in Fluids and Basin Evolution (ed. Kyser, K.). Mineralogical Association of Canada Short Course Volume, 28, 160-196, (2000).
CHAPTER 7. NOBLE GASES IN OIL AND GAS FIELDS: ORIGINS AND PROCESSES
Daniele L. Pinti1 & Bernard Marty2,3
1. Département des Sciences de la Terre, Université Paris SUD XI, Bat. 504, 1e étage, 91405 Orsay
Cedex, France
2. Centre de Recherches Pétrographiques et Géochimiques, 15 Rue Notre-Dame des Pauvres, B.P.
20, 54501 Vandoeuvre les Nancy Cedex, France
3. Ecole Nationale Supérieure de Géologie, Rue du Doyen Roubault, 54501 Vandoeuvre les Nancy
Cedex, France
lations. During the past twenty years, there
has been a large effort by the scientific
community
to introduce new tracing
techniques used in inorganic geochemistry for
application in petroleum geology (Emery and
Robinson, 1993; Cubitt and England, 1995).
Noble gases undoubtedly have a
relevant place among these new techniques.
The main characteristic of these gases (He,
Ne, Ar, Kr and Xe) is indeed to be “noble”, i.e.
chemically inert. Furthermore, they are
INTRODUCTION
Fluids have a fundamental role as
agents for migration and concentration of
hydrocarbons and metals within sedimentary
basins (e.g., Oliver, 1992; Sverjensky and
Garven, 1992; Parnell, 1994; 1998). Tracing
the origin and migration pathways of these
fluids is thus a prerequisite for correct
evaluation of the economic potential of large
basins and for refining exploration, appraisal
and development of hydrocarbon accumu-
3He/4He = 1.4 x 10-6 = Ra
21Ne/22Ne = 0.029
40Ar/36Ar = 295.5
Atmospheric component
Aquifer recharge
In-situ
Deep
Radiogenic
component
3He/4He = 0.01 Ra
4He/40Ar* = 1-100
4He/21Ne* = 9-20 x 106
3He/4He = 8 Ra
21Ne/22Ne > 0.07
40Ar/36Ar = 40000
Mantle-derived
component
Formation Water
Figure 7.1. Schematic diagram illustrating the main components of noble gases which may
be found in sedimentary fluids. The isotopic signature of noble gases He, Ne and Ar are
reported for each component. Modified from Ballentine and O’Nions (1994).
1
preserved in the mantle. Fluids flowing in
extensional
basins
(Ballentine,
1997;
Ballentine et al., 1991; 1996; Ballentine and
O'Nions,
1991)
or
continental
rifts
(Griesshaber et al., 1992) show a clear noble
gas mantle-derived isotopic signature (mainly
excesses of primordial 3He and in rarer cases
129
Xe and 21Ne). In contrast, hydrocarbon
reservoirs located in loading basins (e.g., P o
Basin, Italy; Elliot et al., 1993) do not show
any resolvable mantle noble gas component,
suggesting the thermal structure and tectonic
history of the host basin may control the
occurrence of mantle-derived noble gases
(O'Nions and Oxburgh, 1988). Mantle-derived
noble gases are often associated with a major
gas component such as methane. This does
not mean that natural gas accumulations have
a mantle origin, as has been suggested by some
authors (Gold and Soter, 1980; Gold and Held,
1987). Abiogenic CH4 in continental crustal
fluids may be less than 1% of the total (Jenden
et al., 1993). The occurrence of mantlederived noble gases in subsurface fluids is an
illustration of large-scale fluid flow, possibly
triggered by tectonic activity, which acts in
sedimentary basins and contributes to the
transport and accumulations of ores within the
crust (Ballentine et al., 1991; Sverjensky and
Garven, 1992; Pinti and Marty, 1995,
Torgersen, 1993).
In this chapter we describe the origins
of noble gases in hydrocarbon accumulations
and the physico-chemical processes responsible for their isotopic and elemental
fractionation. These processes are related t o
the physico-chemical conditions of the
hydrocarbon accumulations. Finally, we will
present some case studies to illustrate the
different responses of noble gases in
sedimentary basins affected by different
tectonic and thermal histories.
scarce, hence their common alias, the “rare
gases”. These features make them trace
elements par excellence (e.g., Ozima and
Podosek, 1983). Their elemental and isotopic
compositions can reflect exclusively the origin
of fluids in which they are dissolved or the
physical processes of transport and mixing of
these fluids within the crust (Fig. 7.1).
Three different processes can generate
noble gases in sedimentary fluids and thus give
different information on the origin and
movement of these fluids (Fig. 7.1). Noble
gases can be of atmospheric
origin
(atmosphere contains 0.94% by volume of
noble gases, mainly 40Ar) where they are
dissolved in the water at the recharge site.
Atmosphere-derived
noble gases (ANG
hereafter) successively migrate into basin
aquifers, transported by the groundwater.
Noble gases are more soluble in oil than in
water, so oil-water interaction involves
preferential partitioning of noble gases into
oil, so that their relative concentration in oil
reflects the degree of water flow in oil
reservoirs. This is an important constraint in
tracing secondary oil migration and thus the
spatial
distribution
of
hydrocarbon
accumulations within a basin (Dahlberg, 1995),
as well as on a smaller scale, the possible role
of water on hydrocarbon degradation
(Lafargue and Barker, 1988).
Nuclear
processes
involving
radioactive elements contained in rocks, such
as 235,238 U or 40K can generate isotopes of
noble gases (4He and 40Ar, respectively).
Radiogenic noble gases thus could be used as
fluid chronometers. Unfortunately, fluids are
not a closed system and mixing between fluids
of different provenance makes it difficult t o
constrain the various sources of radiogenic
noble gases, hence the problem in obtaining
absolute fluid ages (Solomon et al., 1996; Pinti
and Marty, 1998; Bethke et al., 1999). The
problem is complicated in the case of
hydrocarbon accumulation by water-oil
solubility partitioning, which can introduce
radiogenic noble gases into hydrocarbons
produced in different layers of oil reservoirs
and subsequently transported by groundwaters.
Finally, noble gases can be primordial
(hereafter “mantle-derived”), i.e. captured
during the formation in the earth and
RARE GAS COMPONENTS IN BASINS:
THEIR BEARING ON HYDROCARBON
ORIGIN AND EVOLUTION
Noble gas isotopes in natural systems
originate from a variety of sources and
processes. For the purpose of tracing sources
of hydrocarbons and associated volatile
elements and the processes having fractionated
them, it is convenient to consider only three
2
derived rocks, which requires specific
precautions when sampling and analyzing
rocks or fluids in order to minimize
contamination by air-derived volatiles. A
notable exception is helium, which escapes
from the Earth’s gravitational field. The two
isotopes,3He and 4He, have residence times in
air of ~ 0.2 Ma and ~ 3 Ma, respectively (e.g.,
Torgersen,
1989).
Consequently,
the
abundance of He in air is low, and He in
underground fluids is dominated by nonatmospheric component(s) such as radiogenic
He or mantle-derived He (see below). For the
other noble gases, it can be considered as a first
approximation that the atmospheric composition remained constant during the geological
duration of typical basin formation and
evolution (Tables 7.1 and 7.2).
Noble gas partitioning between air and
surface water follows Henry’s Law, where the
amount of a dissolved noble gas is proportional
to the partial pressure of this noble gas in the
air. The proportionality coefficient, the
Henry’s coefficient, varies with temperature
and salinity of the water. Consequently, the
amount of noble gases dissolved in water
depends on (i) the mean annual temperature at
the recharge area, (ii) the altitude which
controls the mean atmospheric pressure, and
(iii) the salinity of the recharge water. When
underground water reaches the saturated zone,
exchange with atmospheric noble gases at the
surface ceases, so that atmosphere-derived
noble gases are conservative during underground flow (e.g., Mazor, 1972).
When underground water is pumped at some
stage of the flow path, the abundances of
noble gases can be used to determine the
paleotemperature that existed when water was
at the surface, provided that the water
residence time, the recharge altitude (which,
for time periods typical of water movements
in basins, correspond generally to present-day
surface features) and the salinity of water
when it was at the surface can be estimated
(Stute and Schlosser, 1993). Conversely, if
paleotemperatures can be determined in the
absence of a time constraint for the residence
time of water, and if the above conditions are
reasonably met, then it is possible to estimate
the epoch of water recharge in the recent
Quaternary by simply comparing computed
major end-members: the atmosphere, the
sediments and/or the underlying crust, and the
mantle (Fig. 7.1). Atmospheric noble gases are
primarily introduced in basin fluids by
circulating groundwaters. At recharge areas of
basin aquifers, atmospheric noble gases dissolve
in surface waters in proportions depending on
their respective solubilities. They are then
isolated from the surface when groundwaters
leave the saturated zone, and are transported
along the water flow paths. They are
eventually
transferred
in
hydrocarbon
reservoirs during water-oil-gas exchange.
Natural radioactivity produces a variety of
noble gas isotopes in proportions depending on
residence time and on parent element contents
in reservoir rocks and in the basement. These
radiogenic isotopes therefore have the
potential to address the chronological
dimension of basin evolution. Basins often
develop during extension and stretching of the
continental crust, which often allows the
generation of magmas at depth and the release
of mantle-derived volatiles including noble
gases in those basins. Each of these
components produce noble gases with specific
isotopic compositions that can help in
addressing important problems such as
hydrocarbon-water interaction, fluid residence
time underground, or contributions of mantlederived heat or CO2 to the oil fields. For an
extensive description of sources and processes
contributing noble gases to the natural
environment, the reader should consult the
book by Ozima and Podosek (1983) and other
reviews by Lupton (1983), Farley and Neroda
(1998), Mazor and Bosch (1987), O’Nions and
Ballentine (1993) and Ballentine and O’Nions
(1994).
