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79
© GEUS, 2006.
Geological Survey of Denmark and Greenland Bulletin 9, 79-90.
Available at:
www.geus.dk/publications/bull
Hydrocarbon gases in Palaeogene volcanic rocks from the
Lopra-1/1A well, Faroe Islands
Troels Laier
Hydrocarbon gases were monitored in the drilling fluid during deepening of the Lopra-1 well from
2178-3565 m, in which thermogenic, methane-rich gases had been found previously. The mud gas con-
centration, up to 105 ppm of methane, was generally higher in the hyaloclastite sequence, 2470 m -
terminal depth (TD), than in the overlying lavas of the lower basalt formation. The highest concen-
trations of mud gas in the lower basalt formation were associated with the more porous tuffaceous zones, whereas no simple relationship could be established between measured mud gas concentrations and porosity of the hyaloclastic rocks, which showed less marked porosity variations than the lavas.
Chemical (C2+ < 1%) and isotopic (δ13C1: -34 to -39) compositions of seven samples of mud
gas collected at peak gas concentrations between 2657 m and 3442 m compare well with those of the
hydrocarbon gases which had been seeping more or less continuously into the existing well since 1983, suggesting a common origin of the gases.
Headspace methane concentrations measured in 135 canned samples of cuttings were scattered
between 10 ppm and 6 × 103 ppm, with the exception of six samples from a short interval, 2685-
2745 m, which showed consistently high values > 104 ppm. No particularly gas-rich zones were indi-
cated, however, by the mud gas, nor was any significant change in lithology noted for this interval. It
is possible that the technique of turbo-drilling, that had been attempted over a short interval, 2657- 2675 m prior to collection of the high-level methane samples, may have caused enhanced degassing due to the very fine cuttings produced. Chemical and isotopic composition of headspace gas and mud gas indicated the same type of gas throughout the well, although headspace methane tended to be more enriched with respect to the 13C isotope.
The origin of the Lopra-1 gas is discussed in the light of recent information obtained from source
rock studies of central East Greenland and the Faroe-Shetland Basin.
Keywords
: Faroes, Lopra-1/1A, volcanics hydrocarbon gas, isotopes, headspace methane, cuttings
_______________________________________________________________________________________________________
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
E-mail:
tl@geus.dk
As hydrocarbons were not expected to occur in the basalts
of the Faroe Islands, no monitoring of hydrocarbon gases in the drilling fluid was performed while drilling the 2178 m deep scientific Lopra-1 well in 1981. A few observa- tions made while drilling, however, indicated negligible amounts of gases in the basalts (Waagstein et al . 1984). None of these observations suggested larger accumulations of gas so, after having reached TD in November 1981, the drilling fluid in the well was replaced by fresh water. Soon afterwards, the well began to flow approximately
9.5 litres/min. (K. Højgård, personal communication
1982) and it was decided after a few days to shut-in the well. The Lopra-1 well was not re-opened until March 1983, when temperature logging was to be performed. By that time the wellhead pressure had increased to 19.5 bars (Balling et al. 1984) and an inflammable gas had ac- cumulated within the 190 m long casing at the top of the well (P.H. Nielsen, personal communication 1983). The volume of the gas was estimated to be roughly 9 m3 at 1
bar pressure (P.H. Nielsen, personal communication 1983)
GEUS Bulletin no 9 - 7 juli.pmd
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79
80
and gas chromatographic analysis of a sample collected at
the wellhead showed the gas to consist of methane (72%) and nitrogen (27%) plus traces of higher hydrocarbons (Jacobsen & Laier 1984). After temperature logging, the well was shut-in again and an increase in wellhead pres- sure was noted shortly after. Two months later, the well- head pressure had increased to 10.8 bars (Fig. 1). Collec- tion of a new gas sample showed that the composition of the gas was almost identical to that of the sample collect- ed two months earlier. Furthermore, isotopic analyses in- dicated that the gas was thermogenic in origin, the δ13C
of methane being -39.6 (Jacobsen & Laier 1984).
Encouraged by the hydrocarbon discoveries of Foin-
aven in 1992 and Schiehallion in 1993 in the British sec-
tor 160 km south-east of the Faroe Islands, new investiga- tions of the hydrocarbon traces in the Lopra-1 well were carried out in 1992-1993, including stable isotopic anal- ysis of gases and biomarker analyses of the small quanti- ties of higher hydrocarbons extracted from the water flow- ing
out
of
the
well (Laier
et al.
1997). An additional gas
sample
was taken for isotopic analysis in July 1994, before
the deepening of the Lopra-1 well commenced in July 1996
(Table 1).
