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Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands
,ia
Marine and Petroleum Geoloyy, Vol. 14, No. 3, pp. 257 266, 1997 ~'. 1997 ElsevierScience Ltd All rights reserved. Printed in Great Britain
PII: S0264--8172197100002-.0
0264-8172/97 $17.00+ 0.00
ELSEVIER
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands T. Laier* and H. P. Nytoft Geological Survey of Denmark and Greenland, Thoravej 8, Copenhagen, Denmark O. Jorgensen National Institute of Occupational Health, Lerso Parkall#, Copenhagen, Denmark G. H. Isaksen Exxon Production Research Co., P.O. Box 2189, Houston, Texas, USA Received 9 September 1996; revised 8 December 1996;accepted 21 December 1996 Hydrocarbons in the form of gases, oils and waxes have been observed in the basalts of the Faeroe Islands in the North Atlantic. Gases and traces of oil were observed in the outflowing water of the deep Lopra-1 well drilled in 1981. The hydrocarbon gas was fairly dry, with stable isotopic ratios of ~13C1_3=- 41.4; - 32.4; - 26.5%o respectively, typical of a thermogenic gas. High temperature gas chromatography of the oil showed that it consisted mostly of C13-60 n-alkanes. Biomarker distribution observed by GC-MS indicates that the oil was derived from a mature source rock deposited in an anoxic environment; this suggests that the source rock must lie beneath the known basalts. Waxes exhibiting bright yellow fluorescence under UV light were observed as coatings on zeolite minerals widely distributed on the Faeroe Islands. The waxes consist predominantly of higher n-alkanes shown by HTGC. The fluorescence indicates the presence of aromatic compounds. Biomarker distribution indicated that the waxes were derived at least in part from a source rock containing some terrestrial organic matter as testified by the low amounts of oleanane present. The waxes were probably deposited from traces of oil present in deeply circulating waters in fractures within the basalts. Coals which had been suspected to generate some of the hydrocarbons observed in the Faeroese basalts were also examined. Vitrinite values of Ro=0.5% as well as GC-MS analyses of the Suduroy coal extracts showed that these coals are immature and have not generated significant hydrocarbons. © 1997 Elsevier Science Ltd.
Keywords:hydrocarbons;biomarkers In 1981, a scientific borehole was drilled at Lopra on Suduroy in order to explore the substratum of the Faeroe Islands basalts (Berthelsen et al., 1984). The Lopra- 1 well, however, failed to penetrate the basalts as drilling was terminated at 2178m for technical reasons. Although no direct information was obtained on the sub-volcanic rocks, studies of cuttings as well as fluids from the well gave indications of sedimentary rocks capable of generating hydrocarbons beneath the basalts.
Furthermore, native copper indicating reducing conditions due to the presence of organic material was observed in two samples (1996, 2072 m). Only one of the four waxy samples was readily dissolved in an organic solvent according to Jorgensen (1984). The soluble wax from 2072 m analysed by gas chromatography consisted mainly of n-alkanes C22 37 with an almost symmetrical distribution around C29 (Laier and Nytoft, 1993). For comparison, a wax sampled from an outcrop on Videroy (Figure 1, location 0) was analysed by gas chromatograpy. This sample also consisted mainly of nalkanes C20 43 showing a weak odd over even predominance (Laier and Nytoft, 1993). The insoluble yellow waxes in the Lopra-I well were assumed to originate from the coal-bearing sediments that occur between the Lower and the Middle Basalt Series (Jorgensen, 1984).
W a x y bitumen Secondary minerals were studied in samples of drill cuttings selected every 2 ~ 2 5 m in order to reconstruct the geological and hydrothermal evolution of the area (Jorgensen, 1984). During this study, inflammable waxy material was observed as thin coatings on secondary minerals in four samples at 920, 1524, 2072 and 2120 m depth.
Hydrocarbon gases Only negligible shows of gas were observed during drilling of the Lopra-1 well (Waagstein et al., 1984). However, when the well was re-opened for temperature logging in
*Author to whom correspondence should be addressed at: GEUS Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400, Copenhagen, Denmark. Tel: +45 35 14 20 00. fax: +45 38 14 20 50.
257
258
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et al.
May 1983 the well head pressure had increased to 19.5 bars (Balling el al., 1984) and approximately 9 m -~of gas was released from the well (P. H. Nielsen pers. comm.). The gas consisted of methane (72%) and nitrogen (27%) plus traces of higher hydrocarbons (Jacobsen and Laier, 1984). Based on stable isotopic analyses (c$1~Cl=-39.6; 51~C~=-32.4%0), Jacobsen and Laier, 1984 concluded that the hydrocarbon gases were most likely of thermogenic origin. Being open, the well continued to flow with slightly saline water ( T D S = 900mgl i, 12.81min t) and gas (0.81min i). Sampling at different depths, Jacobsen and Laier (1984) observed an increase in the gas to water ratio with depth, suggesting that gas mainly entered the deeper parts of the Lopra-1 well. Traces of oil were observed in the out-flowing water and a smell of petroleum was noted, particularly in the water samples collected in the deepest part of the well (Jacobsen and Laier, 1984). The gas chromatogram of oil collected from the water showed a biodegraded oil, Pr/Ph = 1.5, probably derived fl'om a marine or lacustrine source rock. Diesel, ca 3000 litres, had been added to the well when the drill string got stuck at a drilling depth of l l00m. However, Jacobsen and Laier (1984) concluded from the gas chromatogram that diesel, if still present in the well, was not the only source ofoil in the out-flowing water.
New hwestigations The hydrocarbon discoveries, Foinaven in 1992 and Shiehallion in 1993, in the British sector 160kin SE of the Faeroe Islands, created new interest in the hydrocarbon traces observed in the Lopra-1 well. Therefore, as part of the current study, gases and oil were re-sampled from the well in order to determine the origin of the hydrocarbons. In addition, wax-coatings observed on zeolites in outcrops were also analysed in samples previously collected by one of the authors. Coals from mines on the northern part of Suduroy were also analysed in order to see if the coals were the likely source of the waxes observed in the Lopra-1 well and elsewhere. The Lopra-I well will undergo extended drilling during the summer of 1996.
Geological setting The Faeroe Islands (Figure 1) arc located in the NE Atlantic between Iceland and Scotland and consist of a pile of tholeitic plateau basalts up to 5kin thick, the upper 3 km of which is exposed (Waagstein et al., 1984). The basalts were extruded subaerialy during the early Tertiary. The volcanic pile can be divided into 3 series (Rasmussen and Noe-Nygaard, 1970), the Lower series being of C26R to C25N magnetostratigrapic age and the upper two series of C24R age (Waagstein, 1988). A coalbearing sedimentary succession, up to l0 m thick, is found between the Lower and the Middle Basalt Series (Figure 1), the total thickness of the coal seams being 1 m. Microfaunal assemblages, date the coals to Late Palaeocene age (Lund, 1989). Sub-basalt geological information is sparse, but the geochemistry of the basalts indicate the presence of continental crust beneath the basalts (Waagstein, 1988). Boldreel and Andersen (1994) concluded from a seismic profile east of the Foinaven Field, that the Mesozoic sediments found beyond the basalt cover continue below
the base of the basalt. The continuation of the Mesozoic sediments beneath the basalts can be traced 10 50kin laterally depending on the thickness and depth of the basalt cover (Kiorboe, 1995). Although the Lopra-1 well failed to penetrate the basalts, Kiorboe and Petersen (1995) concluded from VSP seismics that the base of the basalt in the Lopra area is near 2.34 km depth, i.e. 200 m below TD of the well. The base of the basalt was recognized by a negative reflection coefificient due to lower seismic velocities in the sub-basalt layers, but the modeled sub-basalt velocities for the layers (5.0 5.35 km s ~) are rather high for sedimentary rocks (Kiorboe and Petersen,
1995).
Sampling and methods The material described in this chapter includes all available hydrocarbon samples collected from 1978 (zeolite waxes) to 1992 (oil and gas frorn Lopra-1 well head). However not all material collected during this period was available lk~r new analyses, e.g. the waxy material on cuttings IYom the Lopra-I well.
Bitumens Waxy coatings on zeolites have occasionally been found by one the authors during his studies of secondary minerals in the Faeroese basalts. The waxy coating is typically only recognized under the microscope. The waxy materials from 6 localities (Fiqure 1) were dissolved in dichloromethane and analysed by high temperature gas chromatography (HTGC) and gas chromatography mass spectrometry (GC MS) without further separation of the various fractions due to the low amounts of materials available. The branched/cyclic fraction of one of the samples was isolated by removal ofn-alkanes passing the dichloromethane solution through a column packed with 5 ,~ molecular sieves. This fraction was then analysed on a gas chromatograph equipped with an atomic emission detector (GC AED).
