Geology Reference
In-Depth Information
interpretations of the seismic and gravity data to further
unravel the early history of the CB.
Whilst a distinct seismic reflector recognized below the
CB has been interpreted to record Pan-African orogenesis
(Daly et al.
1991
,
1992
; Kadima et al.
2011a
), it only locally
resolves a tectonic unconformity (for example on lines L50
and L51 in the centre of the basin; Figs.
6.3
and
6.4
: marked
as
). More often the reflector marks a
conformable contact (see for example, line R5 calibrated
against the Mbandaka-1 well; Fig.
6.2a
).
'
Lower unconformity
'
6.3.1 Well Data
Fig. 6.3
Seismic stratigraphy of sequences identified on a section of
profile L51 in the CB and linked further north to the Samba well
(Fig.
6.1b
) and two major unconformities as interpreted in this chapter.
The
first four
sequences beneath the lower unconformity are deformed
on the
left
(SW) side of the profile.
TWT
Two Way Travel-time ( in
seconds).
Vertical black line
: position of CDP 850,
red crosses
: base of
the seismic sequences (corresponding depth in Table
6.2b
)
The original lithostratigraphic scheme of the central CB was
established from logging the two fully cored stratigraphic
wells: Samba and Dekese (Cahen et al
1959
,
1960
) and the
two exploration wells: Mbandaka-1 and Gilson-1 (Esso
Zaire
1981a
,
b
) (location on Fig.
6.1
). The Jurassic to Creta-
ceous sequences that form the uppermost 700-1,200 m are
relatively well described and dated by biostratigraphy (Colin
the Permo-Carboniferous sequences of the Lukuga Group
were recognized in the Dekese well on the basis of their
fossil plants and spores, and their glacial to peri-glacial
characteristics. These were not encountered in the Samba
well but suspected in the Mbandaka-1 and Gilson-1 wells. In
the latter two deeper wells, these sequences overlie sili-
clastics with dolomites (some with stromatolites) assigned
to the Neoproterozoic, although the transition between the
Precambrian and Paleozoic is poorly defined (see detailed
lithostratigraphic subdivision of the four wells in Linol et al.,
depth of
5,000 m s
-1
should represent the Neoprotero-
zoic or older sediments and possibly crystalline basement.
velocities
>
6.3.3 Seismic-Stratigraphic Sequences
Using the seismic profiles acquired by Exxon-Texaco in
1974-1976 (location on Fig.
6.1
), ECL (
1988
) and Daly
et
al.
(
1992
)
distinguished
six
seismic-stratigraphic
sequences (or
) above the acoustic crystal-
line basement, each bounded by regional unconformities.
These sequences are re-described below (using the same
numbering as in Daly et al.
1992
) with reference to the
seismic profile R5 shot along the Congo River and passing
close to the Mbandaka-1 well (Figs.
6.1b
, 2a). They are also
illustrated on the land seismic profile L51 whose northern
extremity approaches the Samba well (Figs.
6.1b
, 3).
On the R5 profile (Fig.
6.2a
), the depth-time curve is
presented for CDP (Common Depth Point) location 2,231,
measured closest to the Mbandaka-1 well (Fig.
6.2b
). The
depth-time curve was constructed by a polynomial interpo-
lation of the CDP data (Table
6.1a
). This allowed calculating
the depth of the base of the different seismic-stratigraphic
sequences and their thicknesses, as identified on the seismic
line R5 (Table
6.2a
). For reference, the bottom (TD) of the
Mbandaka-1 well at 4,350 m deep correspond to a TWT time
of 2,139 ms. Similarly, the depth to the base of the sequences
identified on Line L51 have been calibrated for CDP 850
(Fig.
6.3
, Tables
6.1a
and
6.2b
).
Except for the uppermost sequence (5), the ages of all
sequences is poorly defined. Two major unconformities are
identified and traced in most of
“
Supersequences
”
4,350 m in a soft basement, interpreted to be
halite (Esso Zaire
1981a
).
6.3.2 Velocity Structure Based on Seismic
Refraction Data
Evrard (
1957
) defined for the first time the P-wave velocity
structure of the CB, based on the results of a refraction
seismic survey calibrated with surface and well data at
Samba and Dekese. Whilst Evrard (
1957
) stressed that this
velocity structure is based on the physical characteristics of
the sedimentary rocks and does not directly reflect the strati-
graphy, the following synthesis seems robust: P-wave velo-
cities
3,600 m s
-1
characterize the Mesozoic-Cenozoic
sequences whilst velocities of ca. 3,900 m s
-1
are typical
for the middle to upper Paleozoic sequences. Higher
velocities, between 4,200 and 4,600 m s
-1
, likely represent
the late Neoproterozoic to lower Paleozoic RedBeds, whilst
<
the seismic profiles
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