This paper presents the results of new chemical and
isotopic analyses of both mud gas samples and headspace
gas of canned cuttings collected during a deepening of the Lopra-1/1A well in 1996. The results of the continu- ous mud gas readings are compared with well logs in or- der to identify any particular gas-rich zones, and the pos- sible origin of the hydrocarbon gases in the basalts is dis- cussed.
Sampling and methods
Mud gas
Gas samples were taken at the mud-logging unit on dif-
ferent occasions (Table 2). The gas samples were stored in 100 cm3 steel bottles which had been flushed with mud
gas for two minutes before closing the valves. The sam-
ples were sent to the Geological Survey of Denmark and Greenland (GEUS) within a week for chemical and iso- topic analyses.
The concentrations of hydrocarbon gases in the gas
samples were analysed using a Shimadzu GC9 gas chro-
matograph equipped with a flame ionisation detector (FID). Separation of the gas constituents was performed on a 4 m
1
/
8
" diameter SS column packed with silica gel
70/80 mesh using helium as carrier gas (20 ml/min.) us-
ing a temperature programme beginning at 100°C, in- creasing 20°C/min. to 250°C final temperature.
Headspace gas of canned cuttings
Unwashed cuttings were sampled at approximately 10 m
intervals and stored in 0.5 litre cans to which bactericide had been added. The cans were sent to GEUS for head- space gas and isotopic analyses after the completion of the well. The cans were generally in a good condition when received by GEUS, except for a few cans where the lid was not sealed tightly. The cans were placed upside down, punctured through a septum and 0.2 ml of headspace gas
Fig. 1. Wellhead pressure of the Lopra-1 well during the shut-in
period after temperature logging (P.H. Nielsen, personal commu- nication 1983).
Year CH
4
C
2
H
6
C
3
H
8
iC
4
H
10
nC
4
H
10
N
2
O
2
+Ar CO
2
H
2
13
C
1
13
C
2
13
C
3
D
1
1994 60.2 0.31 0.048
0.014
0.012
39.1 0.06 0.01
-32.1
-26.5
-150
1992 40.6 0.14 0.025 0.008 0.007 59.1 n.d. 0.01
-32.4 -26.5 -148
1983 1.9 0.41 0.064
0.015
0.018
27.5 0.04 0.01
-32.5
n.a. -133
n.a.: not analysed; n.d.: not determined. Concentrations are given in vol%. Stable isotopic ratios are given (in parts per thousand) relative to
the Pee Dee Belemnite (PDB) and Standard Mean Ocean Water (SMOW) standards.
Table 1. Chemical and isotopic composition of gases from the original Lopra-1 well collected at the wellhead
<0.01
<0.01 <0.01
-39.9
-41.5 -39.6
0
4
8
12
Date
Pressure (k
g
/c
m
2
)
83-03-20
83-04-19
83-05-20
GEUS Bulletin no 9 - 7 juli.pmd
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81
Results and discussion
Gases from the original Lopra-1 well
No casing had been set in the old Lopra-1 well except for
the upper 190 m of the borehole. Therefore, the substan- tial amount of gas which had accumulated at the well- head during the 16 months shut-in period after drilling, could in theory have entered from any part of the open hole. However, Jacobsen & Laier (1984) observed an in- crease in the gas to water ratio with depth suggesting that the gas entered mainly in the deeper parts of the well. The flow of gas (0.8 litre/min., ambient pressure and tempe- rature) from the well measured by Jacobsen & Laier (1984) during their field work in June 1983 also suggested that the gas continued to seep into the open borehole of the Lopra-1 well. Taking into account the methane dissolved in the water flowing out of the well (at a water flow rate of 12.8 litres/min.), the total flow of methane is estimated
to have been approximately 0.9 litre/min.; 0.56 litre/min.
in the gas phase (72% CH4) plus 0.3 litre/min. dissolved
in water. So the total flow of methane in June 1983 was
over 1 m3 per day. If the influx of methane had been the
same during the initial 16 months shut-in period, a much
larger quantity of gas would be expected than the 9 m3
(ambient pressure) noted when the well was re-opened in
March 1983. However, the flux may have been slower during the shut-in period due to higher pressure (19.5 bars)
at the wellhead.
Gas was still seeping into the borehole 13 years after
drilling of the Lopra-1 well. The composition of the hydro-
carbon gas had not changed significantly during that pe-
riod (Table 1) and this suggests a major single source of the gas. The source provided mainly methane, as the ni- trogen content in the gas-to-water ratio measured at well-
was withdrawn using a gas-tight syringe to be analysed by
gas chromatography as described above. If a high concen- tration of methane was found, 60 ml of headspace gas was transferred to an evacuated serum bottle for later iso- topic analysis. The volume of headspace gas in the cans, generally between 100-150 ml, was checked by addition of water. No corrections on headspace gas concentrations were attempted to account for the difference in headspace volumes.
