Geochemistry of North and South Brae Areas, North Sea

Transcription

1 Geocheistry of North and South Brae Areas, North Sea R. H. Reitsea SUMMARY: The Brae area of the North Sea has at least two types of crude oils present. Kieridgian shales are the source for both oils. The geologic setting, based on a fan delta odel, is siilar for both and, as expected, the oils are siilar in any ways. No significant differences appear in aturity, carbon isotopic ratios or gas chroatographic analyses. The oils can be distinguished, however, by use of ultiple geocheical paraeters. Quantities and various ratios of pristane, phytane, noral paraffins and gasoline range coponents are characteristic for each area of Brae. Relative aounts of saturates and aroatics and GC/MS patterns are also useful characterization ethods. The Brae Field of the North Sea is within the Viking Graben (Fig. 1). It has been suggested that the source of the oils in several fields in the North Sea is the Kieridge Clay (Fuller 175; Barnard & Cooper 181). This Upper Jurassic Foration is rich in organic carbon and has been reported to contain the kerogen types able to act as source for petroleu (Brooks & Thusu 177). It had been noted previously that the Kieridge Clay is not of a single kerogen type, but is varied laterally; also the hydrocarbons present are not the sae fro one location to another (Barnard & Cooper 181). Variations in aounts of inertinite, vitrinite, waxy sapropel and algal sapropel were reported. These varied with the distance fro the source which was depositing large aounts of land-derived organic atter in an anoxic regie. During exploration and delineation of the Brae area, enough wells were available to perit evaluation of the source of the oil and variations that ight occur over the whole area (Fig. 2). The probles of iportance were whether the oils were fro a single source as previously suggested or fro ultiple, ore ature, ones; also whether the crudes were present in single or ultiple reservoirs resulting fro different histories of the oils. A geologic interpretation of the Brae area has been given by Hars et al (181). The deposition of the sedients in the South Brae area is geologically explained on the basis of a fan delta odel with high input of aterial as streaborne sedients into a arine environent. Lateral variation ight be anticipated depending on the location relative to the fan and its foreset areas. The South Brae area was suggested as being geologically unique aong other Viking Graben prospects. Thus one ight also anticipate a geocheical difference between North and South Brae areas. Conventional geocheical analysis of South Brae sedients deonstrated the source quality of the Upper Jurassic. Organic carbon contents of over 5~ were generally present in all saples. The kerogen was a ixture of algal type together with other aterial of little or no structure (aorphous) and soe coaly aterial which, at ties, constituted a significant part of the organic aterial and in soe saples was present in discrete layers. Extractable hydrocarbons were generally greater than 1000 parts/ 106 as were the aounts of C1_4 hydrocarbons. The aturity of the sedients in the Late Kieridgian was within the early part of the oil generation window in any wells, although the organic-rich aterial continued to depths where aturity easureents by vitrinite reflectance or visual colour suggested very adequate heating of the sedients. The extracts invariably showed no odd carbon preference for the noral paraffins. These results are in agreeent with previous reports for the North Sea Upper Jurassic Cretaceous sedients of other blocks. Source of Brae oils Conventional gas chroatographic analyses suggested reasonable unifority aong oils fro the wells with inor variation between North and South Brae areas (e.g. Fig. 3). The oils are highly paraffinic with a few isoparaffins present in significant aounts to perit their resolution by chroatography. The coparison of noral paraffins on a noralized plot (Fig. 4) again shows the oils to be very siilar. Occasional variations in coposition of paraffins as for exaple with C24, C26 or C31 are not proinent enough to be characteristic. Gas chroatogras of the aroatic fractions of the oils (Fig. 5) fail to provide uch indication of any ajor differences between North and South 203

2 204 R. H. Reitsea / "r" O "l- PLATFORM HORDA BASIN" MORAY :IRTH B. GRAMPIAN HIGH ~ 0 iles 100 I I I! i 0 k 60 MID NORTH SEA HIGH Fro. 1. Location of the Brae Field in the Viking Graben area of the North Sea. Brae oils. The source for these hydrocarbons ust be the sae. There are soe consistent inor differences that perit differentiation, however, particularly in the presence of a unique peak at the higher carbon nubers in South Brae oils. The ost likely source of the hydrocarbons has been reported to be the Kieridge Clay which is present in the Brae area. Coparisons of oils with extracts fro adjacent core aterial showed a strong siilarity in properties of the aterials. Oils fro fro Well 7a/1 N was distinguishable fro extracts fro 361 and 3758 (Fig. 6), but the differences between the extracts in only 67 was as great as between oil and extract. This is not a copletely satisfactory test, because these core saples also contain igrated oil. However, the organic algal richness of the sedient (4.2% organic carbon) at its aturity should have contributed enough hydrocarbons to the extract to alter its coposition if the oil was sourced in a different section. The sae results were obtained on other coparisons of oil/core extract saples fro South Brae. Oils had lost their light ends during extraction processing and extracts often had unresolved coponents particularly near noral C2 to C31, but both contained a doinance of noral paraffins with siilar naphthenic peaks. On the basis of the coparison of the Brae oils fro one area to another and the siilarities between properties of the oils and extracts fro Kieridge Clays one can assue that the oils were sourced fro the Kieridge. However,

