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Ranger Oil F:/cotedivo/westafricaconf/paper99 1
A SEQUENCE STRATIGRAPHIC APPROACH TO EXPLORATION AND
RE-DEVELOPMENT IN THE ABIDJAN MARGIN, COTE DIVOIRE
Ranger Oil, Walnut Tree Close, Guildford, Surrey, GU1 4US, UK.
Juliette Tea, Bleoue NZalasse and Victor Boblai
PETROCI, Imm.Les Heveas 14, Bd. Carde, B.P.V 194, Abidjan, Cte dIvoire.
With contributions from:
Geomark Research, Robertson Research, TimeTrax Ltd, and Western Geophysical
Offshore West Africa 99 Conference and Exhibition
March 23-25, 1999 Abidjan, Cte dIvoire
The Abidjan Margin, Cte dIvoire, has been the subject of exploration activity for forty years
and has generated reasonable hydrocarbon success. From 1974-1992 the Espoir and Belier
fields were produced with 31 mmbo and 20 mmbo of cumulative production respectively. More
recently, as a result of improved fiscal terms established by the Ivorian government, several
independent oil companies have come to the area to develop existing discoveries (Lion,
Panthere, Gazelle, and Foxtrot), re-develop old fields (Espoir with 93 mmbo and 190 bcf of
remaining recoverable reserves) and explore for new hydrocarbon traps.
A sequence stratigraphic model for the Upper Cretaceous to Palaeogene sediments has been
created for the Abidjan Margin. It integrates high resolution biostratigraphy, sedimentological
analysis on the Upper Albian Espoir sandstone and regional seismic data. The sequence
stratigraphic model will be used to predict reservoir presence in the Espoir field re-development,
as well as exploration targets in the shallow and deeper water areas of the Abidjan Margin.
New technologies such as integrated sequence stratigraphic models, deep water drilling, and low
cost shallow water development have given oil companies new opportunities in this region of
The generalised stratigraphy of the Abidjan Margin is shown in Figure 1 and includes the main
hydrocarbon plays in the Albian and Upper Cretaceous identified offshore Cte dIvoire
(Chierici 1996 and Tucker 1992). Further speculative hydrocarbon plays exist in the Tertiary,
lower syn-rift Early Albian-Aptian and pre-rift section.
Pre-rift stratigraphy may include Devonian sandstones productive in the Saltpond Basin, as well
as Permian and Carboniferous sands penetrated on the Tano High offshore Ghana.
Rifting was initiated in the early Aptian and sands of this age are preserved on the Tano High
Ranger Oil F:/cotedivo/westafricaconf/paper99 2
offshore Ghana (Tucker 1992).
The oldest stratigraphy penetrated offshore Cte dIvoire are Lower Albian syn-rift sediments.
During the Apto-Albian times the basin was enclosed with the deposition of lacustrine
claystones as well as turbidite and delta sandstone reservoirs. The Middle Albian turbidites are
the reservoir in the Foxtrot gas field and Upper Albian turbidite/delta sandstones are the
reservoir in the Espoir and Lion oil fields.
After continental break-up at the end of the Albian, a series of delta and submarine fan systems
deposited Upper Cretaceous sands across the Abidjan Margin. These include Cenomanian
SubmarineChannel Sands+ Turbidites
PASSIVE MARGINDOWN WARPING
END OLIGOCENESEA LEVEL FALL
MAIN PHASE OFRIFTING BETWEEN
COTE D'IVOIREAND BRAZIL
Shallow Marine Sands
Sands Present InAccra - Keta andVoltaian Basin
Early Rift TerrestrialSands Passing UpInto Shallow Lacustrine
Incised Channel FillLacustrine / Marine Delta/ Turbidite Sandstones
Figure 1 : Generalised Stratigraphy of the Abidjan Margin, Cte d'Ivoire
Ranger Oil F:/cotedivo/westafricaconf/paper99 3
reservoirs (Panthere gas field), Lower Senonian reservoirs (Belier oil field), Campanian sands
(B-3X gas/condensate discovery), and the Maastrichtian reservoirs (the Ibex and Lion fields).
