Download - Deep-water Niger Delta fold AUTHORS and thrust belt ... · on the structure and tectonics of the deep-water Niger Delta. INTRODUCTION The Niger Delta, located in the Gulf of Guinea,

AUTHORS

Frank Bilotti � Unocal E&E Technology;present address: Chevron ETC, 1500 Louisi-ana St., Houston, Texas 77002;[email protected]

Frank Bilotti is the structural geology teamleader for Chevron ETC. Frank received a Ph.D.in structural geology from Princeton Universityand a B.S. degree in geology and mathematicsfrom the University of Miami. He worked as astructural consultant in Texaco ExplorationTechnology and was most recently a structuralgeology specialist at Unocal E&E Technology.Frank’s current technical interests are in Gulfof Mexico salt tectonics and three-dimensionalrestoration technology.

John H. Shaw � Department of Earth andPlanetary Sciences, Harvard University, Cam-bridge, Massachusetts

John H. Shaw is the Harry C. Dudley Professorof Structural and Economic Geology at HarvardUniversity and leads an active research programin structural geology and geophysics,with emphasis on petroleum exploration andproduction methods. He received a Ph.D. fromPrinceton University in structural geology andapplied geophysics and was employed as asenior research geoscientist at Texaco’s Explo-ration and Production Technology Department inHouston, Texas. Shaw’s research interests in-clude complex trap and reservoir characteriza-tion in fold and thrust belts and deep-waterpassive margins. He heads the Structural Ge-ology and Earth Resources Program at Harvard,an industry-academic consortium that supportsstudent research in petroleum systems.

ACKNOWLEDGEMENTS

This manuscript benefited from very helpfulreviews by Mark Rowan, Bradford Prather,Tom Elliott, and Carlos Rivero. The conceptsfor this work were originally developed as partof Texaco’s regional exploration effort in theNiger Delta. Support for refinement of theseideas and this article was provided by UnocalE & E Technology. John Suppe, Freddy Corredor,and Chris Guzofski provided valuable assis-tance in understanding and constraining theparameters used in the model. We are gratefulto Veritas DGC, Ltd., for granting permission topublish the seismic data presented here.

Deep-water Niger Delta foldand thrust belt modeled asa critical-taper wedge: Theinfluence of elevated basal fluidpressure on structural stylesFrank Bilotti and John H. Shaw

ABSTRACT

We use critical-taper wedge mechanics theory to show that the

Niger Delta toe-thrust system deforms above a very weak basal de-

tachment induced by high pore-fluid pressure. The Niger Delta ex-

hibits 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 (l � 0.90) within the Akata

Formation, a prodelta marine shale that contains the basal detach-

ment 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.

INTRODUCTION

The Niger Delta, located in the Gulf of Guinea, is one of the most

prolific petroleum basins in the world (Figure 1). The Delta con-

sists of Tertiary marine and fluvial deposits that overlie oceanic

AAPG Bulletin, v. 89, no. 11 (November 2005), pp. 1475– 1491 1475

Copyright #2005. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received January 4, 2005; provisional acceptance March 8, 2005; revised manuscriptreceived June 9, 2005; final acceptance June 13, 2005.

DOI:10.1306/06130505002

crust and fragments of the extended African continen-

tal crust. Over the last decade, advances in drilling tech-

nology have opened the deep-water Niger Delta to ex-

ploration. At the deep-water toe of the delta, a series

of large fold and thrust belts (Figure 1) is composed

of thrust faults and fault-related folds (e.g., Damuth,

1994; Morley and Guerin, 1996; Wu and Bally, 2000;

Corredor et al., 2005). Recent discoveries in this fold

and thrust belt include the Agbami, Bonga, Chota, Ngolo,

and Nnwa fields, all of which have structural traps

formed by contractional folds.

The contractional part of the deep-water Niger

Delta is divided into three major zones (Connors et al.,

1998; Corredor et al., 2005): the inner fold and thrust

belt, the outer fold and thrust belt, and the detachment-

fold province (Figure 1). The inner fold and thrust belt

is a highly shortened and imbricate fold and thrust belt,

whereas the outer fold and thrust belt is a more classic

toe-thrust zone with thrust-cored anticlines that are

typically separated from one another by several kilo-

meters (Corredor et al., 2005). The detachment fold

belt is a transitional zone between the inner and outer

fold and thrust belts that is characterized by regions of

little or no deformation interspersed with broad de-

tachment anticlines that accommodate relatively small

amounts of shortening (Bilotti et al., 2005).

The deformation in the contractional toe of the

Niger Delta is driven by updip, gravitational collapse of

shelf sediments. Basinward motion of these shelf sedi-

ments is accommodated by normal faults that sole to

detachments within the prodelta marine strata that lie

above the basement (Figure 2). Slip on the detachments

is transmitted to the deep water, where it is diverted

onto thrust ramps and consumed by contractional folds

in deep-water fold and thrust belts (Figures 2, 3). This

style of gravitationally driven, linked extensional and con-

tractional fault systems is common in passive-margin

deltas (Rowan et al., 2004), including the Gulf of Mexi-

co basin (e.g., Peel et al., 1995). The Niger Delta fold

and thrust belts occupy the outboard toe of the delta in

Figure 1. Map of the offshore Niger Delta showing the location of regional transects (1–10) used in this study, bathymetry, andmajor offshore structural provinces (modified from Connors et al., 1998; Corredor et al., 2005).

1476 Niger Delta Critical Taper Wedge

water depths ranging from 1 to 4 km (0.6 to 2.5 mi)

below sea level (Figure 1) and create a very gentle (<2j),

regional seafloor slope away from the coast.