Isotopic analyses
Headspace gas and mud gas samples were transferred to a
¼" column packed with Porepak Q immersed in liquid nitrogen. Separation of the gas constituents was then per- formed allowing the column to heat to ambient tempera- ture. Methane and ethane were combusted over CuO at 900°C and the resulting carbon dioxide was isolated in flame-sealed glass ampoules using cryogenic traps. Isotopic measurements were performed on a Finnigan Mat 251 mass spectrometer at the University of Copenhagen.
The isotopic ratio is expressed in the usual delta nota-
tion relative to the PDB standard.
13
C( ) = (R
sample
/(R
standard
- 1)) × 1000
where
R =
13
C/
12
C
The analytical precision of the isotopic measurements is ±
0.2 for larger quantities of gas (> 20 µl CO2 STP),
typical of the amounts of methane in all of the analysed
samples, and ± 0.5 for smaller quantities of gas (< 10 µl CO2 STP), typical of the ethane.
Interval (m)
Event
Comments
2360-2450
Sour smell and low
pH of drilling fluid
2657-2675
Turbo drilling
Finer cuttings. Possible enhanced degassing. Potential risk of gas generation due to high
temperature.
2680-2822
One shale shaker
Specific gravity increase, 1.11-1.18 g/cm
3
, of drilling fluid due to solids build-up.
out
of
order
2992
20% dilution of mud
Reduction of specific gravity, 1.18-1.13 g/cm
3
.
3023
Mud centrifuge installed Reduction of specific gravity to 1.06-1.08 g/cm
3
.
3158
String got stuck
5 m
3
diesel pill added.
3091-3565
Side track, Lopra-1A
1% oil in drilling fluid according to daily analysis.
Table 2. Drilling events which may influence hydrocarbon gas measurements
Low pH may cause corrosion and generation of artificial gas.
Indication of bacterial activity. Bacteria may generate/consume methane.
GEUS Bulletin no 9 - 7 juli.pmd
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81
82
NP
HI
-0
.1
2200
2300
2400
2500
2600
2700
2800
2900
3000
3100
10 10
2
10
5
10
3
10
4
3100
3200
3300
3400
3500
3.2
2.6
2.0
Rock density
(g/cc)
Mud gas
CH
4
(ppm)
10 10
2
10
5
10
3
10
4
3.2
2.6
2.0
Rock density
(g/cc)
Mud gas
CH
4
(ppm)
Hyaloclastites,
m
ostly lapilli-tuffs
(subaqueous)
Lower basalt for
m
atio
n
(subaerial)
Fig. 2. Rock density and mud gas recorded in the extended Lopra-1/1A well.
head can be accounted for by the amount of atmospheric
nitrogen initially dissolved in meteoric water (at 6°C) per- colating into the basalts.
Mud gas from the Lopra-1/1A deepening
The original Lopra-1 well, drilled in 1981, was deepened
in 1996. A 7
5
/
8
" casing was first set in the existing 8½"
borehole before the well was extended downwards using a
6½" drill bit. The Lopra-1 well was terminated at 3158 m due to technical problems and a sidetrack was drilled from 3095 m to a TD of 3565 m (Lopra-1A). Any hydrocar- bons detected in the drilling fluid or in the sealed samples of drill cuttings of the extended well section must have come from the new drilling activities. No casing was set in the new well sections, which means that gases detected in the drilling fluid (Fig. 2) represent the sum of gases released by drilling in addition to the gases that may have
GEUS Bulletin no 9 - 7 juli.pmd
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82
83
seeped through the sidewall of the entire open hole. A
KCl-polymer mud was used as drilling fluid and the well logs indicate that no substantial mud cake had been built up in the hole. Therefore, gas seepage from the sidewall of the open hole was probably more significant for the Lo- pra-1/1A well compared to conventional exploration wells in sedimentary rocks.
The mud gas and headspace gas data from the Lopra-
1/1A well are the only information available on hydrocar-
bon gases in the basalts penetrated, as no further tests were performed in the well after drilling. Drilling was optimised to detect hydrocarbon gases in the rocks penetrated, i.e. the specific gravity of the drilling fluid was kept low and the trap for continuous hydrocarbon measurement was placed in an almost closed mudline system. However, the mud gas and headspace gas must be interpreted with cau- tion. It is important to consider any possible effects of the drilling process itself. The drilling events which may have influenced the hydrocarbon measurements and which could give a false impression of the hydrocarbon variation in the rocks, have been listed in Table 2, together with their possible implications.
