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Table of Contents
Introduction .................................................................................................................... 2 Basin Location and Tectonic Settling ............................................................................ 3 Basin Evolution and Stratigraphy .................................................................................. 5
The South Atlantic Ocean ...................................................................................... 5 Field Study 1: ............................................................................................................... 10
Field Location and Discovery ................................................................................. 10 Bounds or Extent of the Field .............................................................................. 10 Discovery of the Rabi-Kounga field .................................................................... 10 Rabi Discovery ..................................................................................................... 10 Kounga Discovery ................................................................................................ 10 Structural or stratigraphic characteristics ............................................................. 11
Characteristics of the Source Rocks ......................................................................... 15 The Melania Shales ............................................................................................. 15
Reservoir Characteristics .......................................................................................... 16 Dentale Reservoir Rock –fig7 and fig13 .............................................................. 16
Production History ....................................................................................................... 18 Pressure Regime ....................................................................................................... 18
Field Study 2 ................................................................................................................ 19 Field Location and Discovery ................................................................................. 19
Márcio Rocha Mello, Barry Jay Katz, AAPG Petroleum systems of South Atlantic margins ......................................................................................................................... 19
Discovery of the Gamba field. ............................................................................. 19 Source Rocks Characteristics ................................................................................... 20
Melania Shales ..................................................................................................... 20 Reservoir Characteristics, Trap and Seal Mechanisms ........................................ 20 Production History ............................................................................................... 20
Discussion .................................................................................................................... 21 Conclusion .................................................................................................................... 25 References ................................................................................................................... 26 Appendix ...................................................................................................................... 28
PART B: CASE STUDIES
Study Area: Gabon Basin
IntroductionA petroleum system is defined by a mature source rock supplying hydrocarbons into a
migration network and, ultimately into a hydrocarbon trap. (Magoon1988).
The Gabon basin (figure 1) is divided into the northern and southern sub-
basin, separated by the N’ Komi fault zone. This project will focus on the southern
sub-basin.
Gabon, the third largest crude oil producing nation in sub Saharan Africa
holds a great potential for hydrocarbon exploration and production in this present
decade, in spite of its recent decline in oil production.
However Gabon has experienced a steady reserves growth. Gabon’s proven
reserves increased from 1.3 billion barrels in 1996 to 2.5 billion barrels in 2002.
The two fields that will be the point of focus is one of Gabon’s largest field
which is the Rabi-Kounga field and the Gamba field which have continued to dwindle [1].
Gabon’s biggest potential lies within the pre-salt section. The pre-salt section
lies below the salt seal and the post-salt refers to sediments deposited above the salt
seal (see Fig 11). The onshore and shallow water areas of the South Gabon Basin have
been successfully exploited but not in the deep water areas. New 2D seismic data
from CGGVeritas is helping to reveal the pre-salt potential before the 10th Gabonese
License Round scheduled between June and December 2010.[2]
1 Petroconsultants2 GeoExpro.com
Basin Location and Tectonic SettlingThe Gabon Basin is located at the limit of two old cratons which have been stable
since ~2Ga. The South Gabon basin stretches from the continental shelf in the east to
the ocean-continent boundary in the west. The sedimentary basins of Gabon (North,
South and Interior sub-basin, figure 1) form part of the salt basin complex that
stretches from southern Cameroon to Angola. They are limited to the north by the
Guinea Ridge and to the south by the Walvis Ridge (Figure 2). Both of these ridges
are volcanic basement features. This whole basin complex and the complementary
basins found on the Brazilian coast of South America were formed during the early
development of the South Atlantic, as Africa first rifted and then separated from
South America. This will be explained in detail in the Basin evolution and history.
The present-day coastline, basin boundaries and structural elements generally parallel
to the Phanerozic alignments mapped on the adjacent cratonic basement.
Figure 1
Gamba Field
Figure 2
Basin Evolution and Stratigraphy
The South Atlantic OceanThe Mesozoic tectonic evolution of the African plate has been characterized by three
main crustal rifting stages. Break-up of Gondwana led first to the Karoo rifting and to
the opening of the Central Atlantic Ocean by separating North West Africa and North
America (180 Ma), and more recently (20Ma) to the opening of the Red Sea-Gulf of
Aden and to the rifting in the East African Rift System. In between, during Late
Jurassic time (~150-140Ma), continental rifting began between South America and
Africa and led to the opening of the South Atlantic Ocean (Fig. 3) and to the
formation of the West and Central African Rift System (Binks and Fairhead, 1992;
Davison, 1999; Kampunzu and Popoff, 1991; Nurnberg and Muller, 1991; Teisserenc
and Villemin,1990).