In this article, we show that the Niger Delta toe-

thrust system, composed of a sloping sea floor, a basal

detachment system, and an internally deforming wedge

of sediments, can be successfully modeled as a critical-

taper wedge like those found in accretionary wedges

at active margins (Davis et al., 1983). The taper of a

wedge is the angle between its free surface and basal

detachment. The theory of critical-taper wedge me-

chanics states that once a wedge reaches some criti-

cal taper, it grows self-similarly as material is added

to or removed from the wedge. Given the strength of

the wedge and its basal detachment, the theory de-

fines the critical taper of the wedge. Conversely, we

use measurements of the sea floor and the basal de-

tachment to define the wedge shape and subsequently

employ measured values of density, fluid pressure, and

basal thrust step-up angles to examine the strength

of the deforming wedge and its basal detachment.

Specifically, we predict the magnitude of fluid over-

pressure in the basal detachment zone that would be

required to produce the observed wedge shape. Fi-

nally, we consider the implications of this model for

the structural styles and evolution of the Niger Delta

fold and thrust belts.

CRITICAL-TAPER WEDGE MECHANICS

Theory

Critical-taper wedge mechanics theory explains the

first-order geometry of fold and thrust belts as a func-

tion of the internal strength of the wedge of deform-

ing material and the strength of the basal detachment

on which the wedge slides (Davis et al., 1983; Dahlen

et al., 1984). For the deformation front to propagate,

the basal detachment of the wedge must be weaker than

the wedge material. The analogy of a bulldozer pushing

a pile of snow or sand is commonly used to illustrate

the first-order mechanics of this system (Figure 4).

Once the critical taper is reached, the wedge grows

self-similarly, and internal deformation of the wedge

maintains the taper. The taper of a specific wedge sys-

tem is determined by the strength of the basal detach-

ment relative to the internal strength of the wedge.

Critical-taper wedge mechanics theory was devel-

oped initially to explain accretionary wedges that form

at convergent plate boundaries (Davis et al., 1983). InFigu

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Bilotti and Shaw 1477

Figure 3. Regional two-dimensional seismic line through the contractional toe of the Niger Delta, showing the tapered, wedge shape of the deep-water fold and thrust beltsmodeled in this article. Note that the bathymetric slope is caused by the underlying deformation, and that the basal detachment is subparallel to the top of underlying oceanicbasement. Data provided courtesy of Veritas DGC, Ltd.

14

78

Niger

Delta

CriticalTaper

Wedge

these tectonic systems, sediments are typically scraped

off the subducting slab and incorporated into an in-

ternally deforming accretionary wedge; these wedges

have a shape generally governed by their internal and

basal strength (Chapple, 1978; Davis et al., 1983; Stock-

mal, 1983). Over the past two decades, critical-taper

wedge theory has been successfully applied to study

many accretionary wedges and orogenic fold and thrust

belts throughout the world (e.g., Zhao et al., 1986;

Breen, 1987; Behrmann et al., 1988; Barr and Dahlen,

1989; Dahlen and Barr, 1989; Dahlen, 1990; DeCelles

and Mitra, 1995; Braathen et al., 1999; Plesch and

Oncken, 1999; Carena et al., 2002). The theory, how-

ever, is a general description of any wedge of material

deforming by brittle frictional processes. The theory

can be used to explain the first-order geometry of fold

and thrust belts regardless of their driving mecha-

nisms. Here, we demonstrate that the critical-taper

wedge theory can successfully model the gravitation-

ally driven fold belts of the deep-water Niger Delta,

extending the application of this theory beyond oro-

genic systems to passive-margin settings as suggested

by Bilotti and Shaw (2001) and Rowan et al. (2004).

Although we will demonstrate that the general

wedge theory is applicable to gravitationally driven

wedges like the toe of the Niger Delta, it is important

to consider differences between these systems and their

tectonic counterparts in developing and interpreting

wedge models. Foremost of these differences is the

manner in which material is added to the wedge. At

submarine convergent margins, the primary source of

material input to the wedge is from sediments scraped

from the subducting sea floor; otherwise, little modi-

fication of the overall shape of the wedge is present. In

the case of the Niger Delta, material is added to the

wedge both at the deformation front, as the basal de-

tachment propagates into the basin, and through active

delivery of sediments to the back of the wedge from the

Niger River. This additional sedimentary input both

modifies the taper of the wedge and provides more

drive to the overall gravitational system. This modifi-

cation of the wedge’s shape is more similar to subaerial

wedges, which are subject to erosion that constantly

modifies the shape of the wedge.

Formulation

The surface slope (a) and the dip of the basal detach-

ment (b) are the first-order parameters modeled in the

critical-taper wedge theory. The sum of a and b, the

wedge taper, is a function of the relative strength of

the wedge and the strength of the basal detachment;

the weaker the basal detachment and the stronger the

wedge material, the narrower the wedge taper. The

wedge strength is defined by frictional coefficients and

fluid-pressure values for both the internally deforming

wedge and its basal detachment. Davis et al. (1983)

derived the equation for the wedge taper (a + b) from

a force balance on a wedge at critical taper. An en-

hancement of the critical-taper equation, with cohe-

sion incorporated, was derived by Dahlen (1990) and

is given as

aþ b � ð1 � rw=rÞbþ mbð1 � lbÞ � Sb=rgH

ð1 � rw=rÞ þ 2ð1 � lÞðsin f=1 � sin fÞ þ C=rgH

ð1Þ

where r is the bulk density of the wedge material; rw is

the density of seawater (in the submarine wedge case);