Gases and oil The Lopra-1 well had been shut in for several months before the sampling of gases and oil took place on the 9th of November 1992. The well head pressure was not known as no manometer was mounted on the well head, but opening the ball valve on the well head slightly one got the impression that the well head pressure was rather low. A 100ml steel cylinder equipped with 2 valves was connected to the well head using a 2 m hose, both cylinder and hose being filled with water. A moderate flow of gas was established by adjusting the the valve on the well head, and the cylinder was flushed with gas for ca 30 s before closing the valves of the cylinder. Two more gas samples were taken during which time the well started flowing with water. Approximately 1 litre of water produced during the sampling of the 2 latter gas samples was transferred to two 1 litre separation funnels and shaken with dichloromethane (20 ml) three times. In the laboratory, the combined dichloromethane extracts were dried over sodium sulphate and the dichloromethane removed by evaporation at 40' C. Separation of the various fractions of hydrocarbons were performed as described below and the paraffinic fraction was analysed by H T G C and GC MS.
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et
al.
259
7° 1
STREYMOY
MYKINES VAGAR 62°
62°
% Irregular intrusive bodies and sills J]]]]]]]~ Upper basalt series r=====]. Middle basalt series Tuff- agglomerate zone Coalbearing sequence Lower basalt series
0 i
, [
,
i
I [ i
i
10 I
i
~o. / ~ _ ; _ ~ I
20 km ,J
J
~o °
\f
1
SUDUROY FAEROE
ISLANDS;
60 °
ETOTLAN °i 17
2o'1
,0- I
~,~
bore h o l e ~
\ o.I
Figure Geological map of the Faeroe Islands. Numbers indicate outcrops where zeolites with wax coatings were sampled. Wax no. 0 was previously analysed (Laier and Nytoft, 1993). The exact location of wax no. 6 sampled on Suduroy is uncertain and is not indicated on the map
260
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et
Coals Samples of coal were collected from 2 coal layers, each ca 0.5 m thick, separated by 1 m of clay in the Rokhagi mine located on the northern part of Suduroy. According to the miners, methane had never been observed in the Faeroese mine and only once had a grey waxy substance been found in one of the mines. The two coal samples were crushed and refluxed with dichloromethane: methanol 93:7 for 4 h. The asphaltenes were isolated from the other hydrocarbons by precipitation with excess npentane and subsequent centrifugation. The remaining pentane-soluble organic matter was separated into polar, aromatic and paraffinic fractions using HPLC and the paraffinic fraction analysed by GC and GC MS. H T G C analyses were performed using a Hewlett Packard gas chromatograph HP-5890, equipped with a 15 m aluminum clad capillary column OVl (QXI/AI). The injector and detector was set at 420'C and the oven was temperature-programmed from 50 to 100'C at 10'C rain 100 to 2 4 0 C at 5 C m i n J and 240 to 400~C at 4'C min ~with an 18 min final hold time. GC MS analyses were performed with a Hewlett Packard HP5890 gas chromatograph equipped with a 25 m HP-5 capillary column and 0.11 mm film. The oven was temperature programmed from 70 to 100'C at 3 0 C m i n ~: and l00 to 300 'C at 4' C min ' w i t h a l 2 m i n final hold time. The mass spectra were obtained in El mode using a Hewlett Packard HP-5971A quadrupole mass spectrometer. GC and stable isotopic analyses of gases were pertbrmed following the methods described by Laier et al. (1992). Stable carbon isotopic analyses of the Lopra- 1 oil fractions and coal extract fractions were performed by Geolab Nor (Norway).
C20
C40
a l. C60
; Lopra-1 i (oil extracted ', from water)
wax no 1
, 11
ixn°3
Results and dicussion The low quantities of oil or waxy material available in most samples restricted the number of different analyses that could be performed on each sample. Therefore, it was not possible to obtain data for full comparison of all samples. Futhermore, no material was left for up-to-date analysis of previously reported samples, e.g. the waxy material encountered in the cuttings of the Lopra-1 well (Jorgensen, 1984), for which only GC data existed (Laier and Nytofl, 1993).
wax
no
4
Lopra- 1 oil and 9ases The oil extracted from water from the Lopra-I well in 1992 appears to be less biodegraded than the oil reported by Jacobsen and Laier (1984), the n-alkanes being much more abundant relative to the hump of unresolved mixture of hydrocarbons below C30 (Figure 2). This difference may be due to the fact that the water sample containing oil traces collected at Lopra-I in 1983 had been stored for 6 months prior to analysis. Biodegradation during storage was probably only very minor for the oil sampled in 1992 as the oil was extracted from water at the well site and analysed within 2 weeks. Analyzing the oil using high temperature gas chromatography (HTGC) revealed abundant long chain n-alkanes>C35 (Figure 2) which could not previously be seen using normal GC. The presence of these long chain alkanes supports the assumption that the oil in the Lopra-I water is not just residues of the diesel, C,~ C23, bp 200-360 C (suppliers intbrmation), added during drilling (Jacobsen and Laier, 1984), but
~.. ~,, l l~J~
........
wax no 5
Figure 2 High t e m p e r a t u r e gas c h r o m a t o g r a m s of the Lopra-1 oil and waxes. N u m b e r s refer to localities s h o w n on Figure 1
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et al.
261
Table 1 Biomarker data obtained from GC and GC-MS analyses
Sample
Isoprenoids pr/ph
Lopra-1 '92 Lopra-1 '83 Wax No. 1 Wax No. 2 Wax No. 3 Wax No. 4 Wax No. 5 Wax No. 6 Coal u. seam Coal I. seam
pr/nC~7
1.8 1.5
0.4 0.8
1.3
0.7
5 15
0.8 1.1
Steranes C29 ~=S/=~S+R
C29
=fl13/~=+~[t[~
C30 Mor/Hop
C3~ S/S+R
C29/C30
Tm/Ts
0.50
0.51
0.09
0.60
0.98
1.30
0.49
0.36
0.16
0.53
0.66
2.22
0.47 0.50 0.44 > 0.9
0.39 0.41 0.40 0.76
0.10 0.11 0.13 0.08 0.63 0.54
0.60 0.68 0.13 0.55 0.05 0.05
0.58 0.54 0.59 0.34 1.34 1.61
1.16 1.53 1.79 0.02 11 8.8
0.12
more likely derived, at least in part, from a source rock in the area. Separation by HPLC showed that the oil consisted of paraffins (68%), aromatics (26%) and polar compounds (6%). No asphaltenes were found. The amounts of oil in the first two water samples collected just after the well was opened in 1992 were 12 mg I ~and 0.5mgl-~, and since oil probably accumulated in the top of the well during the previous shut in period, the concentration of oil would most likely be less than 0.5 mg 1-~ during free flowing water condition.
Biomarkers. Biomarker distributions were monitiored to interpret thermal maturity, source rock organic matter type, and source rock depositional environment. Analyses were made by GC MS, monitoring m/z 191 for tricyclics and hopanes and m/z 217 for steranes (Table 1: Figure 3). One concern in this geological setting is the influence of any maturation effect due to contact metamorphism between source rocks and the Tertiary basalts, or between black oil and the basalts. The effect of later Table 2 Peaks identified in m/z 191 and m/z 217 fragmentograms
shown in Figure 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Terpanes
18~(H)-22,29,30-trisnorneohopane (Ts) 17~(H)-22,29,30-trisnorhopane (Tm) 17/~(H)-22,29,30-trisnorhopane 30-norneohop-13(18)-ene 17~(H),2lfl(H)-30-norhopane 18~(H)-30-norneohopane (29Ts) 17/~(H),2l~(H)-30-norhopane (normoretane) Oleanane 17~(H),21fl(H)-hopane 30-nor-29-homo-17--~(H)-hopane Neohop-13(18)-ene 17/~(H),2l/~(H)-30-norh opa ne 17fl(H),21~(H)-hopane (moretane) 17~(H),21/3(H),22(S)-homohopane 17~(H),21/~(H),22(R)-homohopane Gammacerane 17fi(H),21fi(H)-hopane 17~(H),21/~(H),22(S)-bishomohopane 17~(H),21/~(H),22(R)-bishomohopane 17/~(H),21/~(H),homohopane 17~(H),21/~(H),22(S)-trishomohopane 17~(H),21//(H),22(R)-trishomohopane 17~(H),21/~(H),22(S)-tetrakishomohopane 17~(H),21/~(H),22(R)-tetrakishomohopane 17~(H),21/Y(H),22(S)-pentakishomohopane 17~(H),21/3(H),22(R)-pentakishomohopane 24-ethyl-5c~(H), 14c4H), 17~(H), 20(S)-cholestane 24-ethyl-5cdH), 14/Y(H), 17/~(H), 20(R)-cholestane 24-ethyl-5~(H), 14/~(H), 17/~(H), 20(S)-cholestane 24-ethyl-5c~(H), 14~(H), 17~(H), 20(R)-cholestane
heating of source rocks or oils in proximity to intrusives is not w e l l understood. However, the biomarker distributions support conventional maturation through progressive burial of the source rock. Contact metamorphism would likely have resulted in differing degrees of isomerization among molecular maturity parameters (tricyclics, homo-hopanes, and steranes). Sterane and triterpane maturity parameters (Table 1; Figure 3) suggest a maturity level well within the oil window with respect to hydrocarbon generation. Regular sterane and dia-sterane distributions (e.g. predominance of C~_7steranes; Figure 7) indicative of source rock facies suggest a marine, siliciclastic shale with a predominance of algal organic matter. An anoxic depositional environment is suggested by the relatively high C~/C~4 homohopane ratio (Figure 3). Carbon isotopes (5~C) were determined on the various tYactions of the oil (Table 3) in order to obtain information about the type of source rock from which it was derived. Using Sofer's empirical equation (Sofer, 1984) relating the difference in isotopic values of paraffins and aromatics to the depositional environment we calculate a CV value of -1.31, which indicates a marine source rock. This fits well with the conclusions based on the biomarkers.
