Deep-water Niger Delta Research Articles

The role of geomechanical-based structural restoration in reservoir analysis of deepwater Niger Delta, Nigeria

Recent advancements in fields of geomechanics and structural geology have given rise to geomechanical-based structural restoration method. This study investigated the role geomechanical-based structural restoration in reservoir analysis for Boko discovery in the proximal part of the outer fold thrust belt, deepwater Niger Delta, Nigeria. Results of the study revealed the significance of the geomechanical-based structural restoration method in validating interpretation, re-constructing structural history, and gathering insight into the deformation mechanisms and rock property evolution. The main advantage of this method over the more conventional methods is that the conventional method is not imposed as an input. In addition, several quantitative deformation parameters can be output to enable us understand the structural genesis and potential effect on petroleum system. Finally, quantitative estimates of deformation and stress may be used to respond to challenging questions related to trap history, evolution of reservoir and seal, and permeability heterogeneity within reservoirs in the near future.

Benthic foraminifera from the deep-water Niger delta (Gulf of Guinea): Assessing present-day and past activity of hydrate pockmarks

We present ecological and isotopic (δ18O and δ13C) data on benthic foraminifera sampled from 4 deep-sea stations in a pockmark field from the deep-water Niger delta (Gulf of Guinea, Equatorial Atlantic Ocean). In addition, a series of sedimentological and (bio)geochemical data are shown to back up foraminiferal observations. All stations are located within 1.2km of each other, so prevailing oceanographic conditions can be assumed to be similar at each site. Two of the sites (GMMC-01 and GMMC-02) are located in a pockmark (named “pockmark A”) where current methane seepages were recorded by ROV observations. A third station (GMMC-03) is located in the topographic depression interpreted as a collapsed pockmark (named “pockmark B”). The fourth site (GMMC-04) is a reference station, without evidence of past or present seepages. Our observations show that degraded organic matter with low bio-availability is present at all stations with a preferential burial of organic compounds in topographic depressions (GMMC-03 station). Authigenic aragonite is abundant in surface sediments at stations GMMC-01 and -02. Its precipitation is likely related to high rates of methane oxidation during past seep events in episodically active pockmark A. In contrast, the absence of anaerobic methanotrophic Archaea (ANME) during the sampling period (November 2011) suggests that only moderate sulphide and methane oxidation take place close to the sediment–water interface. Compared to the reference site GMMC-04, living foraminifera at the collapsed and episodically active pockmarks show minor changes in terms of diversity, standing stocks and faunal composition. However, the δ13C signal of living and dead (but well-preserved) foraminiferal species (Ceratobulimina contraria, Melonis barleeanus, Uvigerina peregrina) is depleted in the episodically active pockmark A compared to the other stations. Overgrowth of authigenic carbonate on altered foraminifera generates an important shift to lower δ13C values. Dead faunas carry a complex time-averaged message, integrating taphonomic gains and losses related to the temporal variability of gas emission. They reveal major faunal differences that may be useful to detect gas hydrate seepages in different pockmark stages.

Quantifying the geometry, displacements, and subresolution deformation in thrust-ramp anticlines with growth and erosion: From models to seismic-reflection profile

We quantify the evolution of thrust-ramp anticlines exhibiting growth and/or erosion using the area-depth-strain (ADS) method. We illustrate, validate, and explore the limitations of the ADS method for a series of kinematic models and experimental sand (analog) models. We then apply the method to a seismically imaged thrust-ramp anticline from the deep-water Niger Delta. The ADS method allows determination of the boundary displacements, displacement rates, and layer-parallel strains using the excess areas (areas of marker beds above their regional level) of multiple horizons. The kinematic and experimental models demonstrate that excess areas for pre-deformation units lie on a straight line on an area–depth graph, the inverse slope of which is the boundary displacement and the intercept is the elevation of the basal detachment. For growth strata, the line connecting an area-depth value to the value representing the basal detachment gives the displacement after deposition of that horizon. For partially eroded markers, the calculated displacements are less than the actual displacements; however, originally planar unconformities yield the displacements that accrued after the formation of the unconformities. The experimental sand models show substantial layer-parallel shortening in the anticlines, displacements on the main thrust ramp that are only half the displacement on the basal detachment, and small tectonic dilations. The seismic example is an area-balanced ramp anticline with clearly defined pre-deformation and growth sequences. Fault separation at the center of the ramp in the pre-deformation interval is approximately equal to the total boundary displacement but decreases both upward and downward. The layer-parallel strains have the opposite pattern, with the lowest values at the center of the ramp and increasing values both upward and downward.