l and lb are the Hubbert-Rubey fluid-pressure ratio

Figure 4. Schematic depiction of a critical-taper wedge and fundamental equation relating taper shape to the internal and basalstrength of the wedge (modified from Dahlen et al., 1984). a = surface slope of the wedge; b = basal detachment slope; mb = coefficientof friction at the basal detachment; f = angle of internal friction; l = ratio of pore-fluid pressure to lithostatic pressure in the wedge;lb = ratio of pore-fluid pressure to lithostatic pressure at the base of the wedge; r = density of wedge material; rw = density ofseawater; C = cohesive strength of the wedge; S b = cohesive strength of the basal detachment; g is the gravitational accelerationconstant; H is the local wedge thickness (see text for discussion and Table 1 for range of parameter values).


(Hubbert and Rubey, 1959) for the wedge and the basal

detachment; Sb and C are the cohesive strengths of the

basal detachment and the wedge material, respectively;

g is the gravitational acceleration constant; H is the

wedge thickness; and mb is the coefficient of basal slid-

ing friction. The cohesive strength of the wedge (C) is

more important in the thin toe of the wedge; therefore,

the taper also depends on the distance from the toe of

the wedge. This is because the ratio of cohesive strength

to frictional strength is larger at shallow depths, hence,

the C/rgH term in the wedge equation.

In the next section, we use this formulation to es-

tablish that the Niger Delta fold belts are at critical

taper. Subsequently, we examine the strength of the

basal detachment in the Niger Delta using internal

wedge parameters derived from well data and fault pat-

terns observed in seismic reflection data.

MODELING THE TOE OF THE NIGER DELTA

Measuring the Taper

The overall shape of the distal toe of the Niger Delta

fits the form of a tapered wedge, but to investigate

whether it acts mechanically like a critical-taper wedge,

we make quantitative measurements of the wedge

shape as required by the Dahlen (1990) formulation.

We measure the taper of the fold and thrust belt at the

toe of the Niger Delta in 10 profiles (Figure 5; locations

shown in Figure 1). The sea floor represents the upper

free surface of the wedge, and we use a linear approx-

imation of the seafloor slope to measure the angle a,

the angle below horizontal of the upper surface of the

wedge. For the angle b, we use the observation that

the basal detachment of the fold and thrust belt is gen-

erally parallel to the underlying basement reflector, as

shown in Figure 3. Within the representative 10 pro-

files, we use a linear fit to the basement reflector to mea-

sure the dip of the basal detachment of the wedge, b.

The measured wedge taper (a + b) of the Niger Delta

toe is 2.5 ± 0.4j (Figure 6). This taper (Figure 7) is lower

than all of the accretionary wedges at active margins

reported in Davis et al. (1983). This anomalously low

taper suggests that either the toe of the Niger Delta is

not at critical taper, or it has substantially different me-

chanical properties compared to most active margins.

In the subsequent discussion, we first establish that the

deep-water fold belts are at critical taper and then ex-

plore the possibility of anomalous mechanical proper-

ties for the Niger Delta through critical-taper wedge

modeling.

Is the Toe of the Niger Delta at Critical Taper?

Critical-taper wedge theory predicts a linear relation-

ship with negative slope between the dip of the basal

detachment and the seafloor slope of a contractional

Figure 5. Regional tran-sects of bathymetric slopeand the top of basementsurface, which approxi-mates the basal detach-ment slope through theNiger Delta, based onseismic sections. Thesesections are the sourceof the taper measure-ments. See Figure 1 fortransect locations. Thetop transect is from theshoreline across the ex-tensional province to thedistal edge of the con-tractional toe. The mea-sured transects are re-stricted to the outer foldand thrust belt wherewe expect the critical-taper model to apply.


wedge once a wedge has reached its critical taper. An

active wedge that has not yet reached critical taper

should not exhibit this relationship, nor should it

propagate forward (i.e., grow wider). Instead, a subcriti-

cal wedge would internally deform until it thickened

to its critical taper before it grew wider.

Given these considerations, we propose that the

toe of the Niger Delta has indeed reached critical taper

based on two main observations:

1. The wedge has grown appreciably wider by means

of the basal detachment propagating into the basin.

The fold and thrust belt shown in Figure 3 is more

than 40 km (25 mi) wide, and the entire contrac-

tional toe is on the order of 100 km (62 mi) wide

(Figure 5). A noncritical wedge first builds taper

by internal deformation and then propagates basin-

ward after reaching critical taper. A pronounced

widening of a contractional wedge is an indication

that it has reached critical taper and has continued

to grow.

2. The regionally consistent taper of the wedge as

shown by the negative slope of a graph of a versus bindicates a consistent taper of about 2.5 ± 0.4j over a

range of values for a and b (Figure 6). Critical-taper

Figure 6. Plot of wedge shape measured in 10 transects acrossthe deep-water Niger Delta fold and thrust belts. The transectnumber is labeled on each point. The overall negative slopeof the points is consistent with a critically tapered wedge withtaper (a + b) between 2.3 and 2.9j. Transect locations areshown in Figure 1.