Sedimentary basins associated with the opening of the South Atlantic and
located north of the Walvis Ridge-Rio Grande Rise complex include, Ivory Coast,
Barreirinhas, Benue, Potiguar, Rio de Peixe, Arripe to name a few.
North & South Gabon
Fig.3. The three major tectono-sedimentary South Atlantic domains with the location of the intracontinental and marginal basins in relation to Gondwana mega-discontinuities during the opening of the South Atlantic Ocean, modified after Popoff (1988)
The West African rifted margin is divided into two main domains by the
oceanic Walvis Ridge (see Fig. 3 and 4). South of the ridge there is a volcanic margin
extends from South Africa to Namibia), and is characterized by seaward dipping
reflectors (Gladczenko et al., 1997).
Along the Gabon Margin, onset of rifting started in Neocomian–Berriasian
times (144 Ma-million years ago) based on dating of the oldest rift sediments
corresponding most exclusively to fluviatile basal sandstones. In Cabinda (Fig. 5), a
volcanic layer found on top of the pre-rift sequence has been dated at 140Ma75 Myr,
corresponding to the Jurassic–Cretaceous boundary (Brice et al., 1982). The end of
rifting has been dated and associated with the magnetic anomaly M0 at ~118. Based
on these ages, rifting duration can be approximated to 25 Myr. The Gabon Margin is
segmented by major normal faults parallel to the present-day coastline and by NE–
SW trending strike-slip faults defining zones with partly different tectonic and
stratigraphic histories. The N’Komi fracture zone divides the Atlantic Basin into the
North Sub-Basin and the South Sub-Basin (Fig. 5).
Fig.4 Present-day configuration of the South Atlantic Ocean with sea floor topography derived form satellite data (Smith and Sandwell, 1997).Box shows study area
Fig 5 shows a simplified geological map of the studied area along the South
Western African coast focus on the South Gabon Basin.
From Fig 5 Locations of seismic lines and wells (eg G1 and G2), industry data
provided by Norsk Hydro are reported.
Offshore shallow reflection seismic lines stretch from the coast to the oceanic
crust over 80, 49 and 47 km for the lines 1, 2 and 3 respectively. Wells are mainly
located on the continental shelf.
Some of the southern major fault zones have been identified along the Gabon
Basin by onshore outcrops, reflection seismic, well data and gravity anomalies.
The oldest pre-rift sediments along the Gabon Margin are found in the north,
in the Interior Basin, which is separated from the offshore Atlantic Basin by the
Lambarene Horst as shown in Fig 5.
They consist of Late Carboniferous to Triassic-Jurassic fluviatile and
lacustrine. The oldest sediments are of the Precambrian age. Continental rifting began
in Berriasian time ~144ma and led to the formation of extensional faults and therefore
to series of garbens and horst.
The sedimentary section of Gabon is found in three major sub-basins as previously
showed in figure1.
The Gabon sedimentary section is deposited over a series of horst and garbens that
developed during the rifting of the metamorphic basement complex (Figure 2). These
metamorphics are highly variable ranging from granites and schists to metasediments.
This variability is reflected in the range of magnetic susceptibility seen in these
basement rocks. This magnetic variability leads to uncertainties in the interpretation
of aeromagnetic data especially in the deeper parts of the basin where supporting data
(seismic and well data) is poor or absent.
Fig 6
Fig 6
The identification of structural features on aeromagnetic data is further
complicated by the presence of extrusive volcanics in northern offshore Gabon and by
the boundary between the oceanic and continental crusts. (Figure 6)
As seen in the above figure the Rabi-Kounga oil and gas reservoir belong to
the pre-salt Gamba and Dentale sequence of the Dianongo Basin which form part of
the Lower Cretaceous African-South American rift valley system. Other gas and oil
field like the Gamba-Ivinga can be seen also.