Gases. The gas samples collected in 1992 contained various amount of oxygen (5.6-19.2 vol%) since it had not been possible to flush the sampling equipment thoroughly due to the relatively small volume of gas at the well head. The composition of the Lopra- 1 gas corrected for atmospheric contamination is shown in Table 4 together with the results reported previously (Jacobsen and Laier, 1984). The hydrocarbon content of the gas collected in 1992 is considerably lower compared with that of the gas sampled in 1983. However, the composition of the hydrocarbons has not changed much. The higher nitrogen content of the gas collected in 1992 may be due to a larger contribution of atmospheric gases assuming that the gas in the Lopra-1 well consists of a mixture of a hydrocarbon-rich gas and nitrogen from the circulating Table 3 Stable carbon isotopic ratios of hydrocarbon fractions
Sample Lopra-1 '92 Coal I. seam
Paraffins 513C (%0)
Aromatics (~13C(%0)
Polars (~13C(%0)
- 27.9 28.7
- 27.2 -27.3
27.7 -26.1
262
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et a l.
Lopra-1 oil (extracted from water) m/z 217
Lopra-1 oil (extracted from water) mlz 191 2
28 29
14 i '
0 '
'
27
'
30
19 21
13
22 23
9
Wax No. 3 m/z 191
Wax No. 3 m/z 217 28
5
14
18
~
Wax No. 6 m/z 191
Wax No. 6 m/z 217
28 ,29
14 \%=
18
23
25 26
17
30
~s
Coal Extract mlz 191
Coal Extract mlz 217
2O
Figure 3 m/z 191 (hopanes+tricyclic) and m/z 217 (steranes) fragmentograms of oil extracted from water flowing from the Lopra-1 well, w a x nos 3 and 6 and extract of Rokhagi coal. Bulk samples were used for GC-MS analyses of the waxes. Saturate fractions were used for analyses of the Lopra-1 oil and the coal extract. Identified peaks indicated by numbers are shown in Table 2
Table 4 Chemical and isotopic composition of gases from the Lopra-1 well Year
CH4
C2H6
C3H8
iC4Hlo
nC4Hlo
N2
O2+Ar
CO2
H2
1992 1983
40.6 71.9
0.14 0.41
0.025 0.064
0.008 0.015
0.007 0.018
59.1 27.5
-0.04
0.01 0.01
<0.01 <0.01
~13C1 41.5 39.6
4')13C 2
32.4 32.5
('~13C 3
26.5 n.d.
,~D1 -148 133
n.d.-- not determined. Concentrations are given in vol%. Stable isotopic ratios are given per rail relative to the PDB and SMOW standards.
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et a l. (a)
increase the isotopic values of methane (Coleman et al., 1981) and decrease the C~/(C2+C3) ratio as methane is preferentially degraded by bacteria. Thus, the gas sample collected in 1992 may be considered to be the least affected by degradation.
-100
C02-reduction
-200
:
--- ~/ _o~ /
W(dXCS Waxes from 6 different outcrops (Figure 1) were studied by microscopy. Most waxes appeared as a featureless yellow to pale brown coating on zeolites. They show a bright yellow fluorescence under UV light (Figure 5) suggesting that the waxes were derived from a mature source rock. The waxes were analysed by HTGC, which showed that 5 of the samples consisted almost entirely of C20 C60 n-alkanes with a rather symmetrical distibution (samples 3 5) or a bimodal distribution (samples 1 2) (Figure 2). Wax no. 6 contained no n-alkanes which indicates that this sample has been biodegraded. GC analysis using atomic emission detection showed that lower nalkanes were also present although in very low concentrations (Figure 6) due to the much better sensitivity of the AED compared to the FID detector. The small peaks inbetween the n-alkanes are likely due to branched and/or cyclic alkanes which from the appearance on the chromatogram, may represent series ofhomologs. Unfortunately we have not been able to identify these compounds due to the small amount of sample available. Pristane and phytane were only seen using G C - A E D . Therefore, the pristane/phytane ratio, which may be used as an indicator of the source rock was only calculated for wax no. 2 (Table 1), the only sample analysed by GC AED.
- -t~7--
0 --r-
%
-300
-400
/ I
I
-120
I
I
-80
-40
8130CH4(%0 )
(b) 104
Microbial gas 103 Lopra-1
-% 0+
o"
102
10
I -1 O0
I -80
I
I -60
-40
263
-20
5~aCcH4(~) Figure 4 (a) 6~3Cand 62H of methane from the Lopra-1 well plotted in the classification diagram given by Whiticar et al. (1986). (b) ,93C of methane plotted versus Cd(C2+C3)
water in the basalts. Nitrogen from the atmosphere will exsolve from circulating water oxygen being consumed - - as it moves into warmer regions at depth as testified by the nitrogen gas in the warm spring on Eysteroy (Jacobsen and Laier, 1984). The isotopic ratios of methane through propane (Table 4) suggest that gas is thermogenic in origin. Furthermore, source rock maturity may be inferred from the stable carbon isotopic ratios using the empirical equations derived by Faber (1987). The isotopic values of methane and ethane indicate a maturity around R o = l . 0 % whereas that of propane indicates a value of 1.4% assuming a type II kerogen. A source rock within this range of maturities will generate oil to condensate. The relative methane content of the gas, however, is higher than that observed in most oil associated gases (Figure 4b), which may indicate that fractionation of the gas has taken place either in the reservoir or during migration. The small differences in chemical and isotopic compositions of the hydrocarbons between the gas samples collected in 1983 and 1992 (Figure 4) may be explained by fractionation due to bacterial degradation, which will
Biomarkers. The distributions of triterpanes and steranes were rather similar for wax samples nos 1, 3, 4, and 5 analysed by GC MS (Table 1). The mass fragmentograms of wax no. 3 shown in Figure 3 is representative, although with a better signal to noise ratio, of these 4 samples. GC MS analysis was not performed on wax no. 2 since too little material was available. The waxes appear to be derived from a less mature source rock than that of the Lopra-1 oil, the C2~ steranes not being entirely equilibrated (Table 1). The depositional environment of the source rock was probably anoxic as indicated by the extended hopanes. However, the presence of oleanane suggests that the source rock of the waxes contains at least some terrestrial organic matter of post Late Cretaceous age (Riva et al., 1988). Thus, the source rocks of the Lopra-I oil and the waxes may be different, as is also indicated by significant differences in C27 29 steranes (Figure 7), or oil from an additional shallower source rock has been mixed with a Lopra-I type oil during migration. Wax no. 6 from Suduroy appears to be heavily biodegraded. The n-alkanes have been completely removed, and the biomarkers have also been degraded to some extent (Figure 3). The C27 triterpanes Ts and Tm have disappeared and the concentration of : ( ~ isomers of the C2y C29 steranes has decreased compared to the ~/~/~ isomers which appear as distinct doublets on the m/z 217 fragmentogram (Figure 3.) Wax no. 6 differs from the other waxes not only in being biodegraded but also in that it was probably also derived from another source as it contains gammacerane which indicate a hypersaline depositional environment (ten Haven et al.. 1985)
264
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier e t
a l.
Figure 5 Photographs of wax no. 4 magnified 100 × : visible light (upper) and UV light (lower). The w a x shows bright yellow fluorescence
The hydrocarbon distribution observed for the waxes (Figure 2) is very different from both the Lopra-1 oil and the type of oil one would expect to be generated from a mature type II kerogen. Therefore, the original oils have probably been modified quite extensively to yield the wax products observed in the outcrops; in particular, the light end fraction of the hydrocarbons has been removed. As the waxes are found on secondary minerals (zeolites), it is likely that they were preciptated from traces of oil
present in water circulating in fractures within the basalts. Such a process would result in the accumulation of the most water insoluble hydrocarbons in the solid phase, as observed. One might speculate how the composition of the biomarkers in the waxes relates to biomarker composition in the original oil and how this might influence our interpretation regarding the source rock. Del Rio et al. (1992) studied biomarkers in waxes and oils from producing fields and found no significant differences in
Hydrocarbon traces in the Tertiary basalts of the Faeroe /s/ands: 7". Laier et al.
