N'Dri Koffi
Petroci
Abidjan
Eric W. Barton
Gustavson Associates Inc.
Boulder, Colo.
A recently announced significant oil discovery off Cote d'Ivoire in West Africa has piqued industry interest in the area.
The Lion 11 well, operated by United Meridian International Corp. (UMIC), encountered approximately 205 ft of log indicated net hydrocarbon pay. The three intervals tested at this writing flowed a combined 14,263 b/d of oil through 3/2 in. tubing from intervals deeper than those previously tested in the area.
The well, drilled in the CI-11 block in 70 m of water about 12 km offshore, comes several weeks after UMIC announced a sizable gas/condensate discovery in the same block. UMIC and partners plan further drilling.
Petroci, the Cote d'Ivoire state oil company and working interest partner in the two new discoveries, is offering three large evaluation concessions to the international oil industry (Fig. 1). The 5,000 sq km concessions are situated east of the above mentioned UMIC discovery; commercial production has been established on two of the concessions, and numerous seismically defined prospects are present on all three. The area is referred to as the Abidjan margin.
This current concession round represents the first rime in recent years that the region has been opened for petroleum exploration activity. In a major change, Cote d'Ivoire just reduced its income tax rate for the oil industry to 35% from 50%.
Moussa Fanny, president and director general of Petroci, stated that "we are determined to reestablish oil production in our country to the extent that many terms and conditions are now negotiable."
Cote d'Ivoire offers an extensive geological database, production history from offsetting fields, and oil shows in numerous wells. In addition to the advantageous tax regime for developers and favorable concession conditions, other recent and attractive governmental policy changes are now in effect.
A market outlet for oil is provided through a local 50,000 b/d refinery, and existing infrastructure is considered to be excellent.
A gas market is indicated by a gas fired electrical cogeneration plant currently proposed. The plant is eventually expected to be built on the coast adjacent to the new field and nearby Foxtrot field. Foxtrot has about 500 bcf of recoverable reserves and could supply fuel to the proposed plant for as long as 20 years.
Identical information about Cote d'Ivoire is available in data rooms in Abidjan and Boulder through June 31, 1994.
STRUCTURE
The Abidjan margin is considered to be a typical transform margin emplaced during the earliest Cretaceous by the progressive opening of the South Atlantic Ocean.
The origin and further development of the Cote d'Ivoire sedimentary basin took place as three distinctive stages referred to as prerift, rift, and postrift. The presence of these stages has strongly influenced the type of deformational history of the basin (Fig. 2). The structural evolution can be summarized by:
- A lengthy prerift stage that underwent a complex history involving primarily Paleozoic rocks. These earlier events have only an indirect influence on petroleum occurrence.
- A rift stage that was the site of nonmarine deposition and controlled tectonically by northeast southwest shear and northwest southeast extensional graben formation. Paleozoic, Triassic, and Lower Cretaceous rift infill sequences were affected. This stage is characterized by extensive fracturing and downwarp of the entire margin along the Lagunes fault. As much as 10,000 m of displacement may have occurred here.
Albian rocks are the primary target associated with the rift stage. The complete Albian sequences have neither been entirely seen on seismic sections nor penetrated by drilling. Traps can be differentiated into intra-Albian and uppermost Albian located immediately below the regional unconformity.
- A postrift, or drift, stage that marks the initiation of the Cenomanian transgression following the oceanic opening. This stage lasts well into the Tertiary as the sedimentary prism continues to prograde basinward. In general, gravity deformation along listric normal faults and associated rollovers are typical. Other features include slumps, glides, and growth faults that sole out in the undercompacted shales of the Paleocene.
Cenomanian to Maastrichtian objectives characterize this stage. Structural closures are less common upwards, but more subtle trapping opportunities are provided by the complex arrangement of sand bodies in local deltaic systems.
STRATIGRAPHY
The Albian has been only partly penetrated south of the Lagunes fault, which forms the northern edge of the Cote d'Ivoire sedimentary basin.
The sequence comprises mainly clastic rocks with frequent sandstone intercalations and basement element conglomerates. Provenance area for these terrigenous materials is the topographic scarp created by the Lagunes fault zone. Two main lateral subdivisions appear along the shelf within the Abidjan margin:
- The western part characterized by frequent sandstone intervals;
- The eastern portion consisting of a series of shaly and silty sections with limited evidence of sandstone occurrence in the relatively shallow depths drilled to date.
The Upper Cretaceous section in general includes three sedimentary cycles identified from the three dimensional arrangement o strata. These are the:
- Infill series
- Progradation series, and
- Gravity deposits that control the sand distribution.
The three depositional units are separated by two regional unconformities in the upper Senonian and Miocene. These features are clearly distinguished on seismic and well control.
