a sequence stratigraphic approach to exploration and …€¦ · a sequence stratigraphic approach...

Ranger Oil F:/cotedivo/westafricaconf/paper99 1

A SEQUENCE STRATIGRAPHIC APPROACH TO EXPLORATION AND

RE-DEVELOPMENT IN THE ABIDJAN MARGIN, COTE D’IVOIRE

Justin Morrison

Ranger Oil, Walnut Tree Close, Guildford, Surrey, GU1 4US, UK.

Juliette Tea, Bleoue N’Zalasse and Victor Boblai

PETROCI, Imm.Les Heveas – 14, Bd. Carde, B.P.V 194, Abidjan, Côte d’Ivoire.

With contributions from:

Geomark Research, Robertson Research, TimeTrax Ltd, and Western Geophysical

Offshore West Africa ’99 Conference and Exhibition

March 23-25, 1999 Abidjan, Côte d’Ivoire

ABSTRACT

The Abidjan Margin, Côte d’Ivoire, 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

West Africa.

GENERALISED STRATIGRAPHY

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 Côte d’Ivoire

(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


offshore Ghana (Tucker 1992).

The oldest stratigraphy penetrated offshore Côte d’Ivoire 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

ALBIAN

APTIAN

END OLIGOCENESEA LEVEL FALL

BREAK-UP &ASSOCIATED

-TRANSPRESSION

CONTINENTALDRIFT

PRE RIFT

MAIN PHASE OFRIFTING BETWEEN

COTE D'IVOIREAND BRAZIL

?

Submarine Fan

TE

RT

IAR

YC

RE

TA

CE

OU

S

RECENTTO

MIOCENE

EOCENE

PALAEOCENE

MAASTRITCHTIAN

CAMPANIAN

LOWERSENONIAN

+TURONIAN

CENOMANIAN

JURASSIC

TRIASSIC

PERMOCARBON-IFEROUS

DEVONIAN

PRE-CAMBIAN

BASEMENT

Shallow Marine Sands

Marine Sands

LacustrineTurbiditic Sandstones

Unknown Quality

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, Côte d'Ivoire


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

Côte d’Ivoire into Ghana. It is bounded to the east and west by major strike slip faults, the

Romanche and St Paul’s 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 Côte d’Ivoire 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, Côte d'Ivoire

PANTHERE

LION

FOXTROT

Cote d'Ivoire

GhanaGAZELLE

KUDU

ELAND

IBEX

NORTHTANO

SOUTHTANO

ABIDJAN

GULF OF G

UINEA

ABYSSAL PLAIN

MARGINAL RIDGE OF R

OMANCHE FRACTURE ZONES O U T H T A N O

S U B B A S I N

SALTPONDPALAEOZOIC

BASIN

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 ENS

A 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

3000

2000

1000

100

SHALLOWBASEMENT

St. PAUL'S

FZ

WEST GRANDLAHOU

DEPRESSION

SOUTHGRANDLAHOU

DEPRESSION

H

C O T E D ' I V O I R E

A B Y S S A L P L A I N

4°00'W

4°00'W

2°00'W

2°00'W

5°00'N

4°00'N

3°00'W

3°00'W

5°00'W

5°00'W

50 Kilometres

OCEANIC-CONTINENTAL BOUNDARY (Mascle et al 96)

ESPOIR

QUEBECSOUTHBELIER

JACQUEVILLETROUGH

BELIER


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 Paul’s 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.


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)

LITHOLOGYGR

DE

PT

H

SONICPALYNOMORPH

ABUNDANCE

PALYNOMORPH ABUNDANCE

PALYNOMORPHDIVERSITY

PALYNOMORPH DIVERSITY

140

100.5 SB99.5 MFS

99 SB

? 101 MFS? 103 SB

98.25 MFS

98 SB

0 00 40 700 50150

DINOCYST

DINOCYST

PRASINOPHYTE ALGAE

PRASINOPHYTE ALGAE

FRESHWATER ALGAE

FRESHWATER ALGAE

MIOSPORE

MIOSPORE

1000

ft

West Espoir

XIII

XI

X

IX

VIII

VII

VI

V

IV

III

II

I

XII

East Espoir

10

0 f

t

00

80

GR GR

80

98 SB

96.5 SB97 MFS


ESPOIR SEDIMENTOLOGY

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

S S

S

S

S

S

S S S

S

S S

S

S

S

S

S

S

M

M

M

S

S

M

M

M

M

M

M

M

Py

Py

M

M

LITH

SAND

SIL

TY

CL

AY

AN

D/

CO

AL

GRAIN SIZE

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

VV Red

PyM

M

M

M

M

M

M

M

M

M

M

M

M

M

M

S

S

S

S

S

S

SS

S

S

Fe

MUDSTONE :

CLAY :

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

10ft

10f

t

98 SB

LITH

SAND

SIL

TY

CL

AY

AN

D/

CO

AL

GRAIN SIZE


REGIONAL RESERVOIRS

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

Oligocene.