The atmospheric component
The atmosphere is well mixed with
respect to noble gases and, with the exception
of helium, noble gases are conservative in this
reservoir. They are contributed by degassing of
the mantle via magmatism, primarily through
volcanism along mid-ocean ridges, by the
release of volatile elements by the crust during
weathering reactions and, recently, by use of
fossil fuels. Generally, the abundance of
atmospheric noble gases in natural system is
high relative to that in crustal or mantle-
3
Table 7.1. Noble gas concentrations in dry air (modified from Ozima and Podosek, 1983)
Gas
Dry air
He
Ne
Ar
Kr
Xe
Molecular/atomic weight
(12C = 12 g/mol)
28.9644
4.0026
20.179
39.948
83.80
131.30
Molecular/atomic fraction
1
5.24±0.05 x 10-6
1.818±0.004 x 10-5
9.34±0.01 x 10-3
1.14±0.01 x 10-6
8.70±0.10 x 10-8
Table 7.2. Isotopic composition of noble gases in air (modified from compilation of Ozima &
Podosek, 1983). Uncertainties refer to the last digits.
Isotope
Helium
3
4
Neon
20
21
22
Argon
36
38
40
Krypton
78
80
82
83
84
86
Xenon
124
126
128
129
130
131
132
134
136
Isotopic ratio
Normalization
Abundance (% atom.)
1.386±0.01
10 6
1
714800
0.000140
~100
100
0.296
10.20
9.80
0.0290
1
90.50
0.268
9.23
0.3384±6
0.0553
100
1
0.1869
295.5
0.3364
.0632
99.60
0.6087±20
3.9599±20
20.217±4
20.136±21
100
30.524±25
1.994
12.973
66.23
65.97
327.6
100
0.3469
2.2571
11.523
11.477
57.00
17.398
0.3537±11
0.3300±17
7.136±9
98.32±12
15.136±12
78.90±11
100
38.79±6
32.94±4
2.337
2.180
47.15
649.6
100
521.3
660.7
256.3
217.6
0.0951
0.0887
1.919
26.44
4.070
21.22
26.89
10.430
8.857
paleotemperatures to regional variations of
temperatures through time (Pinti et al., 1997).
If
the
water flow
encounters
hydrocarbon reservoirs during its path, then
water and hydrocarbons will exchange noble
gases. In the ideal case, equilibrium exchange
may be reached, and noble gases will partition
among the two (water plus oil) or three (water
plus oil plus gas) phases according to their
respective solubilities as well as the volumes of
phases involved in the exchange (such
exchanges are formalized in the following
4
stones), the Th/U and K/U ratios do not vary
greatly, so that the production ratios of noble
gas isotopes within the rocks are well
constrained (Table 7.5). However the release
efficiencies are very different for the lightest
noble gas helium, relative to other noble gases
such as argon. The isotope 40Ar* is produced
by electron capture during the decay of 40K
with limited energy recoil, and 40Ar* generally
resides in minerals rich in potassium such as Kfeldspars and micas. The activation energy
necessary to release this isotope is quite high,
so that 40Ar* is retained for temperatures
below 250-300°C, depending on the mineral
(e.g., Ballentine et al., 1994).
Such
temperatures are below those of most basins,
so that large amounts of 40Ar* may be
diagnostic of fluids contributed from the
underlying crust (e.g., Torgersen et al., 1989;
Ballentine et al., 1991; Hiyagon and Kennedy,
1992; Pinti and Marty, 1995). In contrast,
radiogenic helium is readily extracted from
rocks even at low temperatures for three
reasons. Firstly, it is produced from the decay
of 235-238U, which is often deposited on grain
surfaces of minerals in basins since uranium
dissolved in flowing waters precipitates in
reducing conditions. Alternatively, U is
present in small-grained accessory minerals
and the produced 4He can diffuse out quite
easily (Dewonck, 2000). Secondly, 4He is
ejected from the site of decay with an alpha
track of 10-30 microns (Torgersen, 1980),
allowing direct recoil into the cement phase or
directly into the pore water. Third, the
‘closure’ temperature for He (the temperature
below which He can migrate significantly) is
generally low depending on the mineralogy of
the U, Th-bearing rocks, frequently below
100°C for apatite, muscovite and sanidine
(Lippolt and Wegel, 1988). A comparison
between the content of helium measured in
basin rocks of different lithologies and the
computed
He
content
produced
by
radioactivity since the time of sediment
deposition shows that basin rocks have lost 2
to 3 orders of magnitude the in-situ produced
radiogenic He (Tolstikhin et al., 1996;
Dewonck, 2000). It follows that the 4He/40Ar*
ratio can be used as a tracer of the region of
production in a basin. Noble gases produced
within aquifer or reservoir rocks will have high
section). The result of these processes is that
atmospheric noble gases can be found in
hydrocarbons in proportions imposed by
different phases. Since the atmospheric input
signal can be inferred quite precisely, it is then
possible to
evaluate
water-hydrocarbon
interactions and estimate the volumes of
different phases involved (Bosch and Mazor,
1988; Zaikowski and Sprangler, 1990;
Ballentine et al., 1991; Pinti and Marty,
1995).
The radiogenic component
Natural radioactivity of 235, 238U, 232 T h
40
and K produces a number of noble gas
isotopes either directly, or through induced
nuclear reactions. Selected reactions of interest
for the present subject are listed in Tables 7.3
and 7.4. Notably, U and Th decay produces
4
He and, via neutron activation of 6Li, tritium
(3H) which decays with a half-life of 12.3
years to 3He. Because the yield of the last
reaction is low, the 3He/4He ratio of the
radiogenic component is low, generally of the
order of 10 -9-10 -8 depending on the relative
abundances of U, Th versus Li and on other
neutron absorbing elements. Such isotopic
compositions characterize fluids evolving in
crustal environments away from zones of
recent magmatism. Other isotopes of interest
produced by natural radioactivity include 21Ne,
40
Ar and 131-136Xe. Because air contains
significant amounts of these isotopes, it is
often difficult to extract the radiogenic signal
from the atmospheric one. One way to do so is
to consider 21Ne, 40Ar and 131-136Xe isotopes in
excess of those present in the atmosphere
(labeled with an asterisk to distinguish them
from those derived from air).
Since we are often dealing with gases
which have
been fractionated
during
underground processes, it is interesting t o
consider the ratios between selected isotopes,
such as those
produced by natural
radioactivity, for example, 4He/21Ne* or the
4
He/40Ar* ratios. The ratios between two
radiogenic isotopes are fixed by the relative
contents of the radioactive parents in the host
rocks, and by the efficiencies of transfer for
each noble gas from the sites of production in
rocks to underground fluids. For given
lithologies (e.g., carbonates, shales, sand-
5
Table 7.3. Main decay reactions producing noble gas isotopes (modified from Ozima & Podosek,
1983). The parent element contents in the production rate formula are in ppm.
Parent
Type of decay
isotope
Radio-nuclides
238
U
?
238
U
Fission
235
U
?
232
Th
?
40
K
Electron capture
Extinct radio-nuclides
129
I
?
244
Pu
Fission
Production
(atom/atom)
Decay
constant
Production rate
mole g-1 a-1
Daughter
isotope
8
3.5x10 -8
7
6
0.1024
1.55125x10 -10
1.55125x10 -10
9.4895x10 -10
4.9475x10 -11
5.5407x10 -10
5.18x10 -18 [U]
2.28x10 -26 [U]
2.11x10 -19 [U]
1.28x10 -18 [Th]
1.73x10 -22 [K]
4
1
7.0x10 -5
4.63x10 -8
8.45x10 -9
-
129
He
Xe
4
He
4
He
40
Ar
136
136
Xe
Xe
Table 7.4. Main nuclear reactions producing noble gas isotopes. The parent element contents in the
production rate formula are in ppm
Target
6
Li
17
O
18
O
24
Mg
25
Mg
19
F
35
Cl
36
Cl
Reaction
(n, ?)3H(? ?)
(?, n)
(?, n)
(n, ?)
(n, ?)
(?, n)22Na(? ?)
(?, p)
(?)
Product
3
He
20
Ne
21
Ne
21
Ne
22
Ne
22
Ne
38
Ar
36
Ar
Production rate (mole g-1 a-1)
(2.69x10 -4[U] + 6.4x10-5[Th])[Li] x 10-23
(3.12x10 -4[U] + 1.7x10-4Th]) x 10-22
(3.5x10 -4[U] + 1.62x10-3[Th]) x 10-21
(4.82x10 -5[U] + 2.68x10-5[Th]) x 10-22
(1.38x10 -4[U] + [Th]) x 10-23
1.92x10 -30[U] [F]
-
4
He/40Ar* ratios (commonly, 100 or higher),
whereas noble gases derived from deeper
regions will have much lower 4He/40Ar* ratios,
close to the radiogenic production range (1-10
for common crust-forming rocks). Such
generalization deserves care, since other
processes such as those described above may
allow release of noble gases in different
proportions (e.g., mineral alteration: Torgersen and Clarke, 1985). However, for hydrocarbon systems, variations in 4He/40Ar* ratios
among different phases or different reservoirs
have proven to reflect large-scale processes, as
will be shown by several examples in the last
section.
which have isotopic signatures different from
those of atmospheric volatiles (Fig. 7.1). Part
of these differences are due to radioactive
production and accumulation of noble gas
isotopes in the mantle (e.g., 4He, 21Ne, 40 Ar,
129 Xe), imposing chronological constraints
on the kinetics of volatile exchange between
the atmosphere and the mantle (Lupton,
1983; Mamyrin and Tolstikhin, 1984; Marty,
1989; Ozima and Zanhle, 1993; Sarda et al.,
1988; Staudacher and Allègre, 1982; Allègre et
al., 1986; Zhang and Zindler, 1989). But some
of these isotopic "anomalies" cannot result
from nuclear reactions within the Earth and
reflect heterogeneities in the Earth-Atmosphere system, which were established early in
the Earth's history.
