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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
re2
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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).
Bilotti and Shaw 1479
(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.
1480 Niger Delta Critical Taper Wedge
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.
Bilotti and Shaw 1481
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).
1482 Niger Delta Critical Taper Wedge
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).
Bilotti and Shaw 1483
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
1484 Niger Delta Critical Taper Wedge
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.
1486 Niger Delta Critical Taper Wedge
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.
Bilotti and Shaw 1487
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.
14
88
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.
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