Enhanced detection of faults using dip-steered multitrace similarity-computation techniques: Example using offshore Niger Delta 3D seismic data

Techniques for improved detection of faults have been applied to 3D seismic data acquired over the central parts of the deepwater Niger Delta. A volume containing the dips and azimuths of the traces was first computed directly from the data. The data were enhanced by applying filters to compute two structurally improved volumes containing localized and subregional dips and azimuths of the traces. Multitrace similarity was then computed using the seismic reflectivity and subregional dip data as input. The attributes highlighted an east-northeast–west-southwest-trending zone of high dips and low-similarity anomalies representing the location of a major zone of strike-slip faulting separating a northwest-southeast-trending zone of thrusting into two compartments. Faults in the northern compartment consist of forethrusts and backthrusts verging in opposite directions, whereas faults in the southern compartment verge basinward. If the quality of data had not been improved by using these techniques, the complex structural framework might not have been seen clearly.

Geometry, kinematics, and displacement characteristics of tear-fault systems: An example from the deep-water Niger Delta

We use three-dimensional seismic reflection data and new map-based structural restoration methods to define the displacement history and characteristics of a series of tear faults in the deep-water Niger Delta. Deformation in the deep-water Niger Delta is focused mostly within two fold-and-thrust belts that accommodate downdip shortening produced by updip extension on the continental shelf. This shortening is accommodated by a series of thrust sheets that are locally cut by strike-slip faults. Through seismic mapping and interpretation, we resolve these strike-slip faults to be tear faults that share a common detachment level with the thrust faults. Acting in conjunction, these structures have accommodated a north –south gradient in westward-directed shortening. We apply a map-based restoration technique implemented in Gocad to restore an upper stratigraphic horizon of the late Oligocene and use this analysis to calculate slip profiles along the strike-slip faults. The slip magnitudes and directions change abruptly along the lengths of the tear faults as they interact with numerous thrust sheets. The discontinuous nature of these slip profiles reflects the manner in which they have accommodated differential movement between the footwall and hanging-wall blocks of the thrust sheets. In cases for which the relationship between a strike-slip fault and multiple thrust faults is unclear, the recognition of this type of slip profile may distinguish thin-skinned tear faults from more conventional deep-seated, throughgoing strike-slip faults.

Seismic-derived seabed topography: Insights from the outer fold and thrust belt in the deep-water Niger Delta

Seafloor mapping of the outer fold and thrust belt in the deep-water Niger Delta using high-resolution 3D seismic data has revealed a variety of geomorphic features related to gravity-driven compressional tectonics, submarine sedimentary processes, and fluid migration as evidenced by bathymetric ridges caused by folding of the underlying sedimentary succession, gravity slide scars, submarine canyons and pockmarks all clearly imaged on the seismic-derived seabed bathymetry. The largest canyons, typically 25–35 km in length with widths of up to 5 km, incise an EW-trending arcuate zone of elevated bathymetry across the area. This ridge is the reference point for dividing the seabed topographic pattern into distal and proximal domains. Generally, seabed topography is gentle and less complex in the proximal domain and the major structures in the area include circular clusters of fluid-escape features primarily along channel margins and in places along discontinuities and ridges in the eastern half of the seabed. The large-scale distribution of these structures in the proximal parts of the study area may be related to fluid venting from shallow and or deeper reservoirs.

Application of volumetric seismic discontinuity attribute for fault detection: Case study using deep-water Niger Delta 3D seismic data

Techniques for detecting faults have been applied to a 3D seismic volume acquired in the outer fold and thrust belt in the deep-water Niger Delta. Firstly, the dip and azimuth of seismic traces in the data were calculated in a volume referred to as the “raw steering” data. The data were further improved by calculating two additional generations of dip volumes representing localized and subregional structural dips referred to as the “detailed” and “background” steering volumes, respectively. A multitrace similarity attribute volume was then calculated with the reflectivity and background dip-steering data as the input. The attribute data detected discrete zones of dip and similarity anomalies, trending WNW-ESE, that represented the location of discontinuities in the area. The anomalies may not have been seen clearly in the reflectivity and similarity data calculated without the application of dip-steering.