Figure 7. Comparison in rangeof bathymetric slope and de-collement (basal detachment)dip for the Niger Delta versusactive submarine fold and thrustbelts at convergent margins(modified from Davis et al., 1983).The Niger Delta has muchlower taper (a + b) than mostother measured fold and thrustbelts. Those with similar tapers(Makran and the toe of theBarbados accretionary wedge)are known to have high basalfluid pressures (Davis et al.,1983), implying the same con-dition for the Niger Delta. l = lb

line assumes mb = 0.85 andm = 1.03.


wedge theory predicts that the properties of the

wedge and basal detachment prescribe a wedge ge-

ometry that is as consistent as the regional material

strength properties. Conversely, if the wedge was

dominated by sedimentary processes, such as the

angle of repose, we would not expect a consistent

relationship between the basal detachment geome-

try and the seafloor slope.

Based on these considerations, we conclude that

the toe of the Niger Delta is a contractional wedge at

critical taper, and we now explore which properties of

the wedge give it its uniquely low taper. To model the

Niger Delta as a critical-taper wedge, we need to pro-

vide constraints on the main parameters of the formu-

lation of Dahlen (1990). The following sections dis-

cuss the constraints on these parameters for the toe

of the Niger Delta.

Wedge Strength Parameters

Internal Coefficient of Friction

We can constrain the internal coefficient of friction

(m) using the geometry of the wedge as well as basic

rock mechanics. Because a critical-taper wedge is on

the verge of failure throughout the wedge, two planes

are oriented at angles ± (p/4 � f/2), measured with

respect to the maximum principal compression vector,

s1, on which the failure criterion is satisfied (Jaeger and

Cook, 1979) (Figure 8). We use this relationship and

measurements of the angle that thrusts step up from

the basal detachment, db, to estimate the angle of in-

ternal friction, f. We use the relationship from Dahlen

et al. (1984, equation 27):

db ¼ p=4 � f=2 � yb ð2Þ

where m = tan f and yb is the angle between the maxi-

mum principal compression (s1) and the basal detach-

ment (Figure 8).

From 49 measurements of the dips of thrust faults

from the fold and thrust belts of the Niger Delta (Cor-

redor et al., 2005), we derive the histogram of values

for db presented in Figure 8. The bimodal graph has

peaks at about 22 and 32j. The corresponding values

for m = tan f (for yb = 2j) are m = 0.90 and m = 0.40,

respectively. The value for m (0.40) corresponding to

db = 32j falls outside of the range of empirically de-

rived rock strength values 0.6 � m � 1.0, known as

Byerlee’s Law (Byerlee, 1978). Therefore, we suggest

that db = 32j does not represent a fundamental step-up

angle in the area but instead reflects a typical value for

imbricated faults (Suppe, 1983; Shaw et al., 1999) or

perhaps faults that propagated as kink bands (Dahlen

et al., 1984). In contrast, the value for m (0.90) corre-

sponding to db = 22j is consistent with Byerlee’s Law.

This suggests that db = 22j represents the funda-

mental ramp step-up angle, and we employ the corre-

sponding frictional value (m = 0.90) in our subsequent

modeling.

Figure 8. (a) Measured basal step-up angles for thrust ramps in the Niger Delta. We interpret the left peak of the graph to representthe basal step-up angle at which the thrusts initially propagate, which defines db. The second peak represents imbricated thrustramps or thrusts that initially propagated as kink bands. (b) Relationship between the basal step-up angle (db) and the angle ofinternal friction (f) for the wedge (modified from Dahlen et al., 1984).


Internal Fluid Pressure

The Hubbert and Rubey (1959) fluid-pressure ratio is

the ratio of fluid pressure to the lithostatic pressure.

This value is from Hubbert and Rubey’s formulation

of effective stress that sought to explain how large

thrust sheets could be translated long distances with

little internal deformation from friction on the under-

lying fault. Through critical-taper wedge theory, Davis

et al. (1983) showed that high fluid pressures were

not necessary to explain the large-scale geometry of

fold and thrust belts; critical-taper wedge theory ex-

plains the first-order geometry of fold and thrust belts

without calling on significant overpressure. However,

the fluid pressure is still an important component of

the effective strength of the wedge material.

For the Niger Delta, we obtain regional values for

the Hubbert-Rubey fluid-pressure ratio, l, generalized

for the submarine case from formation pressures mea-

sured in deep-water wells:

l ¼ ðPf � rwgD=rghÞ ð3Þ

where D is the water depth, h is the depth of the

measurement below the sea floor (Dahlen et al.,

1984), and r is an estimate of the bulk rock density

derived from many well-density logs. This formulation

removes the effect of the overlying water because the

hydrostatic and lithostatic pressure curves are identical

in the water column. Figure 9 shows the distribution

of l values for 13 wells in the deep-water Niger Delta.

The average value is l = 0.54, and we use this value in

subsequent models. For comparison, lambda for hy-

drostatic fluid pressure, lh, is approximately 0.43, in-

dicating that the Niger Delta section is, in general,

slightly overpressured.

Cohesive Strength

The cohesive strength of the wedge material can be an

important property for thin wedges and near the tip of

high-taper wedges. In the toe of the Niger Delta, the

relative significance of the cohesive strength of the

rocks in the wedge, C, is small because the wedge tip

is at the distal end of the basal detachment, which is

typically about 3 km (1.8 mi) below the sea floor

(Figure 5). At depth, the effect of the increasing fric-

tional strength of the material overwhelms the effect

of the relatively small cohesive strength. Because the

result is relatively insensitive to the cohesive strength

of the material, we can safely estimate the cohesive

strength of the wedge from rock-mechanics experi-

ments (Hoshino et al., 1972) to be 10 ± 5 MPa for our

deep-water sediments.