Mantle-derived noble gases
Since the discovery of primordial 3He
in mantle-derived rocks and fluids (e.g., Craig
et al., 1975; Lupton, 1983), it is now well
established that the mantle contains significant
amounts of volatiles, notably noble gases,
3
He
Basins and their hydrocarbon reservoirs often
contain significant amounts of mantle-derived
noble gas isotopes, although these tend to be
6
Table 7.5. Examples of U, Th, K and Li content in core rocks from the Paris Basin, France. The
3
He/4He and the 4He/40Ar* isotopic ratios resulting from radiogenic production in these rocks have
been computed using relevant equations from Ballentine et al. (1991). The U, Th, Li and K data are
from Marty et al. (1993) and Pinti (1993). For comparison, we give also the isotopic ratios in oilfield waters obtained from measurements in oil-brine mixtures and corrected for phase fractionation
(see Pinti and Marty, 1998, for details).
Drillhole
Lithology/Formation
U, ppm Th, ppm Li, ppm K, ppm
R/Ra
4
He/40Ar*
Middle Jurassic (rocks)
J1
Limestone/Bathonian
J7
Limestone/Bathonian
1.00
1.00
1.53
1.00
5
2
300
1500
0.002
0.001
142.7
25.8
Middle Jurassic (oil-field waters)
J4
Limestone/Bathonian
J5
Limestone/Bathonian
-
-
-
-
0.027
0.051
39.3
15.7
1.00
2.25
2.66
5.89
20
31
34500
28800
0.007
0.011
1.5
4.0
-
-
-
-
0.091
0.088
4.3
7.2
1.80
5.87
7.20
6.01
20
86
25900
40500
0.013
0.032
4.3
5.3
Trias (rocks)
T2
T7
Sandstone/Keuper
Sandstone/Keuper
Trias (oil-field waters)
T2
Sandstone/Keuper
T3
Sandstone/Keuper
CRUST
Average
Core, basin center
relative to 4He, suggesting a source external t o
the sedimentary sequence. A pure radiogenic
He component from the crust does not have
high enough 3He /4He ratios in the range
observed for these fluids, and the best
explanation is therefore the addition of a small
mantle He component. Helium sampled in
lavas and hydrothermal fluids at mid-ocean
ridges have constant 3He /4He ratios of 8±1
Ra, which are interpreted as representing the
convective region of the mantle, often
referred to as the upper mantle (e.g., Craig et
al., 1975; Lupton, 1983). The helium isotopic
ratios of mantle xenoliths sampled in
continental areas show ratios close to this endmember (5-8 Ra; Dunai and Baur, 1995;
Matsumoto et al., 1998), indicating that the
sub-continental lithospheric mantle (SCLM) is
also rich in 3He relative to the crust. The 4
orders of magnitude variation of the He
isotopic ratios in nature make this element a
masked by the generally high background of
atmosphere-derived isotopes. The most prominent mantle signal is represented by 3He in
excess of what could have been accumulated by
radioactive production in the sediments or in
the crust. Models for radioactive production
through decay and other nuclear reactions
(Tables 7.3 and 7.4) predict that 3He/4He
ratios characteristic of sedimentary or crustal
origins should range within 0.03-0.001 Ra
(where Ra is the 3He/4He ratio of air, 1.38 x
10 -6; Lupton, 1983). Table 7.5 lists U, Th, Li
and K abundances measured in sedimentary
rock cores from the Paris Basin, France. Using
equations given in Tables 7.3 and 7.4, it is
possible to compute the production of 3He
/4He ratios for each lithology, which are within
the range of radiogenic He isotopic ratios
given above. Helium sampled from wells
tapping oil fields in the respective lithologies
is enriched by one order of magnitude in 3He
7
fluids indicates the generation and the
degassing of magmas at depth. The geographical distribution of excess 3He can largely
exceed that of magmatism since noble gases
can be transported over large distances by
moving underground fluids (O'Nions and
Oxburgh, 1988; Marty et al., 1992).
very sensitive tracer of mantle contributions.
In the example of the Paris Basin given above,
the contribution of mantle He to total He is as
small as ~1%, yet the occurrence of such a
component is resolvable without difficulty.
Crustal fluids contain helium with
isotopic signatures resulting from different
contributions of the mantle, the atmosphere,
and the crust or sediments, and the 3He /4He
ratios have been successfully used to identify
mantle-derived helium in several sedimentary
basins (e.g., Hooker et al., 1985; Oxburgh et
al., 1986; Torgersen et al., 1987; Martel et al.,
1989; Marty et al., 1992; O'Nions and
Ballentine, 1993). The helium isotopic ratios
of basinal fluids show variations between a pure
radiogenic end-member and an upper mantle,
or SCLM end-member (Fig. 7.2a-c). Figure
7.2a-c represents the statistical distribution of
He isotopic ratios in fluids sampled in different
basins worldwide. In the case of Europe, basins
developing in extensional domains such as the
Pannonian Basin, the Vienna Basin, or the
North Sea rifting systems, show the highest
mantle-3He contributions, whereas a loading
basin like the Po Basin in the foreground of
the Alpine orogenesis contains purely
radiogenic helium (Marty et al., 1992, Elliot et
al., 1993). In many gas fields of Canada and
the USA, mantle He is found (e.g., Hiyagon
and Kennedy, 1992; Jenden et al., 1993), and
the highest mantle signals are observed in
hydrocarbon fields trapped in the Green Tuffs
of Northeast Japan (Wakita and Sano, 1983).
The detection of mantle-derived
helium in sedimentary fluids (water and
hydrocarbon) suggests the occurrence of largescale upward fluid migration and helium sources
external to the aquifers. Moreover, the
presence of mantle-derived 3He implies the
occurrence of still active tectonics underlying
the concerned basins, since mantle melting and
the generation of magma are required t o
transport efficiently 3He and other volatiles t o
the Earth's surface. Noble gases in the mantle
are trapped in minerals and, without magma
generation and transport, could not reach the
Earth's surface because diffusion even at
mantle temperatures would not
allow
significant displacement during geologic time
(e.g., Lupton, 1983). It is likely that the
occurrence of mantle-derived 3He in basin
CH4 / 3He
There have been several theories
proposing an abiogenic origin for some of the
hydrocarbons present in the crust and, among
them, the possibility that methane is
continuously released by the mantle and
injected into the deep crust at lithospheric
plate boundaries, ancient suture zones and
other areas of crustal weakness (Gold and
Soter, 1980; Gold and Held, 1987). The
occurrence of mantle-derived helium, up t o
100 % of the total He, has been taken as one
of the strongest arguments in favor of this
hypothesis. Indeed the release of noble gases is
accompanied by that of other volatiles,
including carbon and nitrogen. As proposed by
Jenden et al. (1993), CH4/3He ratios in natural
gases can be used to evaluate the possible
abiogenic origin of hydrocarbons (Fig. 7.3).
Methane is released at mid-ocean ridge
hydrothermal vents together with He, with a
molar CH4/3He ratio of approx. 3 x 10 6 . From
the CH4/3He ratios (where 3He represents the
fraction of helium-3 in excess of that produced
by nucleogenic processes) observed in natural
gases, Jenden et al. (1993) estimated that the
fraction of abiogenic methane should be less
than 200 ppm vol. on average, and concluded
that this source is probably unimportant with
respect to that derived from biogenic activity.
C /3He
C/3He ratios provide an important
constraint on the origin of carbon within
continental systems. The C/3He ratio measured
in mantle-derived gases trapped in mid-ocean
ridge basalts is remarkably uniform between 1
and 7 x 109 (Marty and Jambon, 1987; Trull et
al., 1993; Marty and Tolstikhin, 1998). C/3He
ratios within this range are observed in some
crustal CO2-rich well gases (Chivas et al.,
1987; O'Nions and Oxburgh, 1988), which
points to an upper mantle source for this
carbon. In other continental settings, C/3He
8
(a)
(c)
upper
mantle
radiogenic
Pannonian
Basin
upper
mantle
radiogenic
(b)
upper
mantle
radiogenic
Green tuffs, Japan
Canadian shield
Vienna
Basin
North China Basin
HugotonPanhandle
North Sea
California
Subei Basin, China
Paris Basin
0.01
0.1
1
10
Songliao Basin, China
3He/4He (R/Ra )
c
Po Basin
0.01
0.01
0.1
1
0.1
1
3He/4He (R/Ra )
c
10
3 4
He/ He (R/Ra c)
Figure 7.2. Distribution of helium isotopic ratios of fluids sampled in various hydrocarbon
fields of the world. The 3He/4He ratios (R) are normalized to atmospheric 3He/4He ratio =
1.4 x 10-6. Data are corrected for contribution of atmospheric helium (subscript c). The
fields of 3He/4He ratios typical of radiogenic production in sediments and in the crust
(R/Ra = 0.02±0.01) and of mid-ocean ridge basalts and fluids (R/Ra = 8±1) representing
the upper mantle are represented by the two shaded areas. (a) European basins: The
Pannonian and Vienna basins, formed by active crustal extension, show the contribution
of mantle-derived He (typically, 1% to 50 % MOR-type He). The North Sea fields
developed within rift structures are also characterized by significant contribution of
mantle-derived volatiles. Most helium in the Paris Basin is derived from the decay of
radioactive isotopes in the sediments and in the crust, with a slight contribution of
approximately 1 % mantle He. Helium present in the Po basin, a loading basin in the
foreground of the Alpine orogenesis, is purely radiogenic. Data are from Ballentine and
O'Nions (1991); Marty et al. (1992); Oxburgh et al. (1986); Marty et al. (1993) ; Hooker
et al. (1985) ; Elliot et al. (1993) ; Hilton and Craig (1989). (b) He data for Canadian and
American hydrocarbon reservoirs. Both the Canadian shield and Hugoton-Pandhandle
gases display a range of He isotopic ratios from a purely radiogenic end-member to a
component slightly enriched in 3He of probable mantle origin. Data from Californian
hydrocarbon fields show significant enrichments in mantle 3He, in relation with the active
tectonics of the West Coast. Data are from Welhan et al. (1978); Jenden et al. (1988);
(1993); Hiyagon and Kennedy (1992); (c) Chinese and Japanese hydrocarbon fields. All
data show significant enrichments of mantle-derived 3He which, in the case of the Green
tuffs of the North-Eastern Japan, represent a practically pure arc-type helium component
dominated by mantle-derived helium. Data are from Du and Liu (1991); Du (1992); Xu et
al. (1995a); (1995b); Sano and Wakita (1983); Wakita and Sano (1983); Wakita et al.