Handling natural complexity in three-dimensional geomechanical restoration, with application to the recent evolution of the outer fold and thrust belt, deep-water Niger Delta

Volumetric restoration can provide crucial insights into the structural evolution of three-dimensional (3-D) petroleum systems. A major limitation to its widespread application is the need to include complex architectures and realistic mechanics such as flexural slip. We apply an implicit approach that allows for, including unconformities, thin and/or pinched-out layers in the models but that cannot explicitly localize slip along horizons. To take advantage of this approach while accounting for flexural slip in 3-D restoration, we investigate new geomechanical properties. We consider flexural slip folding as a result of stacked rigid and thin weak layers, which can be modeled using transversely isotropic properties. We compare restorations of an anticline using transversely isotropic properties, isotropic properties, and a stack of rigid isotropic layers with nonfrictional slip between the layers. Our results show that transversely isotropic properties reasonably approximate flexural slip folding. We use these new tools to model the evolution of a complex system located in the Niger Delta toe. The system includes a detachment fold, a fault-bend fold, and a structural wedge formed in series. Growth stratigraphy and erosional surfaces delimit the kinematics of deformation. Regional erosive surfaces, 3-D gradients of fault slip, and vertical variations in mechanical strength motivated the use of our new restoration techniques. Restoring two growth units results not only in reinforcing the interpretation that the area is behaving as a deforming thrust sheet at critical taper, but also in highlighting coeval activity on both the hinterland structures and the toe of the thrust belt.

Enabling 3D geomechanical restoration of strike- and oblique-slip faults using geological constraints, with applications to the deep-water Niger Delta

We present a new approach of using local constraints on fault slip to perform three-dimensional geomechanical restorations. Geomechanical restoration has been performed previously on extensional and contractional systems, yet attempts to restore strike- and oblique-slip fault systems have generally failed to recover viable fault-slip patterns. The use of local measures of slip as constraints in the restoration overcomes this difficulty and enables restorations of complex strike- and oblique slip-systems. To explore this approach, we develop a synthetic restraining bend system to evaluate different ways that local slip constraints can be applied. Our restorations show that classical boundary conditions fail to reproduce the fault offset and strain pattern. In contrast, adding piercing points and/or properly constraining lateral walls enables restoration of the structure and resolves the correct pattern of slip along the faults. We then restore a complex system of tear-faults in the deepwater Niger Delta basin. We use channel offsets imaged by the seismic data to define local fault-slip constraints. Balancing these constraints equally on both sides of the major faults yields the most consistent restoration outcomes. This approach resolves reasonable slip styles on the complex set of thrust, normal, and strike-slip faults in the structure. This suggests that limited geologic fault slip constraints can be effectively incorporated in geomechanical restorations, yielding accurate restoration kinematics and thereby forecasting faults slip patterns within the structures.

Business impact of time-lapse data in deepwater Niger Delta

Data acquisition projects that cost significant sums of money should always be justified based on derived benefits. Time-lapse seismic data requirements need to be driven by the impact of the new data upon the field devel-opment plan (FDP) and integrated reservoir management. This article discusses the business impact of time-lapse seismic data using both a lookahead and a lookback value of information (VOI) analysis in an offshore oil field in Nigeria.

New insights into deformation mechanisms in the gravitationally driven Niger Delta deep-water fold and thrust belt

We use two- and three-dimensional seismic reflection data from the deep-water Niger Delta fold and thrust belt to document evidence for two, discrete, postfaulting deformation mechanisms. An early phase of thrust-propagation folding is followed by folding caused by thickness changes within the basal shale detachment unit. The later phase of folding is caused by a lateral redistribution of the strata within the basal detachment unit. This example of late deformation occurred over a 4–5-m.y. period as a result of the displacement of approximately 590 km3 (370 mi3) of the underlying strata within the detachment unit. In another deformation event in the basal detachment unit, about 160 km3 (100 mi3) moved, laterally creating a synform in the overburden and parallel onlap fill, indicative of the relatively rapid creation of accommodation space. On the basis of seismic reflection data from the delta and a consideration of the volumes and rates of movement of sedimentary rock, we conclude that the poorly imaged succession commonly referred to as mobile shale cannot deform solely by ductile mechanisms as interpretations of shale tectonic provinces have commonly suggested but instead by brittle processes that involve thickening by thrust faulting and subseismic accommodation structures. Processes such as liquefaction, where a complete loss of shear strength is observed, had a minimal function. Therefore, the term mobile shale in this setting is widely exaggerated. An awareness of postfaulting deformation mechanisms will be important for the successful exploration of gravitationally driven fold and thrust belts.