Fig 7
Rabi-Kounga
Gamba-Ivinga
Field Study 1:
Field Location and Discovery
Bounds or Extent of the FieldThe Rabi-Kounga oil and gas reservoir reaches a maximum thickness of 6000 to
7000m and ranges in age from Early Cretaceous to Recent. The Rabi-Kounga Field is
covered by a Production License of 129 km.
Discovery of the Rabi-Kounga fieldThe Rabi-Kounga field discovered by Shell Gabon in August 1985 lies two
degrees south of the equator in a remote, sparsely inhabited part of the primary rain
forest of equatorial Africa.
The Rabi-Kounga Field was discovered within a relatively well known Lower
Cretaceous rift valley province.
Rabi DiscoverySeismic was acquired in 1985 and another closure, Rabi, which was structurally
higher than Echira could be mapped on the Mandji-M'Bari high trend. M'Bari-1(Fig.
8-appendix) is located 7 km to the north, had been drilled some 200 m down-flank
from the Rabi culmination. The well called Rabi-2 discovered the Rabi Field in
August 1985. Taking into account the low velocities found in the post-salt sequence
of Rabi-2, a reinterpretation of the seismic data in late 1985, led to the definition of
another closed structure to the north of Rabi, the Kounga structure (Fig. 8-
appendices).
Kounga DiscoveryThe exploration well Kounga-1 was drilled in 1986 on a closure 1.7 km (1
mile) west of M'Bari-1 and encountered the Base Salt objectives hydrocarbon bearing
over a gross interval of 68 m. A total of 22 m of gas and 46 m of oil were logged (Fig.
9-appendices). Most important of all, Kounga-1 established the continuity with the
Rabi accumulation. Fluid contacts, reservoir pressures, and crude characteristics
similar to those of the Rabi wells were observed. Further appraisal drilling which took
place during 1986 and 1987 (wells Rabi-4 to 13) confirmed that the two areas were
linked and formed only one structure.
In summary, the Rabi-Kounga discoveries derived from marked improvements
in the quality of the seismic data acquired since 1981 and the higher resolution of the
objective horizons obtained from better static corrections and velocity determinations.
Structural or stratigraphic characteristicsBased on seismic interpretation and well data, the Rabi-Kounga Field now
appears at Top Gamba level as a North-South trending anticlinal feature, almost 14
km long and 4 km wide. The eastern flank is dipping into a North-South trending fault
bounding the Rabi-Kounga structure to the East. The western flank is much flatter,
and in this direction, the edge of the field locally remains to be ascertained. The
anticlinal axis is affected by cresta1 faults which are related to the bending of the
Rabi- Kounga structure over an old Dentale core in post-Gamba times. Deep seated
transcurrent movements are locally reflected at Gamba level. The change in direction
of the structural axis in the northern part of the field may be taken as evidence of such
deep-seated transverse faults.
Below the Base Gamba unconformity, the Dentale anticlinal core is
subdivided by NNE-SSW faults into several tilted blocks which rise toward the NW.
The above shows source rocks that are abundant in the pre-salt sequence. It shows the
Kissenda and Melania shales on the Plateau, together with organic shale intercalations
of the Dentale formation in the trough
Source:-GEOEXPRO-Hunting the Pre-SaltThe stratigraphic cross-section shows Pre-salt traps comprise tilted fault blocks sealed by shales or salt and structural closures
associated with salt movement. Drape structures are also expected over the fault blocks (Gamba reservoir). Post-salt traps comprise drapes over salt domes sealed by shales or combination structural/stratigraphic traps of sand-rich channels within turbidite systems.
Unconformities
Melania, Source rk of Rabi-Kounga field and Gamba Field
Reservoir Rk of Rabi-Kounga
Ezanga Salt
Figure 11
Figure 123
Chronostratigraphic chart of the South Gabon Basin based on high-resolution stratigraphy
The Eustatic curve shows the eustatic change (as opposed to local change) as a result
in an alteration to the global sea levels, such as changes in the volume of water in the
world oceans or changes in the volume of an ocean basin in this case the Atlantic
Ocean.