Source rocks have been identified in the syn-rift Albian lacustrine shales and multiple Upper
Cretaceous marine claystones.
TECTONIC ELEMENTS AND STRUCTURAL HISTORY
The Abidjan Margin shown in Figure 2 is predominantly an offshore basin running from the
Cte dIvoire into Ghana. It is bounded to the east and west by major strike slip faults, the
Romanche and St Pauls Fracture Zones respectively. To the north the basin is bounded by the
Lagunes fault system made up of a series of east west normal faults down-thrown to the south.
To the south the basin is bounded by the continental-oceanic crustal boundary recognised by
Mascle et al (1996).
The basin is believed to have begun rifting in the early Aptian (or Barremian) times. Although
the oldest rocks penetrated offshore Cte dIvoire are of Lower Albian age, wells offshore
Ghana have penetrated syn-rift Aptian stratigraphy as well as pre-rift Permian, Carboniferous
and Devonian rocks.
Continental break-up occurred at the end of the Albian, when a series of Albian highs were
created which include the Foxtrot, Espoir and Quebec Highs. These highs are made up of
numerous tilted fault blocks cut by faults trending westnorthwest-eastsoutheast. The highs are
often bounded to the north by a series of en-echelon normal faults down-throwing into the
Jacqueville Trough to the north. The highs are also bounded on their southern flank by a series
Figure 2 : Tectonic Elements of the Abidjan Margin, Cte d'Ivoire
ES O U T H T A N O
S U B B A S I N
G R A N I T I C A N D M E T A M O R P H I CB A S E M E N T
G R A N I T I C A N D M E T A M O R P H I CB A S E M E N T
LAGU EN SA F ULT
EAST TANOSUB BASIN
G R A N D B A S S A M
S U B B A S I N
S T R E T C H E DC O N T I N E N T A L
C R U S T
C O T E D ' I V O I R E
A B Y S S A L P L A I N
OCEANIC-CONTINENTAL BOUNDARY (Mascle et al 96)
Ranger Oil F:/cotedivo/westafricaconf/paper99 4
of en-echelon faults down-throwing to the south into the Grand Bassam Sub Basin.
The en-echelon nature of the Top Albian Highs, their internal faults and the potential offsets
seen on the Lagunes Fault, implies that transfer zones may be present trending northeast-
southwest and related to the major strike slip faults bounding the basin to the east and west
(Romanche and St Pauls Fault Zones).
HIGH RESOLUTION BIOSTRATIGRAPHY
Recent work by TimeTrax on behalf of Ranger Oil and Partners has concentrated on quantitative
biostratigraphic analysis of palynoflora, microfossils and nannofossils from numerous wells in
the Abidjan Margin. The analysis that TimeTrax have evolved (Janice Weston Pers. Com.) is
similar to that published by Armentrout (1996) in the Gulf of Mexico, and is based on the
analysis of the exact same size of sample at regular intervals down-hole. Each sample is
measured for abundance and diversity of the individual species and histograms are then
generated. Periods of maximum abundance and diversity are candidates for Maximum Flooding
Surfaces. Where the abundance and diversity increases gradually up hole, a potential
transgressive systems tract may be present often associated with significant changes in
assemblages. Conversely, if the abundance and diversity dramatically decrease up hole, then a
sequence boundary may be present.
Using the abundance and diversity data in combination with diagnostic species allows Third
Order Cyclicity to be recorded and tied to world wide sea level curves (Haq et al. 1987 & 1988).
The biostratigraphic interpretation of a West Espoir well is shown in Figure 3, where the main
Espoir sands are of Upper Albian age between the 98.25 Ma Maximum Flooding Surface and
the 98 Ma Sequence Boundary.
The recognition of sequence stratigraphic units can be particularly strong when two
biostratigraphic disciplines are combined such as the use of microfossils and nannofossils. This
has been especially useful in differentiating the Upper Cretaceous and Tertiary sequences.
