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119
© GEUS, 2006.
Geological Survey of Denmark and Greenland Bulletin 9, 119-122.
Available at:
www.geus.dk/publications/bull
A reconnaissance study of fluid inclusions in fracture-filling
quartz and calcite from the Lopra-1/1A well, Faroe Islands
Jens Konnerup-Madsen
Fracture-filling calcite and quartz from the Lopra-1/1A well (at 2380 m and 3543 m depth) contains
both aqueous low-salinity fluid inclusions and hydrocarbon-dominated fluid inclusions. Microther- mometry indicates that the aqueous fluids contain 0.2 to 1.4 equivalent weight% NaCl and occasion- ally contain traces of hydrocarbons. Homogenisation to liquid occurred between 90°C
and 150°C.
Modelling based on these fluid inclusion observations indicates that during burial the basaltic section
was subjected to temperatures of 160°C
and 170°C, occasional pressures of 600-700 bars and the
simultaneous percolation of aqueous and hydrocarbon fluids. These fluid conditions may also be
relevant to the formation of zeolite observed in the Lopra-1/1A well.
Keywords
: Basalts, Faroe Islands, fluid inclusions, hydrocarbons, veins, zeolites
__________________________________________________________________________________________________________________________________________
Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K., Denmark.
E-mail: jenskm@geol.ku.dk
Fluid inclusions in cements or minerals filling vugs and
fractures in buried sedimentary and volcanic rocks may provide important information on the chemical and phys- ical nature and origin of mineral-precipitating fluids, on the potential interplay between migrating hydrocarbon and aqueous fluids, and on the temperatures and pres- sures
of precipitation (e.g. Bodnar 1990; Jensenius & Bur-
russ 1990).
A
reconnaissance
study
was
undertaken
of flu-
id
inclusions
in
vug-
and
fracture-filling
quartz
and
calcite
from samples taken from the basalts penetrated by Lopra-
1/1A. The two samples studied are from core 1 (2380 m) and sidewall core 1 (3543 m). The fluid inclusions were
examined
by
ordinary
microscopy,
fluorescence microsco-
py
and
with
a
Chaixmeca
heating
and
freezing
stage.
Types and setting of fluid inclusions in
selected samples
The samples were selected by examining about 40 thin
sections taken between 2204 m and 3543 m depth in the Lopra-1/1A well. Only two samples, from 2380 m and
3543 m depth, were found to contain fracture-filling
quartz and calcite with fluid inclusions suitable for fur- ther study.
Sample 2380 m (Lopra-1, core 1) is a sparsely plagio-
clase-glomerophyric olivine-clinopyroxene basalt with al-
most complete alteration of plagioclase and olivine. The quartz and calcite studied occur in mm-wide veins. The veins are rimmed by chlorite, calcite and quartz that ap- pear to have been precipitated contemporaneously. Ac- cording to Jørgensen (2006, this volume) the zeolites char- acterising this level in the core are laumontite, prehnite and pumpellyite.
Sample 3543 m (Lopra-1A, sidewall core 1) is a near-
aphyric lapilli-tuff with extensively altered plagioclase,
olivine and clinopyroxene phenocrysts in a cryptocrystal- line groundmass. The irregular veins contain laumontite, prehnite, calcite and rare quartz. The veins are rimmed by chlorite. Again, calcite and quartz appear to have been precipitated contemporaneously, although quartz precipi- tation might have been slightly later.
GEUS Bulletin no 9 - 7 juli.pmd
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119
120
0
1
2
3
50
100
150
200
Homogenisation temperature, °C
Sali
n
ity
,
wt% NaCl
Group A
Group B
Data on fluid inclusions in quartz
from Lopra-1/1A altered basalt
Field for
aqueous fluid
inclusions
in calcite
Types of fluid inclusions in quartz and calcite
Two types of fluid inclusions were observed using fluores-
cence and ordinary light microscopy: (1) aqueous two- phase (liquid-vapour) inclusions with about 5 vol.% va- pour at room temperature, and (2) one or two-phase hy- drocarbon inclusions with fluorescence emission colours that vary from orange-yellow to green. With ordinary light microscopy it is difficult to distinguish between the two- phase liquid-vapour hydrocarbon and aqueous inclusions, although the latter seem to be characterised by a (per- haps) slightly lower vol.% vapour than the former. No clear relative chronology between the twOFluid inclusion types could be established.
Examples of typical morphologies and phase propor-
tions of fluid inclusions observed in quartz are shown in
Fig. 1.