Basal Strength Parameters

The basal detachment of the toe of the Niger Delta

lies within the Akata Formation, a thick marine shale

that is thought to contain the source section for some

Figure 9. Histogram of values for theHubbert-Rubey fluid-pressure ratio (l)derived from pore-pressure measure-ments from 13 wells in the deep-waterNiger Delta. The effect of the watercolumn (D) is removed from the dis-played Hubbert-Rubey fluid-pressurevalues, as shown by the equation (inset)and as required by the wedge mechanicsformulation (Dahlen et al., 1984).


of the major oil fields of the deep-water Niger Delta.

A few wells have approached this section, but we

know of no penetrations of the deeper parts of the

formation where the basal detachment of the fold and

thrust belt resides. The mechanical properties of the

basal detachment cannot be measured directly and

must be inferred from rock-mechanics experiments or

derived from the critical-taper modeling.

Three parameters in the generalized wedge equa-

tion of Dahlen (1990) address the strength of the basal

detachment of the wedge: the basal friction, the basal

cohesive strength, and the basal fluid-pressure ratio. In

this analysis, we assume that the basal coefficient of

friction is similar to that derived for the wedge mate-

rial, mb = 0.91, which is consistent with Byerlee’s Law

(Byerlee, 1978). In addition, because the basal detach-

ment is a regionally continuous surface that is actively

sliding, we assert that it is working as a frictional fault

with low or no cohesive strength. We assume in our

modeling that the basal cohesion is equal to zero.

With the values as described above (summarized

in Table 1), we can directly compute a value for a range

of basal fluid pressures from the critical-taper wedge

equation. Figure 10 shows the predicted wedge taper

for values of lb compared to the measured taper values

from the toe of the Niger Delta. Critical-taper wedge

theory predicts Hubbert-Rubey fluid-pressure ratios

of 0.89 < lb < 0.92 for the range of measured transects

and our preferred values for each parameter. The total

range of predicted fluid pressures using the low- and

high-pressure end members of each parameter is 0.82 <

lb < 1.01. Figure 11 shows the sensitivity of the wedge

model to more typical values of lb using the wedge

formulation of Dahlen et al. (1984). These models show

that for reasonable values of wedge strength and basal

strength, the wedge geometry is very sensitive to the

basal fluid pressure. Based on the observed wedge shape,

we are readily able to distinguish the effects of basal

fluid pressure and conclude that the low taper of the

contractional toe of the Niger Delta is the result of

strongly elevated fluid pressure at the basal detachment.

Elevated basal fluid pressures have also been in-

voked to explain the anomalously low taper of some

orogenic fold and thrust belts, including the Barbados

accretionary wedge (Figure 7) (e.g., Behrmann et al.,

1988). We suggest that the elevated basal fluid pressure

is the dominant cause of low-taper fold and thrust belts,

regardless of whether they are orogenic or gravitation-

ally driven. In the case of the Niger Delta, the elevated

basal fluid pressure is also consistent with the anoma-

lously low compressional wave speeds (�2500 m/sec;

�8202 ft/sec) in the Akata Formation, as well as the

general structural styles manifest in this shale tectonic

province (Wu and Bally, 2000; Rowan et al., 2004; Cor-

redor et al., 2005). In the next section, we examine

how the elevated basal fluid pressure and the corre-

sponding weak basal detachment in the Niger Delta

influence the regional structural architecture of the

deep-water fold belts, as well as the structural styles

expressed by individual toe-thrust structures.

PROPERTIES OF LOW-TAPER WEDGES WITHELEVATED BASAL FLUID PRESSURES

Elevated basal fluid pressure in the Niger Delta has had

a substantial influence on the architecture and deforma-

tional history of its deep-water fold belts. The elevated

Table 1. Critical-Taper Wedge Model Parameters

Parameter Value Method of Determination

Surface slope a 0.7–1.3j Measured from 10 regional transects of seismic-derived seafloor map

(see Figure 5)

Detachment dip b 1.2–2.2j Measured from 10 regional transects of seismic-derived basement map

(see Figure 5 and text for justification)

Density r 2400 kg/m3 Deep-water well-density logs (approximate)

Fluid-pressure ratio l 0.54 ± 0.15 Pressure data from 13 deep-water wells (see Figure 9)

Basal step-up angle db 22j Measured from seismic data (lower peak of graph in Figure 8)

Internal coefficient of friction m 0.91 ± 0.06 Calculated from basal step-up angle

Basal coefficient of friction mb 0.91 ± 0.3 Similar to wedge material strength and Byerlee’s Law (Byerlee, 1978)

Wedge cohesion C 5–15 MPa Hoshino et al. (1972); see discussion

Basal cohesion Sb 0–5 MPa Hoshino et al. (1972); see discussion


Figure 10. Plots of seafloor slope (a) versus detachment dip (b) for 10 transects across the toe of the Niger Delta, with curves of constant basal fluid pressure (lb). The plotsdisplay model results for the high (A), middle (B), and low (C) fluid-pressure end members of the model parameters. The observed wedge taper values are consistent with Hubbert-Rubey basal fluid-pressure ratios (lb) between 0.82 and 1.01, which are greatly elevated above the observed fluid pressures in the wedge (see Figure 9).

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fluid pressure presumably localized the basal detach-

ment in the Akata Formation and controlled the over-

all shape of the thrust belts as expressed by its narrow

taper. Furthermore, the low strength of the Akata For-

mation and the weakness of the basal detachment have

dictated the structural styles expressed in the fold belt.