ESPOIR SANDSTONE RESERVOIR CORRELATION
Correlation of West and East Espoir (Figure 4) indicates that the Espoir reservoir sandstones
were predominantly deposited in the 98.25-98 Ma Highstand Systems Tract. It should be noted,
however, that the upper sand reservoir unit XIII was deposited between the 98 Ma Sequence
Boundary and the 97 Ma Maximum Flooding Surface and hence could be either a Lowstand or a
Transgressive Systems Tract. The recognition of the different sequences will help to predict the
changes in sandstone geometries across the Espoir field.
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Figure 3 : Quantitative Palynological Analysis of a West Espoir Well - Albian Section
Figure 4 : Reservoir Correlation from West to East Espoir (Espoir Reservoir Zonation I-XIII)
100.5 SB99.5 MFS
? 101 MFS? 103 SB
0 00 40 700 50150
96.5 SB97 MFS
Ranger Oil F:/cotedivo/westafricaconf/paper99 6
In March 1998 the Espoir group performed a reservoir study on core from four wells in block
CI-26 (wells C1-1X, C1-3X, A-1X, and A-3X). The study was directed at sedimentological
logging, reservoir quality and petrological analysis.
The sedimentary log from a West Espoir well which covers the 98 Ma Sequence Boundary is
shown in Figure 5a, where the lithology changes from sandstone (commonly friable, micaceous,
carbonaceous, with metamorphic fragments, siderite clasts and locally bioclastic debris) up-hole
into mudstone (alternating laminations with differing colour, calcite content and swelling clays).
A sedimentary core log from deeper in the West Espoir well (Figure 5b) shows sandstone beds,
commonly structureless, with occasional dish and pipe structures indicating de-watering. The
sands often exhibit scoured bases and include rip-up clasts.
The Espoir sands have been interpreted as being deposited in a lacustrine delta front to shelf
slope environment. The latter includes Bouma Sequences tbc and tab.
Figure 5a: Sedimentary Log - West Espoir Well -
Top Espoir Reservoir
Figure 5b: Sedimentary Log - West Espoir Well -
Lower Espoir Reservoir
S S S
Thick beds are commonlystructureless or displaydewatering dish structuresand pipes
Possible Bouma sequences
Bases are commonly sharp,scoured and locally grooved
Thin beds are commonlyplanar or current rippled
Abundant plant debris, micaand claystone rip-up clasts
Alternating laminations withdiffering :-colour, calcite content andswelling clays
Local slumps andsynsedimentary fractures
MINOR LIMESTONES & SANDSTONES : Common current ripples, rarewave ripples and common loadstructures
SANDSTONE : Commonly friable, micaceous,carbonaceous,metamorphic fragments,siderite clasts and locallybioclastic
Locally cross-bedded, planarlaminated, current and waveripples or slumped, dishstructures, scouredbases and locally loaded
Ranger Oil F:/cotedivo/westafricaconf/paper99 7
Regional reservoirs have been identified in the Middle Albian, Upper Albian, Cenomanian,
Lower Senonian and Maastrichtian (examples of the regional reservoirs are shown in Figure 6).
The Middle Albian sandstone reservoir can be seen in Figure 6a, with a gross interval of 4381 ft,
net sandstone reservoir of 2989ft and average porosity of 15%.
The Upper Albian sandstone reservoir can be seen in Figure 6b with 935ft of gross section, net
sand of 409 ft and average porosity of 21%. This well also has net pay of 278ft with average
porosity of 22.5% and average water saturation of 28%.
Cenomanian reservoirs have been identified on the Quebec High (Figure 6c), with gross section
of 447ft, net sandstone reservoir of 259ft and average porosity of 20%. A Lower Senonian
sandstone reservoir from the Belier Field is shown in Figure 6d, with 160ft of gross section, 72ft
of net sandstone reservoir and average porosity of 21%. This well had 59ft of net pay, with
average porosity of 21% and average water saturation of 39%.
The Maastrichtian reservoir section shown in a south Belier High well (Figure 6e) has good
reservoir potential with 653ft of gross section, 329ft of net sandstone reservoir and average
porosity of 21%. It should be noted that a 32ft hydrocarbon column was identified in this well.