The specific gravity of the mud could not be kept con-
stant until a centrifuge had been installed (when drilling
had reached 3023 m), after which the specific gravity re- mained at 1.06-1.08 g/cm
3
. During drilling of the lower
basalt formation from 2184 to 2470 m the specific grav-
ity remained almost constant at 1.03-1.05 g/cm3. At great-
er depths, the cuttings became finer and the specific grav-
ity of the mud was increased gradually to 1.18 g/cm3 at
2992 m by increasing the content of solids. An increase in
mud weight suppresses degassing from the rocks, which lowers the gas concentration in the mud. On the other hand, an elevated solids content may increase gas concen- tration, depending on how fast the cuttings degas. With these uncertainties in mind, a more detailed interpreta- tion of the mud gas will be presented below.
Mud gas concentration varied from less than 10 ppm
to 90
000 ppm of methane down to 3120 m depth, show-
ing a general increase from 2400 m to 3120 m (Fig. 2).
Higher hydrocarbons, mostly ethane, constituted less than one percent of the total gas throughout the drilling of the well and will not be dealt with in detail in the following. The intervals with very low mud gas concentrations, < 10 ppm, were observed only in the lower basalt formation above 2470 m. Comparing mud gas concentration versus depth with various well logs as well as changes in various drilling parameters (rate of penetration, mud density etc.) it appears that rock porosity and rock density are the most important rock properties affecting the mud gas concen- tration in the upper 300 m of the extended well (Fig. 2).
Variations in rock density and porosity correlate closely,
so only the most complete log, the density log, is shown on Fig. 2. Assuming that the gas is indigenous to the rock and not an artefact of drilling, the highest gas concentra- tions should be found in the more porous, less dense rocks, as is also indicated by the data for the upper 300 m of the extended Lopra-1/1A well. However, this simple relation- ship is not observed for the deeper parts of the well. From 2610 m to 3080 m the density of the rock generally in- creases and porosity decreases, yet a general increase in mud gas concentration is observed in this same interval, an increase that may be explained by the increased side- wall surface area. This suggests that degassing through the sidewall of the hole contributes more to the mud gas compared to the gas released by the drilling process itself.
The presumed gas seepage through the sidewall of the
open hole makes it difficult to decide which part of the
well actually contributes to the gases recorded in the mud. However, for the deepest part of the well, below 3430 m, changes in the mud gas concentration appear to be corre- lated positively with porosity (and inversely with rock den- sity), which may indicate that gas is present even in the deepest rocks penetrated by the Lopra-1/1A well. Thus, it may be concluded that hydrocarbons are most likely pre- sent in all of the rocks penetrated, particularly in the more porous rocks. However, no particular hydrocarbon-rich zones and no cap rock were identified.
Samples of mud gas were collected at maximum gas
concentrations as far as possible (Table 3). Three of the
samples were taken when mud circulation was resumed after changing the drill bit, and the rest of the samples were taken at maximum concentration while drilling. The
Table 3. Samples of mud gas collected from
the Lopra-1/1A well on various occasions
Event
Resumed mud circulation
after changing of drill bit Resumed mud circulation after changing of drill bit Gas collected after short drilling break Gas collected at peak con- centration while drilling Gas collected at peak con- centration while drilling Gas collected at peak con- centration while drilling Gas collected after having drilled 6 m after trip
2657
2822
2946
2989
3107.5
3343
3442
Date
96/08/28
96/09/07
96/09/09
96/09/10
96/09/19
96/10/23
96/10/26
Sample
No.
1
2
3
4
5
6
7
Depth
(m)
GEUS Bulletin no 9 - 7 juli.pmd
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84
concentrations of hydrocarbon gases in the mud gas sam-
ples were usually lower compared to mud gas concentra- tions recorded in the mud by the time of sampling (Table 4)
although the chemical composition of the gas was the
same. The reason for this is not clear, but insufficient flush- ing of the 100 cc steel bottle during sampling could be one reason. The chemical and isotopic composition of the mud gas (Table 4) is not very different from that of the wellhead gas collected from the old Lopra-1 well (Table 1) suggesting a common origin for the gases.
