Geology Reference
In-Depth Information
Table 6.1
CPD data
(a) For location 2231 along the line R5, close to the Mbandaka-1 well
Times (ms)
1
316
601
691
1,058
1,555
2,041
2,500
5,000
VRMS (m/s)
1,756
1,859
2,085
2,345
3,282
3,865
4,133
4,555
8,000
Depth (m)
1
294
623
786
1,621
2,824
4,023
5,422
15,000
(b) Location 850 along line R51
Times (ms)
0
625
750
1,050
1,450
1,850
2,225
2,975
5,000
VRMS (m/s)
2,000
2,500
2,690
3,145
3,565
4,075
4,480
4,950
6,000
Depth (m)
0
701
999
1,609
2,506
3,615
4,758
7,059
14,422
Table 6.2
Depth for the base of seismic sequences and their thickness, obtained after calibration using the CDP data
(a) Location 2231 along the line R5, close to the Mbandaka-1 well
Seismic sequences
5
4
3
2
1
0
a
Depth (m)
877
2,270
2,844
3,960
4,350
a
Thickness (m)
877
1,393
370
1,320
390
(b) Location 850 along line R51
Seismic sequences
5
4
3
2
1
0
Depth (m)
1,120
2,990
5,410
7,070
8,290
9,400
Thickness (m)
1,120
1,770
2,420
1,660
1,220
1,210
a
Base of the sequence No defined
6.3.3.2 Sequence 1
Sequence 1 has a layered seismic pattern characterized
by highly continuous and divergent reflectors. In the
Mbandaka-1 well its top occurs at ca. 3,960 m, and its
bottom at 4,350 m. From this bottom upward to 4,133 m
depth, the sequence is represented by a 217 m thick succes-
sion of dark-grey calcareous silty shales that grade into
argillaceous and dolomitic limestones, with interbedded
salt crystals and sparse anhydrite at the base (Esso Zaire
1981a
). In the Gilson-1 well, it is represented between
4,503 m depth and the bottom of the well (4,665 m)
by alternating beds of sandstone, siltstone, shale and
dolomite, with massive dolomite at the base (Esso Zaire
1981b
). Sequence 1 is correlated with the Ituri carbonates
of the Lindian Supergroup of Verbeek (
1970
) and represents
the M9 sequence of Linol (
2013
) in the Mbandaka-1
well, and the G10 sequence of Linol (
2013
) in the Gilson-1
well. It could possibly be correlated also with the
carbonate sequence (BII) of the Mbuji-Mayi basin
(Delpomdor et al.
2013a
,
b
). Using the CDP 2231 data, its
thickness at the position of the Mbandaka-1 well would
be ~ 670 m. In profile L51, at CDP 850, Sequence 1
is ~1,220 m thick with its base at a depth of ~ 8,290 m
(Fig.
6.3
, Table
6.2b
).
(relatively distant one from another). However their age
control is also poor (especially for the lower one) and it is
possible that there are more than 2 regional unconformities
(e.g. Daly et al.
1992
identified 6 unconformities).
6.3.3.1 Sequence 0
Along several profiles, a reflection-free seismic pattern that
corresponds to the acoustic basement is interpreted to
represents crystalline basement. The latter is overlain by a
low-amplitude, discontinuous and transparent seismic
patterns forming the first sequence (Sequence 0 of Daly
et al.
1992
). The reflectors diverge towards the centre of
the CB, suggesting tilting during deposition; and the
sequence is thicker in the centre of the basin than at its
border. Sequence 0 is apparently discontinuous, with rapid
variations in thickness and its base generally is not well-
imaged. Sequence 0 was not penetrated by any cored wells.
It could represent siliciclastic sediments (essentially con-
glomeratic) if correlated with the Liki-Bembian Group out-
cropping to the NW of the basin (e.g. Daly et al.
1992
) and
may thus represents the basal Precambrian sedimentary unit
of the CB. Alternatively, it may also be correlated with the
lower Neoproterozoic clastic sequence (BI) of the Mbuji-Mayi
basin (Delpomdor et al.
2013a
,
b
; and Chap.
4
, this Topic) and
the Roan Group of the Katangan and Zambian basins.
The base of sequence 0 is not identified in the R05
seismic profile, but suspected in profile L51, at ~ TWT
3,710 ms, which is calibrated using the CDP 850 data
at ~9,400 m depth, for a thickness of ~ 1,100 m (Fig.
6.3
;
Tables
6.2a, b
).
6.3.3.3 Sequence 2
A second transparent seismic pattern with discontinuous
reflectors and one or two strong to medium continuous
reflectors in the middle overlies Sequence 1. In the R5
profile (Fig.
6.2a, b
; Table
6.2b
), it corresponds to an interval
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