Further potential sandstone reservoirs have been noted in the Aptian, Palaeocene, Eocene and
SEISMIC SEQUENCE STRATIGRAPHY
Regional seismic lines running north-south through the Central Abidjan Margin can be seen in
Figures 7a &7b. The West Espoir Field (Figure 7a) is located in the first tilted fault block at the
shelf break (Grillot et al. 1991), with a fault block on trend with the Foxtrot Field to the south
and the Jacqueville Trough to the north. The Top Albian Unconformity (96.5 Ma Sequence
Boundary) can be observed eroding the Espoir sandstone reservoir both to the north and south of
the Espoir field.
The Western seismic line in Figure 7b runs north-south into deeper water. It can be observed
from this line that Apto-Albian syn-rift reflections are present at the southern end of this line,
considerably further offshore than previously recognised. Secondly, there is a clear Cenomanian
basin, here named the Grand Bassam Sub Basin, which is present to the south of the shelf break
created during continental break up at the end of the Albian. Deepwater hydrocarbon plays
present in the Grand Bassam Sub Basin include Apto-Albian syn-rift sandstones in the intra-
basinal highs, Cenomanian and younger sand drapes over intra-basinal highs, Albian sands in
tilted fault blocks and multiple sandstone pinch-outs on the southern side of the shelf break (in
the Cenomanian, Lower Senonian and Maastrichtian).
Ranger Oil F:/cotedivo/westafricaconf/paper99 8
Gross 160ftNet Sand 72ftAv. Porosity 21%
Net Pay 59ftAv. Porosity 21%Av. Sw 39%
4045BOPD(33API)+ 1.7 mscf
Porosity x Sw
GR Porosity0 0150 0.3
Gross 447ftNet Sand 259ftAv. Porosity 20%
Porosity x Sw
Gross 935ftNet Sand 409ft
Av. Porosity 21%
Net Pay 278ftAv. Porosity 22.5%Av. Sw 28%
Seven DSTs run between 6530-7040ftWith Flow rates up to
Porosity x Sw
Gross 4381ftNet Sand 2989ftAv. Porosity 15%
Porosity x Sw
Gross 653ftNet Sand 329ft
Av. Porosity 21%
FIT #2 at 5743ftGas & Oil, (41API)
Net Pay 32ftAv. Porosity 21%Av. Sw 47%
Figure 6b : Upper Albian Sandstone
Figure 6c : Cenomanian Sandstone Figure 6d : Lower Senonian Sandstone
Figure 6e : Maastrichtian Sandstone
Figure 6 : Regional Reservoirs of the Abidjan Margin, Cte, dIvoire
Figure 6a : Middle Albian Sandstone
Ranger Oil F:/cotedivo/westafricaconf/paper99 9
Figure 7a : Reprocessed 1980 Seismic through the Central Abidjan Margin & Espoir Field
Figure 7b : Western 1997 Seismic Line across the Grand Bassam Sub Basin
WEST ESPOIRJACQUE VILLE
TROUGHFOXTROT FAULT BLOCK
Base Miocene67 SB68 SB79 SB83 SB93 SB96.5 SB99 SB
Base Miocene67 SB68 SB79 SB83 SB93 SB96.5 SB
INTRA BASINAL HIGHGRAND BASSAM
Ranger Oil F:/cotedivo/westafricaconf/paper99 10
An outline of the Cretaceous Chronostratigraphy of the Abidjan Margin is shown in Figure 8.
The chronostratigraphy has been based on quantitative biostratigraphic analysis of shallow water
wells, tied to seismic in shallow and deep water areas of the basin.
Numerous sequence boundaries have been identified which are related to erosion at the basin
margin, shelf break and occasionally at the intra-basinal highs. The 96.5 Ma sequence boundary
(taken as the end of the Albian by TimeTrax) is a major unconformity which marks the
continental break-up and the change from an isolated basin to more open marine conditions. A
second significant unconformity is the 83 Ma sequence boundary which marks a dramatic
deepening of the Abidjan Margin.
