Structural development of the Niger Delta outer fold and thrust belt. WB Jones (william.jones@pgs.com) & JG Clark (john.clark@pgs.com) PGS Reservoir UK, 17 Marlow Road, Maidenhead, Berkshire, SL6 7AA

Introduction

10,000 km2 of PGS multi client 3D seismic data have been interpreted over the outer fold and thrust belt (toe thrust zone) of the Niger Delta southern lobe, Gulf of Guinea. The survey covers OPL 244, 245, 256, 257 and JDZ Blocks 1, 2, 3, & 4 (Nigeria and Sao Tome & Principe). Four horizons have been interpreted in the deltaic succession and also the Cretaceous oceanic crust which forms the basement.

The purpose of this paper is to illustrate the nature of the toe thrust zone and its relationship with the underlying mud prone Akata Formation and Cretaceous oceanic crust.

Charcot Fracture Zone

Chain Fracture Zone

Fernando Po Fracture Zone

Figure 1: Location and Geological setting of the study area (in red) within Niger Delta Basin. Note, inner thrust zone is within zone of shale diapirism in above diagram.

Regional Geology and Stratigraphic Development

The Niger Delta is a Cenozoic wave dominated delta system that can be regionally divided into two lobes in the west and south separated by the Charcot Fracture Zone (Figure 1). The Chain Fracture Zone limits the western lobe to the north and the Fernando Po limits the southern delta lobe to the south east.

The Niger Delta stratigraphy can be separated into three major Megasequences of the Benin, Agbada and Akata Formations. The Benin Formation is the most proximal of the three composed of pre-dominantly continental fluvial deposits that do not extend offshore into deepwater. The Agbada Formation is a mixed clastic succession composed of sands and shales deposited in deepwater systems with sands deposited in fan successions (Figure 2). The boundary of the Agbada and Akata Formations is broadly tied to a major change in seismic facies from the stratified continuous reflections of the Agbada to the incoherent reflections of the Akata. The Akata Formation is a sequence of pro delta muds deposited in a pelagic system formed as a result of the Sokoto transgression during the Paleocene (Corredor 2005 et al). The muds have become highly over pressured which has been attributed to chemical compaction. The Akata Formation exhibits a velocity inversion as a result of over pressure reducing the interval velocity of the succession.

A strong reflector is observed here within the Akata Formation (Figure 2A). The event has been previously attributed to a detachment level of the overlying thrusts (Corredor et al 2005). Though, it may also relate to a lack of overpressure in a particular unit resulting from fluid expulsion facilitating compaction. Another possibility is an alternative lithology (limestone, cemented sandstone or igneous)

though more work is required to support this model (Figure 2A). The reflector is spatially coincident with a compressional system characterised by a thickening of the Akata to the south, suggesting the two features may be genetically related (Figure 2A).

S10 km

Negative flower

structure

Strike-slip fault

High amplitudereflector

Cretaceous ocean floor

Compressionalpericline(system)

Class I

Figures 2/2A: 2 illustrates interpreted horizons (legend with inferred ages) and their relationship to the toe thrusts, with underlying Akata Formation and oceanic crust. 2A illustrates an intra Akata high amplitude reflector and its spatial coincidence with a potential slump structure.

Structural Geology and Relationship to Oceanic Crust

Two large fold and thrust belts (inner and outer fold and thrust belts) accommodate gravity driven extension on the shelf of the southern delta lobe into the deep water (Figure 1). This subsequently creates complex styles of compressional deformation (Figure 2/2A).

We have separated the compressional faulting into two broad classes;

• Class I toe thrusts: Individual thrusts range from 2-8km apart and are blind thrusts, with a single basal detachment, creating broad closures (Figure 2).

2

2A

• Class II toe thrusts: Steeply dipping, higher spatial frequency (2-3km). Break the seabed in the eastern JDZ part of the study region. They also form tighter folds and are more susceptible to erosion.

The mixed clastic Agbada Formation can be structurally separated into 2 units. The lower (deeper) unit has been deformed by the folding and thrusting at approximately the end of the Miocene with much of the thrust packages exhibiting relatively conformable thicknesses (Figure 2). Recent fault movement is evident in the more basinal (westerly) regions of the data, due to an increase in thickness of the faulted succession. The succession is thrusted by both fore and back thrusts indicating resistance to basal movement of the sediment. The younger upper unit onlaps the thrust blocks and infills the asymmetric piggy back basins on the falling limb of the thrust faults (Figure 2). The top of the upper unit is the present day seabed.