Individual toe-thrust structures involve large compo-

nents of shear within their hanging walls, forming shear

fault-bend folds (Suppe et al., 2004) as documented

by Corredor et al. (2005). Moreover, detachment folds

formed by ductile thickening of the Akata Formation

are common in the deep-water Niger Delta, reflecting

distributed deformation in the weak Akata Formation

as it slides above a basal detachment (Bilotti et al., 2005).

The deep-water Niger Delta also exhibits other,

perhaps more enigmatic, structural characteristics that

we also attribute to its fluid overpressure. Specifically,

the widespread occurrence of backthrusts and the physi-

cal separation of the inner and outer fold belts can be

attributed to the weak basal detachment and corre-

spondingly low wedge taper based on the critical-taper

concept, as discussed below.

Figure 11. Model bathymetric profiles showing the effect of varying each parameter over its range of model values (see Table 1).These graphs use Dahlen et al. (1984, equation 40), which includes the distance from the wedge tip but not separate cohesionstrength parameters for the wedge and the basal detachment. Within our control of the parameters, lb and S0 show the largest rangeof predicted bathymetric slope.


In thrust wedges with very low taper, the max-

imum principal compressive stress (s1) is necessarily

subhorizontal and very nearly parallel to the basal de-

tachment (yb � 0) as well as the sea floor. When this is

the case, as it is in the Niger Delta, only a very small

component of s1 acts on the basal detachment. This

configuration yields no mechanical preference for devel-

oping fore- or backthrusts. This relationship is quan-

tified by yb, the angle between s1 and the basal de-

tachment (Figure 8), which is also a measure of the

difference between the dips of fore- and backthrusts.

Regionally, in the Niger Delta, we estimate yb to be

1–5j. As expected, the basal step-up angle of the back-

thrusts ranges from 15 to 25j (excluding thrusts that

seem to be folded or imbricated), similar to the range

of primary step-up angles for forethrusts shown in

Figure 8. Furthermore, once the faults are formed, the

low bathymetric slope means little difference exists

between the overburden shoreward or offshore from a

given fault, making forethrusts and backthrusts equally

mechanically efficient at accommodating shortening.

In contrast, wedges with large tapers and steep surface

slopes have backthrusts that dip much more steeply

than forethrusts (Figure 12). This makes the forethrusts

much more efficient at accommodating shortening, and

therefore, most of the thrust displacement in wedges

with large tapers occurs on forethrusts. The lack of pref-

erence for forethrusts over backthrusts in many parts of

the Niger Delta results from its low taper and weak

basal detachment (Figures 12, 13). Backthrusts are most

common in the lowest taper zones in the outer fold and

thrust belt, as expected based on the critical-taper

wedge theory.

Another consequence of a low-taper wedge is that

sedimentary deposits can readily enable the wedge to

reach critical taper, even in the absence of deformation.

The seismic line in Figure 14 shows such a case. The part

of the outer fold and thrust belt shown in the seismic

image is separated from the inner fold and thrust belt

by more than 35 km (21 mi) of essentially flat-lying

seismic reflectors. This relatively undeformed zone,

however, has the necessary critical taper and is sliding

stably without requiring internal deformation. Close

inspection of the shallow strata reveals that the taper of

the wedge in this area is generated by a young wedge of

sediments coming from the continental shelf. This il-

lustrates how normal sedimentary input to a part of the

wedge can achieve the necessary taper, which has the

effect of retarding deformation in the underlying part

of the wedge. This phenomenon is presumably most

Figure 12. Schematic models illustrating the effects of taper angle on the tendencies for fore- and backthrusting in a deformingwedge. In high and moderate taper wedges (left), forethrusts typically dip at a lower angle than backthrusts and, thus, are moreeffective at accommodating shortening. In extremely low taper wedges (right), fore- and backthrusts should have essentially the samedip values and are equally effective at accommodating shortening. Areas of the Niger Delta where backthrusts are prevalent (Figure 14)are generally associated with regions of very low taper, consistent with this theory.


Figure 13. Seismic reflection profile across the toe of the Niger Delta in a region dominated by backthrusts. Data provided courtesy of Veritas DGC, Ltd.

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Niger

Delta

CriticalTaper

Wedge

Figure 14. Seismic reflection profile through the northern part of the Niger Delta fold and thrust belt, showing a large (30 km [18 mi] wide) undeformed zone lying to the east ofa series of toe-thrust structures. The critical taper of this undeformed zone is provided by a lobe of sediments that thin toward the deep water, making it unnecessary for this zoneto internally deform to maintain taper. The inferred weakness of the basal detachment caused by elevated fluid overpressure also facilitates sliding of this undeformed zone withoutinternal deformation. Data provided courtesy of Veritas DGC, Ltd.

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apparent in low-taper, passive-margin wedges such as

the Niger Delta because sedimentary deposits alone

could not likely achieve critical taper in wedges with

higher surface slopes.

CAUSES OF ELEVATED BASALFLUID PRESSURE

Several possible causes exist for the elevated fluid

pressure that controls the shape of the Niger Delta

thrust wedge, including rapid burial, tectonic forces,

and increased fluid volume caused by hydrocarbon

maturation. Disequilibrium compaction in shales is

common in sedimentary basins (e.g., Osborne and Swar-

brick, 1997) because permeability declines abruptly

with burial of shales, preventing fluid expulsion and

mechanical compaction. However, once mechanical

compaction stops, subsequent burial causes the pore-

fluid pressure to rise only as fast as the lithostatic gra-

dient (Osborne and Swarbrick, 1997). To produce fluid

pressure as high as we predict in the critical-taper

wedge model (l = 0.90), fluid retention would need

to begin at about 500 m (1640 ft) below mud line.