In general, the fluid inclusions are characterised by
immature morphologies and occur in irregular groupings
or in curved internal planar arrangements, suggesting pe- riods for their entrapment which do not markedly post- date the growth of the host mineral. The liquid-vapour ratio in individual groupings varied slightly, most proba- bly and mainly as a result of necking down of the inclu- sions after entrapment, because liquid-only inclusions could occasionally be observed together with the aqueous two-phase liquid-vapour inclusions. All inclusions indic- ative of having been influenced by necking down were avoided during the heating and freezing stage work.
Microthermometry results on aqueous
fluid inclusions
The results of microthermometry of fluid inclusions in
quartz and calcite are summarised in Fig. 2.
Fluid inclusions in quartz
Incipient melting of ice was observed at temperatures
around -32°C, indicating the presence of additional ions such as Ca2+, Mg2+ and/or Fe2+ in solution rather than chlo-
rides of Na+ and/or K+ (Konnerup-Madsen 1979). Final
melting temperatures were observed in the range -0.1°C
to -0.9°C, corresponding to salinities from 0.167 to 1.49 equivalent weight% NaCl, respectively (average: 0.62 equi- valent weight% NaCl) (Bodnar et al. 1989), but with no clear difference between the two samples. Temperatures of homogenisation occurred between 94°C and 150°C and bimodality in temperature is suggested from the data (see Fig. 2, groups A and B). Group A and group B inclusions gave average homogenisation temperatures of 108°C and 141°C, respectively.
Group B inclusions in quartz showed in three cases clear
indications (ragged outline of meniscus between vapour
and liquid) of the formation of a clathrate hydrate after initial ice melting, indicating the presence of trace amounts of volatiles such as hydrocarbons in the entrapped group B fluids. However, although no temperature of dissolu- tion of the hydrate could be obtained and hence the iden- tity of the volatile component could not be established, its formation suggests that the higher temperatures of homogenisation obtained for group B inclusions may re- flect trace concentrations of hydrocarbons in the vapour phase of these inclusions.
Fig.
1. Examples of typical morphologies of aqueous liquid-vapour
fluid inclusions in quartz from core 1 (2380 m) from Lopra-1/1A.
Fig. 2. Salinity versus liquid homogenisation temperatures of aque-
ous inclusions in quartz from Lopra-1/1A.
10 µm
GEUS Bulletin no 9 - 7 juli.pmd
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121
Fluid inclusions in calcite
Only very few measurements were made on inclusions in
calcite, as most inclusions occurred along well-defined healed fracture-planes so are secondary in origin. Final melting temperatures varied between -0.2°C and -0.7°C, corresponding to salinities of 0.33 to 1.16 equivalent weight% NaCl. Homogenisation temperatures varied from 101°C to 186°C (Fig. 2). However, the higher homoge- nisation temperatures might conceivably reflect partial de- crepitation (stretching) of the inclusions during heating (e.g. Bodnar & Bethke 1984).
Two-phase (liquid-vapour) hydrocarbon inclusions were
observed in fluorescence microscopy in both calcite and
quartz. The abundance of hydrocarbon inclusions appears to be relatively higher in calcite and only very few were observed in quartz. The emission colours, from orange- yellow to green, may be interpreted roughly in terms of compositions corresponding to API gravities of 20-35 (Lang & Gelfand 1985). No successful heating and cool- ing runs were, however, obtained on the hydrocarbon in- clusions in the two samples.
Interpretation of fluid inclusion data
An interpretation in terms of pressures and temperatures
for entrapment of the fluid inclusions in quartz is shown in Fig. 3.
Isochores corresponding to group A and B inclusions
(Brown 1989) in quartz are shown in Fig. 3, assuming
them to be pure aqueous fluids with salinities as indicated by the final ice melting temperatures (Fig. 2). No indica- tions of the entrapment of boiling aqueous fluids were observed during this study and the homogenisation tem- peratures for the fluid inclusions observed are thus con- sidered to be minimum temperatures of fluid entrapment and host mineral formation. A comparison with geother- mal gradients of 20°C/100 bars and 20°C/226 bars that are considered relevant for Lopra-1/1A and that reflect hydrostatic and lithostatic conditions, respectively, has been made in Fig. 3. If hydrostatic conditions prevailed, group A inclusions would indicate entrapment at around 140°C at pressures of around 600 bars.
Microthermometry indicated that Group B inclusions
may contain traces of hydrocarbons and the isochores
shown in Fig. 3 are therefore not strictly applicable be- cause they assume an aqueous-only composition. As trace concentrations of hydrocarbons are present in group B inclusions, pressures at homogenisation will be consider- ably higher than indicated by the isochores drawn in Fig. 3.