3 www.sciencedirect.com
Sources of Interest
Rifting along the Gabon margin
The diagram above is a schematic section running across the Dianongo Basin from
the edge of the crystalline basement outcrops in the East, to the Atlantic coast in the
West. (illustrates the geological setting)
There is only one accumulation where the Dentale Formation is productive, and that is
Rabi-Kounga, see above. The trapping is insured by the salt (explained later) and
Dentale reservoirs are oil bearing because they are included inside a close high.4
The Rabi-Kounga oil and gas reservoirs belong to the pre-salt Gamba and
Dentale sequence of the Dianongo Basin which form part of the Lower Cretaceous
African-South American rift valley system.
The sedimentary prism reaches a maximum thickness of 6000 to 7000m and
ranges in age from Early Cretaceous to Recent. The geological setting is illustrated in
fig 5. The pre-salt source rock sequence is composed of fluvio-lacustrine sand and
shale deposits ranging in age from Neocomian to Aptian, (refer to chronostratigraphic
chart of the South Gabon Basin, fig 12), which have been deformed into tilted blocks
by basement tectonics and syn-sedimentary faulting. The sequence which includes the
Dentale sand reservoirs of the Rabi-Kounga Field and the organic rich Melania shale
source rocks represents the fill of a continental rift valley.
The evaporitic deposits and their thin underlying clastics of Aptian age
(Gamba Formation) which uncomfortably overlie the rift valley sediments reflect the
start of a marine transgression. Their deposition corresponds to a transition phase
when rifting ceased and continents started to separate.
Characteristics of the Source Rocks
The Melania Shales
• Deposited in quiet euxinic rift lakes formed during the late rift phase of the
Cretaceous, these organic rich lacustrine sources shales contain oil prone Type
1 kerogen shales.
• The Melania Formation contains source rocks of very high quality with an
average of 6.1% TOC by weight.
• While the maximum thickness is just about 800 meters
• In the east when approaching the basement outcrop Type I kerogen, passes
into Type III. As illustrated in the Figure 5/1 appendix, it has either been
eroded, or is immature or over-mature in many areas so that it is of interest as
a source of oil only in the Gamba-Ivinga high area.
4 Gabon and Douala Basin, PETROCONSULTANTS
• In short, although it is of very high quality, the potential of this source rock is
in fact limited in this study of the South Gabon Sub-basin
• The organic rich intervals have an organic carbon content that averages 8-10%
but may reach 20%.
Reservoir Characteristics
Dentale Reservoir Rock –fig7 and fig13
The Barremian age dentale sandstone provides the main reservoir for the
hydrocarbons contained in the Rabi-Kounga Field. In the Rabi-kounga field the
sandstone bodies are interpreted as being laid down in stacked channels. Individually,
the reservoirs are discontinuous, but there are many of them. They are in some cases
in communication with each other, giving a thickness of about 30m (Boeuf et al.,
1991). The sandstone is generally poorly consolidated at the relatively shallow depth
of 1100m at which they occur in this field giving excellent reservoir characteristics
with:
1. Average porosity close to 30%;
2. Permeability up to 1D Darcies
But in the southern offshore area, reservoir quality deteriorates, and thickness
decreases. The Dentale Formation (Fig.7 and 13 appendix) consists of sands and
shales and is interpreted as a fluviatile to lacustrine-fluvio-deltaic sedimentary
complex. The sands, mainly channel sands, are discontinuous but abundant in the
upper part of the sequence which becomes shalier at depth – lower Dentale and
Cardita formation. The source of hydrocarbon is in the intra-Dentale brownish shales
which are rich in lignitic and coal debris.
Gamba Reservoir Rock
The gamba formation is a good reservoir objective only if its thickness exceeds about
20m. At shallow depths as in Rabi-Kounga the porosities range from 20to 30% and
permeabilities may vary from 100 to over 1000 mD
The Gamba sands are generally massive, well sorted and homogeneous and are
interpreted as deposited in a lacustrine/coastal environment. Over the field, their
reservoir characteristics are generally very good. Some tightly cemented dolomitic
layers occur at the top of the Gamba sands
Migration Pathways, Faults and Traps and Seals Mechanisms
There is a complex pathway documented in the West Africa Salt Basin. This
involves the migration of pre-salt sourced oil to post-salt reservoirs. Such migration
pathways are indicated by comparison of the fingerprints of such oils and source
shales which appear to match. The Gamba sands act as a migration route. Thus to
reach the post-salt section the upper Gamba shales could be thin or the Gamba must
be fault juxtaposed with the younger section.
