Structural styles in deep offshore West Africa

April 1, 1998
Oil-filled channel sands drilled in 1,000 meters of water. This is one of four successful wells out of 12 tests in deep offshore Niger Delta. [95,911 bytes] Diagrammatic seismic section across the Abidjan Basin showing structural style (from UMC 1997) [28,411 bytes] Diagrammatic seismic profile from deep offshore Rio Muni indicating a high angle slope and ponded sediments at depth. [25,984 bytes] Diagrammatic structural cross-section of the salt province and interior basin showing faulting and

Isolated discoveries show potential of geology

Fred Akanni
Contributing Editor
Lagos
PART II: This is Part II of a two-part series on the geology of West Africa deepwater. Part I, appearing in March, focused on the discontinuity between shallow and deepwater structural styles. Part II features the geological components in deepwater.
It is tempting for the explorationist to frame a deepwater exploration strategy around the distal extensions of known plays in shallower water off West Africa.

Companies have been known to walk away from concessions if they cannot find in extensions some prospective sequences that are related to those that have been proven shoreward. This was one of the reasons Chevron refused to participate in the 1991 bidding for deepwater concessions in Nigeria.

For purposes of this presentation, deepwater West Africa refers to water depths beginning in 200 meters and extending out to 2,500 meters, and containing most of the continental slope.

Similarly, Elf, Marathon, and Occidental Petroleum were all looking for distal extensions of known plays when they acquired some of the deepwater leases in Gabon in 1992. Using a different approach, Shell went for deepwater Gabon features that were analogous to deepwater structures in the Gulf of Mexico.

Abidjan Basin

Despite the lack of relationship, there remains a great deal of prospectivity in deepwater. For example, there are 26 untested structures in deepwater Abidjan Basin, according to United Meridien (UMC), which mapped the prospects after a speculative survey. The company says that the structures probably hold 3.6 billion bbl of oil.

That is a huge figure, if the prospectivity of the nearshore and land segments of the basin are to be used as a peg. The structural profile of the deepwater Ivory Coast is not markedly different from the architecture of the basin on the shelf and onshore, and may have been developed by mere progradation.

The pre-rift tilted blocks are not prominent in the deep offshore seismic sections. At least, they do not show up above 4 seconds (two-way-time, about 13,000 ft of straight hole). Whereas onshore or shallower offshore, they can be mapped even above 10,000 ft.

The deep Albian sediments are updip in a high angle against a set of southward-fading growth faults. They are capped by a Turonian sequence, thinning out to the south, and showing growth on the north side. The Cenomanian sequence is almost absent in the south. It's a small bulge that disappears into the Turonian to the south.

Above the Cenomanian-Turonian sequence is the Campanian shale (a massive, downwarping, reflection-less sequence on seismic), generally gliding down toward the west. Younger sediments overlie it, featuring channels and mass-flow deposits.

Such channel deposits, underlain by the massive, Campanian shale and overlain by another reflection-less sequence is a hydrocarbon target, as are the up-dipping Albian sediments at depth. The 30 million bbl Espoir field is located in the up-dipping Albian sediments in about 350 meters of water.

Deepwater Niger Delta

Eastward of the Abidjan Basin, the continental shelf of the Niger Delta is up to 80 km wide and 200 meters deep. The continental slope (roughly the deepwater segment) extends from the shelf break to depths of 2,000-3,000 meters.

Extensive deformation of the slope by gravity and tectonic processes has created numerous intra-slope basins and diapirs. Growth faults, diapirs and imbricate thrusts are linked with each other to form a regional gravity-tectonic regime hundreds of meters below the surrounding seafloor.

There are three regional styles from the continental shelf to the continental rise:

  • Upper extensional zone of listric faults beneath the outer shelf
  • Translational zone of diapirs and shale ridges beneath the upper slope
  • Lower compressional zone of toe-thrusts beneath the lower slope and rise.
Linked together on a regional scale, these styles suggest that large portions of this thick sedimentary prism are slowly moving downslope by gravity-sliding or gravity-gliding, in a manner analogous to giant mass movements or mega-landslides.

This deformation has created local intraslope basins up to 25 km wide, which are generally filled with mass transport deposits up to several kilometers thick.

Perhaps the most obvious feature on seismic cross sections in this area is that the sedimentary structures occur in competent beds, above a decollement plane. The detachment from the underlying rocks is along a deformed zone occurring within a thick, Upper Cretaceous-Tertiary shale section. This decollement plane changes shape and direction and in places becomes shale ridges, separated by ponded slopes.

