Geoscience Reference
In-Depth Information
predicted by such model. All the main geological and geophysical features of the
region are correctly predicted.
Oil as a metamorphic product
The main reason for the great commercial interest in sedimentary basins is,
of course, the deposits of oil, gas and coal which they may contain. Organic
deposits must undergo metamorphism - elevated temperatures and pressures for
considerable times - before they become hydrocarbons. The subsidence history of
a sedimentary basin combined with oil-maturation history will show where in the
basin the oil is likely to be found. In view of the immense economic importance
of hydrocarbons, a short discussion of their formation is included here.
Organic remains deposited in a sedimentary basin are gradually heated and
compacted as the basin subsides. These organic deposits are called kerogens
(Greek
keri
,'wax' or 'oil'). There are three types.
Inert kerogen
,which is con-
tained in all organic material, transforms into graphite;
labile kerogen
,which
is derived from algae and bacteria, transforms into oil, though a small propor-
tion transforms directly into gas; and
refractory kerogen
is derived from plants
and transforms into gas. At elevated temperatures, oil also transforms into gas
byaprocess called oil-to-gas cracking. These are complex organic chemical
reactions with time and temperature controlling factors. Of course, none of the
reactions could take place if the organic material were not buried in sediment
and protected from oxidation. In the Guaymas basin in the middle of the Gulf of
California, the planktonic carbon-rich silts have been heated to such a degree by
the hydrothermal systems (Section 9.4.4) that the kerogens have been transformed
into hydrocarbons. The sediments there smell like diesel fuel.
The rates at which the chemical reactions proceed are described mathemati-
cally by
d
C
d
t
=−
kC
(10.25)
where
C
is the concentration of the reactant (i.e., the kerogen) and
k
is the rate
coefficient of the reaction. The Arrhenius equation (see also Eq. (6.24)) defines
the temperature dependence of
k
as
A
e
−
E
/
(
RT
)
k
=
(10.26)
where
A
is a constant (sometimes called the Arrhenius constant),
E
the activation
energy,
R
the gas constant and
T
the temperature.
Data on the laboratory and geological transformation of kerogens into oil
and gas are shown in Fig. 10.50(a), which demonstrates the time dependence of
Eq. (10.25), showing the difference between heating labile and refractory kero-
gens at geological (natural) rates and heating at a fast rate in the laboratory. The
calculations were performed using
A
10
13
s
−
1
208 kJ mol
−
1
=
1.58
×
and
E
=
279 kJ mol
−
1
for refrac-
tory kerogens. For the kerogen-to-hydrocarbon reactions to take place within a
10
18
s
−
1
for labile kerogens and
A
=
1.83
×
and
E
=