Juxtaposition by faulting is also common in both the pre-salt and post salt.
Erosional juxtaposition at regional unconformities also occurs primarily in the presalt.
The Gamba sands may rest uncomfortably in Melania shales, which may in turn rest
uncomfortably on Lucina shales. The Gabon Margin is segmented by major normal
faults parallel to the present-day coastline and by NE–SW trending strike-slip faults
defining zones with partly different tectonic and stratigraphic histories.
From the above it is obvious that although such migrations are documented, very specific local circumstances must prevail for them to occur.
Salt (Ezanga Salt) provides the seal for the Dentale and Gamba reservoir.
Traps related to the Dentale Formation are characteristically arcuate, narrow
enlogated and broken by faults into separate tilted compartments. They may also
suffer from unsatisfactory intra-Dentale sealing conditions. Seals are not sufficiently
well developed in the more prospective upper part of the succession to hold large oil
Fig 15 -
accumulations against fault-traps except perhaps where an absence of the overlying
Gamba sandstone allows pressure sealing to develop below the salt.
Potential hydrocarbon traps are manifested as horst, tilted fault blocks and
stratigraphic traps (pinchouts, truncations and aggradational carbonate build-ups).
These traps are potentially sealed by syn-rift shale (Vebo Shale) and Aptian age
transitional-phase evapourites (Ezanga Salt)
Production HistoryRabi-Kounga Field Production Characteristics
The above shows the field Production characteristics
Production testing yielded maximum rates of 448 m3/d on a 5/8 inch choke from the
oil zone, and 280000 m3/d on a 1/2-inch choke from the gas interval.
Pressure Regime
The technical challenges of obtaining a high oil recovery in the Rabi-Kounga
reservoir environment, therefore, centre on making the best possible use of the very
large fluid expansion potential available by avoiding excessive free gas production
which may result in resaturation losses.
In Rabi, the potential for such losses is compounded by the presence of a continuous
overlying sand unit (Gamba Sand Member) which enables fluid communication to
occur between compartments. Through the conduit of the Gamba Sand Member, any
pressure depletion can possibly be communicated via the gas cap without direct oil
drainage.
Field Study 2
Field Location and Discovery
The second field study is the Gamba Field.
The field was discovered in 1967 and had since produced over 200 million
barrels of oil. Little information is present about this field because very little
documentation was done on this field5. It produced oil from sandstones of the Gamba
formation. The Rabi-Kounga field produce from this formation but its source rock is
the same as the Gamba field which is the Melenia source rocks.
The oil is trapped in an aerially large, low relief anticlinal complex that
drapes the basement high of the Gamba Horst adjacent to Hinge Zone 1. (See figure
GAB 1-appendices and fig 2) The anticlinal complex is broken into three fields by
east-west faults that offset the Gamba Horst. Vertical seal to the Gamba sands is
provided by both Gamba shales and the overlying Ezanga salts. The Rabi-Kounga
field has seals which is also Ezanga salts (evporites)
Discovery of the Gamba field.The discovery of the Gamba field involves geological field work, aerial
photography, air magnetometry, gravity and a small amount of seismic reflection
(440Km) and refraction surveys (131Km). On the basis of the results of
aeromagnetics and refraction seismic two highs were identified. The first well,
Kissenda-1, was dry. No reservoirs or significant shows of hydrocarbons were
encountered. However, the second well drilled close to the coast, in the later part of
1963 discovered the Gamba Field (see fig Gab 1). The oil was found at a much
shallower level than expected, in a transgressive sand at the base of a Lower
Cretaceous salt sequence (Gamba Sst., see Figs 7 and 9 appendix).
5 Márcio Rocha Mello, Barry Jay Katz, AAPG Petroleum systems of South Atlantic margins
Source Rocks Characteristics
Melania Shales
The oil is sourced primarily from the organic rich lacustrine Melania shales
(see figure 16) that subcrop the base Gamba unconformity over large areas of the
Gamba Horst (refer to Fig 11).The organic rich but also thin Gamba shales may have
contribute a minor part of the oil.
Refer to the Rabi-Kounga field source rock for the oil window and kerogen type as it
has the same source rock as the Gamba field
Gamba Shales
These are organic rich shales of the upper members of the Gamba formation.