Fault displacement in the thrust faults is taken up by reverse faulting at depth but progressively by hanging wall anticlines at shallower levels. The thrust planes detach within the underlying overpressured Akata Shale (Figures 2/2A). The thrust belt appears to be mainly controlled by the critical taper wedge contained between the surface and the Akata detachment plane, which exhibits an anomalously low taper compared with most orogenic belts (Bilotti & Shaw 2005). This implies a very weak basal detachment in the Akata shale due to overpressure.

The previously mentioned large compressional system (Figure 2A) is an example of a Type II imbricate thrust system documented by Corredor et al in 2005. Two decollement levels within the Akata (at the top and the other near the base) connected by a master ramp. This creates large anticlines cored by the structurally thickened Akata Formation. This model suggests that this feature formed later than the original thrust complex refolding older thrusts.

Recent discoveries in the outer fold and thrust belt, including Agbami, Bonga, Chota, Ngolo, Nnwa, Obo and Zabazaba are all structural traps related to compressional structures.

Four important observations have been made on the relationship of the oceanic basement to the overlying delta succession:

• NE – SW trending depressions (troughs) have been observed in the oceanic crust (Figure 3). They are parallel to the oceanic fracture zones and seem to offset the sedimentary pile. Above one such zone in the NW of the study region thrust orientation changes with basinal thrusting to the South East and thrusts verging towards the hinterland on its North West side.

• NNW – SSE trending mid ocean ridge parallel normal faults (Figure 3). These faults are indicative of oceanic crust and have also been identified on the western lobe of the Niger Delta (Briggs et al 2009).

• Class I toe thrusts appear to be unrelated to underlying oceanic crust and more coincident with the orientation of the tapering wedge as they show a WNW – ESE orientation (Figure 3).

• There is an ENE – WSW trending basement ridge in the JDZ (Figures 4 & 5). The ridge is spatially coincident with overlying ENE- WSW trending imbricate thrusts (Class II) suggesting that they are genetically related.

Basement Trough NE-SW strike

Mid Ocean Ridge parallel normal faultsNNW- SSE striking

Thrust trend superimposed on

ocean crust fault trendWNW- ESE

Basement trough NE- SWComplex faulting in North

50 km

Figure 3: TWT map of the basement (blue/purple = deep, red/green= shallow) Ridge parallel faults in west and JDZ. Velocity inversion in Akata Formation has left in print of thrusting in Agbada overburden due to thickening of Akata Formation beneath thrust faults.

10 km

W E

Unconformity on top of tilted fault blocks

Top of Upper Cretaceous sediments downlapping onto oceanic crust

Class IIClass I

Figure 4: West to East strike line across region illustrating large anticlinal structure in the west (possible slump feature) with Akata Formation upwelling. Imbricate thrusts of the eastern JDZ to the east.

Area of 3D interpretation

Top Akata

Top Oceanic Crust

Class II

Basement Ridge

Figure 5: NW to SE dip line illustrating imbricate thrust which are coincident with underlying basement ridge

Conclusions

An interpretation of 10,000km2 of 3D seismic data within the toe thrust zone of the Niger Delta, Gulf of Guinea has been completed. The data straddles Nigerian and JDZ acreage on the southern lobe of the Niger Delta.

Four horizons plus the oceanic crust have been interpreted dividing the sedimentary succession into Agbada Formation (upper and lower units) and Akata Formation. The oceanic crust appears to play a role in the formation of Class II (imbricate) thrusts by providing a ridge at depth restricting basinal movement of the sediment. Oceanic crust has less of an impact on the Class I toe thrusts as they appear to be more WNW- ESE in orientation compared to the mid ocean ridge parallel faults in the oceanic crust which are NNW- SSE. This suggests investigating the topography of the basement is important in understanding formation of structures in the overlying sedimentary succession.

Acknowledgements

We would like to thank Jaume Vendrell and Henry Dodwell for their assistance during the undertaking of this work.

References

Corredor, F, Shaw, J.H & Bilotti F.2005. Structural Styles in the deep- water fold and thrust belts of the Niger Delta. AAPG Bulletin, v. 89, NO.6 (June 2005), pp 753-780

Bilotti, F. & Shaw, J.H 2005. Deep-water Niger Delta fold and thrust belt modelled as a critical taper wedge: The influence of elevated basinal fluid on structural styles. AAPGBulletin, v. 89, NO. 11 (November 2005), PP. 1475-1491

Briggs, S.E, Cartwright, J. & Davies R.J.2009. Crustal structure of the deepwater west Niger Delta passive margin from the interpretation of seismic reflection data. Marine and Petroleum Geology 26 (2009) 936- 950