Permeability in mudstones at 500-m (1640-ft) depth

is generally too great to cause fluid retention; to pro-

duce fluid pressure in these relatively permeable rocks,

sedimentation would have to strongly outpace fluid

expulsion. Predicted sedimentation rates to produce

fluid retention at 500-m (1640-ft) depth are in excess

of 3000 m/m.y. (10,000 ft/m.y.) (Mann and MacKenzie,

1990), certainly unreasonable rates to sustain at the

toe of the Niger Delta. Although it is likely that some

fraction of the total basal overpressure in the Akata

Formation is caused by disequilibrium compaction, this

process alone is probably not capable of elevating fluid

pressure to the level we infer.

Horizontal tectonic forces can also elevate fluid

pressure (Osborne and Swarbrick, 1997). Because we

see very young or active thrusts in the Niger Delta

wedge (Corredor et al., 2005), the maximum principal

compressive stress is subhorizontal today, and some

fraction of the overpressure we model may be caused

by horizontal stress. In this case, the upper limit of

overpressure is the minimum principal stress (Swar-

brick et al., 2002), which is vertical (i.e., the lithostatic

stress) in the case of fold and thrust belts. With suf-

ficient horizontal differential stress, it is possible to

elevate fluid pressure to the values we model with

critical-taper wedge mechanics. Because we lack data

on the magnitude of the maximum compressive stress,

we are unable to quantify this effect.

Finally, Frost (1996) proposed that the matura-

tion of the source facies of the Akata Formation pro-

duced a sufficient change in fluid pressure to start

structural growth of the toe-thrust belt. The change in

volume caused by the generation of oil from type II

kerogen could be as large as 25% (Meissner, 1978;

Swarbrick et al., 2002). If hydrocarbon maturation is in

fact linked to the elevated basal fluid pressures we

model, there may be both a temporal and spatial re-

lationship between the shape of the Niger Delta thrust

wedge and the maturity state of the Akata source rocks.

CONCLUSIONS

We have shown that the contractional toe of the Niger

Delta acts as a critical-taper wedge similar to the fold

and thrust belts found at active margins. The toe of

the Niger Delta is unique in that it has a very low

taper that results from a very weak basal detachment.

By using measured properties of the wedge material

and making reasonable assumptions about the prop-

erties of the basal detachment strength, we calculate

that the basal detachment in the Akata Formation is

strongly overpressured, with a Hubbert-Rubey fluid-

pressure ratio lb � 0.90. That is, 90% of the weight of

the outer Niger Delta is supported by pore fluids in

the Akata Formation. This result explains the wide-

spread occurrence of detachment folds, shear fault-

bend folds, backthrusts, and the large, relatively un-

deformed regions that separate fold belts in parts of

the Niger Delta. The elevated fluid pressure that we

model is likely caused by the combined effects of dis-

equilibrium compaction, tectonic stresses, and per-

haps, increased fluid volume caused by hydrocarbon

maturation.

REFERENCES CITED

Barr, T. D., and F. A. Dahlen, 1989, Brittle frictional mountainbuilding: 2. Thermal structure and heat budget: Journal ofGeophysical Research, v. 94, p. 3923–3947.

Behrmann, J. H., K. Brown, J. C. Moore, A. Mascle, E. Taylor,F. Alvarez, P. Andreiff, R. Barnes, and C. Beck, 1988, Evolutionof structures and fabrics in the Barbados accretionary prism.Insights from Leg 110 of the Ocean Drilling Program: Journalof Structural Geology, v. 10, no. 6, p. 577–591.

Bilotti, F., and J. H. Shaw, 2001, Modeling the compressive toe of


the Niger Delta as a critical taper wedge (abs.): AAPG AnnualMeeting Program, v. 10, p. A18–19.

Bilotti, F., J. H. Shaw, R. M. Cupich, and R. M. Lakings, 2005,Detachment fold, Niger Delta, in J. H. Shaw, C. Connors, andJ. Suppe, eds., Seismic interpretation of contractional fault-related folds: AAPG Studies in Geology 53, p. 103–104.

Braathen, A., S. G. Bergh, and H. D. Maher, 1999, Application of acritical wedge taper model to the Tertiary transpressional fold-thrust belt on Spitsbergen, Svalbard: Geological Society ofAmerica Bulletin, v. 111, no. 10, p. 1468–1485.

Breen, N. A., 1987, Three investigations of accretionary wedgedeformation: Ph.D. Thesis, University of California–SantaCruz, Santa Cruz, California, 132 p.

Byerlee, J., 1978, Friction of rocks: Pure and Applied Geophysics,v. 116, p. 615–626.

Carena, S., J. Suppe, and H. Kao, 2002, Active detachment ofTaiwan illuminated by small earthquakes and its control offirst-order topography: Geology, v. 30, no. 10, p. 935–938.

Chapple, W. M., 1978, Mechanics of thin-skinned fold-and-thrustbelts: Geological Society of America Bulletin, v. 89, p. 1189–1198.

Connors, C. D., D. B. Denson, G. Kristiansen, and D. M. Angstadt,1998, Compressive anticlines of the mid-outer slope, centralNiger Delta (abs.): AAPG Bulletin, v. 82, no. 10, p. 1903.

Corredor, F., J. H. Shaw, and F. Bilotti, 2005, Structural styles inthe deepwater fold-and-thrust belts of the Niger Delta: AAPGBulletin, v. 89, no. 6, p. 753–780.

Dahlen, F. A., 1990, Critical taper model of fold-and-thrust beltsand accretionary wedges: Annual Review of Earth and Plane-tary Sciences, v. 18, p. 55–99.