Headspace gas of sealed cuttings
Methane concentration in the headspace gas of the 135
canned samples of cuttings from the Lopra-1/1A well var- ied from 7 ppm to over 33 000 ppm, the highest concen- trations being measured at 2273 m and in the interval from 2685 to 2745 m (Fig. 3). The sample at 2273 m was collected just after drilling through a 2-3 m thick layer of lapilli tuff with a distinctly higher porosity compared to the massive lavas above and below. A higher mud gas con- centration was also noted for this porous layer (Fig. 2). The methane concentration in the samples from the 2685- 2745 m interval is more than one order of magnitude higher than that of most other samples from the well. This interval corresponds to the upper part of a section of dominantly lapilli tuff (2610-2880 m) and could indi- cate that these rocks contain more gas than those at other levels. However, the mud gas concentration did not shift to higher values as one might have expected if the gas content was generally much higher in the rocks of this particular interval (Fig. 3). Furthermore, the well logs show no correlation between the increase in headspace meth- ane and any change in rock properties, such as porosity or density. A shift from rotary drilling to turbo-drilling using a diamond bit took place at 2657 m, and turbo-drilling
continued to 2675 m where rotary drilling was resumed.
Cuttings produced by turbo-drilling are generally much smaller than cuttings produced by normal rotary drilling. It is possible that the decrease in cutting size may have led to enhanced degassing and thereby higher headspace meth-
ane concentrations. Furthermore, one of the two shale shak-
ers was out of order due to motor failure during drilling
from 2680 to 2822 m. This may be one of the reasons for the observed build up of solids in the drilling fluid, which, combined with the finer cuttings produced by turbo-drill- ing, may have been the cause of the significantly higher methane concentrations in the headspace gas.
Alternatively, the much higher concentration of meth-
ane measured in the headspace gas shortly after turbo-
drilling could be explained by generation of artificial gas due to the very high temperatures which often occur us- ing this drilling technique. If, however, the gases were gen- erated by some artificial process, an increase in mud gas methane concentration should be expected, which is not the case (Fig. 3). Furthermore, the stable isotopic ratio of the methane is not markedly different in the zone with high methane concentrations compared to headspace methane from other levels (Fig. 3).
Ethene, with concentrations up to 39 ppm, was ob-
served in a number of headspace samples, particularly from
the upper 300 m of the extended well where the pH of the drilling fluid was relatively low (7-8 compared with a typical value of 10) according to the daily drill reports. Unsaturates like ethene are very uncommon among light hydrocarbons in natural gas, so this constituent was most likely formed artificially, either during drilling (Faber et
Facing page:
Fig. 3. Summary of mud gas and headspace results of the extended Lopra-1 well ( A ) and Lopra-1A sidetrack ( B ). Larger circles represent C1/C2 ratio corrected for artificial gas indicated
by ethene (C2=). Numbers are stable isotopic values of methane.
Table 4. Chemical and isotopic composition of mud gas samples
Depth
(m)
2657
2822 2946 2989 3107.5 3343 3442
n.d.: not detected. n.a.: not analysed.
CH
4
ppm
C
2
H
6
ppm
31.6
31.7 19.0 81.6 74.4 17.3 28.4
C
3
H
8
ppm
4.06
3.02 1.56 6.78 9.34 3.44 5.86
iC
4
H
10
ppm
0.44
n.d.
n.d.
0.36
0.92 0.66 0.98
nC
4
H
10
ppm
0.63
n.d.
n.d.
0.40
1.36 1.10 1.45
13
C
1
()
-38.6
-38.2 -36.7 -37.3 -38.1 -34.2 -36.0
13
C
2
()
-31.8
-35.0
-32.3
-33.4
-33.6
C
1
/C
2
140
152 126 128 137 158 151
n.a.
n.a.
4420
4820 2400
10440
10200
2740
4290
GEUS Bulletin no 9 - 7 juli.pmd
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85
A
Mud gas (ppm)
10 10
2
10
3
10
4
10
5
10 10
2
10
3
10
4
10
5
Headspace gas (ppm)
CH
4
CH
4
2200
2400
2600
2800
3000
3200
3000
3200
3400
3600
-38.2
-36.7
-33.7
-30.0 -33.8 -35.1 -28.6 -34.0
Mud gas (ppm)
10 10
2
10
3
10
4
10
5
10 10
2
10
3
10
4
10
5
Headspace gas
CH
4
CH
4
0.1 1 10 10
2
Headspace gas
C
1
/C
2
C
2
=(unsat.)
13
C
-35.8
-35.0
-35.1
-39.2
-34.2
-36.0
Depth (
m
)
Depth (
m
)
13
C
-36.6
0.1 1 10 10
2
Headspace gas
C
1
/C
2
C
2
=(unsat.)
B
1 10 10
2
10
3
1 10 10
2
10
3
-38.6
-37.3
-38.1
-36.6
-36.7
GEUS Bulletin no 9 - 7 juli.pmd
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85
86
al.