They are organic rich marine shale but are usually less than 50m thick so have limited
generating potential as stated before but may be of local significance.
Reservoir Characteristics, Trap and Seal MechanismsThe structural development at the Gamba level is probably due to both post-
depositional compactions across the horst, together with minor reactivation of the
horst, probably in the Albian.
The migration of oil into the structure probably began in the latest Cretaceous
and has continued through to the present day with the main oil generating phase
occurring in the Paleogene.
As stated before the vertical seal of the Gamba sands is provided by both
Gamba shales and the overlying Ezanga salts. These are the same types of seal found
in the Rabi-Kounga field of the Gamba and Dentale formations. Gamba formation has
porosity from 20-30% and permeability from 100mD to 5D.
Production HistoryThe field, which came on stream in February 1967, has now produced some 42
million stm3 and still accounts for a production of some 1500 stm3/d.
DiscussionTo start the discussion of the findings the comparison of the fields in the
Gabon Basin would be compared to another field in the same basin and a different
basin.
The Rabi-Kounga and Gamba field has many similarities. The both fields have
the same source rock which is organic rich lacustrine Melania shales. Also noteworthy
is that the two fields has the same reservoir rock which is the Gamba sands
Gamba Reservoir inProperties Rabi-Kounga Field Gamba FieldPorosity 20-30% 20-30%
Permeability 100 -1000mD 100mD to 5D
Seals Salt Salt, vertical
The above compares the Gamba reservoir sand in 2 different fields
Current production in Rabi-Kounga field= 280,000 m3/d, Gamba field=1,500stm3/d
The field that also has similar characteristics to the Rabi-Kounga is the La
Luna field in the Maracaibo Basin in North- Western Venezuela. The trends that are
similar include the depositional history, lithology, sedimentary bodies. The time
period of the La Luna formation is the Cretaceous period in which the Melania Shales
in Gabon basin also originated from. To compare the depositional environment of the
La Rosa field sands to that of the Dentale reservoir is that the La Rosa field was
deposited in a fluvio-deltaic environment which consisted of continental deposits
overlaying an unconformity which cut Eocene units. The Dentale reservoir sands
depositional environment is related, in that it is a fluviatile to lacustrine-fluvio-deltaic
sedimentary complex which also cut the Eocene unconformity. The thickness of the
dentale reservoir is about 30m thick unlike that of the La Rosa sands range form 5-60
feet.
The comparison of porosity and permeability in the Gabon sub-basin and the
Maracaibo basin is summarise as follows
Basin South Gabon-Sub basin Maracaibo basinProperties Melania source rk La Luna source rk
Age Cretaceous Cretaceous
Mode ofDeposition Quiet euxinic rift lakes
Shelf to slope environment under anoxic conditions
Kerogen type
type I to III Type II
TOC 6.1% 11.2%
The Source Rocks
Basin Gabon South Sub-basin Maracaibo basin
Properties Dentale formation
Gamba formation (Rabi-kounga)
La Rosa Barua
Porosity 30% 20-30% 25-30% 8-20%
Permeability up to 1000mD 100 -1000mD 100-1126mD 50-200mD
lithology sandstone sandstone sandstone sandstoneDepositional Environment
fluviatile to lacustrine-fluvio-deltaic
lacustrine/coastal fluvio-deltaic formed during the mountain building process
Te
rtia
ry
Pa
leog
ene
Ne
og
ene
Mio
cen
eP
aleo
cene
Eoc
ene
PERIOD
Te
rtia
ry
Cre
tace
ous
LA
TE
EPOCH LITHOSTRATIGRAPHY
Olig
ocen
e
Pliocene
Pleistocene
CE
NO
ZO
ICM
ES
OZ
OIC
ER
A
Major eustatic sea level fall
East Africa Basin, Albertine graben sandstone
East Africa Basin, Albertine graben, sandstone
Gabon Basin, Melania shales
Miocene to recent time, several eustatic sea level changes occurred
Basin, Source Name, Type
Maracaibo basin, La Luna Shales
Michigan Basin limestone with interbedded thin shales
Gabon basin unconformites
EOCENE unconformity in Maracaibo basinAnd Gabon basin
Cre
tace
ous
ME
SO
ZO
IC
Jurassic
Triassic
PA
LE
OZ
OIC
PRECAMBRIAN
Permian
Pennsylvanian
Mississippian
Devonian
Silurian
Ordovician
Cambrian
Car
boni
fero
us
EARLY
PERIOD EPOCH
ER
A
Lithostratigraphy
East Africa Basin con’t
Eastern Venezuela Basin, Querecual formation, Mudstone
South Gabon Sub basin, Gamba and Dentale sand/shale sequence of the Dianongo Basin Form part of African-South American rift valley system.