265
Table 5 Vitrinite measurements of Rokhagi mine coals from
Suduroy Sample Upper seam Lower seam
Ro(avarage) Ro(max) 0.50 0.50
0.57 0.56
Ro (min)
variance
0.40 0.40
0.001 0.001
c20
c4o P
Figure 6 GC-AED chromatogram of wax no. 2. Lower section represents part of the chromatogram showing the branched alkanes recorded after most of the n-alkanes were removed by molecular sieve 5A. Residual n-alkanes are marked with dots
69%); polars (27 37%); aromatics (4-5%) and paraffins (i 2%) determined by HPLC. The n-alkane distribution showed a strong odd over even predominance which indicate a low maturity. Low maturities were also indicated by vitrinite measurements (Table 5) of both coals. The biomarker distribution is also typical for that of an immature coal being rich in unsaturates such as the hopenes (Figure 3), high pristane to phytane ratios, high Tm/Ts ratios and the C3~ homohopanes and C2~ steranes being far from equilibration (Table 1). Furthermore, C2~ steranes are more abundant in the coal extracts compared to both the Lopra-I oil and the waxes (Figure 7). Oleanane and its precursors (oleanenes) (Rullk6tter et al., 1994) were specifically looked for, but none of these compounds was detected in the GC MS analyses of the two samples.
Origin of hydrocarbons C27
C28 C29 Figure 7 Ternary diagram showing the distribution of the C27 through C29 ~#/~20(R+S) steranes (m/z 218) from the Lopra-1 oil, waxes and coal extract
biomarker compositions in the two phases. Thus, it is likely that the biomarker compositions of the waxes are similar to those of the original oils.
Distribution of waxes. Waxes were found widely distributed in outcrops on the Faeroe Islands (Figure 1). Furthermore, the wax observed at 2072 m in the Lopra1 well (Jorgensen, 1984) which Laier and Nytoft (1993) considered to be indigenous, indicates that waxes occur in areas with deep circulation of water. Waxes may precipitate at depth or at the surface, but at present it is not possible to determine whether the waxes found at outcrop were exposed by erosion or whether they formed near or at the surface. The waxes appear to be only mildly biodegraded which is suggestive of fairly recent deposits. Coal Extracts of coal from the upper and lower seams in the Rokhagi mine on Suduroy consisted of asphaltenes (56
The analytical results on waxes and coals presented herein do not support previous speculations that the waxes were derived from the known coal occurrences on the Faeroe Islands (Jorgensen, 1984). Biomarker distributions indicate that the source rocks of the waxes were deposited in anoxic environments quite different from those in which coals are formed. The same holds true for the source rock of the Lopra- l oil. The Lopra-1 oil and the original oil which formed the wax coatings on the zeolites were most likely derived from mature source rocks beneath the known basalts, although part of the Lopra-1 oil, particularly the light end hydrocarbons, was probably derived from the diesel added during drilling (Jacobsen and Laier, 1984). In view of the fact that the base of the basalts is just 200 m below the T D of the Lopra-I well (Kiorboe and Petersen, 1995) it is not unlikely that oil migrated up through fractures in the basalt and into the well. Given the maturity indicated by the biomarkers, the source rock itself may exist below the seismic marker indicating the base of the basalts. The waxes coating the zeolites, as sampled at outcrops, were derived from a mature source rock containing some terrestrial input as shown by the presence of oleanane and the weak odd over even predominance for the higher n-alkanes (Figure 2) (Carlson et al., 1993; Wavrek and Dahdah, 1995). This suggests that the source may be younger and stratigraphically shallower than that of the Lopra-I oil. A second possibility is that the hydrocarbons of the waxes were a mixture of Lopra-I type oil plus oil from a shallower source rock containing terrestrial organic matter. The first possibility is considered to be the most likely given the differences in C27 29 steranes (Figure 7) and in C2~/C30 hopane ratios (Table 1) The age of the source rock is more difficult to discern. The presence of oleanane in the waxes is indicative of a source rock deposited after the appearance of angiosperms in Late Cretaceous. No oleanane was observed in the Lopra-I oil, suggesting that it was derived from an older source or a source accumulated in a distal palaeodepositional environment with little or no input of ter-
266
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et a l.
restrial organic matter. Hitchen and Stoker (1993) speculated that the oil traces observed in the Lopra-1 well might have been derived from the Volgian which forms a rich oil-prone source rock west of the Shetland Islands (Bailey el al., 1987). The discovery of the Foinaven and Shiehallion fields in this area confirms the predictions made by Bailey el al. (1987) that large amounts of oil have been generated. A seismic line (Boldreel and Andersen, 1994) close to the two British discoveries indicates that the rift basin beneath the Eaeroe Shetland Channel may be symmetrical, so it is possible that Late Jurassic Earliest Cretaceous source rocks extend west into the Faeroe basin. Kimmeridge to Ryazanian source rocks in the deepest part of the rift basin beneath the Faeroe Shetland Channel are.presently highly mature (dry gas window) (Bailey et al., 1987). Assuming that the Upper Jurassic organic-rich rocks extend far enough into the Faeroe Basin to produce the oil observed in the Lopra-1 well, we conclude that the burial depth of these rocks becomes shallower to the NW of the Faeroe Shetland Channel and at some point enters the oil window.
Conclusions The oil encountered in the Lopra-I well was generated from a marine, siliciclastic source rock with a predominance of marine algal organic matter. The maturity level is intepreted to be well within the oil window for a classical type II (marine algal kerogen) source rock. The oil has likely migrated into the well through fractures in the basalt. The gas in the well is of thermogenic origin and may have been generated by the same source rock as the oil, as indicated by the stable carbon isotopic composition of methane, ethane, and propane. The relative methane concentration is, however, higher than expected for an oil-associated gas. This indicates that the gas has undergone fractionation. Waxes seen as coatings on zeolite grains in outcrop are at least in part derived from a post Late-Cretaceous source rock with mixed marine algal and terrestrial higher plant organic matter. These waxes have probably been deposited from minor amounts of oil present in deep circulating waters penetrating the Palaeocene basalts. Humic coals on Suduroy are immature (R,,=0.5%). Their maturity level and chemical composition show that they did not generate the waxes nor the oil in the Lopra1 well.
Acknowledgements The H T G C FID and the GC AED analyses were performed in Agip's laboratory in Milan during our stay in 1991. Dr A. Riva is gratefully acknowledged for helping us to perform the analyses.We thank Dr J. Ineson for helping us improving the English text.
References Bailey, N. J. L., Walko, P. and Sauer, M. J. (1987) Geochemistry and source rock potential of the west of Shetlands. In Petroleum Geology of North West Europe (Eds. J. Brooks and K. W. Glennie), Graham and Trotman, London, 711-721. Bailing, N., Kristiansen, J. I. and Saxov, S. (1984) Geothermal measurements from the Vestmanna-1 and Lopra-1 boreholes. In The Deep Drilling Project 1980-1981 in the Faeroe Islands (Eds O. Berthelsen, A. Noe-Nygaard and J. Rasmussen), Foroya Frodskaparfelag, Torshavn, 137-147. Berthelsen, O., Noe-Nyegaaard, A. and Rasmussen, J. (1984) The
Deep Drilling Project 1980-1981in the Faeroe Islands. Feroya Frodskaparfelag, Torshavn, 9-10. Boldreel L. O. and Andersen M. S. (1994) Tertiary development of the Faeroe-Rockall Plateau based on reflection seismic data. Bulletin Geological Society Denmark 41,162-180. Carlson, R. M. K., Teerman, S. C., Moldowan, J. M., Jacobson, S. R., Chan, E. I., Dorrough, K. S., Seetoo, W. C. and Mertani, B. (1993) High temperature gas chromatography of high-wax oils. In Proceedings, Indonesian Petroleum Association, 22nd Annual Convention, October 1993, 483-507. Coleman D. D., Risatti J. B. and Schoell, M. (1981) Fractionation of carbon and hydrogen isotopes by methane-oxidizing bacteria. Geochimica Cosmochimica Acta 45, 1033-1037. Del Rio J. C., Philp R. P. and Allen, J. (1992) Nature and geochemistry of high molecular weight hydrocarbons (above C40)in oils and solid bitumens. Organic Geochemistry 18, 541-553. Faber, E. (1987) Zur Isotopengeochemie gasfSrmiger Kohlenwassestoffe. ErOle, Erdgas, Kohle 103, 210-218. Hitchen, K. and Stoker M. S. (1993) Mesozoic rocks from the Hebrides Shelf and implications for hydrocarbon prospectivity in the northern Rockall Trough. Marine and Petroleum Geology 10, 246-254. Jacobsen, O. S. and Laier, T. (1984) Analysis of gas and water samples from the Vestmanna-1 and Lopra-1 wells, Faeroe Islands. In The Deep Drilling Project 1980-1981 in the Faeroe Islands (Eds O. Berthelsen, A. Noe-Nygaard and J. Rasmussen), Feroya Frodskaparfelag, Torshavn, 149-155. Jorgensen, O. (1984) Zeolite zones in the basaltic lavas of the Faeroe Islands. In The Deep Drilling Project 1980-1981 in the Faeroe Islands (Eds O. Berthelsen, A. Noe-Nygaard and