1988) or as a result of corrosion (Laier 1999). Samples
of cuttings generally contain iron filings from the drill- ing. Unsaturates can be generated by a catalytic reaction of the Fischer-Tropsch type, and may occur in canned cuttings with relatively low pH, as was demonstrated for the Swedish deep gas well at Lake Siljan (Laier 1999). The concentration of ethene decreased when pH had been raised to over 10, by adding caustic soda and bactericide to the drilling fluid below 2500 m (Fig. 3).
The methane-to-ethane ratio of the gas may be useful
in determining the origin of the gas in the volcanics, but a
correction should be made for the artificial gas which may influence this ratio. The methane-to-ethane ratio of arti- ficial gas varied between 3-5 in the Siljan well (Laier 1999), which is much lower than that of the gases in the Lopra- 1/1A well (Fig. 3). Equal amounts of ethene and ethane were noted in the artificial gas of the Siljan well (Laier 1999); assuming a similar ratio of 1:1 for the artificial gas component in the Lopra headspace samples, a correction has been made (Fig. 3). Headspace samples with methane concentrations below 100 ppm have generally lower meth- ane-to-ethane ratios, but this cannot be taken as evidence of a different origin of the gas. This is more likely due to uncertainties of the headspace method as well as the ana- lytical uncertainty for the very low ethane concentrations. The methane-to-ethane ratio varied between 102 and 103
for headspace gas samples with higher methane concen-
trations (> 100 ppm) that have less analytical uncertainty (Fig. 3). The methane-to-ethane ratio remained fairly con- stant, around 102, in samples from the side track Lopra-
1A. Thus, given the uncertainties of the headspace meth-
od, it may be concluded that the hydrocarbon gas of the
sealed cuttings mainly has the same origin as that of the
mud gas and the previous wellhead gas.
Origin of the gasses
The chemical and isotopic data for the gasses from the
original Lopra-1 well and the extended well sections have been plotted in the well-known classification diagram for hydrocarbon gases (Fig. 4). The mud gas of the extended well plots fairly close to the wellhead gases of the old Lo- pra-1 well, suggesting that they have a common origin. Headspace gases, however, exhibit a much larger variation
and are generally slightly more enriched with respect to
carbon-13. Such enrichment could result from either bac- terial
oxidation
of
methane
(Coleman
et
al
.
1981)
during
storage or fractionation due to diffusion. The most car-
bon-13 enriched gases are found among the headspace samples with the highest methane concentration (Fig. 3), therefore bacterial oxidation is unlikely to have been re- sponsible for the carbon-13 enrichment. If degassing from the cuttings occurred mainly via diffusion, the gas may have fractionated due to degassing prior to storage in the tight cans.
The hydrocarbon gases are thermogenic in origin, and
the relatively low C2+ content indicates either a gas-prone
source rock or a high maturity of the source rock. Com-
paring the stable isotope values (δ13C) of methane through
propane (Tables 1, 4) with the δ13C versus source maturity
lines published by Rooney
et al.
(1995), a highly mature
marine source rock (type II kerogen) is most likely for the Lopra gases. This is more obvious for the wellhead gases than for the mud gases as can be seen from Fig. 5. The larger variation in δ13C among the mud gas samples com-
pared to the wellhead gases, particularly for ethane, may
be due to the much smaller quantities of hydrocarbons
making the mud gases more susceptible to sampling and
analytical errors.
Rooney
et al.
(1995) estimated that the reservoir gases
used to construct the δ13C versus maturity line for the
type (II) kerogen had been generated in the temperature
range 170-190°C, the upper part of which is not very different from the maximum palaeotemperature estimat- ed for the base of the Lopra-1/1A well (Glassley 2006, this volume). Furthermore, the reservoir gases used by Rooney et al . (1995), which had isotopic values in the same range as those of the Lopra-1 gases, were also very dry (M.A. Rooney, personal communication 2000). On the other hand, using δ13C versus source maturity lines
constructed by other researchers such as Faber (1987) and
Berner & Faber (1996), a somewhat lower maturity is
10
5
10
4
10
3
10
2
10
1
-80
-60
-40
-20
0
2
3
Bacterial
Thermal
type II
type III
Artificial
Well-
head
13
C
CH4
C
1
/(C
2
+ C
3
)
Mud gas
Fig. 4. Plot of C1/C2 + C3 vs. δ13CCH4 for Lopra-1/1A gases. Small
solid circles represent headspace gas. (Diagram modified from Faber
et al. 1999.)
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87
predicted for the source of the Lopra-1 gases (Fig. 5).
Therefore, the maturity of the source rock that generated the gases is still not certain.
