Geology Reference
In-Depth Information
250
12.5
Thermal Evolution Constrained by
Vitrinite Data
200
150
The low heat-flow observed at the present time is not
favourable to the maturation of the organic matter, except
if higher heat-flow or deeper burial conditions have existed
in the past. Vitrinite particles dispersed in the sediments can
provide such constraints on the past thermal conditions.
Their optical reflectance (R
o
) is related to the transformation
rate of the organic matter, which can be modelled by a first
order kinetic process (Ungerer
1990
; Burnham and Sweeney
1989
). In addition to time and temperature, R
o
can be also
affected by pressure (Carr
1999
), but we have not accounted
for this effect. The determination of R
o
from the temperature
history is described in Appendix 3.
Several unpublished and published R
o
measurements
exist in the CB, and more specifically in the deep boreholes
(Table
12.1
). In the M
100
50
0
−50
−100
−150
−200
−250
1000
1250
1500
1750
2000
2250
2500
Distance (km)
Fig. 12.8
Topography calculated from the normal stresses acting on
the upper boundary of the model of viscous mantle creep.
Red line
is for
50 Ma of model evolution (see Fig.
12.7b
),
green line
is at 75 Ma and
the
black line
is at 125 Myr. Topography changes through lateral
migration of convective instabilities set up by the transition in litho-
sphere thickness at model distance of 1,400-1,500 km
bandaka 1 borehole, several unpub-
lished values can be found in oil exploration reports, but
their quality and their reliability are difficult to assess.
Recently, new measurements have been published on the
Dekese 1 and Samba 1 well cores (Sachse et al.
2012
). All
these measurements are displayed on a same plot of the
vitrinite reflectance versus depth (Fig.
12.10
) and several
models of the thermal evolution are also shown for these
three boreholes.
The thermal evolution can be reconstructed with the same
thermal models used for the calculation of subsidence (see
Appendix 1), which includes the effect of sediment deposi-
tion and erosion. The important aspects for the maturity are
the heat-flow history at base of the basin, which depends in
turn on the interaction between the sediment and the litho-
sphere thermal regime (
'
a
100
50
0
−50
−100
−150
−200
20
40
60
80
100
120
140
160
180
200
220
), but also on
the petrophysics of the sediments, which depends on the
compaction of the pores with increasing depth. In the case
of erosion, one should also account that the compaction state
is
“
thermal blanketing
”
Time (Myr)
b
200
100
to the conditions of the maximum lithostatic
pressure. The porosity distribution is therefore a key param-
eter, as it determines the density, the thermal conductivity or
the radiogenic heat-production of sediments. For instance,
higher porosity leads to lower thermal conductivity, but for a
given heat-flow value, to higher thermal gradients than for
more compacted rocks. Here we use a 1D finite difference
model (Lucazeau and Le Douaran
1985
) modified to account
for one or several erosion stages (Pagel et al.
1997
). The
background heat-flow is obtained by considering one or
several thinning stages of the lithosphere; therefore, it is
not prescribed as in many other models, but fully constrained
by the lithospheric history. As in the previous section, the
modelling follows the most recent stratigraphic interpreta-
tion (Linol et al.
2013c
,
d
) and the previous extrapolation to
“
frozen
”
0
−100
−200
−300
20
40
60
80
100
120
140
160
180
200
220
Time (Myr)
Fig. 12.9
Profiles of topography taken through time at a model dis-
tance of (
a
) 1,250 and (
b
) 1,750 km (see Fig.
12.8
). The elevation of the
surface alternates by a magnitude of 100-200 m over periods of
20-40 Ma depending on location
in subsidence as cratonic basins evolve. It should have also a
small effect on the thermal regime compared to the thinning
process associated with rifting.
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