Marakushev_subbed.qxp 26/3/09 04:38 Page 36
Formation of Oil and Gas Fields
Figure 1: Scale of Inversions of the Earth’s Magnetic Field
Separation of water solutions from oil is accompanied by an increase
in the oxidising potential and concentration of hydrocarbons (CH
3
→
CH
0 20 40 60 80 100 120 140 160
2
). This process for aqueous hydrocarbons is shown in Figure 2.
Ma
Pleistocene
The diagram is constructed for a temperature of 250°C and a
Pliocene Oligocene Eocene
Paleo-
cene
Upper Cretaceous Lower Cretaceous Jurassic
Miocene pressure of 48 bar (such conditions are typical of shallow crustal
depths and compositional variations in hydrocarbons, with a gradual
The width of the strips corresponds to the time interval between inversions of the geomagnetic field.
increase in the oxygen potential towards the surface). The oxidising
potential also has a substantial impact on hydrocarbon generation in
Figure 2: µ
H2
–µ
O2
Diagram of Redox Facies of Oil-series Hydrocarbons
(Aqueous n-alkanes) in the Hydrothermal System
mantle magma chambers and increases with the development of
alkaline magmatism.
-360
Eicosane (CH
2C
16
H
34
+ 2H
2
O =
2,1
)
= 4C
Hexadecane (CH
8
H
18
+ O
2
The generation of hydrocarbons and their different degrees of
2,125
)
8C
20
H + 2H
2
O =
6C
8
H
18
+ 2H
2
O = = 10C decomposition occur within a wide range of redox conditions both
16
H
34
+ O
2
= 8C
6
H
14
+ O
2
beneath the crust, where methane and ethane are largely generated,
Hexane (CH
Octane (CH
4C
2,25
)
(kJ)
2,33
)
6
H
14
+ 2H
2
O =
2
O
-370
= 6C
and within the crust, where the molecularly heavier hydrocarbons
4
H
10
+ O
2
TInP
= 3C 4C
(ethane derivatives with the corresponding carbon isotopic
= C 2C
= R
6
H
8
Butane (CH
2 16 H 8 H 11 2,5
)
H
composition) are formed. This correspondence is illustrated in Figure 3,
µ
O 18 18
=
34 =
+ H
+ H
= 2C 3C
2H
2 O =
2 = 2H in which data on the undoubtedly endogenic hydrocarbons (Murchison
2
4
H
6
H
10
2 +O
6C
4
H
10
+ 2H
2
O =
-380 = 4C 5C
14 =
+ H
2
= 8C H
chondrite) and the oldest (Archean Abitibi formation, Canada) and
3 8
+O
2
16
20 2
4C
H
H 3
H
8
+ 2H
2
O =
34
42
Propane (CH
= 6C
most recent (oceanic ridges) massive sulphide deposits are compared
+ H
= 2,67
)
3C = 4C
2
H
6
+ O
2
2
4
H
2C = 3C
10 3
Ethane (CH
with data on the hydrocarbons in sedimentary rocks. In terms of
+ H
H
3
H 3
)
8 8
+ H
2
H
6 variations in carbon isotopic composition, hydrocarbons from
2
=
2
=
-390
sedimentary rocks are similar to the so-called thermogenic
-60 -58 -56 -54 -52 -50
µ
hydrocarbons that are generated by buried biomass (plants and, largely,
H2
= RTInP
H2
(kJ)
micro-organisms) contained in sedimentary rocks at high temperatures
Temperature = 250°C; pressure = 48 bar.
(thermal anomalies). Thermogenic hydrocarbons may also be related to
the influx of endogenic hydrocarbons into sedimentary sequences by
Figure 3: Diagram of the Isotopic Composition of n-alkanes Related to
Their Carbon Numbers (C1–4)
hydrothermal solutions that ascend from abyssal magma chambers and
form thermal anomalies. The combination of these hydrocarbon types
δ
13
C, ‰ with different isotopic variations is governed by the heterogeneity of
0
hydrocarbon pools in sedimentary sequences. Weighty arguments in
favour of endogenic oil origin are represented by the abundant
-10
outflows of hydrocarbons onto the ocean bottom along the global
CH
4
C
2
H
6
C
3
H
8
C
4
H
10
system of rift zones in the World Ocean without any connections with
-20
3
sediments. The presence of oil manifestations in the kimberlitic
-30 1
diamond-bearing pipes is also very typical.
2
-40
Oil pools formed within the biosphere are likely characterised by a
4
complex nature. The presence of the thermogenic component in oil
-50
pools is confirmed by the presence of the so-called biomarkers in oils
or complex organic compounds that are structurally similar to
-60
C
components produced only by living organisms (lipids, steroids and
1
C
2
C
3
C
4
Increase of molecular weight of HC
porphyrins). Most researchers consider the presence of biomarkers in
oils as an indicator of its sedimentary migration (thermogenic origin)
1. Meteorites (Murchison chondrite). 2. Kidd Creek sulphide deposit in the old (2.7 Ga) Abitibi greenstone belt
from remnants of organisms that lived on Earth during previous
in Canada (gases from rocks cored by drilling at a depth of 2.1km consisting of hydrogen, helium, methane,
ethane, propane and butane). 3. Hydrocarbons from the Juan de Fuca Ridge. 4. Sedimentary rocks. geological epochs. This fact can serve as evidence for the
heterogeneity of oil pools being genetically related to the ascent of
to oxidising reactions of water generation: 4CH
3
+ O
2
= 4CH
2
+ hydrocarbons within the Earth’s biosphere, where the concentration
H
2
O, and so on. The oxidising processes eventually result in the of subsurface micro-organisms can be significant. The formation of oil
complete decomposition of hydrocarbons, with the formation of pools is related to the large-scale replacement of sedimentary rocks of
graphite (4CH + O
2
= 4C + 2H
2
O). The scheme is complicated by the platformal and shelf depressions (including crystalline rocks of the
numerous intermediate reactions leading to the formation of solid basement) by oil, with the partial capture of the bacterial and plant
carbonaceous components (bitumen, kerogen and others) during constituents of the rocks. Thermogenic trends of carbon isotopic
the sedimentary process, including the formation of black shales composition can also be observed in endogenic oil if the sedimentary
(siliceous–carbonaceous, clayey–carbonaceous and carbonate– rocks with abundant micro-organisms are replaced.
carbonaceous shales) developed in near-surface parts of crustal
depressions. This is exemplified by the Green River shales High concentrations of metals in oils (sometimes of commercial
(42,300km
2
in area) in the US. significance) represent an additional feature indicating their
36
EXPLORATION & PRODUCTION – VOLUME 7 ISSUE 1
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