The Cenomanian sequence is deposited over the eroded surface of the Albian rift series. It consists of shale with fine grained sandstone interbeds on the shelf area and contains very fine-grained debris flows on the slope. Channel systems including coarser grained sandstone are likely to be present in deeper offshore areas. Platform carbonates are present mainly in the lower section of the Cenomanian. The sequence generally thickens and deepens from west to east.
The distribution of the Lower Senonian sediments is related to the presence of depocenters that correspond to clastic material accumulated at the top of the slope. A north south axis of maximum thickness converging on the depocenters corresponds to the transport fairways of fluvial material coming from relatively small local rivers. Thickening to the east also characterizes the Lower Senonian.
Within the Maastrichtian of the Abidjan margin, deposition was initially controlled by paleotopography. The sequence is composed of primarily of grey, micromicaceous, calcareous, glauconitic shales. Along the eastern extent of the margin there is a clastic series with coarse grained sandstones, conglomerates, and occasional bioclastic limestones. Elsewhere one finds evidence of find to medium' locally coarse grained sandstones.
The Maastrichtian represents the youngest interval with hydrocarbon potential. The depocenter for Maastrichtian sediments is perhaps 20 km south of that of the Albian, reflecting the regressive nature of the passive continental margin.
The Paleocene Pliocene section is composed of several progradational sequences, and its thicker development is usually located slightly north of the present shelf sedimentary prism. Thinning of the section seaward is due to both the limited influx of terrigenous detrital material, and also to the truncation of the Eocene margin before Miocene deposition.
ESPOIR OIL FIELD
Espoir field is on the continental shelf edge about 19 km offshore and 60 km southwest of Abidjan in 105900 ft of water. The hydrocarbons occur in Albian sandstones at 1,800 2,136 m below sea level (Fig. 3).
The size and shape of the field is largely controlled by, faults and by the post Albian unconformity. The field is elongated northwest-southeast for about 13 km and is 3 km wide at its broadest part. The overall field area is about 25 sq km. Seven reservoir zones originally contained about 420 million bbl of oil in place.
The field was brought on stream in August 1982 with four wells in East Espoir and one well in West Espoir at an initial 18,000 20,000 b/d. Seven more development wells were drilled. The field was shut in in October 1988 after having produced 31 million st tk bbl, only 7% of the OOIP.
BELIER OIL FIELD
Belier field is in the eastern part of the Abidjan margin 15 km offshore and 30 km southeast of Abidjan in an average 60 m of water. The hydrocarbon accumulation lies in Lower Senonian sandstones at 1,885 2,005 m subsea (Fig. 4).
Belier is an anticline, elongated north south for about 4 km, and 3 km wide. Its overall extent is about 8 sq km. The vertical closure is about 140 m at top Albian and 120 m at the level of the Lower Senonian reservoir. A major east west trending normal fault downthrown to the south crosses the middle of the structure. Some other minor faults are present.
The reservoirs generally consist of coarse to very coarse grained friable sandstone with thin layers of dark grey shale and silt-stone. Porosity averages 1820%, and permeability varies from 200 500 md.
These sandstones are interpreted to be submarine fan deposits. The field axis comprises predominantly channel deposits, while the sandstone lobes are more extensive laterally. The reservoir is absent on the western flank of the structure, where levee mudstones are found.
Twenty four development wells were drilled (22 production wells and two water injection wells). Production started under natural depletion in 1980. Pressure maintenance by water injection was later initiated in four wells that had been oil producers. Production at the end of 1988 was about 18 million st tk bbl or 28% of OOIP.
FOXTROT GAS FIELD
Foxtrot field is immediately north of the continental shelf edge 15 km offshore and 80 km west southwest of Abidjan. Water is 90 100 m deep over the field's center. Hydrocarbons occur in Upper Albian sandstones lying at depths below sea level of between 2,100 2,503 m (Fig. 5).
The field's size and shape are controlled by the post-Albian unconformity. The field extends north south for 9 km, its widest part being over 8 km. The overall area is about 50 sq km. The field comprises a large paleo high cut by, four major northwest southeast trending normal faults, downthrown to the south. The Late Albian sediments dip 10 12 to the north northwest and are truncated by the post Albian unconformity. Cenomanian and Lower Senonian shales lying immediately above the unconformity provide a seal to the hydrocarbons.
The reservoir consists of a monotonous sequence of sandstone, finely interbedded with dark grey shale, siltstone, and some stringers of microcrystalline limestone. The sandstone is generally fine to very fine-grained, rarely medium to coarse grained, angular to subrounded, moderately sorted, with argillaceous and calcareous cement.
Porosity averages 21%. The permeability is generally low but increases in the uppermost section, where it averages 15 20 md with some values up to 100 md or more.
Total gas in place in the Foxtrot structure as a whole is estimated to be around 940 bscf. The field has not been developed.