Illinois Basin, New Albany Shale.(deposition to migration)
Basin, Source Name, Type
North Sea Basin, Kimmeridgiam shales
Western Canada Basin, Evie shale
Maracaibo Basin con’t
Michigan Basin con’t
North Slope Alaska, Shublik, sandstoneMiddle to upper Triassic,
The above compares the basin’s source rock ages and type (shales or sandstone)
as they appear on the Geological timescale. Most of the source rocks formed in
the Cretaceous period.
Next the effects of sea level changes on the stratigraphy, sediment supply and deposition of various basins are to be compared to the South Gabon Sub-Basin.
Basin Effect of sea level changes on:Stratigraphy Sediment supply Deposition
Gabon
South Sub
Basin
Extensive
erosion and deep
channeling of
the older units
Lowstand events
introduced coarse
clastics into former
deep marine,
Submarine channels
sands, base-of slope
fans clastics and
deep-marine, and
basin floor fan
complex clastics
Coarse sediments
deposited on Eocene
unconformity
Michigan
basin
As time
progressed the
floor of each sea
became the
basin of its
successor
causing each sea
to be smaller
than its
predecessor
20-40 minor
regression and
transgressions
Different sea
types brought
different
sediments eg silt,
salt, organic
material, mud
Periods warm clear
seas, muddy seas,
desert conditions and
shallow and swampy
seas, these different
type of seas deposited
different type of
sediments
Illinois Basin
Withdrawal of
sea resulted in
valley system
with canyons
200 or more feet
deep and with
extensive karst
topography
Structurally open
basin filled with
thicker, finer
deep-water
sediments
Solution features
developed upon the
readily dissolved
Lower Ordovician
terrain
Maracaibo Basin
Lowstand
system tracts
and
Lowstand events.
Sediment supply
due to
Formation of deltas in
sea with drop in
eustatic sea level.
transgressive
system tracts.
transgression and
Fluvial erosion
Due to Regression
With respect to the Gabon Basin the, type of kerogen formed in the Melania source
rock in euxinic rift lakes are type I which suggested that the source is lacustrine,
hydrogen rich and generates oil accompanied by or followed by gas. In the east
(onshore) when approaching the basement outcrop Type I kerogen, passes into Type
III. This suggests that there is a change in the source of material being deposited. That
is from a lacustrine source (organic material deposited in an anoxic lake) to a woody
terrestrial source.
The types of faults present are normal faults parallel to the present-day coastline and
by NE-SW trending strike-slip faults defining zones with partly different tectonic and
stratigraphic histories.6
If a well was to be drilled it would be best to drill it where the reservoir is
compartmentalized by faults and effectively sealed at the top and the faultlines to
prevent migration to another low pressure reservoir.
ConclusionThe South Gabon Sub Basin holds great potential in its pre-salt in the Rabi-Kounga
oil and gas reservoirs which consist of the Gamba and Dentale sequence of the
Dianongo Basin which formed part of the Lower Cretaceous African-South American
rift valley system.
6 www.sciencedirect.com
References
Petroconsultant, 1989. Gabon Basin. The 8th Licensing Round, Gabon.
Dupre´, S., Bertotti, G., and Cloetingh, 2006. Tectonic history along the South Gabon Basin: Anomalous early post-rift subsidence. Marine and Petroleum Geology 24 (2007):151-172.
Discovery and Development of the Rabi-Kounga Field: A Giant Oil Field in a Rift
Basin Onshore Gabon. 1988. Gamba, Gabon:Shell Gabon Co.
Journal, CGGVeritas. 2009. Hunting the Pre-Salt. Geo ExPro 6
Appendix
Figure 16
1
3
2
Gamba/Invinga/Totou field
Rabi-Kounga
Petroconsultants
Figure 5/1
Recommended