The presence of a source rock is, however, very likely as
was indicated by the continued and fairly significant flux
of gas into the original Lopra-1 well over a 13-year peri- od. The gases entering the open Lopra-1 well most likely came from the more porous layers of tuffs beneath the massive lavas of the lower basalt formation. No layers of coal were penetrated by the extended Lopra-1/1A well, so the source is most likely located below the hyaloclastites that were extruded subaqueously (Waagstein 2006, this volume).
Traces of higher hydrocarbons
The traces of higher hydrocarbons measured in mud sam-
ples derived mainly from various additives (Bojesen-Koe- foed & Nytoft 2006, this volume) and cannot give any clues as to the possible origin of the Lopra gases. Fluid inclusion studies by Konnerup-Madsen (2006, this vol- ume) do, however, indicate that hydrocarbon fluids were present
in the volcanics at one time. But analysis of a fluid
extracted from one calcite sample containing fluid inclu-
Fig. 5. Cross-plot of isotope values (δ13C) of methane vs. ethane of
wellhead and mud gas samples from the Lopra-1/1A well. Maturi-
ty lines, from Faber (1987) and Rooney et al . (1995), were based on reservoir data, whereas those of Berner & Faber (1996) were obtained from laboratory experiments. Numbers attached to well- head samples refer to sampling year.
Palaeogene basalt/
Nansen Fjord Formation and Faroes lower formation
Cretaceous-Palaeogene
sediments
Precambrian
basement
East Greenland
Inland
Ice
Kangerlussuaq
Nansen Fjord
B
l o
s s
e v
i l
l e
K
y s
t
Scoresby Sund
Faroe
Islands
100 km
Lopra
36°
33°
30°
66°
67°
68°
69°
70°
36°
33°
30°
27°
70°
69°
68°
67°
24°
27°
Fig. 6. Pre-drift reconstruction of central
East Greenland and the Faroes block, modified from L.M. Larsen et al. (1999). The arrow indicates the location of organic-rich sediments having Ro = 1.2% (M. Larsen, personal communication 2000).
Mud gas
83
94
III = -3
0 (Ber
ner & Faber 1996)
-40
II =
-25
(Ber
ner
& Faber 1
996)
-30 -20
II = -
30
-50
-40
-30
-20
III
(
Roo
ney
et al.
199
5)
II
(Roo
ney
et al.
1995)
III = -25
92
III
Wellhead
13
C
Ethane
(
)
13
C
Metha
n
e
(
)
GEUS Bulletin no 9 - 7 juli.pmd
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87
88
sion could not be related unambiguously to a particular
source (Bojesen-Koefoed & Nytoft 2006, this volume).
Waxes associated with zeolites reported from other parts
of the Faroe Islands (Laier
et al.
1997) appeared to have
been generated by a less mature source than that of the Lopra-1/1A gases. Thus, the presence of traces of higher hydrocarbons supports the assumption that hydrocarbons migrate or migrated in the volcanics of the area, but no direct connection between these higher hydrocarbons and the Lopra gases have been documented.
Source rocks of the Faroe Islands area
The scientific Lopra-1 drilling was termined in Novem-
ber 1981 at 2178 m for technical reasons without having fulfilled its primary objective of penetrating the substra- tum of the lower basalt formation. The deepening of the Lopra-1/1A well in 1996 to 3565 m penetrated hyalo- clastites, mostly lapilli tuffs, under the base of the lower
basalt formation at about 2470 m, but gave no definite
clue as to the type of rocks occurring beneath the volcan- ics. The fact that the volcanics below the lower basalt for- mation were deposited subaqueously suggests that sedi- ments were present in the area prior to the volcanic erup- tions. Such sediments would probably be similar in age to the sediments exposed beneath the basalts in the Kanger- lussuaq area in East Greenland reported by M. Larsen et al.
(1999), sediments that probably give us the best clue as to
what type of source rocks might exist beneath the volcan- ics at Lopra, since the palaeodistance between the Faroes block and central East Greenland was only 100-120 km before the break-up of the northern North Atlantic (Fig. 6). The close connection between the two areas is empha- sised by the similarity in chemical composition of the oldest basalts on either side of the northern North Atlantic (L.M. Larsen et al. 1999).
The organic-rich rocks of the Kangerlussuaq area range
in age from late Aptian to late Paleocene. They proved to
be post-mature with respect to hydrocarbon generation
Fig. 7. Oil and gas fields west of Shetland.
Far
oe-Shetla
nd Cha
nn
el
Fa
ro
e-
Sh
et
lan
d
Ch
an
ne
l
6°00'W
8°00'W
4°00'W
2°00'W
62°00'N
61°00'N
100 km
Foinaven
Shetland
Islands
Orkney
Islands
Clair
Schiehallion
Far
oe-Shetla
nd Cha
nn
el
Faroe Islands
Lopra-1/1A
GEUS Bulletin no 9 - 7 juli.pmd
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89
at most localities examined by M. Larsen
et al.