J. Rasmussen), Foroya Frodskaparfelag, Torshavn, 71-91. Kiorboe, L. (1995) Paleocene to early Eocene development of the Faeroe Basalt Plateau and the Faeroe Basin area. Ph.D. thesis, University of Aarhus, Denmark, 217 pp. Kiorboe, L. and Petersen, S. A. (1995) Seismic investigation of the Faeroe basalts and their substratum. In The Tectonic, Sedimentation and Palaeoceanography of the North Atlantic Region (Eds R. A. Scrutton, M. S. Stoker, G. B. Shimmield and A. W. Tudhope), Special Publication Geological Society London No. 90, 111-122. Laier, T., Jergensen, N. O., Buchardt, B., Cederberg, T. and Kuijpers, A. (1992) Acculation and seepages of biogenic gas in northern Denmark. Continental Shelf Research 12, 11731186. Laier, T. and Nytoft H. P. (1993) Biomarkers and stable isotopes of hydrocarbon traces of the Lopra-1 well. Geological Survey of Denmark, Datareport 3, 39 pp. (in Danish). Lund, J. (1989) A Late Palaeocene non-marine microflora from the interbasaltic coals of the Faeroe Islands, North Atlantic. Bulletin Geological Society Denmark 37, 181-203. Rasmussen, J. and Noe-Nygaard, A. (1970) Geology of the Faeroe Is.ands. Geological Survey of Denmark, First Series 25, 142 pp. Riva, A., Caccialanza P. G. and Quagliaroli, F. (1988) Recognition of 18b(H)oleanane in several crudes and Tertiary-Upper Cretaceous sediments. Definition of a new maturity parameter. Organic Geochemistry 13, 671-675. RullkStter, J., Peakman T. M. and ten Haven H. L. (1994) Early diagenesis of terrigeneous triterpenoids and its implications for petroleum geochemistry. Organic Geochemistry21, 215-233. Sofer, Z. (1984) Stable carbon isotope compositions of crude oils: Application to source depositional environments and petroleum alteration. American Association Petrolroleum Geology Bulletin 68, 31-49. ten Haven H. L., de Leeuw J. W. and Schenck P. A. (1985) Organic geochemical studies of a Messinian evaporitic basin, northern Apennines (Italy) I--Hydrocarbon biological markers for a hypersaline environment. Geochimica Cosmochimica Acta 49, 2181-2191. Waagstein, R. (1988) Structure, composition and age of the Faeeroe basalt plateau. In Early Tertiary Volcanism and the Opening of the NE Atlantic (Eds A. C. Morton and L. M Parson) Special Publication Geological Society London No. 39 225238. Waagstein, R., Hald, N., Jorgensen, O., Nielsen P. H., NoeNygaard, A., Rasmussen, J., Sch6nharting, G. (1984) Deep drilling on the Faeroe Islands. Bulletin Geologcal Society Denmark32, 133-138. Wavrek, D. A and Dahdah, N. F. (1995) Characterisation of high molecular weight compounds - - Implications for advancerecovery tecnologies. In Proceedings, International Symposium on Oil Field Chemistry, San Antonio, 14-17 February 1995, SPE Paper 28965, 207-212. Whiticar M. J., Faber, E. and Schoell, M. (1986) Biogenic methane formation in marine and freshwater environments: CO2 reduction vs acetate fermentation - - isotope evidence. Geochimica Cosmochimica Acta ,50, 693-709.
Marine and Petroleum Geoloyy, Vol. 14, No. 3, pp. 257 266, 1997 ~'. 1997 ElsevierScience Ltd All rights reserved. Printed in Great Britain
PII: S0264--8172197100002-.0
0264-8172/97 $17.00+ 0.00
ELSEVIER
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands T. Laier* and H. P. Nytoft Geological Survey of Denmark and Greenland, Thoravej 8, Copenhagen, Denmark O. Jorgensen National Institute of Occupational Health, Lerso Parkall#, Copenhagen, Denmark G. H. Isaksen Exxon Production Research Co., P.O. Box 2189, Houston, Texas, USA Received 9 September 1996; revised 8 December 1996;accepted 21 December 1996 Hydrocarbons in the form of gases, oils and waxes have been observed in the basalts of the Faeroe Islands in the North Atlantic. Gases and traces of oil were observed in the outflowing water of the deep Lopra-1 well drilled in 1981. The hydrocarbon gas was fairly dry, with stable isotopic ratios of ~13C1_3=- 41.4; - 32.4; - 26.5%o respectively, typical of a thermogenic gas. High temperature gas chromatography of the oil showed that it consisted mostly of C13-60 n-alkanes. Biomarker distribution observed by GC-MS indicates that the oil was derived from a mature source rock deposited in an anoxic environment; this suggests that the source rock must lie beneath the known basalts. Waxes exhibiting bright yellow fluorescence under UV light were observed as coatings on zeolite minerals widely distributed on the Faeroe Islands. The waxes consist predominantly of higher n-alkanes shown by HTGC. The fluorescence indicates the presence of aromatic compounds. Biomarker distribution indicated that the waxes were derived at least in part from a source rock containing some terrestrial organic matter as testified by the low amounts of oleanane present. The waxes were probably deposited from traces of oil present in deeply circulating waters in fractures within the basalts. Coals which had been suspected to generate some of the hydrocarbons observed in the Faeroese basalts were also examined. Vitrinite values of Ro=0.5% as well as GC-MS analyses of the Suduroy coal extracts showed that these coals are immature and have not generated significant hydrocarbons. © 1997 Elsevier Science Ltd.
Keywords:hydrocarbons;biomarkers In 1981, a scientific borehole was drilled at Lopra on Suduroy in order to explore the substratum of the Faeroe Islands basalts (Berthelsen et al., 1984). The Lopra- 1 well, however, failed to penetrate the basalts as drilling was terminated at 2178m for technical reasons. Although no direct information was obtained on the sub-volcanic rocks, studies of cuttings as well as fluids from the well gave indications of sedimentary rocks capable of generating hydrocarbons beneath the basalts.
Furthermore, native copper indicating reducing conditions due to the presence of organic material was observed in two samples (1996, 2072 m). Only one of the four waxy samples was readily dissolved in an organic solvent according to Jorgensen (1984). The soluble wax from 2072 m analysed by gas chromatography consisted mainly of n-alkanes C22 37 with an almost symmetrical distribution around C29 (Laier and Nytoft, 1993). For comparison, a wax sampled from an outcrop on Videroy (Figure 1, location 0) was analysed by gas chromatograpy. This sample also consisted mainly of nalkanes C20 43 showing a weak odd over even predominance (Laier and Nytoft, 1993). The insoluble yellow waxes in the Lopra-I well were assumed to originate from the coal-bearing sediments that occur between the Lower and the Middle Basalt Series (Jorgensen, 1984).
W a x y bitumen Secondary minerals were studied in samples of drill cuttings selected every 2 ~ 2 5 m in order to reconstruct the geological and hydrothermal evolution of the area (Jorgensen, 1984). During this study, inflammable waxy material was observed as thin coatings on secondary minerals in four samples at 920, 1524, 2072 and 2120 m depth.
Hydrocarbon gases Only negligible shows of gas were observed during drilling of the Lopra-1 well (Waagstein et al., 1984). However, when the well was re-opened for temperature logging in
*Author to whom correspondence should be addressed at: GEUS Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400, Copenhagen, Denmark. Tel: +45 35 14 20 00. fax: +45 38 14 20 50.
257
258
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et al.
May 1983 the well head pressure had increased to 19.5 bars (Balling el al., 1984) and approximately 9 m -~of gas was released from the well (P. H. Nielsen pers. comm.). The gas consisted of methane (72%) and nitrogen (27%) plus traces of higher hydrocarbons (Jacobsen and Laier, 1984). Based on stable isotopic analyses (c$1~Cl=-39.6; 51~C~=-32.4%0), Jacobsen and Laier, 1984 concluded that the hydrocarbon gases were most likely of thermogenic origin. Being open, the well continued to flow with slightly saline water ( T D S = 900mgl i, 12.81min t) and gas (0.81min i). Sampling at different depths, Jacobsen and Laier (1984) observed an increase in the gas to water ratio with depth, suggesting that gas mainly entered the deeper parts of the Lopra-1 well. Traces of oil were observed in the out-flowing water and a smell of petroleum was noted, particularly in the water samples collected in the deepest part of the well (Jacobsen and Laier, 1984). The gas chromatogram of oil collected from the water showed a biodegraded oil, Pr/Ph = 1.5, probably derived fl'om a marine or lacustrine source rock. Diesel, ca 3000 litres, had been added to the well when the drill string got stuck at a drilling depth of l l00m. However, Jacobsen and Laier (1984) concluded from the gas chromatogram that diesel, if still present in the well, was not the only source ofoil in the out-flowing water.
New hwestigations The hydrocarbon discoveries, Foinaven in 1992 and Shiehallion in 1993, in the British sector 160kin SE of the Faeroe Islands, created new interest in the hydrocarbon traces observed in the Lopra-1 well. Therefore, as part of the current study, gases and oil were re-sampled from the well in order to determine the origin of the hydrocarbons. In addition, wax-coatings observed on zeolites in outcrops were also analysed in samples previously collected by one of the authors. Coals from mines on the northern part of Suduroy were also analysed in order to see if the coals were the likely source of the waxes observed in the Lopra-1 well and elsewhere. The Lopra-I well will undergo extended drilling during the summer of 1996.