Internal geometry and growth history of a thrust-related anticline in a deep water fold belt

Although 3D seismic imaging has become a fundamental tool for the analysis of extensional fault geometries and their development, it has not been utilised to the same degree to investigate the structure and growth history of thrust faults. Here, the structural evolution of a fold and thrust belt is investigated through the analysis of an isolated fold imaged in the deep water Niger Delta. A quantitative description of thrust fault displacement through transfer zones is presented to investigate the internal structure of a complex fault-related fold, while syn-kinematic growth packages are used to chart fold growth through time, so as to also understand the early growth history. Results reveal that a number of initially separate folds coalesced along the axial trends into a single structural culmination. Numerous thrust faults with similar and opposing dip, link and transfer displacement within the structure as it changes vergence along strike. The distribution of bulk deformation along the fault-related fold is simple, systematic and similar to that observed for extensional fault systems. Deficits in fault heave, both in the form of perturbations on non-linking or aggregated profiles, and displacement minima within transfer zones, are compensated to some degree by an increase in fold-related strain. Despite this accommodation, fold amplitude or crest elevation is largely unaffected by variations in fault heave. The observations for this isolated fold should be widely applicable to fold and fault development in more complex areas.

Insights into the mechanisms of fault-related folding provided by volumetric structural restorations using spatially varying mechanical constraints

We use a new, mechanically based volumetric structural restoration tool to investigate the mechanics of fault-related folding using natural examples imaged in three-dimensional (3-D) seismic data. The restoration technique is based on a finite element approach that simultaneously restores folding and faulting while allowing rock properties to spatially vary during restoration. We apply these techniques to two types of structures, detachment and shear fault-bend folds, where mechanical layering is a significant factor in their development. Our examples include a detachment anticline from the Caspian Sea and a shear fault-bend fold from the deep-water Niger Delta, both of which contain syntectonic growth horizons that help to constrain the restorations. Restorations of the detachment fold most closely match displacement fields specified in the kinematic forward models when materials are defined as incompressible and rigid, yet the variation of mechanical strength in restorations is perhaps more compatible with the growth of natural structures as recorded by syntectonic growth strata. This analysis shows that the restorations of the detachment fold favor a combination of both kink-band migration and limb rotation folding mechanisms. Numerical simulations of the growth shear fault-bend fold also closely match the displacement field prescribed by the kinematics of shear fault-bend fold models when weak basal units and bedding-plane slip surfaces, enabling flexural slip, are incorporated in the model. The results demonstrate that these techniques can be used to provide full 3-D restorations that closely match established two-dimensional kinematic theories, yet allow constraint of 3-D displacement fields and strain patterns in complex structures.

Improved understanding of the petroleum systems of the Niger Delta Basin, Nigeria

Biomarker and n-alkane compound specific stable carbon isotope analyses (CSIA) were carried out on 58 crude oil samples from shallow water and deepwater fields of the Niger Delta in order to predict the depositional environment and organic matter characteristics of their potential source rocks. Using a source organofacies prediction approach from oil geochemistry, the presence in the western deepwater oils relatively abundant C 27 steranes, C 30 24- n-propyl cholestane, low oleanane index, relatively low pr/ph ratios, gammacerane, and positive to nearly flat C 12–C 30 n-alkane compound specific stable carbon isotope profiles, suggests that the source facies that expelled these oils contain significant marine derived organic matter deposited under sub-oxic and stratified water column conditions. This contrasts with the terrigenous organic matter dominated source rocks accepted for shallow water Niger Delta oils. Oils in the shallow water accumulations can be separated into terrigenous and mixed marine-terrigenous families. The terrigenous family indicates expulsion from source rock(s) containing overwhelmingly higher plant source organic matter (average oleanane index = 0.48, high C 29 steranes) as well as having negative sloping n-alkane isotope profiles. Oxic source depositional conditions (pr/ph > 2.5) and non-stratified conditions (absence to low gammacerane content) are inferred for the terrigenous family. The mixed marine-terrigenous family has biomarker properties that are a combination of the deepwater and terrigenous shallow water oils. Bitumen extracts of the sub-delta Late Cretaceous Araromi Formation shale in the Dahomey Basin are comparable both molecularly and isotopically to the studied western deepwater oil set, but with an over all poor geochemical correlation. This poor geochemical match between Araromi shale and the western deepwater oils does not downgrade the potential of sub-delta Cretaceous source rock contribution to the regional oil charge in the deepwater Niger Delta.