(1999).
However, a vitrinite reflectance value of Ro = 1.2 meas- ured on a late Paleocene lacustrine mudstone (TOC = 7%) 22 km north of Nansen Fjord (Fig. 6) shows that source rocks with hydrocarbon generation potential do exist be- neath the basalts in this area.
An estimate of organic matter maturation related to
burial depth alone may be obtained from the Nansen Fjord
area which was covered by approximately 6 km of volcan- ic rocks prior to uplift (L.M. Larsen et al. 1999). The to- tal thickness of the Faroe Islands volcanics may exceed the 6.5 km presently known from exposures and drillings (Waagstein 2006, this volume); however, a more precise estimate of the thickness is difficult to give. It seems real- istic to assume that possible source rocks below the Faroe Islands volcanic succession still have some potential for hydrocarbon generation, given the close connection be- tween the two areas prior to continental split-up. Thus, the more or less continuous seepage of methane-rich gas into the open Lopra-1 well during 1983-1994 may orig- inate from a highly mature source rock located beneath the volcanics in the area.
Hydrocarbon migration from the
Faroe-Shetland Basin
Hydrocarbons may also have migrated into the Faroe Is-
lands area from the Faroe-Shetland Basin, though little is known of the possible migration pathways. The Foinaven and Schiehallion oil fields, 160 km south-east of the Lo- pra-1 well, are the nearest known hydrocarbon occurrences around the Faroe Islands (Fig. 7), but hydrocarbons are likely to have been generated closer to the Faroe Islands in basalt-covered rift basins beneath the Faroe Shelf (Fig. 8).
Subsequently, these hydrocarbons may have migrated to-
wards the Faroe Islands area via sandy turbidites located under the volcanics or via intrabasaltic sandstones. Re- deposition of coarse-grained sediments onto the Faroe Shelf during mid-Paleocene uplift of East Greenland has been inferred by M. Larsen et al. (1999) but such sediments are not indicated on the geological profile of Fig. 7, which shows only strata recognised on seismic sections.
Since much of the Faroe-Shetland Basin is highly ma-
ture thermally, its lack of large gas accumulations has been
explained by the presence of very oil-prone source rocks that have only little potential for later gas generation (Scotchman et al. 1998). This assumption was based on kinetic studies of Middle and Upper Jurassic kerogens from a number of exploration wells west of Shetland. If that is the case, the assumption that the gases observed in the Lopra-1 well came from the Faroe-Shetland Basin is not supported by observations from this basin obtained with- in a reasonable distance from the Faroe Islands.
Conclusions
Thermogenic
gas,
mostly
methane,
exists
in
the
Lopra-
1/1A well in the hyaloclastites and in the more porous
tuffaceous zones of the overlying lower basalt formation.
Chemical and isotopic data suggest that gases in the
hyaloclastites were responsible for the more or less con-
tinuous seepage of gas into the original Lopra-1 well since 1983.
The location and the type of source rock that generat-
ed the gases observed in the Lopra-1 well are still uncer-
tain. However, recent information on the pre-volcanic, organic-rich sediments of central East Greenland shows that these still have a potential for generating hydrocar-
T
w
o-wa
y ti
m
e
(Sec)
0
2
4
6
SE
NW
0
5 0
1 0 0
1 5 0
2 0 0 k m
Lopra-1/1A
20 km to W
F A R O E S S H E L F
F a ro e - S h e t l AND B a s i n
Westray
Ridge
Rona
Ridge
Schie-
hallion
13 km
offset
East Faroe High
Triassic
Paleocene
Uppe
r
Cr
etace
ous
Eocene-Oligocene
Neogene
L. Cretaceous
U. Jurassic
Volcanic
? Basement
Fig. 8. Geological profile modified from
Spencer et al. (1999). Location shown on Fig. 7.
GEUS Bulletin no 9 - 7 juli.pmd
07-07-2006, 14:19
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90
bons (Ro = 1.2%) after having been buried below 6 km of
volcanic rocks. Given the close connection demonstrated between central East Greenland and the Faroes block pri- or to continental break-up, it is probable that the source rock for the gases in Lopra-1/1A exists below the volcan- ics of the Faroe Islands area.
The possibility of migration of gas from the Faroe-
Shetland Basin towards the Faroes cannot be excluded,
although no evidence in support of this hypothesis has yet been found.
Acknowledgements
The two reviewers, Melody A. Rooney and Ger van Graas,
are thankfully acknowledged for their careful review of the manuscript and valuable comments which helped improve the clarity of the paper.
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