been averaged for each location. The averaging was done to be able to see a more regional trend in the maturity. The G C - M S maturity data are then further compared to the maturity of the kerogen as determined from Tmax and vitrinite reflectance.

Short-range migration Interbedded shales and siltstones from location numbers 3-5 (Fig. 1) have been analysed by GC-MS. The lithological variations and sample heights for one of the profiles, Hcgrinden, are shown in Fig. 3. Table 1 summarises some Rock-Eval and extraction data for the same profile. These data show that the high concentrations of hydrocarbons encountered in some OG 6:1/4-Q~

of the siltstones has not been generated from the kerogen in the siltstones. Two of the siltstones have hydrogen poor type kerogen and also relatively high production indices for the actual maturity level. Migrated hydrocarbons may also be seen in at least one of the shale samples. Typical mass chromatograms representing steranes (m/e = 217) and terpanes (m/e = 191) are presented in Fig. 4. In all three locations the mass chromatograms show similar distribution of steranes and terpanes, irrespective of the lithology. In addition to the major 17a(H), 21~(H) hopanes, abundant tricyclic terpanes (* in Fig. 4) were seen in the m/e = 191 traces. The sterane traces (m/e = 217) contain abundant rearranged steranes, and similar molecular weight distributions of the regular steranes for each of the samples, the C27 and C29 analogs being the most abundant. Mass c h r o m a t o g r a m s r e p r e s e n t i n g tri- and monoaromatic steranes were acquired from the aromatic hydrocarbon fractions (Fig. 4). Apart from an increase in the abundance of lower molecular weight components with increased maturity the distribution of triaromatic steranes (m/e = 231) is similar for all the samples. The distributions in the

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Fig. 3. Interpretative lithological section with stratigraphical subdivision from H~grinden at southern Barents~ya. TOC values are plotted for the whole profile to show the general trend. In addition, production indices are plotted tor the samples that were analysed by GC-MS. The sample number (height in profile) assigned to each sample in the field is retained and not adjusted for the correct height in the profile.

monoaromatic sterane traces (m/e = 253) was seen to change slightly, one single peak becoming predominant at higher maturity. It can, however, not be totally excluded that this change reflects different organic environments of the three locations, but we believe it is due to a decreased overall concentration of monoaromatic steranes at higher maturity, resulting in other types of aromatic components appearing more prominent. As an example, Table 2 lists various molecular ratios from the H~grinden profile. The hydrocarbons in the siltstones appear from the biomarker parameters to be of slightly higher maturity than the ones extracted from the adjacent shale samples. This is true even when the Tmax values assign the siltstones themselves as less mature than

the shales. This might indicate that the hydrocarbons in the siltstones are generated from a source rock slightly more mature than the shale closest to the silt layers. As can be seen from the molecular ratios in Table 2, showing data from the Hogrinden profile, the lowermost shale samples have values most similar to those for the hydrocarbons in the siltstones. This probably indicates that this shale has been responsible at last partly for the hydrocarbon generation. There is no evidence that a systematic migration from the shale layers to the adjacent interbedded siltstone layers is a major process. Since the maturity is only slightly higher in the siltstones as compared to the shales, the highly mature shales of the Western Spitsbergen are ex-

517

GC-MS in the Middle Triassic of Svalbard Table 1. Rock-Eval and extraction data from H~grinden Lithology

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103.0 112.0 181.0 183.8 193.5 196.8 200.0

430 429 418 431 429 427 431

261 179 293 411 428 251 484

50 108 31 49 19 111 12

0.16 0.17 0.09 0.10 0.08 0.26 0.06

1270 321 1734 889 1897 1144 1901

1.9 0.7 4.1 1,7 5.0 1.1 11.9

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cluded as the source of the hydrocarbons at the H0grinden location. However, at the basis of maturity comparisons the mature shales in the Central area cannot be ruled out as possible source rocks for the hydrocarbons encountered. The overall very similar profiles of the various mass chromatograms would suggest that short range migration is the process most likely to be responsible for the hydrocarbons in the siltstones. Short range migration is believed to cause only minor alterations in the sterane and terpane mass chromatograms. Similar short range migration from Lower Triassic shales to siltstones on Bj0rnOya was postulated by Bjor0y et al. (1983b). This work was based partly on G C - M S analyses, and concluded that migration took place from shales to siltstone within the formation.

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To study the possible long range migration processes from the mature shales in the Central area of Spitsbergen towards more easterly locations, samples from nine different locations (Fig. 1) were analysed by GC-MS. Only minor variations were detected in the mass chromatograms of saturated terpanes and steranes, all being similar to the examples given in Fig. 4. Aromatic steranes were not analysed in the samples from these additional locations. Some variation in the relative amounts of tricyclic terpanes is seen, but this seems to be consistent with maturity variations, and thus is unlikely to be a result of different types of organic matter. This implies that the type of organic matter in the Middle Triassic is very similar throughout the analysed area, and that no characteristic qualitative differences in the biomarker distribution can be applied to exclude certain sequences. No trend was seen as to variations in the biomarker profiles of the samples that showed signs of weathering effects from the GC traces. The same molecular ratios as presented for HOgrinden in Table 2, were calculated for most of the analysed profiles to study possible long range migration. Fig. 5 shows minimum, maximum and average values for each of the parameters plotted for the individual locations. Most of the ratios are known to represent maturity dependent isomerisation reactions. The reactions in both steranes and terpanes, seem to be close to equilibrium (Seifert and Moldowan, 1978; Seifert et al., 1980), and the variations seen are only minor. However, there does appear to be a trend in maturity. A parallel trend could also be seen in the relative amount of tricyclic terpanes, a ratio known to be affected by type of organic matter as well as maturity. The profiles analysed represent a maturity range from immature through moderately mature to oil window maturity. From the biomarker ratios the maturity order of the extractable hydrocarbons was found to parallel that of the shales, as assigned by Rock-Eval Tmax measurements and

Maturity order from T max -

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Fig. 5. Molecular parameters from GC-MS mass chromatograms (minimum, maximum and average values) plotted for each location. The numbers after each profile name represent number of samples analysed by GC-MS in the actual profiles.