Structural evaluation of column-height controls at a toe-thrust discovery, deep-water Niger Delta

Industry discoveries to date in the deep-water toe-thrust play have limited column heights and lack any unequivocal thrust-fault-dependent hydrocarbon columns. The causes of the limited success are controversial, the main issue being whether the encountered columns are caused by leaking thrust faults or other causes. The controversy is in large part caused by the presence of no-seismic-image zones that, even on three-dimensional (3-D) prestack-migrated data, obscure the possible thrust cutoffs. We use dipmeter, seismic, stratigraphic, and fluid pressure data to construct an integrated geometrically and kinematically balanced cross section through a recent toe-thrust discovery in which a prominent no-seismic-image zone on the southwest flank of the Alpha structure appeared to be a thrust fault zone holding about 100 m (328 ft) of thrust-fault-dependent oil column. To constrain the structure within the unimaged forelimb, dip panels and fold axial surfaces were constructed from dipmeter data recorded in both a vertical hole and a sidetrack well through the no-seismic-image zone. Stratigraphic tops from both wells were projected through the no-seismic-image zone using the dip panels and axial surfaces, maintaining the observed stratigraphic thicknesses. These data and analyses tightly constrain potential fault locations and offsets within the no-data zone. The resulting structural model shows that the no-seismic-image zone in the forelimb is not a thrust but instead an overturned limb without any fault offset of the pay section. The results show that the trapped hydrocarbons are confined completely within the four-way dip closure, and that the trap's potential is not limited by the thrust faults' seal capacities. In addition, top-seal analyses show that top-seal integrity is not a limiting factor for hydrocarbon column heights at the present time and has not been a limiting factor throughout the geological history of the structure. Taken together, these observations indicate that the column heights are most likely controlled by access to charge instead of trap integrity, consistent with an observed lack of thermogenic hydrocarbons in nearby four-way dip closures. These conclusions imply that where sufficient access to thermogenic charge is present, there may be additional thrust-fault-dependent hydrocarbon columns.

Relief and shortening in detachment folds

We present new thickness-relief methods for determining shortening S( z), mean shortening S ¯ ( z ) and curvimetric shortening S c( z) as a function of height in well-imaged structures. Thickness-relief measurements allow us to constrain the fractions of the deformation that are layer-parallel pure shear, simple shear, and flexural, and to measure the excess area A e of fold cores caused, for example, by flow of evaporites. The key measurements are made in the thickness domain, which allows for more certain determination of regional stratigraphic gradients, which is required for reliable separation of structural relief from primary stratigraphic thickness variations. We apply these methods to a diverse set of active detachment folds from the fronts of the Nankai trough Japan, Cascadia accretionary wedge Oregon, southern Tianshan China and Agbami anticline deep-water Niger delta. Three of these folds can be approximated by pure-shear detachment fold models because more than 95% of the shortening is by layer-parallel heterogeneous pure shear plus horizontal compaction. For this reason curvimetric shortening is one to two orders of magnitude less than total shortening. In contrast, Agbami anticline shows layer-parallel stretching above the excess area of the fold core. In all examples the layer-parallel simple-shear component is negligible or absent.

Significance of ODP results on deepwater hydrocarbon exploration – Eastern equatorial Atlantic region