In the Central Abidjan Margin, potential sandstone reservoirs exist in two Middle Albian third
order cycles (103-99 Ma), Upper Albian (98.25-98 Ma Highstand Systems Tract), three
Cenomanian third order cycles (96.5-95.5, 95.5-94 and 94-93 Ma), Campanian third order cycle
(83-80 Ma) and three Maastrichtian / Upper Campanian third order cycles (75-71, 71-68 and 68-
(Haq et al)GRAND BASSAM SUB BASIN GRAND BASSAM
SHELF MARGINESPOIR HIGH JACQUEVILLE
RELATIVE CHANGE OFCOASTAL ONLAP
1.0 0.5 0
93 9394 94
Lacustrine / Paralic Claystone
Figure 8 : Outline of the Cretaceous Chronostratigraphy of the Abidjan Margin, Cte d'Ivoire
Ranger Oil F:/cotedivo/westafricaconf/paper99 11
The Abidjan Margin, Cte dIvoire is a proven hydrocarbon basin with small to medium sized oil
and gas fields discovered to date. There are numerous proven reservoirs including the Albian,
Cenomanian, Lower Senonian and Maastrichtian sandstones. Further potential sandstone
reservoirs exist in the Aptian, Palaeocene, Eocene and Oligocene.
Future hydrocarbon activity will be directed at developing existing reserves and exploration for
new oil and gas reserves. The development activity will include the Espoir Field re-development
in block CI-26 with estimated remaining recoverable reserves of 80 mmbo and 175 bcf. Other
ongoing and future developments include the Foxtrot, Gazelle, Ibex, Eland and Kudu oil and gas
The future exploration will be directed at small to medium sized hydrocarbon prospects in
shallow water (30-100 mmbo recoverable) and large prospects in the deep water (200-1000
It is noted that a greater understanding of the Cretaceous sequence stratigraphy of the Abidjan
Margin will improve reservoir prediction for both development and exploration projects in the
The authors of this paper would like to thank the management of Ranger Oil and PETROCI for
allowing publication of this data. This paper was reviewed by Dave Pratt and David Ellis at
Ranger Oil and Janice Weston at TimeTrax. We are indebted to Steve Hedley and Roy Rees
Williams who produced the diagrams for this paper. We would also like to thank the support of
our partners: Addax Petroleum Cte dIvoire Limited, Gentry International (Cte dIvoire) Inc.,
Clyde Expro plc, Pan Canadian, Societe Nationale dOperations Petrolieres de la Cte dIvoire,
Svenska Petroleum Exploration AB, T.C. Petroleum Inc., and Tullow Cte dIvoire Limited.
Armentrout J.M. (1996), High resolution sequence biostratigraphy: examples from the Gulf of
Mexico Plio-Pleistocene, Eds: Howell, J.A and Aitken J.F., High Resolution Sequence
Stratigraphy: Innovations and Applications, Geological Society Special Publication No. 104,
Chierci M.A. (1996) Stratigraphy, palaeoenvironments and geological evolution of the Ivory
Coast-Ghana Basin. Elf Memoir 16 pages 293-303.
Grillot. L.R., Anderton P.W, Haselton T.M. and Dermargne J.F. (1991), Three-Dimensional
Seismic Interpretation: Espoir Field Area, Offshore Ivory Coast AAPG Memoir 42, Ed: A.
Brown, pages 214-217.
Ranger Oil F:/cotedivo/westafricaconf/paper99 12
Haq, B.U., Hardenbol, J. & Vail, P.R. (1987) Chronology of fluctuating sea-levels since the
Triassic. Science, Vol.235, pages 1153-1165.
Haq, B.U., Hardenbol, J. & Vail, P.R. (1988) Mesozoic and Cenozoic chronostratigraphy and
cycles of sea-level change. In: Sea-level changes: an Integrated Approach. Special Publication,
Society of Economic Paleontologists and Mineralogists, Tulsa, Vol.42, pages 40-45.
Mascle J., Lohmann, G.P., & Clift, P.D. (1996) Introduction. In: Proceedings of the Oceanic
Drilling Program, Vol.159, pages 5-16.
Tucker J.W. (1992) Aspects of the Tano Basin Stratigraphy Revealed by Recent Drilling in
Ghana. Geologie Africaine, Ler Colloque de Stratigraphie et de Paleogeographie des Bassins
Sedimentaires Quest-Africains, Libreville-Gabon 6-8 Mai 1991, Memoire 13, Ed: Curnelle,