(1990).
9
10
between 10 8 and 10 12 reflect loss of mantle C
due to reactions in the crust, and/or mixing
with other carbon end-members having
different origins.
Together with independent estimates
of the 3He flux in sedimentary basins, the
C/3He ratio measured in basin fluids allows t o
set constraints on the flux of mantle-derived
carbon. The flux of He isotopes into
underground aquifers can be computed if a flow
line for which the velocity of water is
independently estimated can be identified
along which the increase of 4He accumulation
exceeds that predicted by in-situ (e.g., pore
rock) radiogenic production (e.g., Andrews and
Kay, 1982; Torgersen and Clarke, 1985). Stute
et al. (1992) estimated the flux of 3He
1014
CH4 3He
1012
entering the deep aquifers of the Pannonian
Basin to be 2-5.4 x 10 3 atoms m -2 s-1. Using
this estimate and the measured C/3He ratios in
basin fluids, Sherwood-Lollar et al. (1997)
estimated that in the Pannonian basin, the
mantle-derived carbon flux into this basin
alone is only 4-5 orders of magnitude lower
than that of the entire mid-ocean ridge
system, and suggested that such a carbon
source
may
have
been
significantly
underestimated previously. This comparative
approach can also be used to constrain the
origin of CO2 in gas fields, an important topic
since dilution of hydrocarbons by CO2
compromises the commercial value of
hydrocarbons and the origin of CO2 in such
fields is a matter of debate. The present
Crustal end-member
R/Ra = 0.01 CH 4/ 3He = 3 x 1013
0.1 ppm
1 ppm
1010
10 ppm
1000 ppm
108
1%
10%
Sub-continental mantle end-member
R/Ra = 6 CH4/3He = 1 x 106
106
0.01
0.1
1
10
R/Ra
Figure 7.3. 3He/4He ratio (expressed as R/Ra) vs. the CH4 /3He ratio in Japanese natural
gas from the Green Tuff Region (Wakita and Sano, 1983) and from the Kamchatka
Peninsula (Kamenskiy et al., 1976). A mixing model for origin of helium and methane is
presented. Numbers indicate the proportion of magmatic methane in these gases. The
magmatic end-member is represented by fumarolic gas compositions of Volcano Usu,
Japan. Modified after Jenden et al. (1993).
10
authors have recently carried out a CO2/3He3
He/4He study on gas fields in South-East Asia
to show that the ~30% CO2 contained in
natural gases were likely to be mantle-derived
and not derived from decarbonation of marine
limestone as previously thought.
Kennedy, 1999). In this regard, anomalous
enrichment in atmospheric Xe observed in
North-American oil accumulations, possibly
derived from adsorption on shales and clays
forming source rocks, may provide constraints
on the mechanisms of expulsion and primary
migration of hydrocarbons (Torgersen and
Kennedy, 1999).
Neon and Xenon
Other isotopic anomalies (i.e. isotopic
ratios that cannot be explained by mixing
between atmospheric and crustal/sedimentary
end-members) require addition of mantlederived noble gases. Neon in mantle-derived
samples has an isotopic
composition
suggesting the occurrence of a solar Ne endmember, clearly different from that of
atmospheric neon (Ozima and Zanshu, 1988,
Sarda et al., 1988, Honda et al., 1991, Hiyagon
et al., 1992). The 20Ne isotope is not produced significantly by nuclear reactions in the
crust, and 20Ne/22Ne ratios in basin fluids
higher than the atmospheric ratio have been
taken as firm evidence for the occurrence of
mantle-derived Ne in those fluids (e.g.,
Ballentine and O’Nions, 1991; Ballentine,
1997). Interestingly, the Ne/He ratios in
mantle-derived gases stored in basin fluids
appear higher than those of mantle-derived
gases sampled at mid-ocean ridges. Such an
enrichment is apparently not due to fractionation during crustal processes and may
rather reflect the generation of small melt
fractions and partial degassing beneath
continents (Ballentine, 1997).
Likewise, the occurrence of 129Xe
excess due to the decay of now extinct 129 I
(t 1/2 = 17 Ma) in a few CO2-rich well gases
suggests a mantle origin for xenon and
presumably other volatile elements in these
gases, and has strong implications for models
of Earth's degassing (e.g., Phinney et al.,
1978, Staudacher, 1987). This has not been
used as a tracer for hydrocarbon systems and is
mainly due to the difficulty in resolving Xe
isotopic anomalies from air-derived xenon,
which is abundant relative to other noble
gases. Such relative abundances result from Xe
enrichment during solution of atmospheric
noble gases (see next section) and the
abundance of xenon in sediments, since xenon
is efficiently adsorbed on sedimentary minerals
(e.g., Fanale and Cannon, 1972; Torgersen and
NOBLE GAS FRACTIONATION
BETWEEN NATURAL FLUID PHASES
An important feature of noble gases,
which applies to oil exploration, is that their
relative abundances are modified in multiphase
fluid systems where, for example, water and
gas, or water and oil phases co-exist. The
physical chemistry of noble gases is well
known and partitioning of noble gases between
fluid phases may be predicted. This can give
constraints on the volumes of fluids involved
in this partitioning. This is a key parameter t o
understand the spatial distribution of oils
within a basin, as well as the effects of water
washing on oil degradation (Lafargue and
Barker, 1988). Here we present two simple
models of partitioning of noble gases between
a groundwater and a natural gas phase and
between water and oil.
Natural gas-water equilibrium
The model presented here is based on
the “solubility equilibrium model” developed
by Goryunov and Kozlov (1940) and refined
successively by Zartman et al. (1961), Bosch
and Mazor (1988), Zaikowski and Sprangler
(1990), Hiyagon and Kennedy (1992), Pinti
and Marty (1995), Ballentine et al. (1996),
Torgersen and Kennedy (1999). The model
assumes that the atmospheric noble gases were
initially contained in a water phase, which was
saturated with air at surface conditions. The
dissolved amount of gas depends on the
relative noble gas solubilities, which in turn are
inversely proportional to the ambient temperature (Fig. 7.4; Table 7.6). The initial
amount of atmospheric noble gases (ANG),
which can be transported from the surface into
oil reservoirs, can thus be predicted with
sufficient precision, assuming a reasonable
range of recharge temperatures (generally
between 0°C and 25°C) and of water salinities.
The latter may vary from 0 g/L of NaCl for
11
Solubility of noble gases in a light oil
(0.82 kg/m3
41 APIo)
Ki, atm.kg mol
10000
He
1000
Ne
Ar
100
Kr
Xe
10
1
0
20
40
60
80
100 120
140
Temperature, oC
Figure 7.4. Solubility of noble gases in light oil calculated from the equations of
Kharaka and Specht (1988). Note that light (He, Ne) and heavy (Kr and Xe) noble
gas solubilities have different behaviors with the increase of temperature.
fractionation has been produced by solubility
partitioning in a multiphase system (see Figs.
7.6 and 7.7).
The ANG are partitioned between the
water phase and a natural gas phase, until
equilibrium is reached. It is assumed that a
parcel of water of volume W equilibrates with
a parcel of natural gas of volume G. The preequilibration amount of noble gases in the
water parcel are similar to those of airequilibrated surface water (ASW) or airequilibrated seawater (ASS), whereas the gas
phase is initially devoid of noble gases. By
mass balance:
an Air-Saturated Freshwater (ASW hereafter)
to 35 g/L of NaCl for a Air-Saturated Seawater
(ASS hereafter). In the “solubility equilibrium
model”, elemental ratios between two noble
gas isotopes (20Ne/ 36Ar, 84Kr/ 36Ar
or
130
Xe/ 36Ar) rather than absolute concentrations are used. The reason is that the
variations in temperature and salinity change
significantly the absolute amount of noble
gases dissolved in the groundwater, but have
little effect on the relative solubilities of two
noble gas species. The ratios between two
noble gas species are always referenced to the
atmospheric-derived 36Ar isotope, which has
an intermediate solubility in oil and water
compared to those of light noble gases He and
Ne and those of heavy noble gases Kr and Xe
(Figs. 7.4 and 7.5). Thus, we should observe
distinct variations in the elemental ratios
20
Ne/36Ar on one hand and 84Kr/ 36Ar and
130
Xe/ 36Ar on the other, if the elemental
W · (Ci)ASW = W · (Ci)w + G · (Ci)g
(1)
where Ci is the concentration of the ANG
species i; superscript “ASW” refers to the
water phase prior to equilibration with gas.
Superscript “w” and “g” denote water and gas
12
Solubility of noble gases in freshwater
Ki, atm.kg / mol
10000
He
Ne
1000
Ar
Kr
Xe
100
0
20
40
60
80
100
Temperature, oC
Figure 7.5. Solubility of noble gases in freshwater (salinity zero) calculated by using
relevant equations from Smith and Kennedy (1983).
(Ci/36Ar)g = (Ci/36Ar)ASW · [(G/W) + (KArw)-1] ·
[(G/W) + (Kiw)-1]-1
(4)
phase after equilibration. As previously stated,
we prefer to work with elemental ratios of
ANG
species
rather
than
absolute
concentrations. In this case Eq. (1) become:
Two limiting cases may be defined: i) as G/W
approaches infinity, (Ci/36Ar)g approaches
(Ci/36Ar)ASW, and ii) as G/W approaches zero,
(Ci/36Ar)g approaches (Ci/36Ar)ASW (Kiw/KArw).