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).


Gross 160ftNet Sand 72ftAv. Porosity 21%

Net Pay 59ftAv. Porosity 21%Av. Sw 39%

DST#1 6502-6563ft

4045BOPD(33°API)+ 1.7 mscf

GR

Porosity x Sw

Porosity0 0

0

150 0.3

0.3

GR Porosity0 0150 0.3


93 SB

GR

Porosity x Sw

Porosity0 0

0

150 0.3

0.3

Gross 935ftNet Sand 409ft

Av. Porosity 21%

Net Pay 278ftAv. Porosity 22.5%Av. Sw 28%

Seven DST’s run between 6530-7040ftWith Flow rates up to

4959BOPD

GR

Porosity x Sw

Porosity0 0

0

150 0.3

0.3


GR

Porosity x Sw

Porosity0 0

0

150 0.3

0.3

Gross 653ftNet Sand 329ft

Av. Porosity 21%

FIT #2 at 5743ftGas & Oil, (41°API)

Net Pay 32ftAv. Porosity 21%Av. Sw 47%

100

ft

100

ft 100f

t

100f

t10

00ft

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, Côte, d’Ivoire

Figure 6a : Middle Albian Sandstone


Figure 7a : Reprocessed 1980 Seismic through the Central Abidjan Margin & Espoir Field

Figure 7b : Western 1997 Seismic Line across the Grand Bassam Sub Basin

6146m

1 s

ec

on

d

S N

WEST ESPOIRJACQUE VILLE

TROUGHFOXTROT FAULT BLOCK

Base Miocene67 SB68 SB79 SB83 SB93 SB96.5 SB99 SB

1

se

co

nd

S N

Base Miocene67 SB68 SB79 SB83 SB93 SB96.5 SB

1

se

co

nd

1 s

ec

on

d

INTRA BASINAL HIGHGRAND BASSAM

MARGIN


CRETACEOUS CHRONOSTRATIGRAPHY

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-

67 Ma).

ALBIAN

CENOMANIAN

TURONIAN

CONIACIAN

SANTONIAN

CAMPANIAN

MAASTRICH-TIAN

STANDARDSTRATIGRAPHY

SEQUENCECHRONOSTRATIGRAPHY

(Haq et al)GRAND BASSAM SUB BASIN GRAND BASSAM

SHELF MARGINESPOIR HIGH JACQUEVILLE

TROUGH

INTRABASINHIGH

FOXTROTFAULTBLOCK

ESPOIRFIELD

RELATIVE CHANGE OFCOASTAL ONLAP

Landward Basinward

1.0 0.5 0

ST

AG

ES

EX

TEN

T O

FS

TA

GE

STR

AT

OTY

PE

SE

QU

EN

CE

BO

UN

DA

RY

AG

E

U

U

U

U

U

74

84

88

89

92

96

U68 68

LagunesFault

71 71

75 75

77.5 77.5

79 79

80 80

83 83

85 85

87.5 87.5

88.5 88.5

90 90

91 91

93 93

94 94

95.5 95.5

96.5 96.5

9899 99

103 103

106 106

107.5 107.5

U

M

M

M

L

L

L

L

L

L

L

Marine Claystone

Sandstone

Lacustrine / Paralic Claystone

Limestone

Onlap

Erosional truncation

Fault

Figure 8 : Outline of the Cretaceous Chronostratigraphy of the Abidjan Margin, Côte d'Ivoire


CONCLUSIONS

The Abidjan Margin, Côte d’Ivoire 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

fields.

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

mmbo recoverable).

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

Côte d’Ivoire.

ACKNOWLEDGEMENTS

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 Côte d’Ivoire Limited, Gentry International (Côte d’Ivoire) Inc.,

Clyde Expro plc, Pan Canadian, Societe Nationale d’Operations Petrolieres de la Côte d’Ivoire,

Svenska Petroleum Exploration AB, T.C. Petroleum Inc., and Tullow Côte d’Ivoire Limited.

REFERENCES

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,

pages 65-86.

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.


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,

pages 153-159.

a sequence stratigraphic approach to exploration and …€¦ · a sequence stratigraphic approach...

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