Thermal Regime and Evolution of the Congo Basin as an Intracratonic Basin - Geology and Resource Potential of the Congo Basin - page 229

Table 12.1 Value of R o obtained in the Congo Basin deep boreholes
Well Depth (m) Ro (%) Number Stratigraphic age Formation References

Samba 1 657-825 0.64 3 Mid-Cretaceous Loia Sachse et al. ( 2012 )

Dekese 1 1,050-1,530 0.69 28 Permian Lukuga Sachse et al. ( 2012 )

M ' bandaka 1 1,985 0.61 10 Trias Haute-Lueki Unpublished

M ' bandaka 1 2,725 1.0 1 Paleozoic Unpublished

M ' bandaka 1 4,030 2.56 1 Neo-Proterozoic Unpublished

First, we reconstruct the thermal history of sediments as
described previously in the subsidence analysis (no erosion).
This is illustrated for the evolution at M

R0 (%)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

' bandaka 1. This “ no

0

erosion scenario shows almost no variation of the surface

heat-flow (Fig. 12.11e ), although the isotherms become pro-
gressively shallower, because of the lower thermal conduc-
tivity of the uppermost sediments. In this scenario, only the
Neo-Proterozoic can reach temperatures greater than 120 C,
with a maximum of 133 C at the bottom of the basin. The
present-day calculated Ro profile (blue line with label

”

500 MDB1

SMB1

DKS1

1000

1500

“ no

erosion in Fig. 12.10 ) is much lower than all observed val-

ues, which clearly shows that the organic matter has
recorded higher temperatures in the past. This is also a
regional effect as the three boreholes where Ro has been
determined are affected. The possible causes of this discrep-
ancy are a lower conductivity, a higher heat-flow or a deeper
burial. The lower conductivity hypothesis is very unlikely: it
requires higher porosity for long periods of time. The exis-
tence of higher heat-flow is possible, but conflicts with the
apparently stable geotherm. The most likely interpretation is
that these rocks have been buried at deeper levels than their
present depth and then have been exhumed during some
erosional period.
If we assume that all R o values in M

”

2000

2500

3000

3500

4000

4500

bandaka 1 are
reliable, one should add at least two burial/erosion periods.
The large difference between the two deepest R o values (1.0
and 2.56 %) is difficult to explain with low temperature
gradients: we need both to add 8,000 m burial
(Fig. 12.11b ) and to increase the heat-flow during the Neo-
Proterozoic rifting by a greater thinning of the mantle
(

'

5000

Fig. 12.10 Vitrinite reflectance data and predicting models with vari-
ous thermal and burial histories. One of the blue lines corresponds to
the hypothesis with no erosion (same profile for all boreholes): the
burial and temperature history is shown in Fig. 12.11a for well MBD1.
The other blue line corresponds to the two stages erosion (8,000 m in
Paleozoic and 4,000 m in Jurassic with an additional heat-source during
Neo-Proterozoic): the burial and temperature history is shown in
Fig. 12.11b for well MBD1. The yellow line corresponds to one
4,000 m stage of erosion during Jurassic: the burial and temperature
history is shown in Fig. 12.11c for well DKS1. The green line
corresponds to one stage of 3,000 m erosion during Cretaceous with
an additional heat-source: the burial and temperature history is shown
in Fig. 12.11d for well SMB1

10 assuming the two layers stretching model of
Royden and Keen 1980 ). The first erosion stage would be
related to the inversion of the basin during the tail end of the
Pan-African orogeny and subsequent extension between
Laurentia and Gondwana during break-up (Daly et al.
1992 ; Kadima Kabongo et al. 2011a ), and therefore the in
excess 8,000 m of sediment should have been deposited
(and/or thickened) and eroded during the lower Paleozoic
stratigraphic gap. A second 4,000 m burial/erosion stage
(Fig. 12.11b ) is needed to explain the intermediate value
(R o ¼ 1 %) at 2,725 m. We can relate this stage to the
fragmentation of the Gondwana land, which corresponds to
the lower-middle Jurassic sedimentation gap in the strati-
graphic record. Because of the initial thermal anomaly, the

ʴ ¼

the Neo-Proterozoic (Kadima Kabongo et al. 2011a ). We do
not intend to present here an exhaustive analysis, but few
different scenarios (Fig. 12.10 ) that can explain the matura-
tion data and satisfy the thermal and geological constraints
described previously.