All other possible G/W ratios have ANG
patterns in the gas phase lying between these
two limiting cases.
W · (Ci/36Ar)ASW = W · (Ci/36Ar)w + G ·
(Ci/36Ar)g
(2)
At subsurface pressures relevant to natural gas
and oil reservoirs, the solubilities of noble
gases in liquids obey Henry’s Law. Thus:
(Ci/36Ar)g = (Ci/36Ar)w ·(Kiw/KArw)
Oil-water equilibrium
The crucial assumption is the same as
for the natural-gas-water equilibrium model.
ANG are contained initially in the groundwater, with elemental concentrations close t o
those obtained by air-equilibrium at nearsurface conditions (ASW). It is assumed that
the ANG are partitioned in equilibrium
between the water and an oil phase. Consider
the equilibration of an ANG species i between
a water parcel of volume W and an oil parcel
of volume O. By mass balance:
(3)
where Kiw is the Henry’s Law constant for
solubility of the ANG species i in water. Kiw is
a function of the water temperature and
salinity conditions at depth (where the
equilibration is produced). Solubility data for
noble gases in water (Fig. 7.5; Smith and
Kennedy, 1983) reveal that up to 100°C, Kiw
decreases with increasing mass from i = Ne to i
= Xe. Combining Eq. (2) and (3):
13
0.25
Gas/water equilibrium
20 Ne / 36Ar
(0)
0.2
Gas
25oC
(0.001)
15
0.15
residual water
(0.01)
(0.1)
Freshwater
0.1
2
3
4
5
0oC
5
130Xe/36Ar x 10-4
Freshwater
(20Ne/36Ar)ASW = 0.14
(130Xe/36Ar)ASW = 4.1 x 10-4
Gas reservoir
Figure 7.6. Gas-water equilibrium model for noble gases introduced from a recharge
area. The diagram shows the relative modifications of the 20Ne/36Ar and 130Xe/36Ar
ratios from an initial ASW value (here taken for a salinity = 0 and a temperature of
10°C) during equilibrium of the water parcel with a gas phase at depth (calculated for
a reservoir temperature of 100°C). Numbers in parentheses indicate the G/W ratios
involved in noble gas partitioning. During gas-water equilibrium, the less soluble Ne
will be preferentially partitioned in the gas phase compared to Ar, whereas the more
soluble Xe will remain in the water phase. Thus, the 20Ne/36Ar increases and
130Xe/36Ar decreases during equilibrium partition (line labeled "gas"). Inversely, if
ANG are measured in the residual water phase, 20Ne/36Ar decreases and 130Xe/36Ar
increases after equilibrium partition with the gas phase.
14
Table 7.6. Constants for calculating mole fraction solubility of noble gases in aqueous NaCl solutions
(Smith and Kennedy, 1983).
Gas i
Helium
Neon
Argon
Krypton
Xenon
A1
-41.4611
-52.8573
-57.6661
-66.9928
-74.7398
A2
42.5962
61.0494
74.7627
91.0166
105.2100
A3
14.0094
18.9157
20.1398
24.2207
27.4664
W · (Ci/36Ar)ASW = W · (Ci/36Ar)w + O ·
(Ci/36Ar)o
(5)
B1
-10.0810
-11.9556
-10.6951
-9.9787
-14.5524
B2
15.1068
18.4062
16.7513
15.7619
22.5255
B3
4.8217
5.5464
4.9551
4.6181
6.7513
Equation is: lnX = D 1 + D 2/Z + D 3lnZ where
Dj = Aj –MBj for NaCl molarity M.
X is the noble gas solubility expressed as mole
fraction. To calculate solubilities of noble
gases (K) in atm g/mole: K = 1/[(1000/18.06) ·
X]
where superscript “o” denotes the oil phase.
Equilibrium partitioning of noble gases
between oil and water should obey Henry’s
law:
(Ci/36Ar)o approaches (Ci/36Ar)ASW(KAro/KArW) ·
(Kio/KiW)-1.
There is a third model, the so-called
“Equilibrium degassing of oil” (Bosch and
Mazor, 1988), which corresponds to degassing
of an oil phase which was previously
equilibrated with a water phase. The
equilibration degassing follows Eq (3) and (4)
with the water phase substituted by an oil
phase and with the ASW phase substituted by a
WEO phase. “WEO” denotes a waterequilibrated oil phase, prior to any separation
of gas. Therefore:
(Ci/36Ar)o = (Ci/36Ar)w ·(Kiw/KArw) ·(Kio/KAro)-1
nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn(6)
(Ci/36Ar)g = (Ci/36Ar)WEO · [(G/O) + (KAro)-1] ·
[(G/O) + (Kio)-1]-1
(8)
where K io is Henry’s Law constant for the
solubility of ANG species i in oil, depending on
the temperature and the oil density. Solubility
data of noble gases in oil as a function of the
ambient temperature and oil density are
available from Kharaka and Specht, (1988).
These data (Fig. 7.5; Table 7.7) show that up
to 150°C, KiO progressively decreases with
increasing mass of i. This decrease is similar,
but more drastic that the decrease of K iW. This
means that the relative solubility of noble
gases in oils is much higher than in the water.
For example, the ratios of solubilities of He,
Ne, Ar, Kr and Xe in light oil and in pure
water at 25°C are 3, 2, 7, 12 and 24
respectively. The same ratios at 100°C are 4,
4, 9, 21 and 20, respectively (Kharaka and
Specht, 1988). Combining Eq (5) and (6):
There are two limiting cases: i) total degassing
(G/O approaches infinity). In this case
(Ci/36Ar)g approaches (Ci/36Ar)WEO; and ii)
infinitesimal degassing (G/O approaching
zero). In this latter case (Ci/36Ar)g approaches
(Ci/36Ar)WEO (Kio/KAro).
ANG as a new aid for oil exploration and
reservoir evaluation
Crucial information for refining oil
exploration or developing reservoir evaluation
can be obtained from the measurement of
ANG in natural gas and oil accumulations, or
in groundwater having had equilibrated with
them (Figs. 7.6-7.7).
In the first case, the measurements of the
ANG in the hydrocarbons accumulations aid in
estimating: i) the amount of water which was
in contact with the hydrocarbon accumulation;
ii) the hydrocarbon residence time in the
reservoir (i.e. the timing of the secondary
hydrocarbon migration), by integrating the
mean G/W or O/W ratio with the residence
time of water that flowed through the
accumulation (Pinti and Marty, 1995). In the
(Ci/36Ar)o = (Ci/36Ar)ASW · [(O/W) +
(KAro/KArW)] · [(O/W) + (Kio/KiW)]-1
(7)
Two limiting cases may be defined: i) as O/W
approaches infinity, (Ci/36Ar)o approaches
(Ci/36Ar)ASW, and ii) as O/W approaches zero,
15
0.20
25oC
residual water
Freshwater
20Ne / 36Ar
0.15
15
0oC
1
0.10
(0.1)
Oil
0.05
(0.01)
(0.001) (0)
Oil/water equilibrium
0
0
5
10
15
20
25
30
130Xe/36Ar x 10-4
Freshwater
(20Ne/36Ar)ASW = 0.14
(130Xe/ 36Ar)ASW = 4.1 x 10-4
Gas reservoir
Figure 7.7. Oil-water equilibrium model for noble gases introduced from a recharge
area. The diagram shows the relative modifications of 20Ne/36Ar and 130Xe/36Ar
ratios from an initial ASW value (here taken for a salinity = 0 and a temperature of
10°C) during equilibrium of the water parcel with an oil phase at depth (calculated for
a reservoir temperature of 100°C). Numbers in parentheses indicate the O/W ratios
involved in the noble gas partitioning. During oil-water equilibrium, Ne will be
preferentially retained in the water phase compared to Ar, whereas the more soluble
Xe will be transferred to the oil phase. Thus, the 20Ne/36Ar decreases and 130Xe/36Ar
increases in oil during equilibrium partition (line labeled "oil"). Inversely, if ANG are
measured in the residual water phase, 20Ne/36Ar increases and 130Xe/36Ar decreases
after equilibrium partition with the oil phase. Note the larger elemental fractionation
of noble gases when an oil phase is involved compared to the case of gas-water
equilibrium (Fig. 7.6). This is due to the greater noble gas solubility difference
between oil and water.
second case, from the measurements of ANG
in the residual water, we can obtain three
crucial pieces of information for refining an
oil survey: i) the nature of the hydrocarbons
encountered by the water in the area; ii) the
volume of the hydrocarbon accumulations (if
the volume of the water has been correctly
estimated);iii) the position of the hydrocarbon
16
Table 7.7. Coefficients a and b for calculating solubility (K) of noble gases in crude oils, depending
their average density (Kharaka and Specht, 1988). The standard error s on the log(K) is also
reported.
Gas i
a
b
s
3
Heavy oil (0.89 kg/m ; 27.5 API gravity)
Helium
3.250
-0.0054
0.0370
Neon
3.322
-0.0063
0.0386
Argon
2.121
-0.0003
0.0092
Krypton
1.607
0.0019
0.0029
Xenon
1.096
0.0035
0.0077
Light oil (0.0.82 kg/m3; 41 API gravity)
Helium
3.008
-0.0037
0.0247
Neon
2.912
-0.0032
0.0425
Argon
2.030
0.0001
0.0054
Krypton
1.537
0.0014
0.0169
Xenon
0.848
0.0052
0.0148
Equation is: log (K) = a + bt where t is the temperature of the oil reservoir in degree Celsius.
accumulation (if we know the main direction
of water flow).