3 Geocheistry of North and South Brae Areas, North Sea /7A- 7A-14 N 2O ~ 25 8 ~, 1 I A-8 S FIG. 2. Location of wells in the Brae area. as will be pointed out, there is not just one 'Kieridge organic type', and correlation ust be ade with the rock that is producing the doinant type of oil. 25 Coparison of North and South Brae oils The ost obvious contrast between North and South Brae oils is in the lighter appearance of the North Brae oils and their higher API gravity. The coposition of the oils is also distinct. The plot of saturate, aroatic, and non-hydrocarbon fraction quantities (Fig. 7) shows the close cluster of values for South Brae oils (Wells 3, 6, 8, 10). North Brae oils show greater aounts of saturate copounds and fewer aroatics and NSO coponents. This suggests a longer igration path for the North Brae oils. The sae observations result fro the absence of asphaltenes in North Brae oils, although South Brae oils also contained only 1-2% of a pentane insoluble fraction. FIG. 3. Coparison of gas chroatogra~, of saturate fractions of oils fro North Sea Brae (7A-14N) and South Brae (7A-8S). An iportant variation between North and South Brae oils, which was shown by gas chroatography (Fig. 4), was the relative aounts of pristane and phytane. These differences were investigated in ore detail to copare the coposition of the two areas and of the interediate 'Middle' Brae oils (Fig. 8). The ratios of pristane to C17 and phytane to Cle are greater for South Brae oils. The pristane-

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5 Geocheistry of North and South Brae Areas, North Sea 207 7A-1 N 7A- M DST 1 il i 7A-17N DST 1 7A-10S Fro. 5. Coparison of gas chroatogras of the aroatic fractions of oils fro North, Middle and South Brae. phytane ratio (Fig. ) is about 1 for South Brae and greater than 1 for North Brae. Middle Brae oils have a siilar ratio to South Brae oils. Another ethod of analysis used to distinguish oils, is the coparison of triterpanes and aroatic sterane coponents. Triterpanes (Fig. 10) showed soe differences particularly in the aounts of C31 and higher terpanes in North Brae Well 14, as expected fro the lighter gravity of Well 14 oil. Aroatic steranes in North and South Brae oils (Fig. 11) showed characteristic copounds despite the strong overall siilarities. The consistency in ajor details of the oils fro North and South Brae is not found in the pristane:c17, phytane:cls and pristane:phytane ratios included in Figs. 7 & 8. Whereas Middle Brae and South Brae Well 10 extracts are siilar to the oils, the contrast between pairs of oils and extracts fro North Brae wells and fro Well 11 in the south is very considerable. This sae contrast is shown in Fig. 12 for stable carbon isotopic ratios of oil fractions and kerogens. Oils fro eight wells fro both north and south areas fail into a rather narrow band. Extract fractions fro Well 11 also fail within this sae range of lighter isotopic carbon ratios. The extract fro Well 10 in the south area shows a distinct range of values for the several fractions separated fro the extract. These two ranges for the extracts can be extrapolated roughly by the procedure of Stahl (178) to the two ranges of kerogens found in the Kieridge Clays. Kerogens with stable carbon isotopic ratios of around -2 to -28~ were found for North Brae Wells 14 and 17 and for South Brae Well 11, whereas the ratio for kerogens found in the Kieridge near the reservoirs of Wells 8, 10 and 13 in South Brae had a value of about Fro these correlations it is possible to conclude that the oils in the Brae area are sourced in a kerogen with an isotopic ratio of around This kerogen is represented in saples fro the North Brae wells and Well 11 in the south. The kerogen in 'Middle' Brae wells and South Brae Well 10 is a different type which

6 208 R. H. Reitsea 7A-1 N OIL EXTRACT 361 EXTRACT 3758 FIG. 6. Coparison of core extracts and oil fro Brae well 7A-1N.