Dahlen, F. A., and T. D. Barr, 1989, Brittle frictional mountainbuilding: 1. Deformation and mechanical energy budget: Jour-nal of Geophysical Research, v. 94, no. B4, p. 3906–3922.

Dahlen, F. A., J. Suppe, and D. Davis, 1984, Mechanics of fold-and-thrust belts and accretionary wedges: Cohesive Coulomb theory:Journal of Geophysical Research, v. 89, no. B12, p. 10,087–10,101.

Damuth, J. E., 1994, Neogene gravity tectonics and depositionalprocesses on the deep Niger Delta continental margin: Marineand Petroleum Geology, v. 11, no. 3, p. 320–346.

Davis, D., J. Suppe, and F. A. Dahlen, 1983, Mechanics of fold-and-thrust belts and accretionary wedges: Journal of GeophysicalResearch, v. 88, no. B2, p. 1153–1172.

DeCelles, P. G., and G. Mitra, 1995, History of the Sevier orogenicwedge in terms of critical taper models, northeast Utah andsouthwest Wyoming: Geological Society of America Bulletin,v. 107, no. 4, p. 454–462.

Frost, B. R., 1996, Structure and facies development in the NigerDelta resulting from hydrocarbon maturation (abs.): AAPGBulletin, v. 80, no. 8, p. 1291.

Hoshino, K., K. Inami, S. Iwamura, H. Koide, and S. Mitsui, 1972,Mechanical properties of Japanese Tertiary sedimentary rocksunder high confining pressures: Japanese Geological SurveyReport, no. 244, 200 p.

Hubbert, M. K., and W. M. Rubey, 1959, Role of fluid pressure inmechanics of overthrust faulting: Geological Society of AmericaBulletin, v. 70, p. 115–166.

Jaeger, J. C., and N. G. W. Cook, 1979, Fundamentals of rockmechanics, 3d ed.: London, Chapman and Hall, 593 p.

Mann, D. M., and A. S. MacKenzie, 1990, Prediction of pore-fluidpressures in sedimentary basins: Marine and Petroleum Ge-ology, v. 7, p. 55–65.

Meissner, F. F., 1978, Petroleum geology of the Bakken Formation,Williston basin, North Dakota and Montana, in 24th AnnualConference, Williston Basin Symposium: Montana GeologicalSociety, p. 207–227.

Morley, C. K., and G. Guerin, 1996, Comparison of gravity-drivendeformation styles and behavior associated with mobile shalesand salt: Tectonics, v. 15, no. 6, p. 1154–1170.

Osborne, M. J., and R. E. Swarbrick, 1997, Mechanisms forgenerating overpressure in sedimentary basins: A reevaluation:AAPG Bulletin, v. 81, no. 6, p. 1023–1041.

Peel, F. J., C. J. Travis, and J. R. Hossack, 1995, Genetic structuralprovinces and salt tectonics of the Cenozoic offshore U.S. Gulfof Mexico: A preliminary analysis, in M. P. A. Jackson, D. G.Roberts, and S. Snelson, eds., Salt tectonics: A global perspec-tive: AAPG Memoir 65, p. 153–175.

Plesch, A., and O. Oncken, 1999, Orogenic wedge growth duringcollision—Constraints on mechanics of a fossil wedge from itskinematic record (Rhenohercynian FTB, central Europe):Tectonophysics, v. 309, p. 117–139.

Rowan, M. G., F. J. Peel, and B. J. Vendeville, 2004, Gravity-drivenfold belts on passive margins, in K. R. McClay, ed., Thrust tec-tonics and hydrocarbon systems: AAPG Memoir 82, p. 157–182.

Shaw, J. H., F. Bilotti, and P. A. Brennan, 1999, Patterns ofimbricate thrusting: Geological Society of America Bulletin,v. 111, no. 8, p. 1140–1154.

Shaw, J. H., E. Novoa, and C. Connors, 2004, Structural controls ongrowth stratigraphy in contractional fault-related folds, in K. R.McClay, ed., Thrust tectonics and hydrocarbon systems: AAPGMemoir 82, p. 400–412.

Stockmal, G. S., 1983, Modeling of large-scale accretionary wedgeformation: Journal of Geophysical Research, v. 88, p. 8271–8287.

Suppe, J., 1983, Geometry and kinematics of fault-bend folding:American Journal of Science, v. 283, p. 684–721.

Suppe, J., C. D. Connors, and Y. Zhang, 2004, Shear fault-bendfolding, in K. R. McClay, ed., Thrust tectonics and hydrocar-bon systems: AAPG Memoir 82, p. 303–323.

Swarbrick, R. E., M. J. Osborne, and G. S. Yardley, 2002,Comparison of overpressure magnitude resulting from themain generating mechanisms, in A. R. Huffman and G. L.Bowers, eds., Pressure regimes in sedimentary basins and theirprediction: AAPG Memoir 76, p. 1–12.

Wu, S., and A. W. Bally, 2000, Slope tectonics—Comparisons andcontrasts of structural styles of salt and shale tectonics of thenorthern Gulf of Mexico with shale tectonics of offshoreNigeria in Gulf of Guinea, in W. Mohriak and M. Talwani,eds., Atlantic rifts and continental margins: American Geo-physical Union, p. 151–172.

Zhao, W. L., D. M. Davis, F. A. Dahlen, and J. Suppe, 1986, Originof convex accretionary wedges: Evidence from Barbados:Journal of Geophysical Research, v. 91, p. 10,246–10,258.
