Thermal Regime and Evolution of the Congo Basin as an Intracratonic Basin - Geology and Resource Potential of the Congo Basin - page 231

scenario obviously requires too much burial and erosion
with no other geological evidence, which suggests that the
maturation data at M

lithosphere thickness, and given the large thickness of the
lithosphere, it takes too much time for any lithospheric
perturbation to relax that it cannot be detected at the present
time. The surface geothermal gradient is constrained by deep
temperatures in oil wells, and unless the sediment series
were such undercompacted in the past that they decreased
thermal conductivity, there is no reason that it varied signifi-
cantly. A further problem is that datasets of the three differ-
ent boreholes lead to different interpretations of the burial/
erosion history. All these interpretations assume that heat
transfers in the basin are limited by the inefficiency of
conductive heat transfers in the lithosphere. Alternatively,
Makhous and Galushkin ( 2003 ) have suggested that the
maturation jumps in the Illizi basin (Algeria) can be
attributed to heat advection by volcanism and/or hydrother-
mal fluids. Such processes in the CB are possible but need to
be documented: they would help to decrease significantly the
amount of burial/erosion when only conductive heat
transfers are considered.

bandaka are either unreliable or are not
representative of regional processes (local fluid circulations?).
The other R o data have been obtained at shallower depths
(Sachse et al. 2012 ), between 925 and 1,530 m at Dekese and
between 657 and 735 m at Samba. Sachse et al. ( 2012 ) use
the easy R o method (Burnham and Sweeney 1989 )
implemented in the PetroMod commercial software to esti-
mate the burial in these two boreholes. However, they used a
much higher value of the present-day heat-flow
(62 mW m 2 ) based on the average of measurements in
the Katagan belt (Sebagenzi et al. 1993 ) near the Zambia
territory where Chapman and Pollack ( 1977 ) obtained simi-
lar high values that they attributed to an incipient arm of the
East African rift system. Moreover, Sachse et al. ( 2012 )
progressively increased the heat-flow before the rifting to
70-80 mW m 2 . Consequently, they proposed a minimum
erosion of 900-1,500 m, which is much less from our esti-
mate for M

'

bandaka 1. This is not a methodology problem
related to the calculation of R o , as we can reproduce their
results with a similar hypothesis, but rather it is related to the
heat-flow assumptions. Therefore, we examine alternative
interpretations based on a more realistic thermal regime.
The first scenario corresponds to burial/erosion during the
lower-middle Jurassic period as assumed in the scenario for
M

'

Conclusions
Several pieces of information were used to estimate the
past and present thermal regime of the Congo Basin. The
present-day heat-flow was estimated from the deep
temperatures and geophysical logs in two oil wells drilled
in the central part of the basin. The resulting heat-flow
value is low and consistent with one published value in
the south of the basin (Mbuji-Mayi region). A stable
geotherm was inferred from the present-day surface
heat-flow and the seismic lithosphere thickness: it agrees
well with the xenoliths pressure/temperature conditions.
On the other hand, the magnitude and the duration of the
tectonic subsidence agree well with the thermal subsi-
dence

bandaka. This scenario can be applied to the Dekese 1
borehole where the maturation data come from the Permian
“

'

4,000 m of burial/erosion
(Fig. 12.11c ). The second scenario is suggested by the mat-
uration of the mid-Cretaceous

Lukuga ” formation. It requires

formation at Samba 1.
In order to record high temperatures, the

“ Loia ”

formation
should have been buried and exhumed later, probably during
the late Cretacous/Paleocene gap. In this case, we need again
4,000 m of burial/erosion to raise R o to the observed values.
In order to reduce the magnitude of the burial, we tried to
introduce a thermal anomaly that does not perturb the mantle
lithosphere (otherwise we cannot end up with the present-
day geotherm) by imposing a high temperatures at the bot-
tom of the crust, just as if some magma issued from the sub-
lithospheric mantle were trapped at that time. This heat-
source produces a significant increase of the surface heat-
flow and an uplift of the isotherms during the burial
(Fig. 12.11d ). It still requires a significant amount of burial
and erosion (3,000 m), but it can match the maturation data
from the

“ Loia ”

of a thick lithosphere rifted during the

NeoProterozoic by a thinning factor 1.4, and the

residual gravity anomaly also agree with a crustal thin-
ning of the same magnitude. Overall, these results suggest
that the lithosphere slowly relaxed to the present thermal
regime. Several anomalies with respect to this long-term
evolution are observed during the subsidence history:
some of them can be related to the far field tectonic
stresses causing reactivation or inversion of the subsi-
dence evolution, others with shorter periods (20-40 Ma)
are more likely related to the instabilities caused by
thickness variations at the base of lithosphere. None
have such significant effect that it could affect the high
maturation observed in the basin. A possible cause of this
maturation is that organic matter was buried at sufficient
depths in the past and then exhumed during erosion. With
the appropriate thermal conditions, it requires however a
minimum of 4 km of burial and erosion, and taking into
account some of the data, it could exceed 10 km! In
addition, the burial/erosion inferred at one borehole is

ʲ ¼

three different boreholes (green curve in

Fig. 12.10 ).
All these scenarios show however that it is very difficult
(or even impossible) to explain the maturation data in the CB
with realistic hypothesis for both the thermal regime and the
burial/erosion history. It is difficult to change significantly
thermal parameters: the lithospheric geotherm is constrained
by surface heat-flow, xenoliths P/T equilibrium and seismic