Seaward decrease in the amplitude of the structures as well as a decrease in sediment ponding in the synclines, suggest that the zone of maximum deformation moved seaward through time.

At the local level, examples of specific features of deepwater gravity flow depositional systems are clearly interpretable with 3D mapping. In OPL 221 (eastern deep, offshore Niger Delta), Mobil has described systems that consist of confined-flow, channel-form elements grading downslope into less confined-flow, lobe-form to sheet-form elements.

In this lease, the gravity flow systems appear to be confined to slope-valley topography. They consist of: upper slope, small-scale channel-form elements converging downslope into single channel-form and nested channel-form elements, with linear to sinuous map patterns. These grade further downslope into slope basin lens-form to sheet-form components, mapped as lobe-form or fan-form elements.Three types of channel-form elements are observed: erosional, erosional-depositional, and depositional.

The depositional channel-form elements have geometries strongly suggestive of channel-levee-overbank complexes formed by deposition from turbid-flow events. Within the fan-form sheets, lateral accretion of depositional elements suggests compensation sedimentation of amalgamated depositional events.

In one case, the gradation from mapped channel-form to lobe-form to fan-form occurs over less than 5 km. The seismic profile of the deepwater Niger Delta, in cases, poses challenging interpretation problems because of the seismic signatures. Sometimes, they resemble structures that are common in the nearshore and land portions of the basin, even when the stratigraphy is profoundly different.

The tripartite lithostratigraphic succession is absent in deepwater Niger Delta, so there is no drilling through freshwater sands and parallics to deepwater shales. But, structures similar to those associated with this succession have been mapped in the deepwater and have led to wrong exploration targets, in some cases.

Agip targeted a 2,000-meter thick sand/shale sequence by drilling through the crest of a four-way anticlinal closure in Abo-1 (OPL 316, 500 meter water depth). They were deceived by the amplitude contrasts that suggested stratal interfaces.

Instead of a sand/shale parallic sequence, they drilled through a shale swell. Statoil's Boi-1 well (OPL 213), another four-way closure, also turned out a duster. Shell's successful Bonga-1 was drilled on the flank of a clay-cored anticline. Shell thinks that the most extensive reservoir in this structure is a sheet sand.

Of the 12 prospects that have been tested in deepwater Niger Delta since 1995, only three have proved successful. It is taking a while for operators to understand what to make of the structural styles in the deepwater Niger Delta.

RDR/Cameroon/FP

The Rio-del-Ray-Cameroon -Fernando Po region in deepwater is a finger of the Niger delta that is genetically and structurally related to the prolific southeast offshore. Mobil's 300 million bbl

Zafiro field, in deep offshore Equatorial Guinea, is indicative of the prospectivity of this area.

The oil pool is stored in reservoirs trapped within the Qua Iboe , a shale-filled channel deposited in a slope setting principally in the Miocene.

The seismic signatures of the reservoir describe a mounded, external seismic geometry with chaotic internal reflections. Combining core studies (over 1,000 ft of conventional cores) with wireline logs reveal blocky, fining-upward and coarsening-upward trends. Mobil has proposed a deep-marine slope or base-of-slope setting.

Aptian Salt Basin

For convenience, the deepwater Rio Muni Basin in Equatorial Guinea, Gabon coastal basin, Lower Congo Basin, Kwanza and Benguela Basins in Angola are collectively described as the Aptian Salt Basin. They share common structure and stratigraphic characteristics.

The Aptian Basin is an elongate, North-South marginal sag basin with established oil production. In the deep offshore, it has been one of the most prolific in West Africa.

The deep offshore profiles exhibit large-scale gravity driven structures in a compressional system. The seismic cross-section describes progressive folding in the seaward direction.

In several dip sections, the pre-rift, tilted blocks are capped by a synrift sequence, which exhibits growth offshore. On top of this are the salt dome features, more on the seaward side. Landward, the rift sequence is truncated

by a younger queue of growth faults, which are better developed progressively in the seaward direction.

These growth faults are concave to the sea. The deeper parts of their footwalls are

filled with upward-dipping sediments, the shallow portions have four-way closures.

The growth section of the synrift sequence is overlain

by mounded sediments. The mounded sediments consist of embedded channel deposits and turbidites. This is the type of setting in which the huge Girassol field (1 billion bbl) is emplaced.

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