520

L. ScHou et al.

other bulk parameters. The least mature hydrocarbons were extracted from the shales of lowest maturity, and the most mature hydrocarbons from the most mature shales. This is taken as an indication that the analysed hydrocarbons have been generated in shales relatively close to the siltstones where they were found, i.e. short range migration. If long range migration from more mature shales was an important process contributing to the high abundance of hydrocarbons in shales of low maturity, then the difference in maturity of the kerogen and the hydrocarbons within one profile would be expected to be more pronounced.

CONCLUSIONS (1) The organic matter in the Middle Triassic sediments are very similar for all the analysed locations. (2) No systematic trend was seen in the GC-MS data of weathered samples compared to unweathered samples. (3) The relative amount of tricyclic terpanes was seen to vary systematically with maturity. (4) In the profiles with interbedded shales and siltstones slightly higher maturity of the hydrocarbons was seen in the siltstones. This would indicate that hydrocarbons have migrated into the siltstones from source rocks of slightly higher maturity [hypothesis (b) or (c)] than that of the siltstones themselves. No systematic migration between interbedded shales and siltstones was seen. (5) Very similar overall profiles of the sterane and terpane mass chromatograms were seen in all the analysed samples. Therefore, long range migration [hypothesis (b)] cannot be excluded based on source characteristic parameters from GC-MS analyses. (6) A comparison of the maturity of the extractable hydrocarbons and the maturity of the corresponding kerogens excludes the hypothesis suggesting long range migration from a higher maturity level [hypothesis (b)]. (7) The same maturity considerations support hypothesis (a), that suggests that the hydrocarbons have been generated in the shales close to the siltstones where they were encountered.

Acknowledgements--American Petrofina Norway A/S, British Petroleum Norway A/S and Elf-Aquitaine Norway A/S are the sponsors of this ongoing project on Svalhard, and the authors thank them for the permission to use the data for this presentation. Statoil gave invaluable logistic support for the field work. The internal standard applied in the GC-MS analyses was supplied by Masspec Analytical. Finally, we wish to acknowledge the very competent technical assistance given by the laboratory staff at IKU. REFERENCES

BjorCy M., Bue B. and Elvsborg A. (1983a) Organic geochemical analysis of the first two wells in the Troms 1 area (Barents Sea). In Advances in Organic Geochemistry 1981 (Edited by Bjorc~y, M. et al.), pp. 16-27. John Wiley, Chichester. Bjor¢y M., M¢rk A. and Vigran J. O. (1983b) Organic geochemical studies of the Devonian to Triassic succession on Bj¢rn¢ya and the implications for the Barent Shelf. In Advances in Organic Geochemistry 1981 (Edited by Bjor¢y M. et al.), pp. 49-59. John Wiley, Chichester. Forsberg A. and Bjoroy M. (1983) A sedimentological and organic geochemical study of the Botneheia Formation, Svalbard, with special emphasis on the effects of weathering on the organic matter in the shales. In Advances in Organic Geochemistry 1981 (Edited by Bjor¢y M. et al. ), pp. 60-68. John Wiley, Chichester. Leythauser D., Mackenzie A. S., Schaefer R. G., Altebaumer F. J. and Bjoroy M. (1983) Recognition of migration and its effects within two core holes in shale/ sandstone sequences from Svalbard, Norway. In Advances in Organic Geochemistry 1981 (Edited by BjorOy M. et al.), pp. 136-146. John Wiley, Chichester. Mackenzie A. S., Hoffmann C. F. and MaxwellJ, R. (1981) Molecular parameters of maturation in the Toarcian shales, Paris Basin, France. I. Changes in aromatic hydrocarbons. Geochim. Cosmochim. Acta 45, 13451355. MCrk A. and Bjoroy M. (1984) Mesozoic source rocks on Svalbard. N. Eur. Margin Syrup. 1983, Norsk Petroleumsforening. MOrk A., Knarud R. and Worsley D. (1982) Depositional and diagenetic environments of the Triassic and Lower Jurassic succession of Svalbard. In Arctic Geology and Geophysics (Edited by Embry A. F. and Balkwill H. R.), pp. 371-398. Can. Soc. Pet. Geol. Mere., No. 8. 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, 77-95. Seifert W. K. and Moldowan J. M. (1980) The effect of thermal stress on source rock quality as measured by hopane stereochemistry. In Advances in Organic Geochemistry 1979 (Edited by Douglas A. G. and Maxwell J. R.), pp. 229-237. Pergamon Press, Oxford. Seifert W. K., Moldowan J. M. and Jones R. W. (1980) Application of biological marker chemistry to petroleum exploration. Proc. lOth World Petroleum Congr., Bucharest, Romania, September 1979, Paper SP 8, pp. 425-440. Heyden, London.