Geological setting The Faeroe Islands (Figure 1) arc located in the NE Atlantic between Iceland and Scotland and consist of a pile of tholeitic plateau basalts up to 5kin thick, the upper 3 km of which is exposed (Waagstein et al., 1984). The basalts were extruded subaerialy during the early Tertiary. The volcanic pile can be divided into 3 series (Rasmussen and Noe-Nygaard, 1970), the Lower series being of C26R to C25N magnetostratigrapic age and the upper two series of C24R age (Waagstein, 1988). A coalbearing sedimentary succession, up to l0 m thick, is found between the Lower and the Middle Basalt Series (Figure 1), the total thickness of the coal seams being 1 m. Microfaunal assemblages, date the coals to Late Palaeocene age (Lund, 1989). Sub-basalt geological information is sparse, but the geochemistry of the basalts indicate the presence of continental crust beneath the basalts (Waagstein, 1988). Boldreel and Andersen (1994) concluded from a seismic profile east of the Foinaven Field, that the Mesozoic sediments found beyond the basalt cover continue below
the base of the basalt. The continuation of the Mesozoic sediments beneath the basalts can be traced 10 50kin laterally depending on the thickness and depth of the basalt cover (Kiorboe, 1995). Although the Lopra-1 well failed to penetrate the basalts, Kiorboe and Petersen (1995) concluded from VSP seismics that the base of the basalt in the Lopra area is near 2.34 km depth, i.e. 200 m below TD of the well. The base of the basalt was recognized by a negative reflection coefificient due to lower seismic velocities in the sub-basalt layers, but the modeled sub-basalt velocities for the layers (5.0 5.35 km s ~) are rather high for sedimentary rocks (Kiorboe and Petersen,
1995).
Sampling and methods The material described in this chapter includes all available hydrocarbon samples collected from 1978 (zeolite waxes) to 1992 (oil and gas frorn Lopra-1 well head). However not all material collected during this period was available lk~r new analyses, e.g. the waxy material on cuttings IYom the Lopra-I well.
Bitumens Waxy coatings on zeolites have occasionally been found by one the authors during his studies of secondary minerals in the Faeroese basalts. The waxy coating is typically only recognized under the microscope. The waxy materials from 6 localities (Fiqure 1) were dissolved in dichloromethane and analysed by high temperature gas chromatography (HTGC) and gas chromatography mass spectrometry (GC MS) without further separation of the various fractions due to the low amounts of materials available. The branched/cyclic fraction of one of the samples was isolated by removal ofn-alkanes passing the dichloromethane solution through a column packed with 5 ,~ molecular sieves. This fraction was then analysed on a gas chromatograph equipped with an atomic emission detector (GC AED).
Gases and oil The Lopra-1 well had been shut in for several months before the sampling of gases and oil took place on the 9th of November 1992. The well head pressure was not known as no manometer was mounted on the well head, but opening the ball valve on the well head slightly one got the impression that the well head pressure was rather low. A 100ml steel cylinder equipped with 2 valves was connected to the well head using a 2 m hose, both cylinder and hose being filled with water. A moderate flow of gas was established by adjusting the the valve on the well head, and the cylinder was flushed with gas for ca 30 s before closing the valves of the cylinder. Two more gas samples were taken during which time the well started flowing with water. Approximately 1 litre of water produced during the sampling of the 2 latter gas samples was transferred to two 1 litre separation funnels and shaken with dichloromethane (20 ml) three times. In the laboratory, the combined dichloromethane extracts were dried over sodium sulphate and the dichloromethane removed by evaporation at 40' C. Separation of the various fractions of hydrocarbons were performed as described below and the paraffinic fraction was analysed by H T G C and GC MS.
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et
al.
259
7° 1
STREYMOY
MYKINES VAGAR 62°
62°
% Irregular intrusive bodies and sills J]]]]]]]~ Upper basalt series r=====]. Middle basalt series Tuff- agglomerate zone Coalbearing sequence Lower basalt series
0 i
, [
,
i
I [ i
i
10 I
i
~o. / ~ _ ; _ ~ I
20 km ,J
J
~o °
\f
1
SUDUROY FAEROE
ISLANDS;
60 °
ETOTLAN °i 17
2o'1
,0- I
~,~
bore h o l e ~
\ o.I
Figure Geological map of the Faeroe Islands. Numbers indicate outcrops where zeolites with wax coatings were sampled. Wax no. 0 was previously analysed (Laier and Nytoft, 1993). The exact location of wax no. 6 sampled on Suduroy is uncertain and is not indicated on the map
260
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et
Coals Samples of coal were collected from 2 coal layers, each ca 0.5 m thick, separated by 1 m of clay in the Rokhagi mine located on the northern part of Suduroy. According to the miners, methane had never been observed in the Faeroese mine and only once had a grey waxy substance been found in one of the mines. The two coal samples were crushed and refluxed with dichloromethane: methanol 93:7 for 4 h. The asphaltenes were isolated from the other hydrocarbons by precipitation with excess npentane and subsequent centrifugation. The remaining pentane-soluble organic matter was separated into polar, aromatic and paraffinic fractions using HPLC and the paraffinic fraction analysed by GC and GC MS. H T G C analyses were performed using a Hewlett Packard gas chromatograph HP-5890, equipped with a 15 m aluminum clad capillary column OVl (QXI/AI). The injector and detector was set at 420'C and the oven was temperature-programmed from 50 to 100'C at 10'C rain 100 to 2 4 0 C at 5 C m i n J and 240 to 400~C at 4'C min ~with an 18 min final hold time. GC MS analyses were performed with a Hewlett Packard HP5890 gas chromatograph equipped with a 25 m HP-5 capillary column and 0.11 mm film. The oven was temperature programmed from 70 to 100'C at 3 0 C m i n ~: and l00 to 300 'C at 4' C min ' w i t h a l 2 m i n final hold time. The mass spectra were obtained in El mode using a Hewlett Packard HP-5971A quadrupole mass spectrometer. GC and stable isotopic analyses of gases were pertbrmed following the methods described by Laier et al. (1992). Stable carbon isotopic analyses of the Lopra- 1 oil fractions and coal extract fractions were performed by Geolab Nor (Norway).
C20
C40
a l. C60
; Lopra-1 i (oil extracted ', from water)
wax no 1
, 11
ixn°3
Results and dicussion The low quantities of oil or waxy material available in most samples restricted the number of different analyses that could be performed on each sample. Therefore, it was not possible to obtain data for full comparison of all samples. Futhermore, no material was left for up-to-date analysis of previously reported samples, e.g. the waxy material encountered in the cuttings of the Lopra-1 well (Jorgensen, 1984), for which only GC data existed (Laier and Nytofl, 1993).
wax
no
4
Lopra- 1 oil and 9ases The oil extracted from water from the Lopra-I well in 1992 appears to be less biodegraded than the oil reported by Jacobsen and Laier (1984), the n-alkanes being much more abundant relative to the hump of unresolved mixture of hydrocarbons below C30 (Figure 2). This difference may be due to the fact that the water sample containing oil traces collected at Lopra-I in 1983 had been stored for 6 months prior to analysis. Biodegradation during storage was probably only very minor for the oil sampled in 1992 as the oil was extracted from water at the well site and analysed within 2 weeks. Analyzing the oil using high temperature gas chromatography (HTGC) revealed abundant long chain n-alkanes>C35 (Figure 2) which could not previously be seen using normal GC. The presence of these long chain alkanes supports the assumption that the oil in the Lopra-I water is not just residues of the diesel, C,~ C23, bp 200-360 C (suppliers intbrmation), added during drilling (Jacobsen and Laier, 1984), but
~.. ~,, l l~J~
........
wax no 5
Figure 2 High t e m p e r a t u r e gas c h r o m a t o g r a m s of the Lopra-1 oil and waxes. N u m b e r s refer to localities s h o w n on Figure 1
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et al.
261
Table 1 Biomarker data obtained from GC and GC-MS analyses
Sample
Isoprenoids pr/ph
Lopra-1 '92 Lopra-1 '83 Wax No. 1 Wax No. 2 Wax No. 3 Wax No. 4 Wax No. 5 Wax No. 6 Coal u. seam Coal I. seam
pr/nC~7
1.8 1.5
0.4 0.8
1.3
0.7
5 15
0.8 1.1
Steranes C29 ~=S/=~S+R
C29
=fl13/~=+~[t[~
C30 Mor/Hop
C3~ S/S+R
C29/C30
Tm/Ts
0.50
0.51
0.09
0.60
0.98
1.30
0.49
0.36
0.16
0.53
0.66
2.22
0.47 0.50 0.44 > 0.9
0.39 0.41 0.40 0.76
0.10 0.11 0.13 0.08 0.63 0.54
0.60 0.68 0.13 0.55 0.05 0.05
0.58 0.54 0.59 0.34 1.34 1.61
1.16 1.53 1.79 0.02 11 8.8
0.12
more likely derived, at least in part, from a source rock in the area. Separation by HPLC showed that the oil consisted of paraffins (68%), aromatics (26%) and polar compounds (6%). No asphaltenes were found. The amounts of oil in the first two water samples collected just after the well was opened in 1992 were 12 mg I ~and 0.5mgl-~, and since oil probably accumulated in the top of the well during the previous shut in period, the concentration of oil would most likely be less than 0.5 mg 1-~ during free flowing water condition.