The presence of only a few parts per thousand methane in
solution would shift homogenisation pressures to values of 400-600 bars at the observed temperatures of homog- enisation (Hanor 1980). The actual isochoric path for group B inclusions should therefore be shifted to a setting essentially parallel to that shown but starting at the bub- ble-point curve for the actual aqueous-hydrocarbon sys- tem at around 400 bars (Fig. 3, point A). If this interpre- tation is valid, both groups of inclusions in quartz indi- cate minimum entrapment of fluids slightly different in composition at conditions of about 400 bars and 140°C. Assuming hydrostatic conditions, probable entrapment of both group A and B aqueous fluids low in salts (average 0.61 equivalent weight% NaCl) and containing occasional traces of hydrocarbons occurred at around 600-700 bars at temperatures of 160°C to 170°C. However, more data would be needed to substantiate this conclusion.
Concluding remarks
Although it is of a reconnaissance nature, the present study
of fluid inclusions in fracture-filling quartz and calcite indicates that the basaltic sections represented by the sam- ples examined were subjected to temperatures of 160°C to 170°C and pressures of 600-700 bars at stages during their burial. During these burial conditions, precipitation of quartz and calcite in fractures (and vugs?) occurred in the presence of low-salinity aqueous fluids containing oc- casional traces of hydrocarbons. Similar P-T-fluid-char-
0
500
1000
1500
2000
0
100
200
300
Temperature, °C
Pr
essur
e
,
bars
Hydrostatic
gradient
20°C/226 bars
Hydrostatic
gradient 20°C/100 bars
Bubble-point curve for
H
2
O-0.2 mole % CH
4
A
group A
group B
Isochore
for aqueous inclusions
Fig. 3. Pressure-temperature diagram with isochores for groups A
and B inclusions in quartz from Lopra-1/1A. The open and filled circles show pressure and temperature at homogenisation for pure aqueous and aqueous-0.2 mole%CH4 fluids in group B inclusions,
respectively. Bubble-point curve from Hanor (1980). See text for
further comments.
GEUS Bulletin no 9 - 7 juli.pmd
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122
acteristics may also be of relevance to the formation of
e.g. zeolites in these rocks. Although no clear evidence for the simultaneous existence and migration of hydrocarbon and aqueous fluids was observed, such simultaneity is sug- gested by the occasional presence of hydrocarbons in the entrapped aqueous fluids and the hydrocarbon-dominat- ed inclusions observed especially in calcite.
References
Bodnar, R.J. 1990: Petroleum migration in the Miocene Monterey
Formation, California, USA: constraints from fluid-inclusion
studies. Mineralogical Magazine 54 , 295-304.
Bodnar, R.J. & Bethke, P.M. 1984: Systematic stretching of fluid
inclusions. Fluorite and sphalerite at one atmosphere confining
pressure. Economic Geology 79 , 141-146.
Bodnar, R.J., Sterner, S.M. & Hall, D.L. 1989: SALTY: a FOR-
TRAN program to calculate compositions of fluid inclusions
in the
system
NaCl-KCl-H2O.
Computers
&
Geosciences
15
,
19-41.
Brown, P.E. 1989: FLINCOR: a microcomputer program for the
reduction and investigation of fluid inclusion data. American
Mineralogist 74, 1390-1393.
Hanor, J.S. 1980: Dissolved methane in sedimentary brines: po-
tential effect on the PVT properties of fluid inclusions. Eco-
nomic Geology 75 , 603-617.
Jensenius, J. & Burruss, R.C. 1990: Hydrocarbon-water interac-
tions during brine migration: Evidence from the composition
of hydrocarbon inclusions in calcite from Danish North Sea oil fields. Geochemica Cosmochemica Acta 54 , 705-713.
Jørgensen, O. 2006: The regional distribution of zeolites in the
basalts of the Faroe Islands and the significance of zeolites as
palaeotemperature indicators. Geological Survey of Denmark and Greenland Bulletin 9 , 123-156 (this volume).
Konnerup-Madsen, J. 1979: Fluid inclusions in quartz from deep-
seated granitic intrusions, south Norway. Lithos
12
, 13-23.
Lang, W.H. & Gelfand, J.C. 1985: The evaluation of shallow po-
tential in a deep field wildcat. Log Analyst
26
, 13-22.
Manuscipt received 15 December 1999; revision accepted 29 June 2001.
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