As briefly discussed in the previous
section, enrichments of Xe in oil and gas
accumulation cannot always be explained by
preferential partitioning between a water and
an oil/gas phase (Torgersen, 1989). Pinti and
Marty (1995) showed that 132Xe/ 36Ar ratios of
up to 100 times the ratio in air observed in
Triassic oils of the Paris Basin could be
explained by Rayleigh distillation following oil
degassing. However, Torgersen and Kennedy
(1999) measured 132Xe/ 36Ar as high as 576
times the ratio in air in hydrocarbons from the
Elk Hills Naval Petroleum reserve in
Bakersfield, Ca. These ratios represent the
largest relative Xe-enrichments ever observed
in terrestrial fluids. The authors concluded that
these enrichments are derived from adsorbed
Xe on the source rocks, generally shales. The
Xe enrichment implies that oil has migrated
from source rocks as a separate phase, and
indicates a minimal role for water in
hydrocarbon expulsion and migration. Thus,
studies of ANG elemental fractionation may
give valuable information on the expulsion
mechanism and the primary migration of
hydrocarbons from a source rock.
UNSCRAMBLING HYDROCARBON
PROCESSES: CASE STUDIES
The
isotopic
and
elemental
composition and fractionation of noble gases
in sedimentary fluids is largely controlled by
the tectonic and thermal history of the host
basin. Cenozoic extensional basins contain
fluids enriched in mantle-derived noble gases
such as 3He derived probably by processes of
underplating (O'Nions and Oxburgh, 1988;
Torgersen, 1993). Intense tectonic activity
probably is the trigger for large-scale vertical
transport of these deep-seated gases into
subsurface traps such as hydrocarbon
accumulations (Torgersen and O'Donnell,
1991). It is expected that these basins, at the
early stage of formation, are also affected by
large-scale horizontal fluid-flows induced by
large hydraulic gradients. Increased groundwater flow should be thus recorded by ANG in
hydrocarbon
accumulations
secondary
migration of oil was synchronous with these
initial episodes of fluid flow.
In older extensional basins or in younger
loading basins, the mantle-derived noble gas
signature should be absent in the sedimentary
fluids or partially masked by the addition of
large amounts of in situ produced radiogenic
noble gases, possibly transported by very old
groundwaters (Pinti and Marty, 1998). In this
17
second case however, the hydrocarbon
ANG signal introduced by meteoric waters if
they were not washed partially or totally out
the basins during previous hydrodynamic
events (Pinti et al., 1997).
We will restrict our discussion to three
case histories of noble gases in sedimentary
basins of Europe, at different stages of their
evolution. These are the Pannonian Basin,
Hungary; the Paris Basin, France and the P o
Basin, Italy (Table 7.8).
accumulations should have preserved the
the total helium), which have been confirmed
by noble gas studies of adjacent gas fields
(from 16 to 39.8% of mantle-derived helium
in Kismarja and Szeghalom-S gas fields;
Sherwood-Lollar et al., 1997). This mantlederived helium is accompanied by large
amounts of radiogenic noble gases 40Ar* and
21
Ne* (Table 7.8). The 40Ar/ 36Ar and
21
Ne/22Ne ratios vary from their atmospheric
values (295.5 and 0.029 respectively; Ozima
and Podosek, 1983) at the shallowest levels in
the field to progressively higher 40Ar/ 36Ar and
21
Ne/ 22Ne ratios of 1700 and 0.045 (Fig. 7.9).
This has been interpreted by Ballentine et al.
(1991) as the result of vertical transport of
deep-seated noble gases (probably produced in
the pre-Miocene basement) up to shallower
levels, and the mixing with local meteoric
waters enriched in atmospheric-derived noble
gases. The radiogenic 4He/40Ar* ratios could
confirm the occurrence of radiogenic gases
produced in a deeper and hotter basement
rather than in the shallower gas reservoir
rocks. Indeed the 4He/40Ar* ratios in fluids
should be close to that of production in the
basement rocks. The results, however, show
4
He/40Ar* ratios ranging from those of a
crustal production (4He/40Ar* = 1-4) up to 6
times higher. The correlation between the
4
He/40Ar* ratios and the atmospheric-derived
20
Ne/36Ar ratios (Fig. 7.10) suggests that the
variations in the 4He/40Ar* ratios are not a
“source effect”. They are indeed the result of
the
solubility
partitioning
between
groundwater and gas of a well-mixed and
unique source of ANG and deep crustalproduced radiogenic gases having 4He/40Ar*
ratios close to crustal values (4He/40Ar* = 110; Table 7.5). During the partitioning of
noble gases between a water and a gas phase,
He, Ne are preferentially released in the gas
phase relative to Ar because of their lower
solubility in liquids than that of Ar. This
produces the observed increase in atmospheric
20
Ne/36Ar ratios as well as in the radiogenic
4
He/40Ar* ratios.
The minimum volume of groundwater required
to supply the atmosphere-derived noble gases
in the methane accumulations of the
Hajduszoboszlo field occupy a rock volume of
some 1000 km 3 (assuming an average basin
Pannonian Basin, Hungary: a Cenozoic
extensional basin
The Pannonian Basin, together with
the Paris Basin, has been extensively studied
for noble gases in aquifers (Oxburgh et al.,
1986; Martel et al., 1989; Stute et al., 1992)
and gas and oil fields (Ballentine and O'Nions,
1991; Ballentine et al., 1991; Stute et al.,
1992; Sherwood-Lollar et al., 1997; SherwoodLollar et al., 1994). The Pannonian Basin
occupies the area of the Great Hungarian Plain
and developed in the Middle Miocene by
extension, with a system of deep basins
separated by shallower basement blocks and
associated volcanism. It consists mainly of
Pliocene to Quaternary sediments deposited
upon a pre-Miocene basement. Oil and gas
fields are distributed widely within the basin,
and the natural gas ranges in composition
from near pure CH4 to near pure CO2 with
different N 2 amounts (Fig. 7.8). The present
hydrogeological regime in the Pannonian
Basin is topographically driven with an upper
flow consisting of low salinity water which is
connected locally to a lower aquifer system
containing saline, possibly connate water. The
system discharges into the Tisza River at a
rate which has been estimated at ca. 35 m3s-1
(Fig. 7.8; Martel et al., 1989).
Noble gas studies focused on the
Hajduszoboszlo field in eastern Hungary
(Ballentine and O'Nions, 1991; Ballentine et
al., 1991). The reservoir is stacked above a
basement high adjacent to the Derescke subbasin over a depth interval between 700 t o
1300 meters. The reservoir rock volume is 1.5
km 3 and it contains 0.37 km 3 of natural gas
methane at reservoir pressure. The most
striking feature is the occurrence of mantlederived 3He in the gas fields (from 2 to 5% of
18
Table 7.8. Geological features of sedimentary basins in Europe, and noble gas signatures in gas and oil
fields. References: Ballentine et al. (1991); Elliot et al. (1993); Pinti and Marty (1995); SherwoodLollar et al. (1997).
Basin
Geological features
Location
Age
Tectonic context
Heat Flow
(mW m-2)
Geothermal gradient
(°C/Km)
Gas/oil type
Pannonian
Basin
Paris Basin
Po Basin
Hungarian Plain
Miocene
Extensional
Northern France
Mesozoic
Extensional
Northern Italy
Plio-Quaternary
Loading
100
60-75
= 40
35
33
18
CH4
Paraffinic
CH4
0.020-0.113
0.0298-0.0379
298-699
1-28
0.027-0.043 Ra
0.0292-0.0293
291-302
-
240
0.81
2400
0.54
MANTLE AND RADIOGENIC NOBLE GAS FEATURES
3
He/4He
0.183-0.384
21
Ne/22Ne
0.0299-0.0431
40
Ar/36Ar
340-1680
4
He/40Ar*
1-25
ATMOSPHERIC NOBLE GAS FEATURES
Volume of HC+water (km3)
0.367
Volume of flowed water
17-50
estimated with ANG (km3)
oolitic limestone (Bathonian) and marly
“alternations” (Bajocian), and the Lower
Cretaceous (Albian) green sandstone. Confined
aquifers have been found in Devonian fractured
gneisses and amphibolites in the southern
basement at Couy. The Middle Jurassic aquifer
is separated from the Triassic by 400-700
meters of low-permeability Lower Jurassic
(Lias) mudrocks and shales, which are the
source-rocks of oils in the Paris Basin. The
Middle
Jurassic
contains
low-enthalpy
geothermal waters (from 50 °C to 80°C) and
oil accumulations. Oils are contained in the
Bathonian and Callovian carbonate sequences.
The Triassic aquifer contains waters at
temperatures up to 120 °C associated with oil
accumulations. Primary migration of oil took
place from the Liassic shales upward to the
Jurassic limestone and laterally to the Triassic
sandstone, during the Paleocene-Oligocene
(Espitalié et al., 1988). Vertical faults affecting the Mesozoic cover of the Paris
Basin and reactivated by tectonic post-Alpine
stresses - have played an important role in oil
porosity of 5%). This is a factor of 670
greater than the reservoir volume (Ballentine
et al., 1991). This clearly indicates that this
young Neogene basin has been affected by
important hydrodynamic episodes during
and/or after the secondary migration of gas
methane. Measured CH4/36Ar ratios from 1.1
to 3.5 x 10 6 in the natural gas accumulations
are close to those predicted for a CH4saturated water at reservoir conditions. This
relationship between the CH4 and ANG at
Hajduszoboszlo field seems to suggest that
groundwater may also have been the carrier
phase for gaseous hydrocarbons from
source rocks to the traps (Ballentine et al.,
1991).
Paris Basin, France: a quiescent
Mesozoic extensional basin
The Paris Basin is a post-Variscan
intracratonic basin where a multi-layered
aquifer system has developed (Fig. 7.11). The
main aquifers are the Upper Triassic (Keuper)
Chaunoy fluvial sandstones, the Middle Jurassic
19
la
Mo
s
Vienna Basin
sin
Ba
se
0
20o
22o
A
Transylvanian
Basin
400 km
Eger
3
HAJDUSZOBOSZLO
Derecske Sub-Basin
3
Danube R.
