2004 Seismic
Interpretation and Prospects
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Seismic Horizons
Regional Geological
Setting
The Rio Muni
Basin forms part of the West Africa margin basin
system formed during continental separation and the
creation of the South Atlantic Ocean which occurred
through the Cretaceous and Tertiary periods. The basin
contains a thick section of mid Cretaceous and younger
sediments lying on an early Cretaceous rifted section
that transitions to oceanic crust.
Block M lies in the distal part of
the Rio Muni basin which is totally unexplored by
drilling. The proximal part of the basin by contrast
has seen extensive recent drilling which has defined
the geological sequence and petroleum systems. The syn-rift
section consists of Late Barremian to Mid-Aptian
terrestrial clastics and lacustrine shales with
extensive rollover structuring to large listric faults
updip. This is overlain by a transitional sequence of
locally developed salt and good quality marine oil
prone source rocks of Aptian age. An Albian (Madiela)
carbonate platform is followed by a
Cenomanian-Turonian sand shale sequence which contains
a major source rock interval. This post-salt sequence
commonly forms extensional rafts detaching on the
Albian-Aptian shale or salt sequence. An extensive
Senonian section onlaps the earlier rafted topography
and contains channel and turbidite sands that form
reservoirs to petroleum deposits. The latest drift
sequence is dominated by a thick wedge of Miocene to
Recent clastics. Commercial oil accumulations have
been typed to both the Aptian and Cenomanian-Turonian
sources. Productive reservoirs are proven in Santonian
and Campanian clastic reservoirs with other possible
reservoirs throughout the section.
In the distal part of the basin,
the pre-drift section is unknown but could logically
be oceanic crust. It is likely that this will be
overlain by Aptian and younger section with both
source and reservoir potential. Regional seismic
profiles and outcrop geology show a thick (up to
10kms) sedimentary sequence extending all the way to
and beyond the islands of Sao Tome and Principe.
Outcrops on these islands have good quality quartz
matrix reservoirs of early Tertiary age (?) with oil
seeps that have been typed to Cretaceous age source
rocks.
Block M is
located approximately 100 kms from the Rio Muni coast,
midway to Principe island and in water depths ranging
1800 – 2300m.
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In 2003,
2000km of 2D seismic was acquired over block M. The
new 2D seismic has been interpreted and tied into the
existing data base and several prospects and leads
have been identified (Figure 1&2)
Prospect 1,
is a giant basement high with four way dip closure,
mapped on the southern side of the significant
regional feature known as the Kribi Fracture zone
which trends north east south west across Block M and
beyond. This basement high covers a very large area
with separate culminations. Several possible
prospective horizons are draped above the basement
high.
This prospect
1 has been mapped at the Cretaceous 1 level, which is
postulated to be Campanian age. This is age-equivalent
to the main reservoir at the Ceiba oil field, which is
located approximately 75 km to the east. The
Cretaceous 1 level has four-way dip closure with 60ms
of relief and it covers approximately 67 sq. km..
The interval
between Basement and the Cretaceous 1 level is
characterised over the area of closure by high
amplitude events which may indicate the existence of
hydrocarbons in this interval. These events occur over
a vertical interval of at least 300 milliseconds (500
meters) and may represent stacked reservoirs. The
water depths over the Prospect 1 area range from 2100m
to 2200m.
This prospect
could easily have been sourced by hydrocarbons
migrating from source rocks adjacent and north of the
basement high. These attain depths of 5,000m or more
below seabed and have sufficient Miocene and younger
burial to be oil generative with reasonable heat-flow
assumptions.
Several other
significant prospects have been identified. To the
east and north of Prospect 1, three compacted channel
features have been identified in the Tertiary section.
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