The structure of the Offshore Niger Delta - Geology …geology.wlu.edu/connors/publications/Connors et al 2009.pdf · 1Dept. of Geology, ... THE STRUCTURE OF THE OFFSHORE NIGER DELTA

The structure of the Offshore Niger Delta

C. D. CONNORS1*, B. RADOVICH2, A. DANFORTH3 AND S. VENKATRAMAN4

1Dept. of Geology, Washington and Lee University, Lexington, VA, USA.

2ION/GX Technology and Dynamic Global Advisors, Houston, TX, USA.

3Independent Consultant, Houston, TX, USA.

4ION/GX Technology, Houston, TX, USA.

*e-mail: [email protected]

Abstract: We present a new analysis of the linked gravity-driven deformation in the Tertiary NigerDelta. On the shelf, the fundamental detachment surface sits at over 11 km subsea and shallows to 7km in the toe of the delta. The inner slope is both translating and accommodating shortening fromupdip extension, and exhibits a ductile and complex shale response. The deepwater contractional toeof the delta is primarily a brittle fold-and-thrust belt of imbricate fault-bend, fault-propagation folds,and shear fault-bend folds. Growth commenced in the late Oligocene, and continues to the present dayon some structures.

Keywords: Niger Delta, gravity spreading, toe thrust, fault-related folding, mobile shale.

It has long been recognized that the progradingNiger Delta causes extensive gravity spreadingwith the offshore manifestation of this gravity-driven deformation consisting of substantialextension on the shelf and inner slope, and con-tractional folding and thrusting in the outerslope and basin floor (Lehner and De Ruiter,1977; Evamy et al., 1978; Doust and Omatsola,1990; Damuth, 1993; Rowan et al., 2004;Corredor et al., 2005). The depth of deformationand complexity of some of the structures has lim-ited the ability of previous work to fully con-strain the detachment depths, structural styles,timing of deformation, and influence of the base-ment. We present here a regional analysis of thestructure and stratigraphy of the offshore NigerDelta from an interpretation of the new regional2D seismic survey that clarifies much of theseprevious uncertainties.

Methods

We interpreted over 4800 line-km of 2D marine seis-mic reflection data from the NigeriaSpan survey,acquired and processed by GX Technology in 2005and 2006. The survey extends from the shelf to basinfloor and covers most of the deformed regions of thedelta depicted in figure 1 except for a minor portionof the eastern and southeastern part of the delta. Theseismic data have optimal characteristics (long offset,long recording time, prestack depth migrated) thatprovide advanced imaging of previously enigmaticmobile shale structures, as well as deep imaging of theprodelta stratigraphy, and detachment levels.

We interpreted several sequence boundaries, thetop of ocean crust and the Moho throughout thedata set. Our interpretation incorporated pub-lished well data to provide ties to sequence

Trabajos de Geología, Universidad de Oviedo, 29 : 182-188 (2009)

boundaries in Neogene and Quaternary strata.We used standard sequence stratigraphic conceptsto interpret sequence boundaries away from wellcontrol and for Paleogene and older strata. Thetop of ocean crust was recognized by high ampli-tude, low frequency reflection above chaotic,hummocky seismic facies. The Moho was inter-preted as a discontinuous high amplitude reflec-tor several km below ocean crust.

Structural interpretation was guided by the conceptthat the tectonics of the Niger Delta is gravity drivenand thus detached. Detachment surfaces were recog-nized where normal faults and thrust faults sole into asurface conformable with foot wall strata or hangingwall strata. Standard fault-related folding concepts

(Shaw et al., 2005) were applied where appropriate,and ductile structures were interpreted in areas whereseismic facies did not show laterally continuous reflec-tors and discrete faults were difficult to discern.

Results

Figure 1 shows the grouping of discrete deforma-tional belts based on mapping of the NigerSpandata set. The most proximal is the ExtensionalBelt, a regional of substantial extensional growthfaults due to progradation of the delta. The regionis limited to the shelf, and is equivalent to exten-sional provinces documented by Doust andOmatsola (1990). Translation from the soling outof these listric normal faults results in contrac-

Figure 1. Structural provinces of the offshore Niger Delta from the interpretation of NigeriaSpan seismic data, overlain on bathyme-try. An Extensional Belt and two distinction contractional belts are recognized and labelled, including a more ductile Inner Fold andThrust Belt and a more brittle Outer Fold and Thrust Belt (Bathymetry from Corredor et al., 2005).