Lk
.
Ba
la
to
n
47o
18o
Debreen
KISMARJA
EBES
Budapest 3
LHP
48 o
3
B
3
SZEGHALOM-S
Bekes Basin
5
7 5
7
Exposed basement
Oil and gas fields
SZEGHALOM-N
3
Szeged
3
Basement isopach (km)
45o
Danube R.
Tisza River
A
B
Depth (m)
200
1000
2000
0
100
km
1
2
3
4
5
6
7
Figure 7.8. Schematic map illustrating the distribution of oil and gas fields in the
Pannonian Basin and the location of the Hajduszoboszlo, Kismarja and Szeghalom-S
gas fields (top). Generalized cross section of the Pannonian Basin (A-B) showing the
geology of the area together with main fluid flow directions. 1, fluid flow directions; 2,
Quaternary; 3, upper Pliocene; 4, Pliocene; 5, middle Miocene; 6, Miocene volcanic; 7
Mesozoic. Modified from Sherwood-Lollar et al. (1997) and O’Nions and Ballentine
(1993).
hydrocarbons (Espitalié et al., 1988). Crossformational fluid flow in the Paris Basin seems
to be confirmed by the common source of
salinity for both Jurassic and Triassic
groundwater, which is halite deposited in the
eastern part of the Triassic aquifer. Crossformational flow between Triassic and Middle
Jurassic is responsible for the transport of
secondary
migration,
constituting
the
preferential pathways for oil flow through the
Lias. An important hydrodynamic component
flowing in the Jurassic and the Triassic aquifers
and contemporary with oil migration seems t o
have affected both the distribution of oil pools
in the Paris Basin (Poulet et Espitalié, 1987)
and the loss of ~90% of totally migrated
20
-800
-1000
-1200
-1400
In situ radiogenic 3He/ 4He
Depth of the reservoir (m)
-600
0.1
0.2
0.3
0.4
0.5
-600
-800
-1200
-1400
0.025
Atmospheric = 295.5
-1000
Atmospheric = 0.0290
Depth of the reservoir (m)
(R/Ra)c
0.03
0.035
0.04
0
21Ne/ 22Ne
500
1000
1500
2000
40Ar/36Ar
Figure 7.9. Variation of the 3He/4He, 21Ne/22Ne and 40Ar/36Ar ratios with the depth of
the gas reservoirs at Hajduszoboszlo gas field, Pannonian Basin. Data are from
Ballentine et al. (1991). The isotopic gradients can be explained by mixing between
groundwaters carrying deep-seated gases (mantle-derived He and radiogenic crustalproduced Ne and Ar) and meteoric waters carrying radiogenic in situ produced helium
and atmospheric Ne and Ar.
tectonically active Pannonian Basin. Furthermore, the radiogenic noble gas component
shows lower amounts in 40Ar* and 21Ne*
probably due to a large production in the
aquifer reservoirs, rather than in the deeper
basement (Table 7.8).
The large in situ production of
radiogenic noble gases is progressively masking
the traces of a large-scale fluid flow which
affected the Paris Basin probably in early
sodium chloride in the Middle Jurassic aquifer
(Worden & Matray, 1995).
Both
groundwater
and
oil
accumulations have been studied for noble
gases (Marty et al., 1993; Pinti and Marty,
1995; Pinti et al., 1997; Pinti and Marty,
1998). The main noble gas feature in the Paris
Basin fluids is the presence of a resolvable
mantle-derived noble gas component, but
weaker than that measured in the younger and
21
6
F(4He/ 40Ar)rad
A
4
2
0
0
1
2
3
4
5
6
7
F(20Ne/36Ar)ASW
Figure 7.10. Co-variation of atmospheric 20Ne/36Ar ratios and radiogenic 4He/40Ar*
ratios at Hajduszoboszlo gas field, Pannonian Basin. Modified after Ballentine et al.
(1991). The F-values are the measured ratios for a particular noble gas component
divided by the same ratio produced through radiogenic production (rad: 4He/40Ar* =
4.9) or introduced by equilibration with the atmosphere (ASW: 20Ne/36Ar = 0.192).
The co-variation between He/Ar and Ne/Ar ratios is due to water/gas equilibrium
partitioning of a common source of radiogenic and atmospheric gases in the basin.
mixing of two fluids: one fluid from the east,
enriched in radiogenic 4He, and one from the
south, associated with the Rouen-Couy fault,
relatively enriched in mantle-derived 3He.
The relationship between the helium
isotope ratios 3He/4He and the total amount of
4
He in the basement, Triassic and Middle
Jurassic groundwaters (Fig. 7.13), suggests that
there are at least three sources of helium
occurring in the Paris Basin. The first source is
fluids circulating in the southern crystalline
basement and characterized by high 3He/4He
ratios (0.12-0.14 Ra) due to addition of
mantle-derived 3He. The second source is
Triassic groundwaters located at the center of
the basin, which are characterized by 3He/4He
ratios intermediate between the basement and
the Middle Jurassic fluids (3He/4He = 0.08 Ra).
The third source are waters located east of the
Middle Jurassic aquifer, with low 3He/4He ratios
Tertiary time. It is likely that this episodic
fluid flow introduced deep-seated mantlederived and radiogenic noble gases in the basin,
and possibly triggered the hydrocarbon
primary and secondary migration within the
basin. This past circulation has been partially
traced by Pinti and Marty (1998). The
geographical distribution of the 3He/4He ratios
measured in the Middle Jurassic and Triassic
groundwaters and oils shows a progressive
westward increase in 3He/4He ratios (Fig.
7.12). The east basin is dominated by sources
of helium with 3He/4He ratios ranging from
0.018 to 0.026 Ra, compatible with a
radiogenic production in the Middle Jurassic
limestones and Triassic sandstones (Table 7.5;
Fig. 7.12). The west basin is dominated by
sources of helium enriched in 3He with respect
the radiogenic values (3He/4He = 0.11 Ra).
Such a distribution can be explained by the
22
Eng l is h
Ch a n ne l
Ardennes
F1
F2
Rouen
Reims
J17
PARIS
J3
T10 T9
Armorican
Massif
T4
T3
T2
W
T8
T7
J5
J18
Nancy
J4
J6
E
J1 T6
J16
Orleans
Tours
Couy-GPF3
Bourges
Massif Central
Tertiary
Cretaceous
Dogger and Lias
Trias
Paleoproterozoic
Massifs (outcrops)
W
Paris
Basin
Auxerre
0
100
F1 Bray fault
F2 Rouen-Couy fault
PARIS
Rouen-Couy
fault
km
Bray fault
E
Tertiary
L. Cretaceous
E. Cret.
Malm
Dogger
Lias
Paleozoic
Trias
0
100
km
Chalk
Limestone
Evaporite
Sandstone
Basement
Shales
Source rock (generated hydrocarbons)
Source rock (expelled hydrocarbons)
Oil pools
Figure 7.11. Map showing the general geology of the Paris Basin and the position of
the oil fields (Middle Jurassic oil reservoirs; white circles; Triassic oil reservoirs;
black squares). The area at the center of the Basin represents the zone of intensive
noble gas survey in groundwaters and oils (top). Generalized cross section of the
Paris Basin with the position of the oil source rocks is also shown. The oil reservoirs
are the Middle Jurassic (Dogger) carbonates and the Triassic (Keuper) sandstones.
Modified after Pinti and Marty (1995).
23
(3He/4He = 0.02 Ra) resulting from the
production of helium in the local reservoirs
rocks (Bathonian-Callovian limestones).
The transport of radiogenic helium,
argon and neon (and associated mantle-derived
helium) from the Trias to the Middle Jurassic
aquifer is exemplified in Fig. 7.14, where the
radiogenic 4He/40Ar* and 21Ne*/ 40Ar* isotope
ratios are reported for both the Triassic and
the Middle Jurassic oil-field brines. The
X Geographical coordinates, km
Y Geographical coordinates, km
560
180
600
640
680
720
N
Bray-Vittel Fault
0.055
0.076
140
Paris
0.048
0.055
0.045
0.055
0.065
0.050
0.109
0.083
0.018
0.071
0.050
0.050
0.020
0.050
0.107
0.076
0.090
0.112
≤ 0.048
0.026
0.077
0.088
100
0.025
0.051
0.051
0.049
0.080
0.027
0.054
0.085
0.088
0.070
0.084
Rouen-Couy Fault
60
0.16
M. Jurassic
Trias
(R/R a)c
0.12
0.08
0.04
Radiogenic in situ produced3He/4He
0.00
560
600
640
680
720
X Geographical coordinates, km
Figure 7.12. Geographical distribution of 3He/4He ratios measured in the oils and
groundwaters of the central Paris Basin (R) normalized to that of air (Ra =1.4 x 10-6)
and corrected for air contamination (top). The direction of groundwater flow is also
plotted. The (R/Ra)c vs. horizontal geographical coordinates for the Middle Jurassic
and Triassic oils and groundwaters of the Paris Basin is also shown. The dashed area
illustrates the range of 3He/4He ratios produced in situ in the Middle Jurassic
limestones and Triassic sandstones. Occurrence of a mantle-derived helium
component westward in the basin is evident. Modified from Pinti and Marty (1998).
24
0.15
M.Jurassic
South
Trias
Basement
(R/Ra)c
West
0.10
Centre
0.05
East
East
0
0
50000
10000
1/ 4He, L/mol
Figure 7.13. Helium concentration vs. (R/Ra)c in the Middle Jurassic, Triassic
and crystalline basement oils and groundwaters. The dashed area indicates a
three end-member mixing between helium sources located east of the Middle
Jurassic aquifer, center of Triassic and southern basement. The noble gas
distribution indicates that the eastern Middle Jurassic and Triassic reservoirs
may be hydraulically disconnected. Modified from Pinti and Marty (1998).