Biomarkers. Biomarker distributions were monitiored to interpret thermal maturity, source rock organic matter type, and source rock depositional environment. Analyses were made by GC MS, monitoring m/z 191 for tricyclics and hopanes and m/z 217 for steranes (Table 1: Figure 3). One concern in this geological setting is the influence of any maturation effect due to contact metamorphism between source rocks and the Tertiary basalts, or between black oil and the basalts. The effect of later Table 2 Peaks identified in m/z 191 and m/z 217 fragmentograms
shown in Figure 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Terpanes
18~(H)-22,29,30-trisnorneohopane (Ts) 17~(H)-22,29,30-trisnorhopane (Tm) 17/~(H)-22,29,30-trisnorhopane 30-norneohop-13(18)-ene 17~(H),2lfl(H)-30-norhopane 18~(H)-30-norneohopane (29Ts) 17/~(H),2l~(H)-30-norhopane (normoretane) Oleanane 17~(H),21fl(H)-hopane 30-nor-29-homo-17--~(H)-hopane Neohop-13(18)-ene 17/~(H),2l/~(H)-30-norh opa ne 17fl(H),21~(H)-hopane (moretane) 17~(H),21/3(H),22(S)-homohopane 17~(H),21/~(H),22(R)-homohopane Gammacerane 17fi(H),21fi(H)-hopane 17~(H),21/~(H),22(S)-bishomohopane 17~(H),21/~(H),22(R)-bishomohopane 17/~(H),21/~(H),homohopane 17~(H),21/~(H),22(S)-trishomohopane 17~(H),21//(H),22(R)-trishomohopane 17~(H),21/~(H),22(S)-tetrakishomohopane 17~(H),21/~(H),22(R)-tetrakishomohopane 17~(H),21/Y(H),22(S)-pentakishomohopane 17~(H),21/3(H),22(R)-pentakishomohopane 24-ethyl-5c~(H), 14c4H), 17~(H), 20(S)-cholestane 24-ethyl-5cdH), 14/Y(H), 17/~(H), 20(R)-cholestane 24-ethyl-5~(H), 14/~(H), 17/~(H), 20(S)-cholestane 24-ethyl-5c~(H), 14~(H), 17~(H), 20(R)-cholestane
heating of source rocks or oils in proximity to intrusives is not w e l l understood. However, the biomarker distributions support conventional maturation through progressive burial of the source rock. Contact metamorphism would likely have resulted in differing degrees of isomerization among molecular maturity parameters (tricyclics, homo-hopanes, and steranes). Sterane and triterpane maturity parameters (Table 1; Figure 3) suggest a maturity level well within the oil window with respect to hydrocarbon generation. Regular sterane and dia-sterane distributions (e.g. predominance of C~_7steranes; Figure 7) indicative of source rock facies suggest a marine, siliciclastic shale with a predominance of algal organic matter. An anoxic depositional environment is suggested by the relatively high C~/C~4 homohopane ratio (Figure 3). Carbon isotopes (5~C) were determined on the various tYactions of the oil (Table 3) in order to obtain information about the type of source rock from which it was derived. Using Sofer's empirical equation (Sofer, 1984) relating the difference in isotopic values of paraffins and aromatics to the depositional environment we calculate a CV value of -1.31, which indicates a marine source rock. This fits well with the conclusions based on the biomarkers.
Gases. The gas samples collected in 1992 contained various amount of oxygen (5.6-19.2 vol%) since it had not been possible to flush the sampling equipment thoroughly due to the relatively small volume of gas at the well head. The composition of the Lopra- 1 gas corrected for atmospheric contamination is shown in Table 4 together with the results reported previously (Jacobsen and Laier, 1984). The hydrocarbon content of the gas collected in 1992 is considerably lower compared with that of the gas sampled in 1983. However, the composition of the hydrocarbons has not changed much. The higher nitrogen content of the gas collected in 1992 may be due to a larger contribution of atmospheric gases assuming that the gas in the Lopra-1 well consists of a mixture of a hydrocarbon-rich gas and nitrogen from the circulating Table 3 Stable carbon isotopic ratios of hydrocarbon fractions
Sample Lopra-1 '92 Coal I. seam
Paraffins 513C (%0)
Aromatics (~13C(%0)
Polars (~13C(%0)
- 27.9 28.7
- 27.2 -27.3
27.7 -26.1
262
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et a l.
Lopra-1 oil (extracted from water) m/z 217
Lopra-1 oil (extracted from water) mlz 191 2
28 29
14 i '
0 '
'
27
'
30
19 21
13
22 23
9
Wax No. 3 m/z 191
Wax No. 3 m/z 217 28
5
14
18
~
Wax No. 6 m/z 191
Wax No. 6 m/z 217
28 ,29
14 \%=
18
23
25 26
17
30
~s
Coal Extract mlz 191
Coal Extract mlz 217
2O
Figure 3 m/z 191 (hopanes+tricyclic) and m/z 217 (steranes) fragmentograms of oil extracted from water flowing from the Lopra-1 well, w a x nos 3 and 6 and extract of Rokhagi coal. Bulk samples were used for GC-MS analyses of the waxes. Saturate fractions were used for analyses of the Lopra-1 oil and the coal extract. Identified peaks indicated by numbers are shown in Table 2
Table 4 Chemical and isotopic composition of gases from the Lopra-1 well Year
CH4
C2H6
C3H8
iC4Hlo
nC4Hlo
N2
O2+Ar
CO2
H2
1992 1983
40.6 71.9
0.14 0.41
0.025 0.064
0.008 0.015
0.007 0.018
59.1 27.5
-0.04
0.01 0.01
<0.01 <0.01
~13C1 41.5 39.6
4')13C 2
32.4 32.5
('~13C 3
26.5 n.d.
,~D1 -148 133
n.d.-- not determined. Concentrations are given in vol%. Stable isotopic ratios are given per rail relative to the PDB and SMOW standards.
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et a l. (a)
increase the isotopic values of methane (Coleman et al., 1981) and decrease the C~/(C2+C3) ratio as methane is preferentially degraded by bacteria. Thus, the gas sample collected in 1992 may be considered to be the least affected by degradation.
-100
C02-reduction
-200
:
--- ~/ _o~ /
W(dXCS Waxes from 6 different outcrops (Figure 1) were studied by microscopy. Most waxes appeared as a featureless yellow to pale brown coating on zeolites. They show a bright yellow fluorescence under UV light (Figure 5) suggesting that the waxes were derived from a mature source rock. The waxes were analysed by HTGC, which showed that 5 of the samples consisted almost entirely of C20 C60 n-alkanes with a rather symmetrical distibution (samples 3 5) or a bimodal distribution (samples 1 2) (Figure 2). Wax no. 6 contained no n-alkanes which indicates that this sample has been biodegraded. GC analysis using atomic emission detection showed that lower nalkanes were also present although in very low concentrations (Figure 6) due to the much better sensitivity of the AED compared to the FID detector. The small peaks inbetween the n-alkanes are likely due to branched and/or cyclic alkanes which from the appearance on the chromatogram, may represent series ofhomologs. Unfortunately we have not been able to identify these compounds due to the small amount of sample available. Pristane and phytane were only seen using G C - A E D . Therefore, the pristane/phytane ratio, which may be used as an indicator of the source rock was only calculated for wax no. 2 (Table 1), the only sample analysed by GC AED.