7 Geocheistry of North and South Brae Areas, North Sea 20 SAT NSO AR FIG. 7. Coposition of oils fro Brae wells. 1 14,~ 17 ~" 3 6 z 6.J.d 13 ~ 10 I o O 0 OO O O i NORTH GI) J 13 ~ uj 8 11 SOUTH o o 8 11 o i Pr/Clr 1.5, i Ph/C18 I KEY OILS O EXTRACTS Flo. 8. Pristane:Cl7 and phytane:cls ratios of Brae oils. corresponds to the carbon isotopic ratio reported for another North Sea Kieridge kerogen (Stahl 178). On the basis of several different analyses, such as lighter gravity, no asphaltenes, lower aroatic and NSO contents and fewer heavy coponents, it is concluded that oils in North Brae wells were the consequence of longer igration than South Brae oils. South Brae oils also have igrated, but probably uch less. The carbon isotopic data further suggest that igration for South Brae oils also occurred, since, on the basis of carbon isotope ratio studies, the oils do not correlate with the kerogens. It is probable that the North Brae igration path is longer and this is supported by the

8 210 R. H. Reitsea if ~ NORTH h- 6.J _1 13 tu ~ 10 0 SOUTH KEY OILS O EXTRACTS 0.5 1,0 Pr/Ph FiG.. Pristane:phytane ratios of Brae oils. /e It tl t.,a-..,t ([. ~ ~ I~ it EXT 376 rn,..,,i,, I... 'L,~J',,.,. I I...) 'l...,~.,,~i~l ~. jl,.k,;.,,,~j,..,,',-,,,-..j L.j'..~..~-'.._.~,,--.~-.-.,',. r--v,... ::.';..."''-... "'"... 2:... ". ": "... _...;. ' A-14N q I rl ' ii OIL ~.,,.,,,..~._.~:...r...,.r~,...~l._~..,.~,.j,.,,~..,oi;,..../~,...?.~',.,,,...,...,...,..,.,...,...,......~, _~ 7A-10S OIL... _~....L/',~... t ~,,._~.~,,,.--,_o ~, ~ J~ 7A-11S, t', lid\._-_;l.----.a-"~... O I L FIo. 10. Coparison of triterpane coposition of North and South Brae oils. greater reoval of the heavier coponents. The source of the two oils appears to be the sae and corresponds to the type of Kieridge sedients found in North Brae wells and in South Brae Well 11. These geocheical observations led to additional visual inspection of kerogen types in the two reservoir zones with the observation that the North Brae type of kerogen con- tains abundant so-called aorphous atter with no structure in contrast to the rich spore-laden kerogen in the South Brae wells. The carbon isotopic ratios suggests that the principal source of Brae oils is ore arine, and is presuably related to the algal type of kerogen found in Kieridge section of the North Brae and South Brae Well 11.

9 /o Geocheistry of North and South Brae Areas, North Sea A-1 ~v~_r ~, ~ ~t~ 7A-11 ~_~.~ 7A-10 ~ ~ 7A-8 - NORTH BRAE SOUTH BRAE FIG. 11. Coparison of aroatic steranes in North and South Brae oils. / SAT L - ^ -- : / 7~'1/~8 OILS. ' AR /t, 1,.so \,/ ASPH ~ / / / ~, KEROGEN, 0.%,! -30 I I I I I C FIG. 12. Carbon isotopic ratios of functional group fractions of Brae oils and extracts fro South Brae Wells 10 and l 1.

10 212 R. H. Reitsea Conclusions Cobining these different geocheical techniques has led to a recognition of the two types of kerogen in Kieridge shales as had been suggested by Barnard & Cooper (181). This variability can occur over a short distance as found in the Brae area. Identification of geocheical differences between kerogens fro South Brae Wells 10 and 11, and variations in the visual organic atter, correlates well with the two types of sedients. Well 10 is possibly closer to the centre of the depositional streagenerated fan, whereas Well 11 is on the edge and closer to a arine environent, where the kerogens could be associated with a shallow water type of deposit. BARNARD, P. C. & COOPER, B. S Oils and source rocks of the North Sea area. In: Petroleu Geology of the Continental Shelf of North-West Europe, Inst. Petrol., London. BROOKS, J. & THUSU, B Oil-source rock identification and characterization of Jurassic sedients in the Northern North Sea. Che. Geol. 20, FULLER, J. G. C. i Jurassic Source Rock Potential and Hydrocarbon Correlation, North Sea. In: Proceedings of Jurassic Northern North Sea Syposiu, Stavanger 175, 11, References HARMS, J. C., TACKENBERG, P., POLLOCK, R. E. & PICKLES, E The Brae oil field area. In: Illing, L. V. & Hobson, G. D. (eds). Petroleu Geology of the Continental Shelf of North- West Europe, Heyden and Son, London. STAHL, W. J Source rock-crude oil correlation by isotopic type-curves. Geoche. cosochi. Acta 42, , FAILER, E., SCHMITT, M. & CAREY, B. D Carbon isotopes in oil and gas exploration. In: Nuclear Techniques and Mineral Resources. International Atoic Energy Agency, Vienna. R. H. REITSEMA, Marathon Oil Copany, P.O. Box 26, Littleton, CO 80160, U.S.A. Conference discussion COMMENT: Seifort has used relative concentrations of tricyclic terpanes to indicate relative igration distances within oil failies. Has Marathon evaluated tricyclic terpane data to indicate both distance and direction of igration for oils in the Brae field. RESPONSE: No. The geographic area reported on in this work was probably too sall for a reliable study.