4
He/40Ar* and 21Ne*/ 40Ar* isotope ratios
clearly show a correlation, indicating a mixing
between the Triassic and the Middle Jurassic
groundwaters. The variation of the radiogenic
noble gas isotope ratios can be attributed to the
initial ratios of the parent elements 238,235 U ,
232
Th and 40K in minerals and rocks, which
vary for different lithologies, or to preferential
diffusion of 4He and 21Ne* relative to 40Ar*
from the mineral to the fluid phase, which
depends on the thermal and tectonic regime of
the basin. The Triassic groundwaters show
4
He/40Ar* ratios of 4-7 and 21Ne*/ 40Ar* ratios
of 2.5-4 x 10 -7. These ratios could correspond
to a source having a K/U ratio of about 35,000
and which releases He, Ne and Ar in water
close to their production ratio. This source
could be the Triassic sandstones, the crystalline
basement, or both. The second source of
radiogenic noble gases has high 4He/40Ar*
ratios of 40 and 21Ne*/ 40Ar* ratios of 65 x
10 -7 and could correspond to the carbonate,
which is characterized by very low K/U ratios.
The complete study of noble gases
allowed tracing the groundwater circulation in
the Paris Basin around the main oil
accumulations (Fig. 7.15). The Middle Jurassic
is dominated by groundwaters flowing from the
two recharge areas located south and east of
the basin. The groundwaters from the eastern
recharge are accumulating radiogenic 4He
produced in the Middle Jurassic aquifer rocks.
The groundwaters flowing from the southern
recharge contain helium produced in the
southern crystalline basement. A third source
of radiogenic helium is transported into the
25
21Ne/ 40Ar* x 10-7
100
J4
M. Jurassic
limestone
K/U = 300
J17
J18
J16
10
J6
Continental Crust
K/U = 1.4 104
T8
T3
J5
T2
T5
T10
T7
Trias sandstone
K/U = 3.4 x 104
1
1
10
100
4He/40Ar*
Figure 7.14. Radiogenic noble gas isotope ratios 4He/40Ar* plotted against
21Ne*/40Ar* ratios in the Middle Jurassic and Triassic oil-field brines of the Paris
Basin. The plot shows a two-component mixing between a deep source of
radiogenic noble gases identified with the Triassic sandstones or the crystalline
basement (dark stars) and a second source which corresponds to Middle Jurassic
carbonates (white star).
throughout the basin, as previously suggested
from oil studies (Worden & Matray, 1995; du
Rouchet, 1981).
Elemental fractionation of atmosphere-derived noble gases in the Middle
Jurassic oils was consistent with an oil/water
phase equilibrium partitioning. In the Triassic,
a successive degassing of the oil, which was
previously equilibrated with groundwater,
complicates this model. Pinti and Marty
(1995) have interpreted this degassing as the
result of gas stripping due to oil washing
(Lafargue and Barker, 1988). Calculations
from Pinti and Marty (1995) indicate that
both Middle Jurassic and Triassic oils have
seen much larger quantities of waters with
oil/water ratios possibly ranging between 0.2
and 0.01, whereas the present-day oil/water
Middle Jurassic aquifer by waters located at the
center of the Triassic aquifer. These fluids
result from the mixing of saline waters located
east and accumulating radiogenic in situ
produced helium with waters flowing from the
south, accumulating helium from the basement.
The Triassic and basement groundwaters enter
the Middle Jurassic aquifer close to the BrayVittel and Rouen-Couy faults (Fig. 7.11), which
are regional tectonic lineaments affecting the
sedimentary cover and basement of the Paris
Basin. These faults may therefore act as
episodic vertical drains for He-rich fluids across
the 400-700 m of the Lias low permeability
shales, which separate the two aquifers of
Middle Jurassic and Triassic. It is likely that
these tectonic features act also as drains for
the primary and secondary migration of oils
26
SW
NE W
E
Eastern
recharge
Southern
recharge
Rouen-Couy
Fault
3He/ 4He ~ 0.09 R
a
4
40
He/ Ar* ~ 28
Bray-Vittel
Fault
M. Ju
rassi
c
3
4
He/ He ~ 0.14 R
Halite
3He/ 4He ~ 0.05 R
a
4
40
He/ Ar* ~16
3
Lias
a
Crystalline
Basement
3He/ 4He ~ 0.02 R
a
4
40
He/ Ar* ~ 39
4
He/ He ~ 0.02 R
a
4
40
He/ Ar* ~4
Trias
3He/ 4He ~ 0.08 R
a
4
40
He/ Ar* ~ 6
Figure 7.15. Schematic cross section of the Paris Basin illustrating the fluid circulation
recorded by noble gases in oils and groundwaters. Arrows indicate fluid flow
directions. Typical values of (R/Ra)c, and 4He/40Ar* ratios are reported for the
identified fluid end-members. Note the cross-formational fluid inputs through the
main tectonic features of the Paris Basin: the Bray-Vittel and the Rouen-Couy faults.
It is likely that the episode of cross-formational fluid flow responsible for the
distribution of mantle-derived helium across the basin and possibly of the associated
oils occurred during early Tertiary time. The flow of meteoric waters enriched in
ANG and the release of radiogenic in situ produced noble gases seem to have not yet
erased the last traces of these ancients inputs of mantle and possibly radiogenic gases
from below. Modified from Pinti and Marty (1998).
(Paleocene-Oligocene, Poulet
1987; Espitalié et al., 1988).
ratios in the Middle Jurassic and Triassic oil
fields average ~ 1 (Oil Company data).
Assuming a mean groundwater residence time
of ~ 4±2 Ma (Marty et al., 1993) in the center
of the Paris Basin where most of the oil
accumulations reside, and an integrated mean
oil/water ratio in oil reservoirs lower by one
order of magnitude than those presently
observed, the residence time of oils in their
reservoirs should also be an order of magnitude
higher than those of the flowing waters
(40±20 Ma). Such a figure is in qualitative
agreement with current estimates for the
timing of oil migration in the Basin
et
Espitalié,
Po Basin, Italy: a loading basin
The Po Basin (Fig. 7.16) is a classical
loading basin, and differs from the other two
basins in its noble-gas signature (Table 7.8).
There is an absence of a resolvable mantlederived noble gas signature in the fluids, as has
been found in other loading basins (Marty et
al., 1992). The sedimentary sequence is
mainly Pliocene to Quaternary deposited on a
complex Mesozoic basement,
including
thrusted nappe structures (Fig. 7.16).
27
EAST
10o
ALPS
12 o
Garda Lk.
Milan
Venice
Adriatic
Sea
Po
Riv
er
AP
EN
NIN
ES
B
A
Basin
e
ass
Pa
nn
on
ia
n
W
es
tA
lp
s
Ba
si
n
l
PO
BASIN
400
a
Se
km
tic
ria
Ad
N
0
Depth, km
Legend
Basin limit
Hydrocarbon
traps
Dosso
Degli Angeli
A
SW
Dosso
Degli Angeli
Bologna
Vienna Basin
Mo
45 o
Bologna
44 o
N
0
km
40
B
Po R.
NE
2
0
Pliocene-Quaternary
2
Pre-Pliocene
4
Mesozoic
6
40
0
km
Figure 7.16. Map showing the distribution of oil and gas fields in the Po Basin,
Northern Italy, and the location of the Dosso degli Angeli gas field (top). Generalized
cross section illustrating the geology of the Po Basin and of Dosso degli Angeli oil
field. Modified from O’Nions and Ballentine (1993) and Elliot et al. (1993).
ratios are atmospheric, within uncertainties.
The measured 20Ne/36Ar ratios depart from
those predicted for air-equilibrated groundwater. This fractionation is consistent with a
seawater/gas
phase
equilibration
under
conditions close to the present-day reservoir
conditions. Measured CH4/36Ar ratios in the
natural gas are close to those predicted for a
CH4 gas separated from seawater under
reservoir conditions. This, as in the case of
Methane-rich natural gas fields occur all over
the Po Basin and in the near-offshore, but
only the gas field of Dosso degli Angeli has
been studied for noble gases (Elliot et al.,
1993). The results from samples over a depth
interval from 3200 to 3500 m are quite
straightforward (Table 7.8). The 3He/4He
ratios are those expected for radiogenic
production in reservoir rocks (R/Ra = 0.035;
Table 7.5) and the 21Ne/22Ne and 40Ar/36Ar
28
the Pannonian Basin, seems to suggest that
water is also the carrier phase of the main gas
component of the hydrocarbon accumulation
in the Po Basin.
REFERENCES
Allègre, C. J., Staudacher, T. and Sarda,
1986, Rare gas systematic: formation of
atmosphere, evolution and structure of
Earth's mantle: Earth Planet. Sci. Lett.,
127-150.
PERSPECTIVES AND CONCLUSIONS
In the above section, we have selected
a few studies that cover contrasting tectonic
regimes. Ballentine et al. (1996) were able t o
quantify interactions between oil and seawater
in the Magnus oilfield, East Shetland Basin,
northern North Sea. He concluded that the
amounts of 20Ne and 36Ar that are found in the
oil phase require a seawater/oil phase ratio of
110, which is approximately the ratio thought
to be present at depth. They accordingly
concluded that the Magnus oil has achieved
complete static equilibrium with the groundwater in the reservoir drainage volume and
placed these results in the framework of the
cementation history of the reservoir. These
findings are at odd with those obtained for the
Pannonian, Vienna, and Paris basins for which
noble gas data strongly suggest water/oil ratios
much higher than presently observed, and
therefore, active hydrodynamics in these
basins.
The use of noble gases to trace sources
of hydrocarbons and processes having lead t o
their accumulation is still in its infancy,
despite the unique information they are able t o
deliver. One of the limits to their applicability
is the inherent difficulty in analyzing very
small amounts of gases (down to 10 -20 moles
for 3He) extracted from chemically complex
hydrocarbons. This requires working with
ultra-high vacuum lines, but purification of
hydrocarbons in such lines is extremely
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