THE STRUCTURE OF THE OFFSHORE NIGER DELTA 183

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C. D. CONNORS, B. RADOVICH, A. DANFORTH AND S. VENKATRAMAN186

THE STRUCTURE OF THE OFFSHORE NIGER DELTA

tional accommodation on the slope in the InnerFold and Thrust Belt, as well as distal contractionin the Outer Fold and Thrust Belt.

Figure 2 is a 275 km, prestack depth migrated andinterpreted seismic line across the offshore NigerDelta that extends from the shelf to the basin flooroutboard of the deformation front associated withgravity spreading. The line sits outboard of most ofthe Extensional Belt, but displays the result of thisproximal gravity-driven extension as translation thatoccurs on an unambiguous detachment surface atover 11 km subsea, near the base of the Tertiary. Inthe contractional toe of the delta this fundamentaldetachment shallows to about 7 km subsea, ratheruniformly in the Upper Eocene to Lower Oligocenesection.

The inner slope is both translating and accommo-dating shortening from updip extension, andexhibits a ductile and complex shale response. Onevery large fold is shown in figure 2. Based on thehigh quality of the NigeriaSpan imaging we inter-pret structures such as these as primarily contrac-tional, asymmetric, sometimes thrusted, detach-ment folds with mobile, Eocene-Early Oligoceneprodelta and marine shale chaotically deformed inthe cores of these structures. An adjacent updipbasin shows evacuation of the prodelta and marineshale out of the syncline forming a weld on theunderlying Lower Tertiary strata. While this rela-tionship implies mobility to the Lower Tertiarystrata we would not characterize this as diapiric.Growth commenced in the late Oligocene, andcontinues to the present day on many of thesestructures. This can be seen in figure 3 where justoutboard of the large detachment fold, older con-tractional structures are refolded during morerecent contraction. These long-lived structuralhighs often show later thrusting, with substantialcut-and-fill geometries and several unconformitieson their crests (Fig. 3).

This structural and stratigraphic style is in starkcontrast to the deepwater contractional toe of thedelta which is primarily a brittle fold-and-thrust

belt of imbricate fault-bend, fault-propagationfolds, and shear fault-bend folds (Fig. 4). Growthstrata on these fault-related folds, constrains tim-ing from late Miocene to the present day, but indi-vidual structures do not generally show long-livedactivity. Instead, thrusting in the more brittle toetends to be a relatively systematic break-forwardsequence (Fig. 4).

Discussion and conclusions

New long offset, long recording time, prestack depthmigrated from the NigeriaSpan regional seismicreflection data set shows clear differences in structur-al style from the more ductile Inner Fold and ThrustBelt to the more brittle Outer Fold and Thrust Belt.Detachment horizons are well constrained below theshelf at around 11 km subsea, near the base of theTertiary. In the contractional toe of the delta this fun-damental detachment lies at about 7 km subsea, inthe Upper Eocene to Lower Oligocene section.Growth is more long-lived in the Inner Fold andThrust Belt, commencing in late Oligocene and manystructures are still active. In the Outer Fold andThrust Belt growth commenced in the late Miocene,with some structures still active. The differences inthe structural styles and timing are probably the resultof several factors related to the underlying prodeltaand marine shales including differences in overpres-sure, original thickness and composition of the shales.

Acknowledgements

The authors are members of the GX Technology NigeriaSpanteam and would like to thank GX Technology CorporationHouston, Texas, an ION Geophysical Company for permission touse and show the seismic data. We would also like to thankNigeriaSpan partners, Department of Petroleum Resources(DPR), Nigeria, and Mabon Ltd and the GX processors BernardLachaux and Tom Brady, team-manager Peter Nuttall, and data-management specialist Tom Mize. The manuscript benefited fromthe helpful review of Jose de Vera. This work was presented at theInternational Meeting of Young Researchers in StructuralGeology and Tectonics (YORSGET-08). We are grateful to theorganizers of YORSGET-08 for hosting such a stimulating con-ference.

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