Scientific ocean drilling has provided access to samples of potential hydrocarbon source rocks in a number of deepwater regions around the globe. The samples are often well constrained stratigraphically and normally free from organic drilling fluid contamination. The focus of this study is the results obtained on one of the Ocean Drilling Program’s (ODP) legs – Leg 159, which was located along the Equatorial portion of the West African margin, a region of considerable hydrocarbon exploration interest. Four drilling sites were included in Leg 159 along the continental margins of Côte d’Ivorie and Ghana. Drilling at these sites recovered sediments of Albian to Pleistocene age. Prior studies revealed the presence of a number of organic-rich zones capable of yielding significant quantities of hydrocarbons within both the Cretaceous and Tertiary sections. These intervals could act as hydrocarbon sources, if suitable maturity levels were obtained. Both oil and gas would be expected as their primary products. A shore-based study which focused on Site 959 and to a lesser degree Site 962 provided an opportunity to expand upon the original dataset and to further characterize the organic matter. Detailed characterization of the bitumen fractions from Site 959, provided not only information on the geochemical character of these specific sediments, but permitted them to be placed into a more regional context by comparing them to oils from the Equatorial portion of the West African margin. These data reveal a similarity, but not necessarily a genetic relationship, between the Cretaceous sediments and the majority of the Côte d’Ivoire oils. The Paleogene extracts display similar geochemical attributes as the deepwater oils from the Niger Delta. Although this study is not attempting to establish a definitive correlation, the data suggest a Tertiary source rock system for the deepwater Niger Delta, where deposition occurred under oxic to sub-oxic conditions. This contrasts with the Côte d’Ivoire where a Cretaceous source was effective and was deposited under sub-oxic to anoxic conditions. The absence of evidence for an effective deepwater Niger Delta Cretaceous source rock could be either a result of poor timing or the lack of migration pathways to the Miocene and younger reservoirs. The limited number of Tertiary-derived oils along the Côte d’Ivoire margin could be a result of the lack of sufficient overburden.

Integration of electromagnetic and seismic data to assess residual gas risk in the toe-thrust belt of deepwater Niger Delta

During the last couple of years, drilling activities in the deepwater thrust belt play of Nigeria (Figure 1) have shown mixed results with some wells finding pay but others coming across low-saturation gas and/or brine sands. In this frontier exploration context, Shell Nigeria E&P Co. (SNEPCo) has been particularly successful in a number of toe-thrust prospects where the key risk is trap integrity, related to both earlier thrust faults and later extensional faults.

Degradation of compressional fold belts: Deep-water Niger Delta

Three-dimensional seismic interpretation of the toe-of-slope region of the west Niger Delta reveals a range of erosional and depositional features that are the result of the degradation of thrust-propagation folds and include (1) backlimb and forelimb failures that cause debris flows and deposits no more than a few tens of meters thick; (2) large slumps with evidence for internal deformation; (3) more enigmatic ovoid depressions, probably formed as a result of slumping and bottom-current erosion; and (4) degradation by deep-water channel erosion and channel-margin slumping. Thrust-propagation folds in the study area have up to 200 m (656 ft) of relief on the sea floor and are at various stages of degradation and burial. The dominant style of degradation of these folds occurs as retrogradational, small-volume failures that form thin deposits at or below seismic resolution. Slope morphology, sedimentology, and the presence of anisotropies affect the type of failure that occurs. The backlimb of one fold is long and shallow (6°), and the failed sediment masses have longer runout distances than on the forelimb, which has a steeper slope (15°), which abruptly shallows, causing the flow velocity to drop. A generic model for fold degradation in this submarine setting is synthesized, and it is predicted that a significant proportion of the sediment deposited in adjacent minibasins has been recycled from the fold crests and is a nonreservoir. Stratigraphic variability means that along-strike and across-fault correlation of deposits and erosional surfaces is difficult at shallow levels and highly problematic in buried, potentially prospective, degraded folds.

Deep-water Niger Delta fold and thrust belt modeled as a critical-taper wedge: The influence of elevated basal fluid pressure on structural styles

We use critical-taper wedge mechanics theory to show that the Niger Delta toe-thrust system deforms above a very weak basal detachment induced by high pore-fluid pressure. The Niger Delta exhibits similar rock properties but an anomalously low taper (sum of the bathymetric slope and dip of the basal detachment) compared with most orogenic fold belts. This low taper implies that the Niger Delta has a very weak basal detachment, which we interpret to reflect elevated pore-fluid pressure ( 0.90) within the Akata Formation, a prodelta marine shale that contains the basal detachment horizon. The weak basal detachment zone has a significant influence on the structural styles in the deep-water Niger Delta fold belts. The overpressured and, thereby, weak Akata shales ductilely deform within the cores of anticlines and in the hanging walls of toe-thrust structures, leading to the development of shear fault-bend folds and detachment anticlines that form the main structural trap types in the deep-water fold belts. Moreover, the low taper shape leads to the widespread development of backthrust zones, as well as the presence of large, relatively undeformed regions that separate the deep-water fold and thrust belts. This study expands the use of critical-taper wedge mechanics concepts to passive-margin settings, while documenting the influence of elevated basal fluid pressures on the structure and tectonics of the deep-water Niger Delta.