- -t~7--
0 --r-
%
-300
-400
/ I
I
-120
I
I
-80
-40
8130CH4(%0 )
(b) 104
Microbial gas 103 Lopra-1
-% 0+
o"
102
10
I -1 O0
I -80
I
I -60
-40
263
-20
5~aCcH4(~) Figure 4 (a) 6~3Cand 62H of methane from the Lopra-1 well plotted in the classification diagram given by Whiticar et al. (1986). (b) ,93C of methane plotted versus Cd(C2+C3)
water in the basalts. Nitrogen from the atmosphere will exsolve from circulating water oxygen being consumed - - as it moves into warmer regions at depth as testified by the nitrogen gas in the warm spring on Eysteroy (Jacobsen and Laier, 1984). The isotopic ratios of methane through propane (Table 4) suggest that gas is thermogenic in origin. Furthermore, source rock maturity may be inferred from the stable carbon isotopic ratios using the empirical equations derived by Faber (1987). The isotopic values of methane and ethane indicate a maturity around R o = l . 0 % whereas that of propane indicates a value of 1.4% assuming a type II kerogen. A source rock within this range of maturities will generate oil to condensate. The relative methane content of the gas, however, is higher than that observed in most oil associated gases (Figure 4b), which may indicate that fractionation of the gas has taken place either in the reservoir or during migration. The small differences in chemical and isotopic compositions of the hydrocarbons between the gas samples collected in 1983 and 1992 (Figure 4) may be explained by fractionation due to bacterial degradation, which will
Biomarkers. The distributions of triterpanes and steranes were rather similar for wax samples nos 1, 3, 4, and 5 analysed by GC MS (Table 1). The mass fragmentograms of wax no. 3 shown in Figure 3 is representative, although with a better signal to noise ratio, of these 4 samples. GC MS analysis was not performed on wax no. 2 since too little material was available. The waxes appear to be derived from a less mature source rock than that of the Lopra-1 oil, the C2~ steranes not being entirely equilibrated (Table 1). The depositional environment of the source rock was probably anoxic as indicated by the extended hopanes. However, the presence of oleanane suggests that the source rock of the waxes contains at least some terrestrial organic matter of post Late Cretaceous age (Riva et al., 1988). Thus, the source rocks of the Lopra-I oil and the waxes may be different, as is also indicated by significant differences in C27 29 steranes (Figure 7), or oil from an additional shallower source rock has been mixed with a Lopra-I type oil during migration. Wax no. 6 from Suduroy appears to be heavily biodegraded. The n-alkanes have been completely removed, and the biomarkers have also been degraded to some extent (Figure 3). The C27 triterpanes Ts and Tm have disappeared and the concentration of : ( ~ isomers of the C2y C29 steranes has decreased compared to the ~/~/~ isomers which appear as distinct doublets on the m/z 217 fragmentogram (Figure 3.) Wax no. 6 differs from the other waxes not only in being biodegraded but also in that it was probably also derived from another source as it contains gammacerane which indicate a hypersaline depositional environment (ten Haven et al.. 1985)
264
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier e t
a l.
Figure 5 Photographs of wax no. 4 magnified 100 × : visible light (upper) and UV light (lower). The w a x shows bright yellow fluorescence
The hydrocarbon distribution observed for the waxes (Figure 2) is very different from both the Lopra-1 oil and the type of oil one would expect to be generated from a mature type II kerogen. Therefore, the original oils have probably been modified quite extensively to yield the wax products observed in the outcrops; in particular, the light end fraction of the hydrocarbons has been removed. As the waxes are found on secondary minerals (zeolites), it is likely that they were preciptated from traces of oil
present in water circulating in fractures within the basalts. Such a process would result in the accumulation of the most water insoluble hydrocarbons in the solid phase, as observed. One might speculate how the composition of the biomarkers in the waxes relates to biomarker composition in the original oil and how this might influence our interpretation regarding the source rock. Del Rio et al. (1992) studied biomarkers in waxes and oils from producing fields and found no significant differences in
Hydrocarbon traces in the Tertiary basalts of the Faeroe /s/ands: 7". Laier et al.
265
Table 5 Vitrinite measurements of Rokhagi mine coals from
Suduroy Sample Upper seam Lower seam
Ro(avarage) Ro(max) 0.50 0.50
0.57 0.56
Ro (min)
variance
0.40 0.40
0.001 0.001
c20
c4o P
Figure 6 GC-AED chromatogram of wax no. 2. Lower section represents part of the chromatogram showing the branched alkanes recorded after most of the n-alkanes were removed by molecular sieve 5A. Residual n-alkanes are marked with dots
69%); polars (27 37%); aromatics (4-5%) and paraffins (i 2%) determined by HPLC. The n-alkane distribution showed a strong odd over even predominance which indicate a low maturity. Low maturities were also indicated by vitrinite measurements (Table 5) of both coals. The biomarker distribution is also typical for that of an immature coal being rich in unsaturates such as the hopenes (Figure 3), high pristane to phytane ratios, high Tm/Ts ratios and the C3~ homohopanes and C2~ steranes being far from equilibration (Table 1). Furthermore, C2~ steranes are more abundant in the coal extracts compared to both the Lopra-I oil and the waxes (Figure 7). Oleanane and its precursors (oleanenes) (Rullk6tter et al., 1994) were specifically looked for, but none of these compounds was detected in the GC MS analyses of the two samples.
Origin of hydrocarbons C27
C28 C29 Figure 7 Ternary diagram showing the distribution of the C27 through C29 ~#/~20(R+S) steranes (m/z 218) from the Lopra-1 oil, waxes and coal extract
biomarker compositions in the two phases. Thus, it is likely that the biomarker compositions of the waxes are similar to those of the original oils.
Distribution of waxes. Waxes were found widely distributed in outcrops on the Faeroe Islands (Figure 1). Furthermore, the wax observed at 2072 m in the Lopra1 well (Jorgensen, 1984) which Laier and Nytoft (1993) considered to be indigenous, indicates that waxes occur in areas with deep circulation of water. Waxes may precipitate at depth or at the surface, but at present it is not possible to determine whether the waxes found at outcrop were exposed by erosion or whether they formed near or at the surface. The waxes appear to be only mildly biodegraded which is suggestive of fairly recent deposits. Coal Extracts of coal from the upper and lower seams in the Rokhagi mine on Suduroy consisted of asphaltenes (56
The analytical results on waxes and coals presented herein do not support previous speculations that the waxes were derived from the known coal occurrences on the Faeroe Islands (Jorgensen, 1984). Biomarker distributions indicate that the source rocks of the waxes were deposited in anoxic environments quite different from those in which coals are formed. The same holds true for the source rock of the Lopra- l oil. The Lopra-1 oil and the original oil which formed the wax coatings on the zeolites were most likely derived from mature source rocks beneath the known basalts, although part of the Lopra-1 oil, particularly the light end hydrocarbons, was probably derived from the diesel added during drilling (Jacobsen and Laier, 1984). In view of the fact that the base of the basalts is just 200 m below the T D of the Lopra-I well (Kiorboe and Petersen, 1995) it is not unlikely that oil migrated up through fractures in the basalt and into the well. Given the maturity indicated by the biomarkers, the source rock itself may exist below the seismic marker indicating the base of the basalts. The waxes coating the zeolites, as sampled at outcrops, were derived from a mature source rock containing some terrestrial input as shown by the presence of oleanane and the weak odd over even predominance for the higher n-alkanes (Figure 2) (Carlson et al., 1993; Wavrek and Dahdah, 1995). This suggests that the source may be younger and stratigraphically shallower than that of the Lopra-I oil. A second possibility is that the hydrocarbons of the waxes were a mixture of Lopra-I type oil plus oil from a shallower source rock containing terrestrial organic matter. The first possibility is considered to be the most likely given the differences in C27 29 steranes (Figure 7) and in C2~/C30 hopane ratios (Table 1) The age of the source rock is more difficult to discern. The presence of oleanane in the waxes is indicative of a source rock deposited after the appearance of angiosperms in Late Cretaceous. No oleanane was observed in the Lopra-I oil, suggesting that it was derived from an older source or a source accumulated in a distal palaeodepositional environment with little or no input of ter-
266
Hydrocarbon traces in the Tertiary basalts of the Faeroe Islands: T. Laier et a l.
restrial organic matter. Hitchen and Stoker (1993) speculated that the oil traces observed in the Lopra-1 well might have been derived from the Volgian which forms a rich oil-prone source rock west of the Shetland Islands (Bailey el al., 1987). The discovery of the Foinaven and Shiehallion fields in this area confirms the predictions made by Bailey el al. (1987) that large amounts of oil have been generated. A seismic line (Boldreel and Andersen, 1994) close to the two British discoveries indicates that the rift basin beneath the Eaeroe Shetland Channel may be symmetrical, so it is possible that Late Jurassic Earliest Cretaceous source rocks extend west into the Faeroe basin. Kimmeridge to Ryazanian source rocks in the deepest part of the rift basin beneath the Faeroe Shetland Channel are.presently highly mature (dry gas window) (Bailey et al., 1987). Assuming that the Upper Jurassic organic-rich rocks extend far enough into the Faeroe Basin to produce the oil observed in the Lopra-1 well, we conclude that the burial depth of these rocks becomes shallower to the NW of the Faeroe Shetland Channel and at some point enters the oil window.
Conclusions The oil encountered in the Lopra-I well was generated from a marine, siliciclastic source rock with a predominance of marine algal organic matter. The maturity level is intepreted to be well within the oil window for a classical type II (marine algal kerogen) source rock. The oil has likely migrated into the well through fractures in the basalt. The gas in the well is of thermogenic origin and may have been generated by the same source rock as the oil, as indicated by the stable carbon isotopic composition of methane, ethane, and propane. The relative methane concentration is, however, higher than expected for an oil-associated gas. This indicates that the gas has undergone fractionation. Waxes seen as coatings on zeolite grains in outcrop are at least in part derived from a post Late-Cretaceous source rock with mixed marine algal and terrestrial higher plant organic matter. These waxes have probably been deposited from minor amounts of oil present in deep circulating waters penetrating the Palaeocene basalts. Humic coals on Suduroy are immature (R,,=0.5%). Their maturity level and chemical composition show that they did not generate the waxes nor the oil in the Lopra1 well.
Acknowledgements The H T G C FID and the GC AED analyses were performed in Agip's laboratory in Milan during our stay in 1991. Dr A. Riva is gratefully acknowledged for helping us to perform the analyses.We thank Dr J. Ineson for helping us improving the English text.
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